Starch properties as affected by sorghum grain chemistry

Abstract

To determine the relationship between sorghum grain polyphenol content, grain structure, and starch properties, starch was isolated from 10 sorghum varieties using an alkali steep and wet-milling procedure. SV2, a tannin-free variety with white pericarp, gave a white starch. Varieties having red or white pericarp and higher polyphenol levels gave pink starches. Hunter colour values (L, a, b) of starches were not correlated with grain polyphenol content. Grain appearance in terms of pericarp colour, or presence or absence of pigmented testa, did not relate to the intense pink colouration of sorghum starches. Starch amylose content was significantly negatively correlated (r = −0.88, p < 0.001) to grain floury endosperm texture. Sorghum starches had higher peak viscosity (PV) in pasting than commercial maize starch. The time taken to reach peak viscosity from the initial viscosity rise was less for sorghum starches than maize starch. However, sorghum starches had a higher rate of shear thinning (Rst) than maize starch. There was a significant positive correlation between grain polyphenol content and starch PV (r = 0.75, p < 0.05). Starch gel hardness was negatively correlated to pasting properties of Rst and paste breakdown (r = −0.78 and −0.77 respectively) at p < 0.01. Peak gelatinisation temperature (Tp) occurred over a narrow range from 66 to 69 °C. Tp was negatively correlated to the floury endosperm portion of the grain (r = −0.77) at p < 0.01. It is concluded that sorghum grain polyphenol content and grain characteristics influence its starch properties. © 2000 Society of Chemical Industry