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Physicochemical and Functional Properties of Tropical Tuber Starches: A Review

The tropical tuber crops contain starch as the major component and thus act as important source of starch. Except cassava and to a smaller extent sweet potato, starch from other tuber crops has not been exploited for industrial applications partly because of difficulty in the extraction of the pure starches and partly because of non-availability of information about the properties of these lesser known starches. This review attempts at collating data available on the physicochemical and functional characteristics of the tropical tuber starches, highlighting their unique properties and potential field of applications. The physicochemical properties like granule shape and size X-Ray diffraction (XRD) patterns, amylose content, or content of non-starches components, show considerable variations among the tuber starches. In addition, factors like genetic origin environmental conditions and age of the plant also influence the properties. The starch granules of Colocasia esculenta and Dioscorea esculenta tubers are very small whereas dose of Canna edulis are very large, XRD patterns of yam starches are generally 'B', while the aroid starches posses 'A' patterns. DSA gelatinization temperatures are low for cassava starch and high for the aroid starches. The functional characteristics like viscosity, swelling power and solubility also depend on a number of factors such as varietal variation, method of extraction, processing conditions and instruments used for analysis. Viscosity is high for cassava and C.edulis starches, but low for most aroid starches. Clarity is good for cassava and yam starches Compared to others. Digestibility also varies among the starches. The diversity available in the tuber starches shows that some of the starches can be used in place of chemically modified starches available on the market. The realization of their important can help in value addition of these neglected crops and also provide starch with special properties for specific applications. Keywords: Tropical tubers; Starches; Physicochemical properties; Rheological properties.

  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

1. Introduction

Tropical root and tuber crops are important food crops serving either as subsidiary or subsistence food in different parts of the tropical belt. They are rich sources of starch [1-4] besides many vitamins, minerals, etc. although there has been some decline in the use of these roots and tubers as food, their industrial application, especially that of cassava, is making rapid advantages. Cereals are the major source of commercial starches in most developed countries. Nevertheless, cassava (Manihot esculenta Crantz) and to a small extent, sweet potato (Ipomoea batatas Lam) are used for starch extraction in countries such as India, Brazil, Thailand, Indonesia, Philippines and China. Physical and chemical modifications of cereal starches are sometimes necessary to overcome certain undesirable characteristics and make them suitable for specific end uses. Though starch tailor-made for different applications can be made by various treatments, there has been a resistance towards use of modified starches in food applications since many chemicals are used for modification. Studies on different starches at Central Tuber Crops Research Institute (CTCRI) and elsewhere have brought to light the wide diversity in the starch characteristics of tuber crops and the possibility of using these native starches instead of chemically modified starches [5, 6]. In this treatise, the isolation and properties of tuber crop starches viz.., cassava (Manihot esculenta Crantz), sweet potato (Ipomoea batatas Lam), aroids (taro,, Colocasia esculenta; tannia, Xanthosoma sagittifolium and elephant foot yam, Amorphophallus paeoniifolius); yams (Dioscorea sp.) and minor tuber crops like arrowroot (Maranta arundinacea), yam bean (Pachyrrhizus erosus), Canna (Canna edulis), coleus or Chinese potato (solenostemon rotundifolius), Curcuma sp., etc. have been discussed, highlighting their advantages and potential uses.

  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

2. Extraction Techniques

Whereas extraction of starch from cassava is simple and the isolated starch is pure white in colour and relatively free from other chemical impurities this is not so with other tuber starches. The settling of starch granules is often hindered by presence of various components like mucilage and latex, leading not only to loss of starch, but also lowering of the quality of extracted starch. The long residence time can also promote microbial growth bringing about breakdown of starch and resultant loss of starch quality. Besides, the presence of these microorganisms affects the colour of the starch limiting its use in food and textile applications. Maximum recovery of starch with good physicochemical and functional qualities coupled with economical extraction of starches from tuber crops other than cassava is thus important. Work carried out at the Central Tuber Crops Research Institute, Trivandrum, India on use of various chemicals in improving the yield of starch from various tubers [7-9] showed that ammoniacal solutions gave the best results (Tab. 1). Aqueous ammonia (0.03 M), when used for starch extraction, not only improved the yield, but also the functional characteristics such as paste viscosity and swelling. Ammonia acts by complexing with the mucilaginous material releasing the starch granules and enabling faster settling of starch in less viscous slurry. Table 1 : Yield and Blue Values for starches extracted with ammonia solution and water

StarchExtraction mediumYield [%]Total amylose [Blue Value]
CassavaWater21.8±0.540.37±0.01
CassavaNH322.2±0.370.37±0.02
ColocasiaWater6.2±1.790.28±0.01
ColocasiaNH316.2±0.370.26±0.02
Dioscorea alataWater17.0±1.430.45±0.01
Dioscorea alataNH318.3±1.00.44±0.01
Dioscorea esculentaWater17.7±1.060.29±0.00
Dioscorea esculentaNH318.7±1.140.28±0.01
Dioscorea rotundataWater18.8±0.850.40±0.01
Dioscorea rotundataNH319.5±1.160.40±0.01
Sweet potatoWater13.0±1.020.34±0.01
Sweet potatoNH310.9±1.100.35±0.01
XanthosomaWater20.0±0.320.38±0.01
XanthosomaNH320.5±1.760.36±0.02

The short residence time also prevents microbiological damage of the starch. Lactic and citric acids improve the yield and colour of starch from sweet potato tubers [10]. Kallabinksi and Balagopalan [11] developed an enzymatic method fro enhancing the recovery (26% increase) of starch from cassava tubers using pectinase and cellulose enzymes. These enzymes alter the integrity of the pectin-cellulostic matrix of cell membranes and thereby facilitate the release of the starch granules. It was found that using the same technique, starch recovery from sweet potatoes could be enhanced by 20% without affecting starch properties [10, 12]. Padmanabhan and Lonsane [13] studied the effect of conventional and conventional-and-enzyme-integrated extraction methods on cassava starch properties. Mathew et al. [14, 15] used a mixed culture inoculum to enhance the yield of starch from cassava tubers. Although the recovery of starch increased, the starch was invariably contaminated with fibrous material. The starchy flour has functional properties slightly different from the native starch, but possessed better attributes like puffing ability which makes it suitable for specific food applications [16, 17].

  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

3. Other components in Starch

The extracted starch is invariably accompanied by various other components viz., fibre, lipids, proteins and minerals, depending on a number of factors such as method of extraction, age of the crop, environmental conditions etc. Some of these impart desirable qualities to the starch, while others affect the quality.

  1. Moisture Content

    The moisture content of dry starch varies from 6-16% depending on the process used for drying the starch. Higher levels of moisture can lead to microbial damage and subsequent deterioration in quality. The maximum moisture content prescribed for safe storage by most of the starch producing countries is 13% [18-20]. Considerable variation in moisture content among tuber starches has been reported by several workers as evident from the values in Tab. 2[21-30, 32]. Climatic factors also play a parting deciding the moisture content. For D.dumeforum starch, Nikala et al. [32] obtained 12% moisture for starch extracted during the wet season, but 13.5% for that extracted during the dry season. The result has been explained on the basis of higher granularity due to lower granule size during the dry season illustrating the effect of environment on the moisture content. Table 2 : Biochechemical contents of different tuber starches
    StarchMoisture [%]Fibre/ash [%]Lipid [%]Phosphorous [%]
    Cassava14.8[21], 10-13[22]0.02-0.49[33], 0.33[34], 0.22[35], 0.32[21], 0.1-0.8[22], 0.01-0.029[36]0.1-1.54[41,33,34,43], 0.96[35], 0.1-0.4[22], 0.07-0.73[36]0.007-0.012[33], 0.0075[35]
    Sweet potato11-17[23,24-26] 9.8-15.3[26]0.05-1.28[23-25], 0.7-1.3[26]0.006-0.26[23,37,46]0.009-0.022[23,24,37,46]
    C.esculenta16.6-17.4[21]0.81-0.92[21]-0.006-0.013[64]
    X.sagittifolium15.5-16.5[21], 12.0[32]0.19[38], 0.19-0.22[21]0.39[38]-
    P. erosus10.9, [27]0.06[27]0.33[27]-
    Arror root10.10[22]Tr[22]--
    A.paeoniifolus10.55[28]0.58[28]0.088[35], 0.1[28]0.045[35]
    Canna edulis11.0[29]0.061[29]0.30[29]0.01[29], 0.05-0.08[66]
    D.alata13.6, 18.2[21]0.22[22], 0.26[21]--
    D.esculenta16.8[21]0.46[21]--
    D.rotundata16.7-18.6[21]0.19-0.46[21]-0.011-0.015[65]
    D.dumetorum12-13.5[31], 16.5[21] 13,5[22]0.16-0.3[31], 0.30[21]37.3-39.6[31]0.003[31]
    D.ballophylla---0.005[35]
    D.abyssinica-0.1[39]1.0[39]-
    Coleus12.2[30], 15.1[21]0.183[30], 0.4[42]--
    Curcuma sp.---0.045[67]

  2. Fibre Content

    The fibre content in starch varies to a great extent depending on the sieve used for removal of the fibrous material, varietal variation and age of the crop, especially for cassava and sweet potato, where the fibre content increases with the maturity. Wide variation in fibre and ash contents in different tuber crops is evident from various reports (Tab 2.). Cassava flour (containing 2-3% fibre) had different properties comparer to the isolated starch (having 0.1-0.15% fibre) and neither defeating nor ethanol extraction brought about any major change in the properties of the starch properties [40-41]. The total dietary fibre in cassava flour was reported to vary from 4.7 to 5.5%. In D.dumentorum starch, the ash content almost doubled during dry season. [31].

  3. Lipid Content

    Lipids from another important component that has a strong effect on the starch properties [43-45, 47-51]. The formation of the starch-lipid or starch-surfactant complexes improves the textural properties of various foods [44, 52-54]. The starch-lipid interaction is particularly important in cereal starches, which harbour lipids to noticeable extent. The tuber starches contain much lower quantities of lipids so that the effect is not so pronounced. The lipid content in live cultivars of cassava varied from 0.11 to 0.22% in starch and 0.27-0.45% in flour [41]. Widely varying lipid contents have been reported for the different tuber starches (Tab 2). Lipid content in starch was also influenced by various pretreatments of the tubers of yams and aroids [8]. Viscosity stability of cassava starch could be enhanced by treatment with surfactants [55]. Since the root starches contain much smaller quantities of native lipids in them, the addition of lipids or surfactants would be desirable to obtain quality improvement and it was found that there is no hindrance for the tuber starches to complex with surfactants or lipids [56, 57]. The specific complexing ability of the amylose with surfactants has been utilized for the determination of amylose content in starches [58-60]. Recently modulated DSC has been used in determination of amylose content in eleven tuber starches [61].

