Effect of lactic acid bacteria or yeast as co-inoculants with Aspergillus oryzae in koji making on the flavor properties of walnut meal-based soy sauce

Co-inoculation with Aspergillus oryzae , Lactobacillus plantarum , and Saccharomyces rouxii significantly improved walnut soy sauce (WSS) quality. 72 volatile compounds (VOCs) and 1972 non-VOCs were identified by headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS) and liquid chromatography–mass spectrometry (LC-MS), respectively. Compared to single A. oryzae fermentation, S. rouxii co-inoculation increased amino acid nitrogen by 36.1% to 1.13 g/100 mL ( p < 0.05), while L. plantarum elevated total acids by 33% and ketones to 54.7% relative content. Triple co-inoculation fermentation achieved the highest umami (7.8 point) and soy sauce-like flavor (8.2 point) with minimal bitterness. Metabolic pathway analysis revealed that key flavor compounds were associated with glycolysis, organic acid, and amino acid metabolism. These findings provide new insights into optimizing the flavor and quality of WSS through mixed-culture fermentation.