Transcriptomic and metabolomic insights into how KCl and MgCl2 drive flavor diversity in low-sodium radish Paocai fermented by Levilactobacillus brevis.

High dietary sodium in fermented foods like traditional Chinese paocai poses significant public health risks and environmental burdens. As a preventative control measure, partial NaCl substitution with KCl and MgCl2 is explored, but its complex impact on flavor and microbial mechanisms remains unclear. This multi-omics study investigated the ion-specific effects of sodium salt substitution with 30% KCl (K30), 30% MgCl2 (Mg30), and a combination of 15% KCl and 15% MgCl2 (KMg15) on the fermentation of radish paocai by Levilactobacillus brevis PL6-1. All low-sodium groups maintained comparable LAB growth, acidification, and overall acceptability, ensuring microbiological safety and stability. The 30% MgCl2 substitution significantly reduced radish hardness, while K+ promoted mannitol synthesis and influenced citric acid metabolism. Combined K+/Mg2+ substitution (KMg15) further increased the fruity compound ethyl propanoate. Transcriptomic analysis revealed targeted gene expression changes in L. brevis PL6-1, rather than widespread alterations, in response to specific ionic environments. Key genes involved in propanediol utilization, hexuronic acid degradation, and stress response were differentially expressed. Notably, the upregulation of the pduE gene may have contributed to the increase in ethyl propanoate content. Functional enrichment showed Mg2+ stimulated broader energy metabolism, increasing metabolic diversity. This research elucidates how specific cations modulate gene expression in L. brevis, shaping distinct flavor profiles. Findings provide a scientific basis for designing innovative, low-sodium fermented vegetables with tailored sensory attributes, enhancing food quality and product sensory quality through effective preventative control measures.