  4. Phosphorous Content

    Another important component invariably present in starch is phosphorous, which is associated in the synthesis of starch in the chloroplasts. Wide variation occurs in the phosphorous contents of different starches (Tab 2). No noticeable variation in P content of cassava starch was observed with age of the crop for six cultivars over a growth period of 2-18 months [62]. The phosphorous content in sweet potato starch is nearly similar to cassava starch [23, 24, 37, 46], but much less than that of potato starch. Takeda et al. [25] also found that sweet potato amylose contains less P (3-6 μg/g) than the amylopectin (117-144 µg/g). Noda et al. [63] did not observe any effect of fertilization on the P content in two sweet potato varieties. The P content in starch of different Colocasia cultivars varied from 0.006 to 0.013% [64]. Studies on the P content in six accessions of D. rotundata showed only very minor variability (0.011-0.015%) [65]. For D.dumetorum starch. Nkala et al. [31] observed that almost all of the phosphorous exists as bonded to starch and in the range 29-32 mg/100 g. Starch from three cultivars of Canna edulis from CTCRI contained P in the range of 0.05-0.08% which is even higher than that found in potato starch [66]. Curcuma starch also contained a high percentage of P (0.045 %) [67]. The high phosphorous content impart high viscosity to starch and also improves the gel strength. High P starches can find use in food applications requiring high gel strength, such as jellies etc. Cookies made using Canna starch are very popular in some Latin American countries.
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

4. Colour and Appearance

Colour is an important criterion for starch quality, especially for use in sago and textile industries. The starch paste should be clear and free from any off-colour for better acceptability [20]. Starch from cassava tubers has a good white colour, if the skin and rind are removed prior to crushing [1, 68]. Chemicals like acids, sculpture dioxide, bleaching agents, etc. are occasionally used to improve the colour of starch, but these chemicals negatively affect the starch quality [69]. Organic acids also improve the colour at lower concentrations, but affect starch settling at higher concentrations. Use of ammonia could considerably improve the colour of the starch from aroids, especially Colocasia [8, 9]. Mechanisation of the starch extraction process, replacing the conventional method of drying in the sun and using centrifugal separation instead of settling tanks can improve colour and appearance of starch [68].

  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

5. Granule Shape and Size

Wide variability in the size and shape of starch granules has been observed (Tab 3). These characteristics have been determined by simple optical microscopes, counters and electron microscopy.

  1. Cassava Starch

    Cassava starch granules are mostly round [5, 33, 45, 70-72] with a flat surface on one side containing a conical pit, which extends to a well-defined eccentric hilum. Some granules appear to be compound [73, 74]. Under polarized light, a well defined cross is observed. Though earlier studies on selected cassava varieties using microscopy did not reveal any noticeable variation in size among them (Tab 4) [81]. Examination of starch from five varieties using the Coulter Counter revealed considerable difference in the distribution pattern of the granule sizes (Fig 1). Thus starch from the H.1687 showed a higher frequency in the size range of 13-16 µm and relatively less in the range of 6-13μm compared to H165, H 97 and S 856 starches. Variety M4 also had a slightly different distribution pattern, but not as prominent as H-1687 starch [41]. Starch granule size was found too increase up to the sixth month with age of the crop for six varieties of cassava, and then remained steady [62]. Defloor et al. [75] concluded that the granule size was lower during the dry season and attributed it to the higher percentage of small granules during this period. Table 3 : Ganular properties of different tuber starches
    StarchGranule shapeGranule size [µm]Reference
    CassavaRound, truncated, cylindrical, oval, spherical, compound4-43, 3-32[5,23,37,41,45,70-75], [77]
    Sweet potatoRound, polygonal, oval, bell, round, polygonal2-72, 11.1-19.4, 43[21,25,27,46,76-78] [79], [25]
    C.esculentaround1-10, 1-1.5, 0.3-3.3[5,64,85-87,89], [23], [22]
    X.sagittifoliumround10-50, 17-20, 2.8-22[5], [23], [22]
    Pachyrrhizus. erousRound, cupoliform or convex-biconcave polyhedral6-35, 8-35[5,31], [23]
    Arror rootRound, polygonal5-50[5]
    A.paeoniifolusRound, polygonal3-30[5,91]
    Canna edulisOval, polyhedral 5-43.5(mean)13-57, 15-20, 60-14535[41], [66], [23,89], [66]
    D.alataOval, shell shaped, ellipsoid16-100, 20-140, 5-50, 17-26, 10-50[5], [89], [23], [22], [80]
    D.esculentaRound, oval, polyhedral, compound2-15, 1-5, 1-5[5], [89], [22]
    D.rotundataOval, polyhedral oval, round triangular10-70, 10-50, 15-70[5], [80], [22]
    D.dumetorumRound, polygonal1-3, 1-16, 3-5, 1-3[89], [30], [80], [22]
    D.cayensisPolyhedral, ellipsoid10-70, 10-50[89], [80]
    Coleusround, oval5-20, 2.5-15[36], [33]
    Curcuma sp.Elliptical14-46, 1.6-4.2[67], [23]
    Table 4 : Effect of varietal difference on the physicochemical properties of cassava starch
    VarietiesGranule Size [µm]Reducing values 1Amylose contentPasting Temp. [ 0C]Visc. 2% Paste3XRD patternAbs. Cryst. [%]Sw. Vol. [ml/g]SolubilityTonset [ 0C]DSC Data Tend [ 0C]ΔH [J/g]
    M-45.4-35.11.80.53060.7058.0A8.9130.522.868.2075.511.8
    Kalikalan5.4-40.51.80.55063.7058.0--38.824.8---
    H-16875.4-40.51.40.54055.6858.0A11.4725.523.667.273.412.6
    h-23045.4-43.21.40.52552.6855.0--30.524.8---
    H-2265.4-43.21.80.50055.6656.0--33.827.6---
    H-975.4-43.21.20.53558.7055.0A8.830.517.269.477.113.7
    H-1658.1-48.61.60.50552.6554.0A11.4737.827.263.474.713.1
    S-856-----A10.16--65.674.910.6
    Itchyapuram Local-------41.824.4---
    1[Ferricyanide No.]
    2[Blue value]
    3[s]

  2. Sweet Potato Starch

    Sweet potato starch is polygonal or almost round in shape [23, 37, 46, 76-78, 82, 83] and has a centric distinct hilum. Polarization crosses are less distinct compared to cassava starch. Granule size of sweet potato is almost equal to that of cassava. The granule size is reported to affect some functional properties such as swelling, solubility, and digestibility. Bowkamp [78] reported a negative correlation between particle size and susceptibility to amylase and acid degradation in sweet potato cultivars. Noda et al. [63] did not find any effect of fertilization on starch granule size, while in another study by the same group, it was found that the average granule size increased during early stage of development and then remained steady in two varieties [84].

  3. Other Starches

    Yam starches have a large variability in shape viz., round triangular, oval and elliptical. Other tuber starches also exhibit different granular shapes (Tab. 3). Colocasia granules are among the smallest of starches observed in the plant kingdom making them useful in various applications. E.g. as a filler in biodegradable plastics, in toilet formulations, aerosol, etc. [85, 86]. Strauss and Griffin [87] have examined a large number of taro cultivars and found maximum granules having size of 5.0µm and least in the size 1.79 µm with a mean value of 3.34 µm. Unlike other tuber crop starches, which do not exhibit any significant variability in size with varietal differences, Colocasia starch was found to exhibit varietal difference. Studies on ten varieties revealed a significant difference in average granule size (Fig. 2, Tab. 5) [64]. The average granule size and the distribution of the granules sizes showed only minor difference between the corms and cormels of four Cultivars of Colocasia [88]. Although variation existed among the cultivars, no significant variability was noticed within a cultivar during the growth period. Such varietal difference was not observed in Dioscorea alata, D.rotundata and D. esculenta starches showed an increase in granule size up to five months and there after remained steady. The small granule size of D.essculenta (2-15 µm) may be useful in applications similar to Colocasia and cereal starches. Starch granule size of D.alata and D. rotundata are much bigger. Farhat et al.[80] have examined the distribution in size of different yam species D. rotundata and D. cayensis starches showed similar patterns with a single symmetrical distribution centred at 32 and 35 µm respectively. D. alata starch, however, exhibited a non-symmetrical wider particle distribution around 31 µm and D. dumetorum starch had uneven distribution. The authors also used polarized light microscopy to confirm the findings and corroborated the results of Rasper and Coursey [90] on granule sizes. Variation in granule size could not be observed among the three yam species viz. d.rotundata, D. alata and D.esculenta.

    Variation in Granule Size Distribution as Indicated by Relative Abundance of 250 Particles in Different Size Ranges in Starch of Five Cassava Varieties in a Coulter Counter

    Variation in Granule Size Distribution as Indicated by Relative Abundance of 250 Particles in Different Size Ranges in Starch of Five Cassava Varieties in a Coulter Counter
    Table 5 : Starch granule size, total and soluble amylose contents in starch from different varieties of Colocasia.
    VarietyGranule Size (µm)Total Amylose Content(%)Soluble Amylose Content(%)
    C-95.1919.48.8
    C-622.9614.56.6
    C-464.2715.19.3
    C-1493.0614.18.8
    C-1893.3014.04.4
    C-2163.5114.56.5
    C-2183.3915.78.3
    C-2203.5517.610.7
    C-2263.1615.79.8
    C-3043.2016.17.5
    Table 6 : Yield of Starch, granule size, and amylose contents of starch from different varieties of Amorphophallus
    VarietyYield (%)Average granule Size (µm)Total Amylose (%)
    Am 27.013.0323.2
    Am 510.012.4923.5
    Am 148.110.3222.9
    Am 1512.311.1223.3
    Am 2711.19.6223.3
    Am 329.910.6223.2
    Am 3412.211.6922.9
    Am 3610.510.3523.9
    Am 4310.510.1921.9
    Am 5114.39.8523.2
    There was only minor variation among ten accessions of Amorphophallus studied (Tab 6) [91] . The largest granulesize was observed for canna edulis starch (>35 µm) [66]. The granule size was in the range of 16-58 µm for Curcuma zedoaria and 14-46 µm for C.malabarica starches [67]. These values obtained are much higher than those reported earlier viz. 1.6-4.2 µm [22]

    Variation in Granule Size Distribution as indicated by Relative Abundance of 250 Particles in Different Size Ranges in Starch of Four Varieties of Colocasia in a Coulter Counter

    Variation in Granule Size Distribution as indicated by Relative Abundance of 250 Particles in Different Size Ranges in Starch of Four Varieties of Colocasia in a Coulter Counter
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

6. Spectral Features

The infrared spectra of starch of different of cassava similar with peaks at 3600-3200 (broad), 2800 (medium), 1660 (weak), 1480-1250 (medium) and a number of peaks between 1150 and 710 cm-1. FTIR of the differences, in spite of their different Crystallinity and granule sizes. The Raman spectra of the tuber starches differed distinctly in their peak pattern in the region 800-200 cm-1 (Fig. 3) [92]. The CP-MAS 13C NMR of the different starches showed typical pattern for the starches and three main peaks were observed. The first peak was at 101-102 ppm corresponding to C1 and appeared as a single or doublet depending on the source of the starch. The next peak appeared at 75-80 ppm corresponding to C2,3 . 5 and was a singlet. The final peak was at 64 and a singlet. The peaks were observed only if the starch granules contained 8-10% moisture. A clear correlation existed between the XRD pattern and the NMR peak pattern. Whereas the starches having pattern 'A' showed a doublet for C1 peak in NMR, those with 'B' pattern had clear singlet C1 peak (Tab 7. Fig 4). Thus the structural difference between the two types of starch is evident. Table 7 : XRD Patterns and chemical shifts for C1 peaks of different tuber starches.

