Physicochemical and Functional Properties of Tropical Tuber Starches: A Review

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  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 Parameter C. Edulis Arrowroot Cassava A.xanthorrhiza A.saggittifolium
    Gelatinisation temp. range [0C] 68-90 75-90 68-90 62-95 78-95
    Peak viscosity [PV] 300 150 50 760 60
    Final viscosity at 950C [H] 460 150 40 520 -80
    Viscosity at 500C 660 170 40 540 130
    Breakdown[P-H] -160 0 10 240 -20
    Set back[C-P] 360 20 -10 220 70
    Consistency[C-H] 200 20 0 20 50

    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).

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