Physico-chemical properties of flour and fine structure of starch in relation to sorghum couscous quality

Adam Niger Aboubacar, Purdue University

Abstract

In several West African countries, attempts are being made to develop small-scale processing units for the commercial production of sorghum couscous. Sorghum cultivars differ in their couscous-making ability. In this study, physico-chemical properties of flour and starch were determined and used to explain differences in couscous quality parameters among eight sorghum cultivars. It was found that couscous yield significantly correlated with the amount of damaged starch in flour (r = 0.89, P $<$ 0.01), and with the proportion of fine flour fraction (r = $-$0.87. P $<$ 0.01), and ash content of flour (r = $-$0.86, P $<$ 0.01). All sorghum flours had similar light (L) color value; flour red and yellow color varied among the cultivars. Couscous color of the cultivars varied from brown to yellow. Significant differences in couscous water absorption index and water solubility index also existed among sorghum cultivars. Stickiness is the most important couscous texture parameter. Sorghum couscous stickiness positively correlated with the amount of damaged starch in flour (r = 0.89. P $<$ 0.01). Couscous hardness correlated with apparent amylose content (r = 0.79, P $<$ 0.01), amylose-to-amylopectin ratio (r = 0.80, P $<$ 0.05), and amylograph peak viscosity (r = $-$0.75, P $<$ 0.05). Total starch in flour correlated (r = $-$0.89, P $<$ 0.01) with couscous chewiness. The proportions of long chain fraction (FrI) and short chain fraction (FrIII) of sorghum amylopectin were found to be associated with the gelatinization and retrogradation behavior of the starch. Sorghum cultivars with high proportion of FrI tended to produce couscous with harder texture. Water-soluble material from couscous contained significant amounts (40 to 90%) of carbohydrate that significantly correlated (r = 0.71, P $<$ 0.05) with couscous stickiness. Stronger relationships were obtained between the proportion of high molecular (HMW) weight component of the soluble starch (r = 0.86, P $<$ 0.01) and its debranched fraction II (r = 0.89, P $<$ 0.01) with couscous stickiness. This HMW starch was concluded to be depolymerized amylopectin. This study is the first to report on flour and starch properties correlating with sorghum couscous quality.

Degree

Ph.D.

Advisors

Hamaker, Purdue University.

Subject Area

Food science

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