Starch sourceNature of C1 NMR peakXRD pattern
CassavaDoubletA
ColocassiaDoubletA
AmorphophallusDoubletA
PacchyrhizusDoubletA
XanthosomaDoubletA
Arrow rootDoubletA
D.alataSingletB
D.esculentaSingletB
D.rotundataSingletB
Canna edulisSingletB
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

7. X-Ray Diffraction Pattern

Starch has a definite nature and the Crystallinity has been assigned to the well-ordered structure of the amylopectin molecules inside the granules. Different starches possess either 'A' , 'B' or 'C' pattern (which has been suggested to be a mixture of 'A' and 'B' patterns).

  1. Cassava and Sweet Potato Starches

    Cassava starch possesses 'A', 'C' or a mixed pattern with three major peaks at 2? = 15.3, 17.1 and 23.5o [33, 75, 89, 93-97]. The absolute Crystallinity values of starch from five varieties of cassava were found to be in the range 8-14% (Tab. 4) [41]. The flours of these five varieties also possessed similar XRD pattern and absolute Crystallinities. Earlier reports gave much higher values for the Crystallinity of cassava starch, viz. 38% [94] and the difference is attributed to the method used for calculation of the Crystallinity. Asaoka et al. [36] examined the diffraction pattern and absolute Crystallinity of some Colombian cassava varieties and found the values to be nearly the same during different seasons 915.3-17.3%0 and all possessing a 'CA' pattern. Sriburi et al [98] reported that extrusion of the starch led to destruction of Crystallinity.

    Figure 3

    Raman Spectra of Different Tuber Starches in Different Regions

    Figure 4

    XRD Patterns of D Rotundata Starch Extracted From tubers Harvested at Different Periods

    Figure 5

    Intensity
    Sweet potato starch also was reported to possess 'A' pattern [25, 83, 89], 'C' [76, 94, 99, 100] or intermediate between 'A' and 'C' [23, 63, 101, 102]. Takeda et al. [25] observed 'A' pattern for two varieties while it was 'CA' for another variety. The absolute Crystallinity for this starch was 38% [94].

  2. Other Starches

    Colocasia, Xanthosoma, Pachyrrhizus, Arrowroot and Amorphophallus starches also possesses 'A' pattern [5, 86, 89] while edible Dioscorea starches (viz. D.alata, D.esculenta and d.rotundata), D. abysincia and D.cayensis starch possessed 'B' patterns. However 'A' pattern has been recorded for D.dumetorum starch [80], while McPherson and Jane [103] reported 'C' pattern of extracted starch is the same throughout the growth period of D. rotundata (Fig 5.). Starch of Canna edulis and Curcuma sp. exhibited 'B' pattern. The absolute Crystallinity of Canna starch was 26% [94]. A detatiled study of the XRD parameters of the starch extracted from Amorphophallus and Xanthosoma tuber subjected to pretreatment using different chemicals has been carried out. The 'd' spacing, angle intensity and peak intensity were found to be similar for control (water) and chemically pretreated samples of Amorphophallus samples. However, the peaks were shifted, indicating partial change in the crystalline phase [8, 9, 104]. For Xanthosoma starch, higher concentration brought about more significant changes especially with potassium metabsulphite pretreatment. It has been reported that the heat-moisture treatment of potato starch brings about change in the XRD pattern of potato starch from 'B' to 'A' [105], but such a change was not noticed with D. rotundata starch [106]. Lorenz and Kulp [107] also observed change in XRD patterns of cassava and arrow root starches by heat-moisture treatments.
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

8. Molecular Weight

  1. Cassava Starch

    The molecular weight of cassava starch from different varieties determined by the ferricyanide, the alkali number or the periodate oxidation method did not give consistent results [ 5, 108] and it was concluded that only minor differences existed among the different varieties (Tab. 4). The molecular weight of amylose from cassava starch was reported to be 232,000 [71], 431,000 [109], 522,000 and 1,250,000 [110]. For amylopectin, the molecular weight was given as 450 x 106 [111]. The average chain length has been reported as 21 glucose units [71,110], and a weight average of 26[101]. Takeda et al. [109] obtained the following values: mean DP 2600, mean chain length 340 with 7.4 chains per molecule. Takeda et al. [112] showed that cassava amylose was composed of nearly equal numbers of branched and unbranched molecules. Suzuki et al. [110] also reported a value of 7710 for weight average degree of polymerization and 3220 for number average degree of polymerization. Shannon and Garwood [74] obtained the molecular parameters of cassava amylose and amylopectin as: mean chain length - amylose 100 to 10,000 amylopectin 20 to 30; DP (glucose residues) amylose-100 to 10,000, amylopectin - 10,000 to 100,000.

  2. Sweet Potato Starch

    Takeda et al. [25] found a trimodal distribution pattern for the sweet potato amylopectin while Hizukuri [101] reported a bimodal distribution. They concluded that sweet potato has a higher proportion of 'A' chains and short 'B' chains compared to potato starch. Seog et al [77] reported alkali number values between 7.66 and 12.13 for six Korean sweet potato varieties compared to 5.33 for cassava starch [93]. Noda et al. [63] used HPAEC-PAD on sweet potato starch and found the amylopectin to have peaks at DP =12 and DP=8. The concentrations of the peaks at DP=6 and DP=7 were 7.1-7.5% and 6.7-7.0% respectively. Variation in chain length has been found in some varieties based on the low viscosity and high reducing values [37].

  3. Other Starches

    Colocasia esculenta and Amorphophallus paeonlifolius starch from different varieties had almost the same range of reducing values, showing that the tuber starches have nearly equal molecular weights. D.rotundata starch exhibited only mior changes in the molecular weight over the growth period [65]. Studies on the yam and sroid starches extracted from tubers subjected to treatment with different chemicals also indicated only very minor differences in the reducing values among the different treatments [8, 9 ,104]. For Coleous starch,, the reducing value was 1.71 [42], while it was between 1.7 to 2.1 for Curcuma starch [67] indicating the same range as for other tuber starches.
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

9. Amylose Content

The linear component of starch viz., amylose imparts definite characteristics to starch. Amylose content varies considerably among different starches and genetic modifications have been carried out to obtain starch of amylose contents varying from 0 to >75%. Pan of amylose can exist as soluble amylose in the amorphous regions of the starch granules.

  1. Total Amylose

    1. Cassava Starch

      The total amylose content in cassava starch has been reported to range from 13.6-23.8% (Tab 8) [33].The Blue Values corresponding to total amylose varied from 0.50 to 0.55 for seven cassava varieties, indicating only very minor variation among the varieties [81]. Only insignificant differences in the amylose content were noticed during the growth period of six cassava varieties [62]. The gel permeation chromatographic analysis of five varieties of cassava carried out after debranching with isoamylose also did not show any noticeable difference among the varieties supporting the earlier results (Fig. 6)[41] . in a comparative study of five varieties from Nigeria Olorunda et al [113]have reported that higher amylose could be correlated with tuber meanliness. Kawabata et al [95] did not observe any noticeable effect of variety on the amylose content. Defloor et al. [75] reported that the amylose content of five cassava varieties harvested at different seasons was not related to the genotype. Asaoka et al. [36] in their study observed noticeable effect of season on amylose content. Examination of the effect of surfactants on the amylose content in cassava starch revealed that though the surfactants reduced the Blue Values [55]. Maximum reduction was obtained with cetyltrimethylammonium bromide, having bulky hydrophilic groups, might be blocking the entry of iodide into the amylose helix. Sriroth et al[114]reported that SO2 treatment didi not affect the amylose content of cassava starch and the DP of the resultant amylose changed only slightly from 1020 to 1050 by the treatment. Table 8 : Phsicochemical and functional properties of Cassava and sweet potato starches

      		CassavaSweet potato
      Amylose content [%]13.6-27[37]; 18.3-23.6[75]; 18[94]; 8-16[89]; 17.9[38]8.5-38[21]; 20.5-25.5[28]; 17.5-38[23,46,116,117]; 13.4-19[79]; 20[94]; 18[89]
      Pasting temp. [0C]58.5-70[70.93]; 55-64[97]; 49-73[146]; 58.5-68.5[86]; 60-70[32]; 68-90; 73-90[132]; 58-70[70]; 57-65[74]; 55-64[97]; 58.5-70[91]; 49-64.5; 62.73[146]; 58.5-68[96]65-80[76]; 65-90[27]; 63-70[94]; 67-75[79]; 58.5-73[28]
      Sw. vol. [mL/g]71[37]; 49[22]; 46-52[97]; 42[93]46[37]; 27.5-33.3(950C)[21,100]; 24.5-27.4(850C)[77]; 60-79(950C)[77]; 32-46(800C)[77]
      Solubility [%]48[37]; 35[22]; 25-28[97]; 31[93]18[37]; 13.2-14.4(950C)[21,100]; 11.4-12.9(850C)[77]; 60-79(950C)[77]; 30-50(800C)[77]
      Tonset[0C]68.50[133]; 64.0[134]; 65-69[35]; 50.7-57.7[129]; 64[120]; 64.10[80]; 65.6[131]; 53.9-62.1[75]; 68[127]; 62.4[132]61.3[135]; 58-64[28]; 67-75[100]; 65.6-68.21[15]; 67.3[137]
      Tmax[0C]71.2[133]; 69.2-73[41]; 54.7-61.3[129]; 78.0[120]; 69.0[80]; 70.8[131]; 78.0[127]; 69.3[132]65.68[133]; 70.2-77[135]; 63-74[26]; 73-79[100]; 72.8-74.3[115]; 72.7[137]
      Tend[0C]74.72[133]; 76.9[134]; 73.7-77.8[35]; 60.2-67.2[129]; 100[120]; 76.4[80]; 75.2[131]; 76.3-83.2[75]; 92[127]; 84.1[132]80.7-88.5[135]; 78-83[28]; 81.4-84.8[100]; 84.6-86.8[115]; 13.6[137]
      ΔH [J/g]12.4[133]; 16.6[134]; 10.8-13.6[35]; 6.8-8.8[129]; 16.0[120];15.6[131]; 9-15[75]; 22[127]; 4.8[132]14.8-18.6[28]; 10-12.3[100]; 15.1-16.3[115]; 13.6[137]

    2. Sweet Potato Starch

      For sweet potato starch also, considerable variation in amylose content has been reported (Tab. 8). Madamba et al [46] working on six varieties of sweet potato from Philippines found only very little variation in amylose content. Garcia and Walter [26] have obtained by potentiometric titration values ranging from 20-25% for some Peruvian cultivars and location did not have any effect. Our studies showed that sweet potato starch has an amylose content of 20-25% depending on the variety. Noda et al. [63] did not observe any effect of fertilization on amylose content. Similarly there was no change in the amylose content of sweet potato varieties during the growth period [84]. Ishiguro et al. [118] studied the retro gradation tendencies of starch isolated from ten sweet potato cultivars having different amylose contents and chain length distribution. Starches having fewer amylose molecules and amylopectin molecules with higher content of short chains (DP 10) retrograded slower compared to others. Curing was reported to have either no effect on amylose content [117] or resulted in a slight increase [119].

