The Effect of Acid Catalysis on Hydroxycinnamate Recovery from Corn Stover Using Hydrothermal and Organosolv Treatments

Corn stover (CS) is a highly abundant type of agricultural biowaste, largely composed of lignocellulosic material. CS may be a particularly rich pool of hydroxycinnamates, represented primarily by p-coumaric acid and ferulic acid; yet, these compounds are bound onto the lignocellulosic matrix, and their release requires an appropriate acid and/or alkaline catalysis. This being the case, this study herein aimed to develop an effective process to boost hydroxycinnamate recovery by employing acid-catalyzed hydrothermal and organosolv treatments. To this end, oxalic acid was tested as a benign, natural acid catalyst, along with the well-examined sulfuric acid. A kinetic assay showed that both the acid catalyst and the use of an organic solvent (ethanol) may greatly impact the rate and level of polyphenol recovery. Under optimized conditions, determined by implementing response surface methodology, it was demonstrated that the organosolv treatment was far more effective than the hydrothermal one, with regard to total polyphenol recovery, while the oxalic acid catalysis was equally efficient as the sulfuric acid one. This treatment afforded 17.8 ± 2.3 mg gallic acid equivalents per g of dry CS mass. However, a thorough insight into the polyphenolic composition of the extracts produced revealed that hydrothermal treatment may enable, apart from p-coumaric and ferulic acid release, the formation of a compound tentatively identified as an ester of p-coumaric acid with a pentose. Furthermore, it was shown that sulfuric acid-catalyzed organosolv treatment provided almost 25 and 34% higher yields for p-coumaric and ferulic acid, respectively, but it strongly inhibited p-coumaric acid-pentose ester formation. These compositional differences appeared to impact the antioxidant activity of the corresponding extracts. It was concluded that the oxalic acid-catalyzed ethanol organosolv treatment of CS may have important potential in a biorefinery context, but improvements are required to further enhance treatment performance. This would lead to replacing corrosive catalysts, such as sulfuric acid, with benign ones, thereby establishing a fully sustainable process for the recovery of bioactive phytochemicals.