Optimization of Blue Honeysuckle Co-Fermentation with Lactobacillus rhamnosus and Saccharomyces cerevisiae to Enhance Total Phenolic Content and Antioxidant Activity

Blue honeysuckle (Lonicera caerulea L.) is a cold-adapted berry rich in bioactive polyphenols. The association of phenolic compounds with the plant cell-wall matrix may limit their extractability. To promote polyphenol release and enhance antioxidant properties, a blue honeysuckle matrix was co-fermented with Lactobacillus rhamnosus 6224 and Saccharomyces cerevisiae W5. Process parameters were optimized using response surface methodology, with total phenolic content (TPC) as the response variable. Under the optimized conditions, TPC reached 1374.79 mg/100 g. Compared with the unfermented control, DPPH radical, ABTS radical, H2O2, and hydroxyl radical scavenging activities increased by 4.27, 15.10, 12.89, and 12.63 percentage points, respectively. LC-MS profiling showed that co-fermentation markedly altered the phenolic profile of the blue honeysuckle matrix and increased the relative abundances of several putatively annotated phenolic compounds, including quercetin, protocatechuic acid, catechin, and caffeic acid. These findings indicate that the optimized co-fermentation process increased TPC, altered phenolic composition, and enhanced in vitro antioxidant activity, offering a potential approach to the value-added processing of blue honeysuckle.