    3. Other Starches

      Colocasia esculenta starch showed a wide range in the amylose content and a noticeable relationship between the amylose content and granule size was observed. The varieties C-9 which had the largest granule size, also possessed the highest amylose content [64]. In a study on a number of cultivars of taro. Strauss and Griffin [87] found that the maximum value for amylose content was 43% and minimum 3% and a mean value of 24.04%. They could not observe any good correlation between amylose content and granule size. Amylose content in yam starches also varied considerably according to various reports. Farhat et al.[80] have obtained the following values for amylose content in starches of different Dioscorea species : D alata 25%, D. rotundata and D. cyanesis 23.8% and D. dumetorum 12.6%. A values of 29.7% was reported for the amylose content of D. abyssinica starch from Ethiopia [39], while Soni et al [35] obtained 24.1% for D. ballophylla starch. Only very little variation in the amylose content was observed with the age of the crop for D. esculenta, D. alata and D.rotundata starches. Xanthosoma starch also had a similar amylose content viz. 15-25% and very little varietal variation was observed. The amylose content of ten cultivars of Amorphophallus paenoiifolius was found to vary very little [91]. Starch from these three accessions of Canna edulis had an amylose content ranging from 24-30%, the highest being observed for purple accession [66]. However, Soni et al. [29] reported a still higher value of 38%. Coleus starch ahd an amylose content of 33% [42] while Abraham and Mathew [30] found the amylose content in Coleus starch to be 34% by colourimetry and 18 % by amperometry. Based on all reports, it can be inferred that among the various tuber crops. Canna edulis and Coleus starches have the highest amylose contents (Tabs 9,10 Tabs 9,10)

      Figure 6

      GPC Pattern of Isoamylase Debranched Starch from Five Cassava Varieties on a Fructogel Column

  2. Soluble Amylose

    Soluble amylose can play a significant role in determining the textural properties. This fraction is easily leached out and hence considered responsible for cohesiveness in cooked tubers [52]. The soluble amylose contents in the tuber crop starches determined using iodometry ranged from 10-40% of total amylose. Soluble amylose content in different varieties of cassava did not vary during the growth period. Leelavathi et al [34] found that the soluble amylose content in cassava starch to be 4.8% out of a total amylose content of 17.9%. A similar trend was observed for Colocasia, D. alata and Xanthosoma starches. For Amorphophallus starches from different accessions, the soluble amylose content ranged from 9-11% forming nearly 45% of the total amylose content [91]. The soluble amylose content in Coleus starch was 12.8% [42], while it ranged from 10-12% for Canna edulis starch [66].

  3. Effect of Surfactants on Soluble Amylose

    The studies on complexation of tuber starches with the surfactants indicate that the soluble amylose is suppressed to different levels with different starches and surfactants (Tab. 11). In case of cassava starch, the surfactants had variable effect on soluble amylose portion. The anionic surfactants, potassium palmitate and potassium stearate and the neutral surfactant glyceryl monosterate had little suppressive action, whereas the cationic surfactants cetyltrimethylammonium bromide and sodium lauryl sulphite had a significant suppressive effect. Increase in concentration of these surfactants led to reduced Blue Values for soluble amylose [55]. Though the cationic surfactant cetyltrimethylammonium bromide reduced the Blue Value for soluble amylose of all the starches (Tab. 11) its effect on Colocasia iand D. esculanta starches was more prominent indicating that the soluble amylose of these two starches may be more anionic in nature or that the amylose helix of these two starches is able to sterically favour the complex formation. Though these two starches have the lowest granule size, no correlation between complexing properties and granular size could be observed for the other starches.
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

10. Thermal Characteristics

Since the earliest study Stevens and Elton [120], many other workers also have used Differential Scanning Calorimetry (DSC) to investigate starch gelatinisation. Table 9 : Physicochemical and functional properties of Dioscorea starches


D.alataD.BallophyllaD.dumetarumD.abyssinicD.rotundataD.cayensisD.esculenta
Amylose content [%]21[22] 25.0[80] 30[89]-13.5[30] 15[22.23] 12.3[80]29.7[42]21.7-24.6[65] 21.1-23.6[80] 27[89]14[89];
Pasting temp. [0C]83[22] 69-88[23]75-80[32]83[30]72.9[42]79-97[65]--
Sw. vol. [mL/g]28-35[8]--10(650C)[42] 17(750C)[42] 23(850C)[42]15-25[65] 18-22[8]-24-27[48]
Solubility [%]13-19[8]--3.5(650C)[42] 5.5(750C)[42] 11.0(850C)[42]10-15[8]-6-01[8]
Tonset[0C]77.21[133] 73.74[134] 70.2[137] 76.5[80]-78.1[80]64.2[42]79.02[133] 72.17[134] 71.5[80]69.4[80] 65.7[134]75.92[133]
Tmax[0C]81.52[1.33] 74.4[137] 78.8[80]-81.3[80]68.2[39]83.12[133] 74.8[80]2.9[80]79.75[133]
Tend[0C]--86.4[80]74.8[42]87.95[133] 80.8[134] 80.5[80]76.7[80]85.68[133] 75.35[134]
ΔH [J/g]---19.2[42]10.28[133] 15.01[134]-13.64[133] 13.25[134]

Table 10 : Physicochemical and functional properties of minor tuber starches


C.esculentaCarna edulisA.paecniifoliusArrow rootA.xanthorrhizaX.SaggittifoliumColeusP.erosusCurcuma Sp.
Amylose content [%]14-19.4[64]; 9-17[89];28[94]; 38[41]; 27[89]; 24.2-27.6[66]21.9-23.9[91]; 24.5[41];16-27[5]1.8[61]; 21.4[61];16-24[5]33[36];17-25[5]25-28[67]
Pasting temp. [0C]81-85[64]65-70[41] 68-90[23] 73-95[132] 70-97[66]75-80[32]75-90[132] 79-92[132]75-90[132] 79-92[132]78-95[138] 85-95[138]---
Sw. vol. [mL/g]25-60[64]10.8-14.6[66] 19[29]21.5-24.4[91] 3.2(750C)[40] 22.3(800C)[40] 31.5(1000C)[40]23[5]23[5]20[5]25[36];25[5]19-30[67]
Solubility [%]--1.4(750C)[40] 17.0(800C)[40] 20.8(1000C)[40]-----11-23[67]
Tonset[0C]65.7[134] 75.92[133]65.35[134] 63[127] 61.6[132]77.8[134]68.5[132]68.5[132]83.11[133] 74.8[134] 66.0[137] 74.0[138]-63.6[134]79.7[67]; 74.3[67]
Tmax[0C]79.75[133]--68.5[132]68.5[132]85.72[133] 69.9[137] 78.0[138]--82.7[67]; 76.8[67]
Tend[0C]75.45[134] 85.68[133]70.85[134] 92[127] 74.5[132]83.53[134]85.0[132]85.0[132]90.4[133] 79.5[134] 81.8[137] 87.0[138]-76.6[134]97.1[67]; 82.1[67]
ΔH [J/g]13.25[134] 13.64[133]16.04[134] 28.8[127] 1.8[132]16.6[134]4.4[132]4.4[132]9.08[133] 15.22[134] 12.9[13.7] 3.98[138]-13.65[134]16.06[67]; 17.47[67]

Table 11 : Effect of cetyl trimethyl ammonium bromide on the Blue Values of different Tuber starches.

StarchTotal amylose [Blue Values]Soluble amylose [Blue Values]
Cassava0.320.18
Cassava + CTAB0.270.13
Colocassia0.280.18
Colocassia + CTAB0.200.07
D.esculenta0.290.14
D.esculenta + CTAB0.220.04
D.alata0.430.18
D.alata + CTAB0.380.11
D.rotundata0.380.18
D.rotundata + CTAB0.350.12
Sweet potato0.380.13
Sweet potato + CTAB0.340.09
Xanthosoma0.380.21
Xanthosoma + CTAB0.330.15

  1. DSC Gelatinisation Temperature

    1. Cassava Starch

      DSC studies on starch extracted from five varieties of cassava possessing different organoleptic quality [41] showed that varietal differences manifest themselves in the DSC patterns (Tab 4. Fig. 7.). The characteristics peak shape could be traced to structural differences among the varieties. The onset of gelatinisation as indicated by Tonset was earliest for H-165 starch (65.35 0C) and latest for H-97 starch (69.350C). However, Tend was highest for M4 showing the largest range for M4 starch. This is also evident from the broad DSC peak for this starch (Fig. 7). The other reported values for Tonset of cassava starch are presented in Tab. 8 and also show considerable variation. A similar variation in values is found for Tend also. Asaoka et al. [129] have examined the gelatinisation characteristics of four cultivars harvested at different seasons and their results indicated that both genetic constitution and environmental conditions affectedt he DSC parameters. Starch of one cultivar (CM 681-2) consistently displayed the highest gelatinisation temperature through all the harvesting seasons (Tab. 12). Defloor et al. [75] studied the DSC characteristics of five varieties harvested during dry and rainy seasons. They found that although within each planting season for each genotype and for each harvest time, significant differences in gelatinisation temperatures were noticed, no systematic changes as a function of genotype or harvest time was noticed. Starch samples harvested at six months stage in dry season had highest onset, peak and conclusion temperatures compared to that from rainy season, but the reverse was true for the 12, 15 and 18 month harvested tubers. Even the temperature of drying affected the DSC gelatinisation temperature of drying affected the DSC gelatinisation temperatures. Heat-moisture treatment enhanced the gelatinisation values of cassava starch considerably [107]. The gelatinisation parameters of cassava starch extracted using SO2 incorporated water are enhanced by 20C from 59.6 to 62.0 for Tonset and 84.7 to 87.20C for Tend [114].

