Antinutrient effects of plant polyphenolic compounds
A series of in vitro investigations was conducted to study the effects that polyphenolic compounds exerted toward rumen cellulolytic micro-organisms. The effect of the complexing of tannins with proteins on the ruminal degradation and intestinal digestibility of protein was also tested. Five polyphenolic compounds of known molecular mass were selected as representative of hydrolysable tannin (tannic acid) or their molecular components (gallic acid and ellagic acid) and components of condensed tannin (catechin and epicatechin). The effects of polyphenolic compounds on the degradation of microcrystalline cellulose (Avicel) and ryegrass and on the formation of gas and VFAs by mixed rumen micro-organisms undergoing rumen-like fermentation (consecutive batch culture, CBC) were studied. On a molar and weight basis, tannic acid was the most inhibitory compound towards cellulose degradation and on the production of VFAs followed by ellagic acid, catechin and epicatechin. Low concentrations (up to 10 mM) of gallic acid caused a significant (P < 0.05) stimulation of cellulolysis, gas and VFA formation. This was probably due to the presence of gallate-degrading bacteria in the rumen. The ability of the ruminal fungus Neocallimastix frontalis to degrade cellulose in two different media, to adhere to cellulose, to utilize glucose and to form colonies from zoospores was assessed, in the presence and absence of polyphenolic compounds. The compounds tested were more inhibitory towards fungi grown in a defined medium rather than in a peptide and rumen fluid-containing medium, presumably due to the interactions between the proteins in the medium and the tested compounds. The adhesion of three fractions of the fungi to cellulose was inhibited by the presence of phenolic compounds in the medium.