Effect of sorghum flour substitution on pasting behavior of wheat flour and application of composite flour in bread

The objective of this study was to investigate the effect of sorghum flour substitution to wheat flour on pasting and thermal properties of the composite flours as well as firmness of the flour gels and application of composite flour in pan bread. Wheat-to-sorghum flour ratio used were 100:0 (wheat control), 0:100 (sorghum control), 80:20, 60:40, 40:60 and 20:80, with the wheat control serving as a benchmark. Both flours had approximately 10% moisture content. Crude protein content of sorghum flour was 14.90%, which was lower than that of wheat flour (16.91%). Sorghum flour contained 60.19% starch (with 14.88% amylose) as compared to wheat flour which contained 73.03% starch (with 23.87% amylose). Crude fat and crude fiber contents of sorghum flour were 1.76% and 2.27%, respectively, which were higher than those of wheat flour. Pasting properties as monitored using a Rapid Visco Analyzer showed a significant decrease (p≤0.05) in peak viscosity with increasing level of sorghum substitution from 1538.33 cP in wheat control to 1265.33 cP in the 20:80 composite. Sorghum control demonstrated significantly lower breakdown (400.33 cP) than wheat control (716.33 cP) and this led to a progressive decrease in breakdown of the wheat-sorghum composites. Among the composite flours, the 20:80 blend exhibited the greatest final viscosity of which the value decreased with increasing ratio of wheat flour. In terms of setback, sorghum control was higher in setback (1478.67 cP) as compared to wheat control (1108.00 cP). Setback of the wheat-sorghum composites, therefore, increased with increasing sorghum proportion. Regarding starch gelatinization, all wheat-sorghum composites had similar temperature and enthalpy of gelatinization (DHG) to those of wheat control. As to retrogradation, an increase in enthalpy for melting of amylopectin crystallites (DHR) was observed in all samples but at a different degree, with the 20:80 composite revealed the smallest increase in DHR with increasing storage time. In the matter of flour gel firmness, freshly prepared sorghum gel was of higher firmness than wheat gel. Gel firmness of the wheat-sorghum composites correspondingly increased with increasing sorghum ratio. For the bread properties, moisture content and water activity of the bread samples were found to decrease with increasing storage time. Specific loaf volume decreased with increasing sorghum flour substitution. Pertaining to the air cells, it was found that there was increasing irregularity in cell size and shape upon substitution with sorghum flour. For crumb texture, hardness, gumminess, chewiness increased with increasing sorghum ratio and storage time. Water-soluble starch content decreased from 6.14% in wheat bread to 1.38% in the 20:80 composite bread.  Color of crust and crumb progressively became darker as the level of sorghum flour substitution increased. Wheat-sorghum composite is a viable alternative to 100% wheat flour at levels up to 40% substitution. In areas that sorghum is locally grown and of lower price, this could reduce the volume of wheat importation.