Enhancing alfalfa conversion efficiencies for sugar recovery and ethanol production by altering lignin composition

Alfalfa (Medicago sativa L.) biomass was evaluated for biochemical conversion into ethanol using dilute-acid and ammonia pretreatments. The two alfalfa lines compared were a reduced S-lignin transgenic cultivar generated through down regulation of the caffeic acid O-methyltransferase gene and a wild-type control. Both were harvested at two maturities. All the samples had similar carbohydrate contents including a mean composition of 316 g glucan and 497 g total neutral carbohydrates per kg dry biomass, which corresponds to a theoretic ethanol yield of 382 l/ton. Ethanol yields for alfalfa stems pretreated with dilute-acid were significantly impacted by harvest maturity and lignin composition, whereas when pretreated with dilute-ammonia, yield was solely affected by lignin composition. Use of a recombinant xylose-fermenting Saccharomyces strain, for converting the ammonia pretreated alfalfa samples, further increased ethanol yields. Ethanol yields for the xylose-fermenting yeast were 232–278 l/ton and were significantly enhanced for the reduced S lignin cultivars.