Ammonia–Mechanical Pretreatment of Wheat Straw for the Production of Lactic Acid and High-Quality Lignin

In this study, wheat straw was fractionated into carbohydrates (cellulose and hemicellulose) by ammonia–mechanical pretreatment for l-lactic acid fermentation. Under optimal conditions (aqueous ammonia concentration: 19% w/w, liquid–solid ratio: 2.1:1 w/w, holding time: 4.80 h), the delignification was more than 60%. After enzymatic hydrolysis, the maximum conversions of cellulose and hemicellulose were 92.5% and 83.4% based on the pretreatment residue, respectively. The wheat straw hydrolysate was used to produce l-lactic acid with Thermoanaerobacter sp. DH-217G, which obtained a yield of 88.6% and an optical purity of 99.2%. The ammonia–mechanical pretreatment is an economical method for the production of fermentable monosaccharide, providing potential for further downstream high value-added applications.