Effect of hydrolytic lignin on formation of protein–lignin complexes and protein degradation by rumen microbes

Several methods have been developed to protect feed protein from rumen microbial degradation. The current study aimed to evaluate the potential use of an industrial lignin, namely hydrolytic lignin, to protect protein from rumen microbial degradation. The hydrolytic lignins assessed in this study were extracted from wheat straw previously subjected to various steam treatment conditions (pressure: 15, 17 and 19 bar; reaction time: 0, 5 and 10 min; use of acidic catalyst: without and with 2% H2SO4 on DM basis). Results indicated that hydrolytic lignin can precipitate protein when measured by a standard bovine serum albumin assay. It was also observed that protein-precipitating capacity of lignin increased with increasing harshness of steam treatment until a point from which no further effect was observed. The effect of lignin upon protein degradation in vitro was clearly detected. Both ammonia nitrogen and iso-acid concentration in vitro were significantly decreased (P<0.01) when lignin was added to fermentation flask containing casein. Unlike tannins, hydrolytic lignins do not inhibit rumen microbial activity. Additionally, it was observed that lignin’s ability to bind and protect protein is a pH-dependent reaction. Protein binding to lignin is markedly reduced at pH<3.0.