Wheat Straw Upcycling: Severity Effects, Release Kinetics and Optimization of an Alkali-Catalyzed Organosolv Treatment for Enhanced Recovery of Hydroxycinnamate Antioxidants

The examination presented herein aimed at developing a benign alkali-catalyzed organosolv treatment for efficacious recovery of antioxidant hydroxycinnamates from wheat straw (WS), which is a widespread agricultural residue, rich in lignocellulosic material. After an initial screening of aqueous mixtures of various common alcohols, 20% (v/v) 1-propanol was selected as the most efficient solvent, while a following trial indicated 1.5% sodium hydroxide as the most appropriate catalyst concentration. Using this system (20% 1propanol/1.5% sodium hydroxide), WS treatment was investigated by carrying out polyphenol recovery kinetics, estimating the effect of severity and performing treatment optimization with response surface methodology. It was shown that the yield in total polyphenols and the ferric-reducing power of the extracts produced were highly correlated with treatment severity, but the antiradical activity was less so. Under optimized conditions, the treatment afforded a total polyphenol yield of 30.0 ± 1.7 mg ferulic acid equivalents g−1 dry WS mass, at 300 min and 80 °C. Analysis of the extracts obtained under optimized conditions with liquid chromatography–tandem mass spectrometry revealed the instrumental role of the alkali catalyst in liberating major bound hydroxycinnamates, namely p-coumaric acid and ferulic acid. The corresponding yields of these two compounds were 4.01 and 2.55 mg g−1 dry WS mass, suggesting WS as a promising source of high value-added phytochemicals, with a significant prospect in food, pharmaceutical and cosmetics industry.