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

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].

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