Inhibition of Structural Carbohydrate Fermentation by Forage Phenolics

The benzoic and cinnamic acids found in forages were tested for their inhibitory effects on digestion of cellulose and hemicellulose by mixed cultures of rumen microorganisms in vitro. Rate and extent of cellulose digestion with the addition of cinnamic acids were determined. The benzoic acids and aldehydes were also compared for their inhibitory potential. Of the benzoic acid derivatives, only the parent compound (benzoic acid) inhibited in vitro dry matter disappearance (IVDMD) of cellulose after 48 h, whereas most of the cinnamic acids were inhibitory. None of these compounds depressed IVDMD of hemicellulose. Extent of cellulose digestion was unaffected by cinnamic acids, but rate of digestion was significantly reduced. The ranking of compounds for inhibitory potential was essentially reversed between 48 and 168 h of fermentation as compared with the ranking for 0 to 48 h of fermentation. There were also shifts in the molar proportions of volatile fatty acids produced between these times. The rapid reduction in concentration of cinnamic acids by microbial fermentation may explain these shifts. Benzoic aldehydes were more inhibitory of structural carbohydrate fermentation than were acids. No clear relationship existed between addition of hydroxyl or methoxyl groups to benzoic or cinnamic acids and their depressing effects on IVDMD. Cinnamic acids, which are more abundant in forages, were more inhibitory than the benzoic acids; but for all compounds, it appeared that hemicellulolytic bacteria were generally more tolerant of these compounds than the cellulolytic bacteria.