Physicochemical properties of sweet potato starches and their application in noodle products

Starches isolated from 3 Chinese sweet potato varieties (XuShu18, SuShu2, and SuShu8) differed in granule size and particle size distribution as well as in protein, lipid and phosphorus contents but theamylosecontents were similar for these starches (19.3-20.0%). The pasting behavior, swelling pattern, andsyneresisproperties were investigated and found to vary. On comparison, the physicochemical properties of the sweet potato starches rather differ from those of potato andmungbean starches. The quality of the starch noodle made from SuShu8 starch was well comparable to that made frommungbean starch, and better than that made from SuShu2 and XuShu18 starches as evaluated by both instrumental and sensory analysis. Correlation between starch noodle quality and gel properties of the original starches was established in order to be able to predict the suitability of a starch for starch noodle manufacture. It was found that differently sized granule fractions showed a difference in ash,amyloseand phosphorus content, as well as in gel firmness and freeze-thaw stability. The small size (<20 µm) granule fractions were found to be more suitable for starch noodle making and the qualities of both dried and cooked starch noodles made from these fractions were significantly better than those made from their original starches and much better than those made from the large size granule fractions. Sweet potato and potato starches and their derivatives (acetylated andhydroxypropylated) were also evaluated for the ability to manufacture high quality White Salted Noodle (WSN) by replacing the commonly used wheat flour up to 20%. It was found that only the use of acetylated starches could significantly improve WSN quality resulting in decreasing cooking loss, and increasing softness,stretchabilityand slipperiness. The cold peak breakdown (CPBD) of the composite flour, as measured in 1.5%NaClsolution, showed a significant correlation with the cooking loss, stretch stiffness andstretchabilityof WSN.Moreover, acetylated starch from potato and sweet potatoes were studied with respect to the degree of substitution (DS) and acetyl group distribution in differently sized granule fractions. The DS of the fractionated starches increased with decreasing starch granule size dimension. The DS of theamylopectinpopulations of differently sized granule fractions showed the same trends as the original starches, while the DS of theamylosepopulations were quite constant. It was confirmed that theacetylationonly occurred in the outer lamellae of the crystalline region, but took place in all amorphous regions of starch granules. The acetyl group distribution is more heterogeneous in theamylosepopulations isolated from small size granule fractions.