Microbial upgrading of wheat straw: selective degradation of lignin by higher fungi and an Actinomycetes.

Physical and chemical treatment influence the straw composition and as a consequence its digestibility in ruminants. The combination of heating and concentrated NaOH solutions severely decreased the ration lignin / carbohydrates, resulting in a highly digestible substrate. As this is at the expense of high dry matter losses and because of the polluting impact on the environment, biological methods for the selective delignification of wheat straw are preferred. White-rot fungi such as Pleurotus pulmonarius, Pleurotus sajor-caju, Lentinus edodes and Phanerochaete chrysosporium were shown to effectively upgrade wheat straw. The digestibility was doubled and the lignin content halved within 4 to 12 weeks solid state fermentation (SSF) of ground wheat straw. SSF of chopped straw resulted in a slower fungal growth, lower lignin losses and a slower increase of straw digestibility. The results could be improved using cylindrical fermentors flushed with oxygen and the effect was pronounced when supplements were added. As an alternative to pasteurization, pre-incubation of the subtrate with lactic acid bacteria was proposed. Although, after such pre-incubation, straw must still be dried to a moisture content, optimal for fungal growth (80 per cent), ensiling was a good alternative. SSF with an actinomycetes, Streptomyces viridosporus, yielded a substrate with a higher lignin concentration and a lower digestibility than the unfermented substrate. The increase in lignin was due to the formation of acid-precipitable polymeric lignin, a lignin degradation intermediate, complexed with carbohydrates and proteins. As Pleurotus pulmonarius was able to consume this compound, the co-incubation with both organisms resulted in a highly digestible and lignin-poor straw. Lignin peroxydase (LiP), which is thought to play an important role in lignin degradation was produced in liquid cultures of Phanerochaete chrysosporium. It is rather delicate process and subjected to many variables. In addition to effects caused by carbon- or nitrogen-limitation, agitation speed and detergents, it was found that the media used to produce spores also showed effects. More important, however, were the effects caused by repeated subculturing, resulting in a quasi disappearance of enzyme activity. The results obtained in liquid media were taken in consideration when studying lignin degradation in solid substrates. However, it is still not clear that LiP alone should play the initiative role in thensformation of barley.