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

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.