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 T_onset and T_peak 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).