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

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 chrysosporium and Phlebia brevispora, in degrading cell wall-bound phenolic acids in maize (Zea maize L) stover and the free phenolic monomers in a liquid culture medium. In the study, 150 mg litre(-1) of p-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 by P chrysosporium, but resistant to degradation by P brevispora. Disappearance of caffeic acid from the medium was related to both fungal degradation and abiotic hydrolysis. In the culture media. P chrysosporium was superior to P brevispora in 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 chrysosporium degraded only 20% of the initial concentration of cell wall PCA, and was less effective in degrading cell wall FA compared to P brevispora. These results indicate that the white-rot fungi were able ro degrade phenolic acids both in pure culture medium and in cell wall matrix of maize stover. Furthermore, P brevispora exhibited stronger ability to degrade cell-wall-bound phenolic acids. Thus, this fungus can be more effective for the enhancement of fibre digestibility.