Enhanced Lactic Acid Production From Soybean Waste Using a Xylose-Fermenting Lactiplantibacillus pentosus Strain

The high carbohydrate content of the cellulose and hemicellulose fractions in agricultural crops, along with their by-products and derivative residues (e.g., soybean straw, hulls, sugarcane bagasse, and corn stover), offers a promising resource for biorefinery industries. These lignocellulosic materials, generated during harvesting and mechanical processing, support large-scale biological production of renewable energy, sustainable chemicals, and alternative food sources.

This study investigated lactic acid fermentation of liquid hydrolysates from soybean straw hydrothermally pretreated with either sodium hydroxide (NaOH) or hydrogen peroxide (H₂O₂), and assessed the impact on cellulolytic enzyme activity and subsequent sugar release. Soybean straw (10% w/v, dry weight basis) was subjected to moderate-severity hydrothermal pretreatment at 121 °C for 60 min in the presence of either 1% (v/v) NaOH or H₂O₂ as representative dilute chemical benchmarks. The resulting pretreated solids were enzymatically hydrolyzed using a cellulolytic enzyme mixture at a loading rate of 50 mg enzyme protein/g cellulose (equivalent to 90 IU/g glucan) at 50 °C for 72 h under shaking incubation at 150 rpm.

Scanning electron microscopy (SEM) revealed that both NaOH and H₂O₂ pretreatments effectively disrupted the surface morphology and biomass structure of the soybean straw, increasing surface roughness and exposure of reactive sites, thereby enhancing enzymatic accessibility and improving the conversion of glucan and xylan to glucose and xylose, respectively. NaOH pretreatment achieved hydrolysis yields of 66.8% for cellulose and 45.7% for hemicellulose, whereas H₂O₂-treated samples yielded only 52.3% (cellulose) and 38.1% (hemicellulose). In comparison, the untreated control yielded only 21.6% cellulose and 16.4% hemicellulose conversion. A positive control using Solka-Floc (a lignin-free cellulose powder) yielded 83.5% glucose and 62.6% xylose conversion. Subsequent lactic acid fermentation of the NaOH-pretreated hydrolysates using a xylose-co-fermenting Lactiplantibacillus pentosus strain produced L-lactic acid at 80.5% of the theoretical yield (12.72 g/L), with productivity of 0.35 g/L/h and a yield of 0.2544 g LA/g of NaOH-pretreated straw solids.

These results demonstrate that NaOH pretreatment is more effective than H₂O₂ in enhancing the enzymatic saccharification of soybean straw. Moreover, the resulting hydrolysates can be efficiently converted into L-lactic acid using a Lactiplantibacillus pentosus strain capable of co-fermenting xylose, supporting the feasibility of this integrated process for value-added bioconversion of lignocellulosic biomass.