Changes in the alkaline-labile phenolic compounds of wheat straw cell walls as affected by SO2 treatment and passage through the gastro-intestine of sheep

Sheep were fed two rations based on untreated (WS) and SO2-treated (SO2-WS) wheat straw, and the effect of chemical treatment and passage through the gastro-intestine on the composition and degradation of ester and ether-linked cell wall (CW) phenolics was studied. The SO2 treatment reduced the content of total ferulic acid (FA) and p-coumaric acid (PCA) by 35% while tripling the level of vanillin and increasing by 40% the concentration of protocatechuic acid. In WS most of the phenolic compounds were CW-bound, but 37% of the vanillic and 88% of the protocatechuic acids were in the alcohol soluble (AS) fraction. The solubilising effect of the treatment was expressed in releasing the phenolics from the CW mainly as AS-lignins. Most of the FA (62%) was ether-linked, whereas most of the PCA (78%) was ester-linked in the CW of WS. The other minor components were either entirely or mostly, etherified units. The SO2 treatment was more effective in cleaving the ester than the ether bonds of the cinnamic acids. Ester-linked FA was more extensively degraded in the rumen than ester-linked PCA. Ester-linked FA and PCA were more extensively degraded in the rumen than the respective ether-linked compounds. Nevertheless, substantial amounts of ether-linked FA, PCA and other phenolics were removed from CW in the rumen, most likely as oligo-lignols. Phenolic compounds were determined in rumen liquor of sheep fed the WS and WS-SO2 rations. FA was not detected and PCA was at a very low (20–40 μM) concentration. Phenyl-propanoic acid (PPA) was the major monomeric phenolic compound detected, at concentrations of 580 and 380 μM in the rumen of WS and WS-SO2 sheep, respectively. It is suggested that hydrogenation of PCA and combined hydrogenation and demethoxylation of FA were responsible for the production of PPA in the rumen.