Tropical Sources of Starches

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3. Other starches

1. Sweet Potato starch

   Unlike cassava, extraction of starch from sweet potato tubers is not easy. The presence of fibrous material and latex prevents easy settling of starch and this leads to extended residence time for the starch in the mother liquor. Since the mother liquor contains lot of sugars, fermentation sets in leading to deterioration of starch properties. Starch often possesses an off-colour if not processed properly due to the phenolics present in the tubers.

   Kallabinski and Balagopalan et al. (38) have used an enzymatic technique to extract starch from sweet potato tubers. The use of cellulase and pectinase resulted in increased yield of starch without affecting the properties of the extracted starch.

   Sweet potato starch also is similar to cassava starch in its lipid and phosphorus content and hence its properties are quite similar to cassava starch.

   Sweet potato starch is polygonal oralmost round in shape ^18,39 and has a centric distinct hilum. Polarisation crosses are less distinct compared to cassava starch. Granule size of sweet potato is in the same range as that of cassava starch. Bowkamp ⁴⁰ reported negative correlation between particle size and susceptibility to amylase and acid degradation in sweet potato cultivars. Noda et al. ⁴¹ found no effect of fertilisation on starch granule size. The same authors found that the average granule size increased during early stage of development and then remained steady in two varieties . XRD pattern of sweet potato is reported as ‘A’ pattern, ‘C’ or intermediate between ‘A’ and ‘C’ ^42-44. Takeda et al. ⁴² observed ‘A’ pattern for two varieties while it was ‘C_A‘ for another variety. The absolute crystallinity for this starch was 38%.

   The molecular properties of sweet potato starch has been examined in detail. Takeda et al. ⁴² found a trimodal pattern for the sweet potato amylopectin while Hizukuri ⁴⁵ reported a bimodal distribution. They concluded that sweet potato has a higher proportion of ‘A’ chains and short ‘B’ chains compared to potato. Seog et al. ⁴⁶ reported alkali number values between 7.66 and 12.13 for six Korean sweet potato varieties compared to 5.33 for cassava starch. Noda et al. ⁴¹ 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.

   For sweet potato starch also, considerable variation in amylose content has been reported, Table 3. Madamba et al. ⁴⁷ found only very little variation in amylose content among six varieties of sweet potato from Philippines. Garcia and Walter ⁴⁸ obtained values ranging from 20-25% by potentiometric titration for some Peruvian cultivars and location did not have any effect. Noda et al. ⁴¹ did not observe any effect of fertilisation or growth period on amylose content.Ishiguro et al ⁴⁹ studied the retrogradation tendencies of starch isolated from 10 sweet potato cultivars having different amylose contents and chain length distribution. Starches having fewer amylose molecules and amylopectin molecules with higher content of short chains (DP 10) retrograded slower compared to others Collado et al. ⁵⁰ have examined the DSC characteristics of 44 sweet potato genotypes from Phillipines and obtained considerable variation in all the parameters. The mean T_onset was 64.6°C and range 61.3-70° C, mean T_peak 73.9°C (range 70.2-77°C) and mean T_end 84.6°C, range being 80.7-88.5°C and the mean gelatinisation range was 20.1° with a range of 16.1 to 23°C. Garcia and Walter ⁴⁸ have examined two varieties cultivated at different locations and found the range to be between 58-64°C for T_onset, 63-74°C for T_peak and 78-83°C for T_end. While selection index did not affect the values, location influenced the parameters Noda et al. ⁴¹ found varietal difference but no effect of fertilisation on the DSC characteristics of two sweet potato varieties. It was also found that during growth period, the T_onset was the lowest at the latest stage of development . The starch from fresh tubers and freeze dried sweet potato tubers gave nearly equal values (67 – 73°), but the small granules gelatinised between 75 and 88°C.

   The pasting temperature of sweet potato starch (Tab. 3) varied between 66.0 and 86.3°C by viscography while microscopic determination gave values between 57-70 to 70-90°C.

   Sweet potato starch behaves almost similar to cassava starch in its viscosity characters, viz., peak viscosity, viscosity breakdown and setback viscosity. The viscosity properties of sweet potato starch measured by various methods have been reviewed by Tian et al. ³⁹ Tab. 3. Collado et al ⁵⁰ studied 44 different sweet potato genotypes at 7 and 11% concentrations using Rapid Visco Analyser and have worked out the correlations among the RVA parameters. They observed wide variation not only in the Peak Viscosity but also the broadness of the peaks. A significant negative correlation between Peak Viscosity and amylose content was noticed.

   The rheological properties of sweet potato starch extracted using an enzymatic process did not vary among the different concentrations of enzyme upto 0.1% ⁵¹. Guraya et al. ⁵² reported the apparent viscosity of a large number of sweet potato varieties to vary considerably from 71-442 cPs and storage led to reduction in viscosity.

   The rheological properties of sweet potato starch have been examined using a Bohlin rheometer ⁴⁸. Storage modulus, G’, Loss modulus, G” and tan d summed over different starch samples were determined. During heating, the G’ and G” increased while phase angle decreased indicating change from sol to gel. The initial increase has been attributed to progressive swelling of starch granules leading to close packing. When the starch granules became very soft, deformable and compressible, decrease in G’ and G” were observed. Elastic nature prevailed over the viscous nature of the paste Data on the swelling power has been compared by Tian et al. ³⁹ and the values vary considerably not only among varieties, but also at different temperatures (Tab. 3). Delpeuch and Favier ⁵³ have reported a two stage swelling while Rasper ⁵⁴ and Madamba et al. ⁴⁷ found a single stage swelling for the same starch. The comparatively lower swelling volume of sweet potato starch has been attributed to a higher degree of intermolecular association compared to cassava or potato starch. Collado et al. ⁵⁰ have examined the swelling volume of starch of a number of Phillipino accessions and found the range to be between 24.5 to 32.7 ml g^-1 with a mean value of 29 9 ml g^-1 showing weaker associative forces compared to legume starches. There was no significant correlation between amylose content and swelling volumes.

   The solubility of starch extracted from seven sweet potato collections from Peru indicated that solubility increased with temperature and reached nearly 10%, while for commercial starch, it was 28% ⁴⁸. The authors found that Selection Index did not have noticeable effect, but location had significant influence at temperatures above 60°C. Collado et al. ⁵⁰ found the solubility to be in the range 12 to 24% (average 16.9%). It was presumed that the bonding forces might be tenuous but comparatively extensive, immobilising the starch within the granules even at high levels of swelling

   In vitro and in vivo digestibility study of the native and gelatinised starch indicated that the starch possesses very good digestibility even in its native state. However sweet potato has been associated with flatulence and the starch has been implicated in contributing to flatulence, but the digestibility studies do not prove the role of starch in contributing to flatulence.