Production of lactic acid from lignocellulosic hydrolysates by thermotolerant bacteria

Abstract

Lactic acid (LA), is an industrially important organic acid with extensive applications in pharmaceuticals and food industries as well as biodegradable plastics. It has garnered significant attention for sustainable production from renewable lignocellulosic biomass resources. This study was aimed at lactic acid production by isolating thermotolerant and inhibitor-tolerant bacterial strains, optimizing lignocellulosic biomass utilization and employing co-cultivation strategies for enhancing lactic acid yields. Among 45 bacterial isolates obtained from compost, soil, and fermented food two strains Bacillus licheniformis DGB and Bacillus sonorensis DGS15 were identified as high lactic acid producers which were thermotolerant and inhibitor-tolerant. They demonstrated robust growth in Bushnell Haas medium and efficiently utilized glucose and xylose at elevated temperatures (45°C 50°C), while tolerating inhibitory compounds (furfural and hydroxymethyl furfural) commonly derived from lignocellulosic biomass pretreatment. Initial fermentation trials yielded 13.2 g/L and 12.8 g/L of lactic acid for DGB and DGS15, respectively. Morphological, biochemical and molecular analyses confirmed their identity, and their thermotolerance and inhibitor resistance ensured suitability for industrial applications. Furthermore, both strains exhibited efficient carbon catabolite repression (CCR) bypass features, enabling simultaneous glucose and xylose metabolism. Rice straw and wheat straw were selected as lignocellulosic feedstocks due to their abundance and high cellulose content. Pretreatment processes involving acid hydrolysis and enzymatic saccharification were optimized using Response Surface Methodology (RSM) to maximize sugar release. Pretreated rice straw yielded 50.8 g/L of total reducing sugars (TRS) from 100 g of biomass, while wheat straw yielded 48.6 g/L. Structural analysis using FTIR confirmed effective delignification, with the disappearance of lignin peaks at 1670 cmand#8315;¹, and SEM analysis revealed significant disruption