Resting of sorghum-soya biscuit dough: effect on dough and biscuit physicochemical properties and consumer acceptability of the baked biscuits

Abstract

Previously researchers (also from the University of Pretoria) reported low acceptability ratings for sorghum and sorghum-soya composite biscuits because consumers perceived the biscuits as hard and gritty. Grittiness in sorghum biscuits may be attributed to limited hydration of starch during dough preparation due to the presence of hydrophobic sorghum protein bodies that surround starch granules. Hence, there is need for further research to improve the understanding of the sensory properties and also physicochemical properties of sorghum-soya dough and/or its biscuits. In the current research, it was hypothesized that increasing the resting time of sorghum-soya composite biscuit dough would reduce the grittiness of sorghum-soya biscuits by promoting water absorption, starch granule swelling and starch gelatinisation. The objective of this study was to evaluate the effect of dough resting time (15 min and 24 h) on the physical properties, proximate composition, texture, thermal properties, starch and protein digestibility and consumer acceptability of sorghum-soya biscuits. A sorghum (70%) and full-fat enzyme active soya (30%) composite flour was mixed with other ingredients (water, sugar, baking powder, vanilla essence, margarine) to make a dough, followed by resting for 15 min or 24 h respectively, before baking. Increasing dough resting time from 15 min to 24 h produced biscuits that were thicker (7.6 mm), with moisture content that increased by 30%, had lower stress and required less force to break (34.8 N). Biscuits made from dough rested for 15 min were 6.5 mm thick, had higher stress and required 51.1 N to break (p < 0.05). The higher moisture content of sorghum-soya biscuits made with dough rested for 24 h suggests that water acted as a plasticizer and reduced the rigidity of the biscuit matrix. Increase in biscuit thickness and diameter with dough resting time may have been influenced by aerating effect of baking powder that produced leavening gases during dough mixing and baking. Enzymatic action of proteinase and amylases on proteins and starch in sorghum-soya dough, respectively, may have been more pronounced during the longer dough resting time, resulting in more browning reactions to occur in biscuits made from dough rested for 24 h. Fifty consumers assessed the acceptability using a 9-point hedonic scale and preference testing of the biscuits from dough rested for 15 min and 24 h. The consumer sensory evaluation test revealed that the sorghum-soya biscuits were equally liked for appearance, colour and texture, despite significant differences in instrumental texture and colour of biscuits with dough resting time (p < 0.05). Biscuits made from dough rested for 24 h received lower liking scores for taste and overall flavour, and were less preferred to biscuits made from dough rested for 15 min. The perceived bitter aftertaste in these biscuits may have been caused by formation of bitter peptides during enzymatic hydrolysis of proteins with dough resting time. Comments suggested that biscuits made from both resting times were grainy. Increasing the dough resting time did not affect starch and protein digestibility of the biscuits. As expected, in vitro starch digestibility confirmed that the estimated glycaemic index (EGI) of sorghum-soya biscuits was lower (approx. 47%) than that of commercial digestive whole-wheat biscuits (64%) and white bread (94%). There was no significant difference (p > 0.05) in thermal properties of sorghum-soya (70:30) dough and/or biscuits with increasing dough resting time. Increasing sorghum-soya dough resting time to 24 h may have promoted hydration of some starch granules but not starch gelatinization. Longer dough resting treatment did not have a positive impact on the flavour, as consumers preferred the flavour of the biscuits from dough rested for 15 min. For this reason, it is recommended to not rest the sorghum-soya biscuit dough for 24 h.