    2. Sweet Potato Starch

      Collado et al. [135] have examined the DSC characteristics of 44 sweet potato genotypes from the Philippines and obtained considerable variation in all parameters. The mean Tonset was 64.60C and range 61.3-700C, mean Tpeak 73.90C (range 70.2-770C) and mean Tend­ 84.60C range being 80.7-88.50C, the mean gelatinisation range being 20.10C with a range of 16.1 to 230C. Garcia and walter [26] have examined two varieties cultivated at different locations and found the range to be between 58-640C for Tonset 63-740C for Tpeakand 78.830C for Tend . While the selection index did not affect the values, location influenced the parameters. Slightly lower values for these parameters have been reporte by other scientists [23, 115, 130, 136]. Noda et al. [63] found varietal difference but no effect of fertilization on the DSC characteristics of two sweet potato varieties. It was also found that during the growth period, the Tonset was the lowest at the latest stage of development. Valetusdie et al. [137] have compared the gelatinisation temperature of starch from fresh tubers and freeze dried sweet potato tuber gave nearly equal values (67-730C), but the small granules gelatinized between 75 and 880C. Similar results were obtained for Xanthosoma and D. alata starches also. They also studied the effect of the tuber extracts or sucrose solutions on the gelatinisation temperature and found that the extracts and sucrose at 12% concentration enhanced the gelatinisation temperature considerably.

    3. Other Starches

      The DSC data on other tuber starches is presented in Tabs. 9 and 10 and Fig. 8. Considerable variability in the values is observed. We found large difference between the values obtained in two different studies for the same starches [133,134] .Compared to cassava and sweet potato starches these starches generally had higher Tonset and Tend values. Highest values were noticed for Colocasia starch and the other starches had values in between. For colocasisa starch, we obtained Tonset values of 83.20C [133] and 79.90C [134]. The corresponding Tend values were 90.0 and 85.00C, respectively, these being the highest among the starches. For Amorphophallus starch, the values were 77.80C for Tonset and 83.50C for Tend. DSC data of the starches from the two Curcuma varieties indicated that the gelatinisation peak of C. malabarica starch was doublet and the peak splitting may be attributed to possible structural differences in the starch [67]. The onset of gelatinisation was earlier for C. malabarica starch whereas Tend was nearly similar for the two starches. The higher Tonset for c. zedoaria starch also indicates the possibility of Curcumin forming complexes with starch molecules. The range of gelatinisation was higher (17-220C) for Curcuma starch and similar to yam and potato starches. This can also be attributed to the presence of phosphate linkages in these starches. The Tonset values of both starches were similar to those of yam starches (75-800C) but higher than that of cassava starch (65-690C). The Tpeak values also showed a similar trend Tendvalues and consequently the range of gelatinisation were found to be higher for Curcuma starch. Heat-moisture treatment enhanced the gelatinisation temperature of arrowroot starch considerably [107]. Table 12 : Physicochemical and functional properties of cassava starch from tubers harvested in different periods
      CultivarHarvested monthTonset [0C]Tend [0C]ΔH [J/g]Swelling PowerSolubilityRVA[SNU]Brabender data [BU]
      600C700C800C600C700C800CPVbreak downSet backPT [0C]PVV0.5
      HMC-1March53.263.88.4------------
      August53.663.56.819.830.639.49.115.819.056740513259-69680340
      November50.760.57.622.632.043.09.617.019.955940313758-62820310
      CM 489-1March55.464.68.8------------
      August54.863.77.217.627.738.77.514.419.659043713760-70720360
      November50.959.87.620.627.734.89.515.919.362947914058-66725340
      CM 681-2March57.667.28.4------------
      August57.767.27.617.031.140.910.017.419.948734414262-72635320
      November54.163.98.019.027.735.511.317.819.655741612461.72660335
      CM 1559-5March55.064.68.4------------
      August54.664.16.820.230.340.99.315.318.456741214460-70750370
      November51.060.27.621.029.343.68.915.618.862447214558.95795335
      The differences in the gelatinisation temperatures among the various tuber starches can be traced to the variation in the starch inter molecular bonds. High temperature of gelatinisation can be an indication of the higher stability of the starch crystallites in the starch molecules, which means that more heating is required to swell the granules. In addition, a number of other factors like varietal differences, environmental conditions and the experimental protocols like level of moisture, sample preparation, rate of heating and instrument used contribute to the differences in values.

  2. Gelatinisation Range

    The range of gelatinisation is also quite different among the different starches. In our study, we obtained the highest range for cassava starch (12.90C) and the lowest for Xanthosoma starch (4.70C) [134]. Higher range has been attributed to a higher level of Crystallinity, which imparts higher structural stability so that the water molecules need longer time to penetrate the crystalline areas [139,140]. Billiaderis et al. [126]. Tester and Morrison [141] and Leszkowiat et al. [142] have suggested that higher transition temperatures indicate more stable amorphous regions and lower degree of chain branching. There does not appear to any relation between the XRD pattern and gelatinisation range, as both cassava and Xanthosoma have 'A' pattern but the range of gelatinisation is far different. An 'A' XRD pattern indicates closer packing and should result in a higher range of gelatinisation. But such an effect is not observed. In addition, the Tonset does not appear to be influencing the range of gelatinisaiton in any regular pattern: granule size and Gelatinisation range also do not show any relationship. In the Brabender Viscographic curves, the yam starches show a longer gelatinisation range, which does not appear in the DSC. The different in the water starch ratio between Viscography and DSC may be the reason for this anomaly.

  3. Gelatinisation Enthalpy

    Gelatinisation enthalpy depends on a number of factors such as Crystallinity intermolecular bonding, etc. For cassava starch, values range all the way from 4,8 [132] to 16J/g (Tab.8 )[120]. The low value of 4.8[132] appears to be mistakenly reported as J/g instead of cal/g. The enthalpy of gelatinisation of five varieties of cassava varied from 10.6-13.8 J/g [41]. Gelatinisation enthalpy was also found to depend on genetic and environmental factors as illustrated by Asaoka et al. [129] and Defloor et al. [75] from studies using different varieties, time of harvest and seasonal variations. SO2treatment was found to enhance the gelatinisation enthalpy of cassava starch from 18.1 to 19.1 J/g [114]. For sweet potato starch, the values for gelatinisation enthalpy have been reported to be between 10-18.6 J/g [23, 36, 100, 115, 135]. Effect of variety and environmental conditions was also evident [26, 63]. During the grown period, the ?H was lowest at the earliest stage of development in two sweet potato cultivars and the enthalpy ranged between 11.8-13.4 J/g [84]. Table 13 : DSC Characterstics of the hydrothermic transition of purified starches and fresh and freeze-dried and parenchyma tuber cells.

    		Tonset[0C]Tmax[0C]Tend[0C]?H[J/g]
    Sweet potato 
    Starch67.372.779.613.6
    Fresh tubers67.473.580.16.8
    Freeze-dried tubers67.873.281.59.3
    Small starch granules75.682.688.315.3
    Tania 
    Starch66.069.981.812.9
    Fresh tubers66.570.583.66.1
    Freeze-dried tubers67.270.284.28.8
    Yam 
    Starch70.274.480.920.9
    Fresh tubers70.174.682.612.3
    Freeze-dried tubers70.474.183.816.2
    The gelatinisation enthalpy for other starches is listed in Tab. 9 and 10 and again illustrates the considerable variability among the reports. There does not appear to be any relationship between the enthalpy and other factors like amylose content, granule size and XRD patterns. Since amylopectin has a more crystalline nature, higher amylopectin content has been considered to be contributing to higher enthalpy of gelatinisation. However, such an effect is not evident in any of the studies. Again the differences do not reflect also demonstrate that unlike the gelatinisation temperatures, which show wide variation, the enthalpy of gelatinisation is within a small range for the tuber starches.

    Figure 8

    DSC traces of different tuber starches

  4. Starch Retrogradation

    DSC has been quite useful in studying retrogradation properties of starches. It has been established that amylopectin can also take part in the retrogradation by the association of the outer chains [49, 143-145]. Retrogradation parameters of tuber starches have been examined by DSC, but the results were quite erratic and hence diffcult to arrive at definite conclusions [134]. The values for Tonset showed a very wide range from 370C to 580C, highest and lowest being for arrowroot and Xanthosoma starches, respectively. It is well known that retrogradation brings about drastic reduction of Tonset of starches. The highest reduction was observed for Amorphophallus and lowest for D. esculenta starches. The range also varied widely from 12.20C for cassava to 430C for D.alata. the wide range in values indicates that the retrograded starch contains recrystallised amylopectins of different Crystallinity . Among the starches D.alata starch appears to have the maximum variability in crystalline structure. Silverio [143] has reported a value of 430C for potato starch. As expected, the enthalpy of gelatinisation also fell during retrogradation, the reduction being 2.5-fold to 7-fold. No relationship could be derived from the properties of retrograded starches with those of the non-retrograded starches [134].
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

11. Gelatinisation and Pasting Temperatures

The gelatinisation temperature, which is indicative of the temperature at which the starch granules gelatinising, can be measured microscopically or by DSC. The pasting temperature at which a perceptible increase in viscosity occurs and which is always higher than gelatinisation temperature, is usually measured using a viscometer. Here also considerable variability exists among the different tuber starches.

  1. Cassava Starch

    Among different tuber starches, cassava starch has the lowest gelatinisation temperatures. Gelatinisation temperatures of starch of seven cassava varieties examines microscopically showed that starch of H-165 and H-1687 gelatinised earlier and the gelatinisation range was relatively higher, viz. over 120C. No relationship between granule size and gelatinisation temperature was observed [75]. The gelatinisation temperatures of cassava starch determined microscopically by various workers ranged from 49-640C [146] to 62-730C [33]. Pasting temperature of H-165 starch determined using a viscograph was slightly lower than those for most of the other varieties, and M4 starch had the highest range of pasting temperature [41]. The values were quite close to DSC values of 66 and 780C for Tonset and Tpeak respectively. The pasting temperature of cassava starch obtained by various workers is presented in Tab.8. When cassava starch was subjected to steam-pressure treatment at different pressures, there was progressive increase in pasting temperature by 2 to 90C depending on the time of treatment and pressure used. The pasting temperature rise was higher for longer time of treatment and higher pressures [147]. Increase in pasting temperatures was also observed on treating the starch with different surfactants [55], the most pronounced effect was noticed on treatment with potassium palmitate and potassium stearate. On esterification of the starch OH groups to give acetate groups. The pasting temperatures were reduced to weakening of associative forces [148]. Srioroth et al. found that SO2 treatment lowered the pasting temperature of cassava starch from 92 to 890C [114]. Examination of gelatinisation temperature of cassava starch is non-aqueaous solvents indicated that the values were enhanced tremendously in glycerol and ethanediol, while in dimethyl sulphoxide (DMSO) and formalin only a slight increase was noticed. The large increase in the first two solvents can be traced to steric factors [149].

  2. Sweet Potato Starch

    The pasting temperature f sweet potato starch (Tab. 8) obtained using a Brabender Visco Amylograph varied between 66.0 and 86.30C while microscopic determination gave values between 57-70 to 70-900C. Noda et al. [63] observed the pasting temperatures of starch from two sweet potato cultivars grown at different fertilizer levels to be 70.8-73.90C.

