Physicochemical and Functional Properties of Tropical Tuber Starches: A ReviewPrev Page | Next Page
- Extraction Techniques
- Other components in Starch
- Colour and Appearance
- Granule Shape and Size
- Spectral Features
- X-Ray Diffraction Pattern
- Molecular Weight
- Amylose Content
- Thermal Characteristics
- Gelatinisation and Pasting Temperatures
- Swelling Power
- Sol stability
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.  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 . 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.
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  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. . 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.  reported that raw starch has poor digestibility (24%) as found earlier by rasper . Szylit et al. obtained a value of 15% cooking and pellettising increased the digestibility . Gallanteral  found that ‘A’ type starches showed high susceptibility to ?-amylase. However, leach and schoch had found that enzymes successfully susceptibility was not related to factors such as external surface area XRD patterns swelling are solubility. Madamba et al found significant varietal effect among sweet potato cultivars on ?-amylase attack. Quidart-i-khuda and De 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;.  also obtained low conversion for cassava starch. Gallart et al.  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. .
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 . 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 . Coleus starch was reported to have 50% digestibility by salivary α-amylase.