Extraction of natural pigments from red sorghum (Sorghum bicolor) husk by ultrasound and microwave assisted extraction: A comparative study through response surface analysis

Sorghum (Sorghum bicolor) is an important source of bioactive compounds, such as the phenolic compounds. The phenolic compounds in sorghum grain are concentrated in its hull; however, its hull is usually being discarded as a waste. In this study, ultrasonication power intensity (916, 1018, and 1120 W/cm²), sonication time (20, 25, and 30 min), and particle size (100, 300 and 500 μm) for ultrasound assisted extraction and, microwave power (50, 60 and 70 W), microwave time (90, 105, and 120 s), and particle size (100, 300, and 500 μm) for microwave assisted extraction were investigated and optimized using Box–Behnken design. For ultrasound assisted extraction, optimized condition was found as 1078 W/cm² power intensity, 23.6 min sonication time and 100 μm particle size, and corresponding values of responses were 8.55 mg LE/g dw total anthocyanin content, 779.0 mg GAE/g dw total phenolic content, 172.0 mg CE/g dw total flavonoid content, 70.0 and 73.4% antioxidant activity by DPPH and ABTS methods, respectively and 11.1 redness (a*). For microwave assisted extraction, optimized condition was found as 63 W power, 101 s time and 100 μm particle size, and the responses were 10.5 mg LE/g dw total anthocyanin content, 606.0 mg GAE/g dw total phenolic content, 195.0 mg CE/g dw total flavonoid content, 79.5 and 82.2% antioxidant activity by DPPH and ABTS methods, respectively and 13.6 redness (a*). For validation of model at optimum conditions, predicted values were insignificantly (p > .05) differ than actual values for all the responses for both ultrasound and microwave assisted extraction. PRACTICAL APPLICATIONS: Sorghum consists of a rare class of natural pigments (3‐deoxyanthocyanidins) in its husk. These natural pigments have antioxidant, anti‐proliferative activity, and thus, can be utilized as natural food colorant in food industry. Extraction and isolation of these natural pigments from sorghum husk are prerequisites for their application in food industry. In order to obtain an efficient extraction of sorghum anthocyanins, resulting in minimal degradation and high antioxidant capacity, the choice of the extraction method is important. This research provides information about standardization of novel extraction techniques (ultrasound and microwave assisted extraction) for effective utilization of sorghum husk for extraction of natural pigments, which can be utilized for application in various food products.