  3. Other Starches

    The data on other starches are given in tabs 9 and 10. Strauss and Griffin [87] found the gelatinisation temperature of different varieties of Colocasia starch to range from 69 to 740C. Pasting temperatures of different cultivars of Colocasia esculenta and Xanthosoma sagittifolivium showed only very slight difference between the varities, but were distinctly higher than those of cassava and sweet potato starches. Perez et al [132, 138] have compared the gelatinisation temperatures obtained by rapid Visco Analyser (RVA) and DSC of Arracaccia, Xanthosoama. Canna edulis arrowroot and cassava starches and obtained values of 68-950C and 56-730C, 85-950C and 74.87 0C, 73.95 0C and 61.75 0C ,79.92 0C and 68.85 0C 73.90 0C and 62.84 0C respectively (Tab. 14). Soni et al[29] found the gelatinisation temperature of Canna starch as 65-70 0C by microscopy. The gelatinisation temperature of starch of three accessions of Canna was 74-85 to 80-95 0C by Barbender Viscograph, but 74-750C by the RVA [66]. Pasting temperature of starch of Amorphophallus paeonifolius extracted from ten accessions were nearly similar and the starch gelatinized in the same range as the other aroid starches [91]. However, the range was lower compared to cassava or the yam starches. For pachyrrhizus starch, we obtained values of 74-790C and only minor variation existed among varieties [5]. The values for Curcuma starch were 810C by RVA[67] while it was 65-850C for Coleus starch determined microscopically[42]. Abraham and Mathew [30] found the gelantinisation temperature of Coleus starch to vary from 75-810C depending on the concentration used. Table 14 : Gelatinisation temperature range and B arbender and Viscoamylographeological parameters of some starches
    Rheological ParameterC. EdulisArrowrootCassavaA.xanthorrhizaA.saggittifolium
    Gelatinisation temp. range [0C]68-9075-9068-9062-9578-95
    Peak viscosity [PV]3001505076060
    Final viscosity at 950C [H]46015040520-80
    Viscosity at 500C66017040540130
    Breakdown[P-H]-160010240-20
    Set back[C-P]36020-1022070
    Consistency[C-H]2002002050
    Yam starches gelatinized over a temperature range of around 200C and gelatinization continued even after 950C showing strong intermolecular linkages. The RVA results on pasting of different yam species indicate the values to range between 75 and 83 0C, the highest being for D dumetorum and lowest for D. cayensis starch [80]. D. abyssinica starch had a gelatinization temperature of 730C [39]. Nkala et al. [31] examined the starch of D. dumetorum dry and wet seasons and found the values to be similar, i.e. 830C. The large range for yam starches may be attributed to the presence of phosphate linkages in these starches (similar to potato starch).
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

12. Viscosity

Use of starch in textile, paper, adhesive and food industries depends on the viscosity of the starch paste. Among different equipment, the Barbender Viscograph and Rapid Cisco Analyser are tmost commonly used. Different tuber starches exhibit considerable variation in their viscosity characteristics.

Figure 9

Brabender Viscocity Patterns of Starch from five Cassava Varieties

  1. Cassava starch

    When starch of different varieties of cassava was compared using a Barbender Viscograph, mainly three peak patterns were observed. These were :
    1. Single stage gelatinisation with high peak viscosity and high viscosity breakdown.
    2. Two-stage gelatinisation with high peak viscosity and medium breakdown.
    3. Broad two-stage gelatinization with medium breakdown.
  2. These patterns seem to be genetically controlled as the patterns were maintained by these starches irrespective of the environmental factors, though there was variation in the viscosity values. No correlation was obtained between the viscosity and granule size. In a study involving five cassava varieties having different cooking quality, it was observed that H-1687 starch had a medium peak viscosity and low viscosity breakdown but high setback viscosity and M4 starch had slightly lower peak viscosity and setback viscosity. On the other hand, H-165 starch had a very high peak viscosity and the breakdown was also quite large. The measurements clearly indicated that for H-165 starch, the setback viscosity was much lower compared to peak viscosity, whereas for H-1687 starch, the reverse was true (Fig.9). the result indicates a possible relationship between cooking quality and starch rheology, as H-1687 has reasonably good cooking quality, while H-165 is poor in its culinary quality. M4 starch, which behaves somewhat similar to H 1687 starch in its rheology, has the best quality [41]. Olorunda et al. [113] found that mealier cassava varieties had slightly higher peak viscosity for their starches. Rickard et al. [33] reported wide variation among the viscosity data for cassava starch from various sources [70, 97,151-154] (Tab. 8). However, Rosenthal et al. [97] reported only variations among a few Brazilian varieties. Padmanabhan and Lonsane [13] found slight reduction in peak viscosity and breakdown in cassava starch extracted by an enzymatic method. The viscosity of cassava starch extracted from inoculum provided fermentation was lowered due to presence of fibrous matter [17]. However, the breakdown was also correspondingly reduced due to the cementing of the granules by fibrous materials and its rheology was similar to cassava flour. In a Redwood Viscometer No. 1, a 2% paste of cassava starch had a residence time (viscosity value) of over 50 s and only minor variation among varieties was observed. Asaoka et al. [129] has compared the viscosity data of cassava starch using Brabender Viscograph and RVA (Tab. 12). When cassava starch was subjected to steam-pressure treatment, the viscosity fell steadily with increase in pressure and time out of treatment. At a pressure of 103 kPa for 150 min, the Redwood viscosity fell from 58.5 s to 30 s. the same trend was observed in the Brabender Viscosity results also, where the peak viscosity dropped from 430 BU to just 30 BU. It was also observed in that the reduction in viscosity was linearly related to the severity of treatment [147]. Lorenz and Kulip [107] observed changes n viscosity by heat moisture led to lowering of peak viscosity but other parameters were enhaced. Though sodium sulphite had a tremendous influence on the swelling volume of cassava starch at low concentrations (0.05%), the effect on viscosity was not noticeable [156]. Recently, it has been found that some Lewis acids ate able to liquefy starch at low concentrations without affecting the basic properties of the starch and this treatment may be effective as a method of thinning starch. Raja and Ramakrishna [157] have examined the effect of different salts on viscosity characteristics of cassava starch. It was observed that the influence of aqueous chloride and sulphate solutions was nearly similar, but the effect of sulphate was more evident. The Disodium orthophosphate induced a different effect compared to sulphate or chloride. Nonionic solutes like glucose and sucrose brought about only minor effect [157,158]. Blending cassava starch with maize and potato starches modified the viscosity properties with the resulting blend having properties in between those of the starches [159]. Sriroth et al. [114] found that SO2 treatment during extraction lowered from 283 to 231 BU. The Breakdown was not affected to any major extent. Extrusion cooking reduced the apparent viscosity of cassava starch [32]. Different surfactants affected the viscosity of cassava starch differently [55]. Whereas sodium lauryl sulphate increased the peak viscosity, especially at higher concentrations, the effect of potassium stearate and potassium palmitate was not so pronounced. Glyceryl monostearate reduced the peak viscosity at higher concentrations. A similar effect was observed for the Redwood 2% viscosity values also. Non-aqueous solvents like ethanediol depressed the Redwood viscosity from 55 s to 17.5 s, while glycerol increased it to 175 s. DMSO and formain also increased the viscosity, but to a much lower extent [149]. In addition to peak viscosity, the breakdown in viscosity is another important criterion that decides the applicability of starch in food and industry. In this respect, cassava starch is considered interior to maize starch, because its viscosity is rapidly lowered on heating under shear leading to a 'long' and cohesive texture for its paste, which is not desirable in food and textile applications. Steam-pressure treatment has been reported to improve the paste stability, but it is accompanied by a corresponding reduction in peak viscosity. However, surfactants were found to have a more desirable effect and are easy to handle as well. On incorporation of potassium stearate or potassium palmitate, even at 0.02mol% concentration/100 g starch, the viscosity was maintained and also stabilized [55]. In view of the presence of large number of hydroxyl groups in starch, cross-linking using di- and trifunctionsl chemicals has been epichlorohydrin, phosphorus oxychloride and sodium metaphosphate. These chemicals bind the starch molecules in the granules, increasing the associative forces, rendering the starch granules stronger and preventing breakdown during heating and stirring [155].

  3. Sweet potato starch

    Whereas extensive work has been done with cassava starch, only very little work has been carries out on the other tuber starches. Sweet potato starch behaves almost similar to cassava starch in its viscosity characters, viz. peak viscosity, viscosity breakdown and setback viscosity. The viscosity properties of sweet potato starch measured by various methods have been reviewed by Tian et al. [23] (Tab. 8). Collado et al. [135] studied 44 different sweet potato genotypes at 7 and 11% concentrations using the Rapid Visco Analyser and have worked out the correlations among the RVA parameters. They observed wide variation not only in the PV but broadness of peak, which has been attributed to another parameter; viz. time elapsed from start of gelatinisation to the time peak viscosity is reached. A significant negative correlation between PV and amylase content was noticed. Noda et al. [84] found that the viscosity values varied from 509-579 SNU for two sweet potato varieties at different fertilization levels. The rheological properties of sweet potato starch extracted using an enzymatic process did not vary among the different concentrations of enzyme used up to 0.1% [12]. Guraya et al. [150] reported the apparent viscosity of a large number of sweet potato varieties to vary considerably from 71-442 mPas and storage led to reduction in viscosity. The rheological properties of sweet potato starch have been examined using a Bohlin rheometer [26]. Storage modulus, G', loss modulus, G'' and tan? summed over different starch samples were determined. During heating, the G' and G'' increased while the phase angles decreased indicating the change from sol to gel. The initial increase has been attributed to progressive swelling of starch granules leading to close packing. When the starch granules became very soft, deformable and compressible, decrease in G' and G'' were observed. Elastic nature prevailed over the viscous nature of the paste in agreement with other reports [166,167]. The authors have been also derived the parameter 'complex viscosity' and found that location had a significant effect on initial and final viscosities. Laboratory samples were having higher initial and final complex viscosities at 950C as well as lower complex viscosities and other viscosity parameters at 350C. the rheological properties of various tuber starches have been compared using the Bohlin rheometer and wide variability in the values of G' and G'' was observed. However, all the starches exhibited uniformity in their elastic behaviour predominating over viscous nature [168].

  4. Other starches

    Data on the viscosity characteristics of these starches are presented in Tabs. 9 and 10. The viscosity of taro (Colocasia esculenta) starchj extracted from ten cultivars was less than that of cassava starch and close to that of cereal starches [88]. Consideranle difference in peak viscosity among the accessions was observed. It was also interesting to observe that starch from the C-9 variety having the highest granule size and amylose content had the highest peak viscosity. There was only nominal breakdown in viscosity even at the highest concentrations pointing to the possibility of using this starch in various applications, which require paste stability. The viscosity values were nearly the same for the corm and cormel starches [88]. Variability in the viscosity properties of Amorphophallus paeonilifolius starch extraceted from ten accessions was quite minor [91]. The viscosity breakdown for the starch samples was very low, similar to other aroid starches and in the same range as the cereal starches. Soni et al. [35] observed a value of 440 BU at 5% concentration and a breakdown of 40 BU, while Wankhede and Sajjan [28] reported peak viscosity of 1570 BU at 10% concentration with a breakdown of 270 BU. There was only a slight difference in the viscosity values of tannia (Xanthosoma saittifolium) starch among the varieties. The breakdown varied from 0 to 30 BU at 7% concentration, showing the stability of the starch paste.

