Effects of Organic Acid Catalysts on the Ethanol Organosolv Treatment of Wheat Bran to Produce Ferulate-Enriched Extracts

Wheat bran (WB) is a major wheat processing byproduct, and it is characterized by significant ferulic acid content and ferulate derivatives, which are biologically important polyphenols. These phytochemicals occur in WB in bound form, and their recovery requires acid- and/or alkali-catalyzed hydrolysis. In the work described herein, ethanol organosolv treatment was employed, catalyzed by organic acids (oxalic, citric), to investigate their catalytic potency in releasing ferulates. Sulfuric acid was also included to compare the effects of mild and strong catalysis. Treatment assessment based on severity showed that the yield of total polyphenols was related to the combined severity factor in an exponential manner, while kinetics revealed that increases in temperature resulted in lower recovery rates but higher yields. Response surface optimization suggested 80 °C and 300 min to be the ideal conditions, where both oxalic acid- and sulfuric acid-catalyzed treatments yielded 11.6 mg ferulic acid equivalents per g dry WB weight; citric acid-catalyzed treatment was significantly less efficient (p < 0.05), giving a yield of 9.2 mg ferulic acid equivalents per g dry WB weight. Liquid chromatography–diode array–tandem mass spectrometry analyses showed that both sulfuric acid and oxalic acid catalysis was pivotal in the generation of certain ferulate derivatives, whereas the effect of citric acid was very weak. Some of the major derivatives tentatively identified were ferulate pentose esters and related compounds, in line with earlier findings. The differences in the compositions of the extracts obtained were reflected in their antioxidant properties, where important differentiation was observed. It was concluded that oxalic acid-catalyzed treatment might be an effective replacement for corrosive sulfuric acid in processes that are aimed at harnessing WB as a raw material for the generation of bioactive ferulate derivatives.