Abstract: Using birch sap as the substrate, fermented broth was prepared by inoculating Lactobacillus paracasei LC37. With DPPH free radical scavenging rate as the core index, the fermentation process was optimized via single-factor experiments combined with response surface methodology. Liquid chromatography-mass spectrometry LC-MS) was employed for untargeted metabolomic analysis of samples before and after fermentation. Key differential metabolites (VIP≥1 and P＜0.05) were screened using the orthogonal partial least squares-discriminant analysis(OPLS-DA) model, followed by KEGG pathway enrichment analysis. Results showed that the optimal fermentation conditions were temperature 37 ℃, time 39 h, initial pH 7.00, and inoculum size 1.3 % (v/v). Under these conditions, the DPPH free radical scavenging rate reached 22.91 %, which was 4.7 times higher than that of the unfermented sample (4.86 %). A total of 505 key differential metabolites were identified before and after fermentation (317 upregulated and 188 downregulated).KEGG analysis indicated significant enrichment of D-fructose-6-phosphate in the pentose phosphate pathway. This metabolite can supplement 6-carbon substrates, activate the oxidative phase, and increase the production of NADPH and ribose-5-phosphate, providing a metabolic basis for the enhanced antioxidant capacity of the fermented broth.

Key words:

fermentation,  , Lactobacillus paracasei, birch sap, process optimization, metabolomic analysis