Biotransformation of Agro-Livestock Residues by Lactobacillus delbrueckii subsp. bulgaricus: Advancing Circular Bioeconomy in Veracruz, Mexico

Abstract

Agro-industrial residues from livestock and sugarcane production—particularly abundant in regions such as Veracruz, Mexico, where both agro-industries co-occur in high density—constitute underutilized carbon and nitrogen streams with high potential for biotransformation within circular bioeconomy frameworks. This study aimed to evaluate whether inoculation with Lactobacillus delbrueckii subsp. bulgaricus SP96 at increasing levels (5, 10, and 15% v/v) achieves simultaneous stabilization, carbohydrate-to-lactic-acid conversion, and nutritional enrichment of a thermally pretreated bovine manure (BM)–agro-sugarcane waste (ASCW) mixture, and to describe the underlying growth kinetics using the Gompertz model. Thermal pretreatment reduced Salmonella spp. and fecal coliforms to levels compliant with NOM-004-SEMARNAT-2002 Class B biosolid standards, yielding a substrate with suitable fermentability (14.56 gL−1 carbohydrates, 0.44% total nitrogen, pH 6.52, and 90.43% volatile solids). Fermentation at 37 °C and 120 rpm for 72 h followed Gompertz kinetics (R2 = 0.92–0.97). The 15% inoculum achieved the highest conversion efficiency and product yield (lactic acid and carbohydrate consumption), whereas the 10% inoculum represented the most balanced operating condition (best kinetic fit, R2 = 0.97). However, differences in lactic acid concentration among treatments were not statistically significant (p > 0.05). The resulting biomass showed high organic matter (93.45%), increased crude protein (8.5%), a C/N ratio of 39.9, and enrichment in P2O5, MgO, Na2O, and B. These findings validate a low-cost, scalable platform for simultaneous waste stabilization and generation of value-added, protein-enriched biomass with potential application as a feed supplement and soil amendment, pending further safety and functional characterization, from agro-livestock residues.