    Figure 10

    Viscocity Reduction in Dioscorea Starches at Different Level of Steam Pressure Treatments

    Figure 11

    RVA Patterns of Different Yam Starches in Comparision to Cassava and Potato Starches
    All the yam starches showed a characteristic pattern of slow rise in the viscosity and even after 950C and some of them gelatinize only during the holding period. In this respect the yam, starch of four varieties of D, esculenta extracted using water and ammonia solution showed only minor difference between the varieties (800-950 BU), but the viscosities of starch extracted using ammonia solution were much higher than those obtained by water extraction. For D, alata starch, there were no clear peak viscosity values. As observed for the D, esculenta starch, there was no noticeable breakdown in viscosity on heating and stirring. The 2% viscosity (Redwood) of six varieties of D, rotundata starch varied from 37.5 to 46 s. As far as the paste viscosity is concerned, the values ranged from 325 to 550 BU at 5% concentration and 680 to 920 BU at 6% concentration [65]. The viscosity breakdown was quite low in spite of the high viscosity levels. The yam starches contain three to four times more phosphorous than cassava and aroid starches. It has been reported that the phosphate linkages in potato starch are responsible for its high viscosity and such effects may also be important in the yam starches. Steam-pressure treatment of the D, alata and D, rotundata starches reduced the viscosity depending on the pressure and time of treatment (Fig. 10). The peak viscosity came down to nil value at 103 kPa for 60 min for both the starches [106] and hence steam pressure treatment can be attempted for the modification of stach. Gebre-Mariam and Schmidt [39] have obtained 781, 756, 1282 BU for peak viscosity, hot paste viscosity and cold viscosity respectively for D. cayensis starch showing low breakdown, but high setback for this starch. Melo et al. [27] have determined the intrinsic viscosity of Pachyrrhizus starch to be 2.23 mL/g, similar to cassava (2.45mL/g) and corn (1.81mL/g) starch. Moorthy [42] found the Redwood viscosity of coleus starch to be 37 s at 750C for a 2%solution which increased to 56 s on cooling to room temperature. Abraham and Mathew [30] obtained 40 s for this starch in Redwood Viscometer while the Brabender values were peak viscosity (6%)-890 BU, peak viscosity (10%)-2670 BU; breakdown being 260 BU and 430 BU, respectively. Farhat et al. [80] have examined the viscosity properties of four yam species (Fig 11). The peak viscosities ranged from 2028 mPas for D dumetorum to 3893 mPas for D. cayensis compared to 8900 mPas for potato and 3134 mPas for cassava starches. The Breakdown in viscosity was much lower compared to cassava and potato starches and in conformity with the results obtained with the Brabender Viscograph. The final viscosity was slightly higher than the peak viscosity indicating a tendency to retrograde. Among these species, all the viscometric parameters of D. dumetorum starch were lower than those of the other three, which is in support of earlier results [39]. Nkala et al. [31] did not observe much difference in the viscosity parameters of D.dumetorum starch (using a rheostat-2 rotary viscometer) during the dry and wet seasons. The RVA profiles of starch of some varieties of D. alata, D. esculenta and D.rotundata harvested at different maturity have revealed that maturity did not affect the rheological properties to any major extent. Pachyrrhizus starch had similar viscosity as arrowroot starch and there was only very little difference among ten varieties examined. Perez et al. [132] have compared the RVA profiles of cassava, arrowroot and Canna edulius starches and found the highest peak viscosity for C. edulis starch followed by arrowroot and cassava had the lowest value. Breakdown was the highest for cassava starch among the three consistency highest for Canna edulis starch. Perez et al. [138] have studied the viscosity characteristics of Xanthosoma and Arracaccia starches by both Brabender Viscograph and RVA and the results indicate that Arracaccia starch had a higher peak viscosity compared to Xanthosoma, but the latter had higher consistency (Tab. 14). Soni et al [29] compared the viscosity parameters of Canna edulis starch with maize starch and the former had three times higher setback. Results in our laboratory, however, do not show such high setback for Canna starch. The peak viscosity values for the starches from three accessions of Canna varied from 3887 to 4187 mPas. The RVA patterns of the starches from the three accessions do not coincide exactly with the Brabender data. The results indicate noticeable breakdown for all the three starches but is not widely different among them [66]. Viscosity studies on Curcuma starch showed that variation exists between the species studied. The C. malabarica starch had a higher peak viscosity of 3387 mPas compared to C. zedoaria starch. Removal of curcumin led to an increase in peak viscosity of C. zedoaria starch almost to the level at C. malabarica starch, further confirming the presence of curcumin starch complex formation in C. zedoaria starch. The breakdown in viscosity was quite low for both starches, especially for C. zedoaria starch (66mPas). This shows that the granules are quite strong and resist breakdown under shear and heat. In this respect also. Curcuma starch resembles yam starches rather than cassava starch, which exhibits considerable breakdown in viscosity. The setback viscosity of both starches indicating a reasonable amount of reassociation of granules during cooling. The comparatively high setback viscosity and low breakdown viscosity of Curcuma starch may be due to the high phosphorous content. Phosphate linkages may be binding the starch franules making them quit stable. In this respect, Curcuma starch resembles potato and C. edulis starches [67].
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

13. Swelling Power

Swelling power provides evidence of non-covalent bonding between starch molecules. Factors like amylose-amy-lopectin ratio, chain length and molecular weight distribution, degree/length of branching and conformation determine the degree of swelling and solubility [33].

  1. Cassava starch

    The swelling power of cassava starch is in between those of potato and cereal starches - a property in conformity with its observed viscosity. The swelling power of cassava starch was found to vary considerably from 42-71 g/g according to various reports (Tab. 8) [33]. The swelling volume of different varieties of cassava varied from 25.5 to 41.8 mL/g of starch (Tab. 4). It was observed that during the growth period, starch of two varieties H-2304 and M4 maintained their swelling volumes within small ranges, while that for some varieties such as H-165 expressed wide variations, which indicate that these varieties are very much susceptible to environmental influences [62] (fig. 12)., and also point to possible relationship between cooking quality and swelling volumes, since M4 starch has a steady swelling volume and the tubers have consistently good cooking quality. Soni et al. [35] have reported a two-stage swelling for cassava starch and attributed it to the two types of force, which require different energy input to cause relaxation. Asaoka et al. [129] have determined the swelling volume of four varieties of cassava harvested during two seasons. They found that the swelling power was higher in samples harvested in November compared to those of harvested in August (Tab. 12). The swelling power had minimum variability among these samples in relation to organoleptic quality. Swelling volumes also depend on the presence of various chemicals and treatments carried out on starch. High amylose content and presence of stronger or a higher number of inter molecular bonds can reduce swelling [169]. Formation of lipid-starch complex can also affect the swelling volumes [45] as also presence of naturally occurring carbohydrates and non-carbohydrates along with starch [43, 45,170]. This has been amply illustrated in the effect of fibre on the swelling volumes of different varieties. The fibre acts as a barrier to free swelling of starch and extraction with ethanol or defatting of flour did not change the swelling volumes showing that the suppressive effect is more due to the fibrous material rather than lipid or sugars present in the flour [41]. The starchy flour extracted from fermented tubers also exhibited the same trend [17]. Sodium sulphite had noticeable effect in suppressing the swelling volume of cassava starch. The swelling volume dropped to very low values at different concentrations and narrowed down to zero at 0.05 and 0.1% concentration of the salt (fig. 13). At higher sodium sulphite concentrations, the swelling volumes increased to nearly the same levelas that of the native starch. The effect has been attributed to the oxidative-reductive depolymerisation brought about by the sulphite ions. Similarly the effect of the sulphite was neutralized by addition of propyl gallate, which is an oxygen scavenger [156]. The effect was not unique to cassava starch and swelling volume of Dioscorea starches was also lowered by sulphite at similar concentrations. Potassium sulphite gave similar results as sodium sulphite showing that the suppressive effect is due to sulphite ions. Other salts like sodium chloride, sodium sulphite and sodium phosphate failed to reduce such a trend. Another salt that brought a similar effect as sodium sulphite was sodium thiosulphate at the same concentrations [158,171]. Surfactants also affect the swelling volume of starches. The swelling volume was reduced by half by potassium palmitate and potassium stearate even at the lowest concentrations, while glyceryl monostearate affected the swelling volume only to a small extent in contrast; sodium lauryl suphate and cetyltrimethylammonium bromide enhanced the swelling volume considerably [55]. Lorenz and Kulip [107] observed that the swelling volume of starch was reduced by heat-moisture treatment. Steam-pressure treatment also lowered the swelling volume by compressing the starch molecules and thus restricting the free swelling of starch [147]. Sriroth et al. [114] observed that in starch extracted using SO2 treatment, the swelling volume was reduced at temperatures below 800C, while at higher temperatures the effect was negligible.

    Figure 12

    Swelling and Solubility Patterns of Starch from different Cassava Varieties during growth period

  2. Sweet potato starch

    The swelling power of sweet potato starch from various varieties and at different temperatures has been collated by Tian et al.[23] and the values vary considerably not only among varieties , but also at different temperatures (Tab. 8). Delpeuch and Favier [169] have reported a two stage swelling, while Rasper[161] and Madamba et al. [46] found as single-stage swelling for the same starch. The lower swelling volume of sweet potato starch has been attributed to a higher degree of intermolecular association compare to cassava or potato starch. Collado et al. [135] have examined the swelling volume of starch of 44 Philippine accessions of sweet potato and found the range to be between 24.5 to 32.7 mL/g with a mean value of 29.9 mL/g showing weaker associative forces compare to legume starches. There was no significant correlation between amylose content and swelling volumes.

