Hydrothermal and Organosolv Treatments for Hydroxycinnamate Release from Corn Stover: Strong versus Mild Alkaline...

Corn stover (CS) is an abundant biomaterial, which is regularly rejected during corn harvesting. This biowaste is a typical lignocellulosic source rich in hydroxycinnamates, which are mainly represented by p-coumaric acid and ferulic acid. These polyphenols are largely bound onto the lignocellulosic complex and can be effectively liberated using alkaline catalysis. On this basis, the work described herein targeted at developing a high-performance process for producing hydroxycinnamate-enriched extracts, by deploying alkali-catalyzed hydrothermal and organosolv treatments. For this purpose, sodium carbonate was tested as a benign, natural alkali catalyst, along with the well-studied sodium hydroxide. The kinetic study demonstrated that both the alkali catalyst and the organic solvent (ethanol) may significantly affect polyphenol recovery, a fact further investigated by carrying out response surface optimization. The hydrothermal treatment was shown to be more efficacious than the organosolv one, with regard to total polyphenol recovery, while the sodium carbonate catalysis was less efficient compared to the sodium hydroxide one. Under optimized conditions, the hydrothermal treatment afforded 74.4 ± 3.6 mg gallic acid equivalents per g of dry CS mass. On the other hand, a more thorough investigation of the polyphenolic profile of the extracts obtained clearly demonstrated that the sodium hydroxide-catalyzed organosolv treatment provided almost 76 and 98% higher yields for p-coumaric and ferulic acid, respectively, compared to the hydrothermal treatment. Extract composition impacted the antioxidant activity, and it was revealed that the higher the p-coumaric acid/ferulic acid ratio, the stronger the antioxidant effect. It is proposed that the sodium hydroxide-catalyzed ethanol organosolv treatment of CS may be a particularly promising technique in a lignocellulose biorefinery frame, although improvements might be necessary to further increase treatment performance. Such a process might contribute to fully valorizing agricultural biowastes for the production of high value-added chemicals, in line with the "lignin first' philosophy.