Flaxseed kernel as an alternative substrate for novel natto fermentation: metabolic characteristics and flavor formation mechanism

BACKGROUND Natto is traditionally produced from soybeans, but exploring novel substrates can expand its nutritional and organoleptic diversity. This study investigated the feasibility of utilizing flaxseed as an alternative substrate for novel natto production and elucidated the key metabolic and volatile flavor formation mechanisms during fermentation. RESULTS Multi‐sensory and chemical analyses, including electronic nose, electronic tongue, free amino acid profiling, gas chromatography–mass spectrometry (GC–MS), and headspace solid‐phase microextraction coupled with gas chromatography–mass spectrometry (HS‐SPME‐GC–MS), demonstrated that fermentation significantly transformed the flavor and nutritional characteristics of flaxseed natto. Fermentation reduced sweetness while markedly enhancing umami intensity and richness, driven by a sharp increase in total free amino acids from 1.52 to 79.56 g kg −1 , particularly glutamic and aspartic acids. Free fatty acid saturation remained essentially stable, with α ‐linolenic acid dominating the lipid profile. Metabolomic screening identified 368 differential volatile organic compounds (VOCs) out of 1017 total detected VOCs. Based on odor activity value (OAV > 1) and aroma concentration index (ACI > 0.001) criteria, eight key active aroma constituents were pinpointed, among which 2‐ sec ‐butyl‐3‐methoxypyrazine and 2‐isobutyl‐3‐methoxypyrazine were identified as the crucial contributors to the characteristic flavor profile of flaxseed natto. CONCLUSION Flaxseed represents a promising alternative substrate for producing novel natto with enhanced umami flavor, high α ‐linolenic acid content, and distinct volatile profiles. These findings provide critical insights into the aroma formation mechanisms of fermented flaxseed products, laying a scientific basis for the high‐value utilization of flaxseed in functional food development.