Effect of Time, Temperature and Size Reduction on Some Physico-chemical Characteristics of Sorghum bicolour Leaf Sheath Extracts

Abstract

Sorghum bicolor leaf sheaths are a common dried ingredient used as a colourant for waakye, a popular Ghanaian dish made from rice and cowpea. The leaf sheaths are also used in traditional medicine due to its impressive bioactive composition. Its potential as a natural food colourant and antioxidant has been established but the effect of different processing conditions on the physicochemical composition and characteristics have not been adequately researched. The present study assessed the effect of size reduction (whole, coarse and fine), temperature (room temperature (28°C) and 98°C) and time (20, 40 and 60 min) on the colour, pH and ascorbic acid content of sorghum leaf sheaths. Samples were steeped in a measured amount of water and analysis conducted on the extracts. Size reduction significantly reduced the lightness (l*) and colour intensity (chroma and hue) of extracts steeped at both temperatures. The pH of all extracts was relatively neutral, ranging from 6.63 to 7.23 and was not significantly affected by size reduction or time. Extraction of ascorbic acid was more effective at 98°C and did not degrade with constant heating within the experimental time. Average ascorbic acid content of extracts was 3.89 g/L. For effective utilization and value addition of Sorghum bicolor leaf sheaths, food producers should consider fine milling and late incorporation into food for optimum colour and phytochemical content preservation.