    Figure 13

    Effect of Different Concentrations of Sodium Sulphite on the Swelling Volume of Cassava Starch at 95 C

  3. Other Starches

    Considerable variation in swelling volume of different varieties of Colocasia has also been reported. The values ranged from 26.5 - 60 mL/g - which indicates a high degree of variability. For C-9 starch, having the highest granule size, the swelling volume was the least [64]. Inverse relationship was noticed between the granule size and swelling volume of 10 accessions of Taro[88] . Swelling volume of starch of six colonal selections of D.rotundata showed only slight differences among the selections. At 1% concentration the values ranged from 15-25 mL/g and at 5%, the range fell considerably due to deficiency of enough water to swell all granules [65] . starch from different varieties of D.esculenta, D.alata, Xanthosoma Sagittifolium and Amorphophallus Paeonilifolius had much lower ranges (Tabs. 9 and 10). D.Ballophylla and Amorphophallus starches had lower swelling volumes compared to Cassava, but among these the Amorphophallus starch had higher value [35]. The swelling volume of D.Abyssinica starch increased from 10-23 mL/g as temperature was increased from 65-85oC. The relatively lower swelling of Dioscorea starch compared to potato starch has been attributed to the higher lipid content in the starch and also higher inter-associative forces compared to potato starch. Swelling volume of Canna edulis starch was observed to be higher compared to maize starch by Soni et al.[29], while Nagahama and Truong [172] reported 23.5 mL/g at 80oC. The swelling volume of Amorphophallus starch extracted from tubers pretreated with different chemicals depended on the chemicals used. All the chemicals lowered the swelling volume, but the highest production was with Glyceryl monostearate [10]. For Xanthosoma starch, Glycerol monostearate (GMS) and ammonia enhanced the swelling volume to a small extent [104]. In general, the aroid starches had rather low swelling volumes. For coleus starch, the swelling volume was around 25 mL/g [42]. For curcuma starch, the value obtained was 19 mL/g for C. zedoaria and 30 mL/g for C.malabaricum starches [67].
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

14. Solubility

Solubility of starch depends on a number of factors such as source, inter-associative forces, swelling power, presence of other components etc.

  1. Cassava Starch

    Cassava starch has a higher solubility than the other tuber crop starches on the higher solubility can be attributed partly to the higher swelling cassava starch undergoes during Gelatinisation. The solubility values ranged from 25-48o(Tab.8)[33]. The solubility of starch of different cassava varieties varied from 17.2-27.2%. However, no direct correlation between swelling and solubility could be observed [5]. Solubility data of starch from different varieties during the growth periods also showed that the starch of varieties H.2304 and M4 had good stability in their solubility whereas the stability of the other varieties are medium or poor [62]. The solubility was enhanced by heat-moisture treatment [107]. Among a few non aqueous solvents studied for solubility of starch, maximum solubility was obtained in DMSO and formalin while in glycerol it was moderate. Starch was insoluble in anisole and methyl cellosolve (Tab.15). The solubility data indicate that starch is more soluble in polar solvents or solvents with affinity towards water [149]. Table 15 : Solubility gelatinisation temperature and viscocity of starch solutions in different solvents

    		Solubility [g/100mL]Gelatinisation Temperature [0C]Viscosity (2%) Redwood [s]Viscosity (2%) after cooling [s]
    Glycerol10130-1451751800
    Ethanediol2110-12517.538
    DMSO2575-8585130
    Formalin2570-8578139
    DMFNil---
    Methyl celluloseNil---
    AnisoleNil---

  2. Sweet potato starch

    The data on solubility of sweet potato starch is given in Tab.8 the solubility of starch extracted from seven sweet potato collections from peru indicated that solubility increased with temperature and reached nearly 10% while for commercial starch, it was 28% [26]. The authors found that selection indentity did not have noticeable effect, but location had significant influence but temperatures above 600C, Collado et al. [135] found that for 44 Philippine genotypes of sweet potato, the solubility varied in the range of 12-24%, mean being 16.9%. The high swelling volume of the starch was reflected in its solubility. It was presumed that the bonding forces might be tenuous but comparatively extensive, immobilizing the starch within the granules even at high levels of swelling.

  3. Other Starches

    The solubility of D.abyssinica starch was found to be enhanced with temperature and the values resembled those of the maize starch[39]. The solubility starch of yams and aroids pretreated with various chemicals was affected to different extent by the chemicals used and also the concentration [10]. The values were between 18-32%. Solubility of the other tuber starches varied from 10-30% and in general the aroid starches had usefully lower solubilities.
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

15. Clarity

The high clarity of starch has much relevants in food and textile applications and depends on the associative bonds between the starch molecules in the granules.

  1. Cassava starch

    Because of its weaker associative forces cassava starch has a better clarity than cereal starches. When derivatives of cassava starch were compared, the clarity of acetylated and propylated derivatives was better, though only to a small extent. The clarity was best when pyridineacetic anhydride was used for acetylation, probably due to the high DS achieved. Esterification tends to weaken the associative forces by reducing the available hydroxide groups[ 148]. In contrast, heat-moisture treatment reduced the clarity by strengthening the associative forces.

  2. Other Starches

    Dioscorea starches have almost equal clarity as cassava starch, indicating their associative forces were similar. Variation was not observed among different varieties of dioscorea, alata , D.esculenta and D.rotundata starches. Steam pressure treatment was found to decrease the clarity of D.alata and D.rotundata starches and the reduction was directly proportion to the pressure used and time of treatment. The reduction in clarity on pressure treatment can be attributed to the strengthening of associative forces [106]. The clarity of aroid starches is, as expected, poor and the values are closure to that of cereal starches. The clarity of starch pastes of ten accessions of colocasia esculenta was nearly equal [64]. Similarly no significant variation among ten different accessions of amorphophallus paeonifolius was observed [91]. The clarity of three accessions of canna edulis was found to be much higher than that of the aroid starches [66]. Generally 'B' starches appear have higher clarity compared to 'A' starches.
  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

16. Sol stability

Sol stability or paste stability reflects the retrogradation tendency of starch paste. Cassava and sweet potato starches have low retrogradation tendency and therefore high pastes stability. Suzuki et al. [109] have suggested that the lower retrogradation tendency of cassava may be due to the higher weight-average molecular weight of the amylose fraction in cassava. The sol stability of cassava starch in various non-aqueous solvents varied from 3h in ethanediol to more than twenty days in formalin. Formalin may be preventing parallel association of the starch molecules, especially the amylose change, by forming complexes [149]. Sol stability was also affected by added surfactants. Sol stability was enhanced by derivarisation as well, though there was no correlation between degree of substitution and sol stability. Dioscorea starches also have good stability, while that of the aroid starches is poor. The paste stability of starch different varieties of D.alata , D.rotundata, D.esculenta , X.sagittifollum , and A.paeoniifilius was nearly same. Heat-moisture treatment [steam-pressure treatment] of dioscorea starches decreased the paste stability and the observed reduction at higher levels of treatment was so high that the starch gel started settling within 2-3 h indicating that the starch molecules come close to each by a compressive treatment leading to the fast settling[106].

17. Digestibility

Digestibility of starch by enzymes is important for evaluating nutritive value and also in industrial applications. Cassava starch is one of the least enzyme resistant root starches [23,33,46]. Using extra cellular amylases kainuma [173] Observed high levels of hydrolysis of raw cassava starch. Other studies also indicated high level of raw cassava starch digestibility by microbial amylases sweet potato starches was found to be more susceptible than cassava starch to degradation by ?-amylose and glycoamylase [169, 173-175]. Digestibility of raw starch of eight sweet potato varieties glycol amylases was compared by noda et al. [84]. No significant correlation between digestibility and amylose content was noticed. The digestibility was >80% after 24 h. It was also found that digestibility was lowered at later stage of development of tubers. Woolfe et al. [37] reported that raw starch has poor digestibility (24%) as found earlier by rasper [161]. Szylit et al.[83] obtained a value of 15% cooking and pellettising increased the digestibility [174]. Gallanteral [89] found that 'A' type starches showed high susceptibility to ?-amylase. However, leach and schoch[176] had found that enzymes successfully susceptibility was not related to factors such as external surface area XRD patterns swelling are solubility. Madamba et al[46] found significant varietal effect among sweet potato cultivars on ?-amylase attack. Quidart-i-khuda and De[177] reported more than 30% hydrolysis of cooked starch to maltose by saliva and taka-diastase, respectively, in contrast to raw starch, which had only less than 10% hydrolysis. Rasper et a;. [178] also obtained low conversion for cassava starch. Gallart et al. [89] found that pelletisation increased the raw starch digestibility by bacterial ?-amylase from 17% to 45%. SEM studies indicated that enzymatic corrosion occurs mainly at the surface of the granules. Deep radial corrosion channels were not observed on starch granules. [96]. In vivo digestibility of different tuber starches was compared with corn and potato starch in albino rats. In the case of raw starch, digestibility of cassava, sweet potato, Colocasia, Xanthosoma and Amorphophallus starches was quite high (65-75%), comparable to corn starch (76%) but that of the Dioscorea starches was low (15-25%) similar to potato starches. The large increase in digestibility on cooking can be attributed to the change in starch structure on gelatinisation [179]. This was confirmed by the observation that the XRD patterns of all the cooked starches were similar, unlike the uncooked starch in which aroid starches have 'A' pattern and yam possesses 'B' pattern. A comparison of the digestibility was also carried out using pancreatic ?-amylase under in vitro conditions and maximum activity was obtained on cassava starch while digestibility was much lower for Colocasia starch indicating that in vivo and in vitro digestibility could not be related either to the amylose content or the soluble amylose contents in these starches. Earlier reports had indicated that popping of sweet potato improved starch availability and nitrogen digestibility. Digestion of Amorphophallus starch by ?-amylase digestibility [104]. Coleus starch was reported to have 50% digestibility by salivary α-amylase[30].

  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

18. Conclusions

The studies on the different tuber crops reveal the vast variability available among them, which is not generally observed in the case of cereal starches. The high viscosity of cassava and Canna starches makes these starches very useful in many food and industrial applications especially where high thickening power is desired. The low viscosity of aroid starches can be exploited in paper industries where lower viscosity and good film forming capacity are preferred. The small granular size of Colocasia and D.esculenta and D.dumetorum starches makes these ideal as filter in biodegradable plastics, and in aerosois and talcum powders. The clarity of cassava, Canna and yam starches is useful in many food applications. Similarly the good gel strength of these starches, especially Canna starch can be utilized in a wide array of food products. The easy gelatinisation of cassava and sweet potato starches can make them suitable in the manufacture of hydrolysis products derived from starch. The range of characters observed makes the tuber starches amenable to different applications based on their properties in place of different applications based on their properties in place of chemically modified starches. An awareness of their potential uses can help in large scale cultivation of these crops and extraction of starch from them. It is also possible to modify the starch properties by simple physical methods like hydrothermal or steam-pressure treatments. Latest developments in biotechnology can also be tried to modify the starches. These include fermentation of starch by the use of selective organisms or enzymatic modifications, which can bring about specific substitutions [105-107,147, 180-182]. Lot of work has been done on fermentation of cassava and its effect on starch quality whereas the use of enzymes in starch derivatisation has not been exploited and offers very good scope for value addition [183-188].

Abbreviations
BUBrabender units
CP-MAS13C NMRCross Polarised Magic Angle
 Scattering 13Carbon Nuclear Magnetic Resonance
CTCRICentral Tuber Crops Research Institute
DMSODimethyl sulphoxide
DPDegree of polymerization
DSDegree of substitution
DSCDifferent Scanning Calorimetry
FTIRFourier Transform Infra Red
GMSGlyceryl monosterate
PKPeak Viscosity
RVARapid Visco Analyser
SNUStirring number Units
XRDX-Ray diffraction
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