Biodegradability of Free Monomeric and Cell-Wall-Bound Phenolic Acids in Maize Stover by Two Strains of White-Rot Fungi

Abstract

Development of fungal bioconversion technology to upgrade feeding value of fibrous crop residues into animal feed requires the understanding of the impact of fungal treatments on cell wall phenolic acids. This study investigated the capacity of two strains of white-rot fungi, Phanerochaete chrysosporiumandPhlebia brevispora, in degrading cell wall-bound phenolic acids in maize (Zea maizeL) stover and the free phenolic monomers in a liquid culture medium. In the study, 150 mg litre−1 ofp-coumaric acid (PCA), ferulic acid (FA), syringic acid (SYA),p-hydroxybenzoic acid (PHBA), vanillic acid (VA), and 200 mg litre−1 of caffeic acid (CA) were added to a liquid nutrient medium and incubated with each of the fungi for 28 days. The results indicated that within 21 days the fungal treatments completely removed the added PCA and FA from the medium, and over 94% of SYA was degraded after 28 days. During the incubation periods, VA and PHBA were also extensively removed byP chrysosporium, but resistant to degradation byP brevispora. Disappearance of caffeic acid from the medium was related to both fungal degradation and abiotic hydrolysis. In the culture media,P chrysosporiumwas superior toP brevisporain removing the phenolic acids. Solid incubation of maize stover with the selected fungi for 28 days resulted in extensive degradation of cell-wall-bound FA and PCA. However, complete degradation of cell wall FA and PCA did not occur in maize stover. In contrast to results in pure liquid medium,P chrysosporiumdegraded only 20% of the initial concentration of cell wall PCA, and was less effective in degrading cell wall FA compared toP brevispora. These results indicate that the white-rot fungi were able to degrade phenolic acids both in pure culture medium and in cell wall matrix of maize stover. Furthermore,P brevisporaexhibited stronger ability to degrade cell-wall-bound phenolic acids. Thus, this fungus can be more effective for the enhancement of fibre digestibility.