Metabolic Features and Proteolytic Pathways of Lactobacillus Harbinensis M1 during Soymilk Fermentation

Lactobacillus harbinensis is a novel species that has the potential to improve the quality of fermented product. L. harbinensis M1 (M1), previously isolated from naturally fermented tofu whey (NFTW), has been shown to provide probiotic potential and to improve the organoleptic properties of fermented soymilk. However, its lifestyle and contribution to health-related functionality are unclear. This study investigated the metabolic features and proteolytic pathways of M1 involved in soymilk fermentation, to uncover its molecular and functional characteristics. During soymilk fermentation, M1 converted soybean carbohydrates into lactic acid (6.91 mg/mL) and short-chain fatty acids (including acetic, propionic and butyric acids). Proteolytic analysis showed that M1 extensively hydrolyzed the soybean protein into oligopeptides and free amino acids. The soluble peptide fraction (189–451 Da) was increased from 4.6% to 86%. The whole genome of M1 was sequenced for metabolic prediction. Genes encoding enzymes required for carbohydrate/pyruvate metabolism and isoflavone biotransformation are present. The proteolytic system was predicted, including 4 genes encoding extracellular proteases, 64 genes encoding intracellular-peptidases and the Opp/Dpp peptide transport system. Finally, M1 improved the radical scavenging activities and growth inhibition effects on HepG2/MCF-7 cancer cells of soymilk. These findings offered a better understanding of the metabolic mechanisms of L. harbinensis M1, which elucidate its role and envisage its application in prebiotic products.