Non-enzymatic Raman sensing and imaging of lactic acid directly from intact microbial cultures with protonation-state...

Lactic acid is a key microbial metabolite with widespread industrial applications in food preservation, cosmetics, pharmaceuticals, polymer industries, and others. The direct detection of lactic acid in complex microbial cultures remains a challenge. Enzymatic assays and high-performance liquid chromatography are traditional methods for detecting lactic acid, but they require extensive sample preparation. In this study, we report a non-enzymatic Raman sensing framework for the direct detection and mapping of lactic acid in microbial culture. We used Lactobacillus plantarum and Lactobacillus acidophilus as representative lactic acid-producing microbes, and detected the characteristic Raman signal of lactic acid in their culture supernatants. An additional multiplex advantage of the approach is that both protonated and deprotonated forms of lactic acid were detected across species under varying culture pH conditions. Raman imaging was also performed directly on microbial cultures, enabling visualization of the spatial distribution of lactic acid. Raman images revealed a diffuse distribution of lactic acid within the unprocessed culture matrix, consistent with metabolic secretion by actively growing cells. This work pipeline highlights the analytical utility of Raman spectroscopy for a non-enzymatic, non-invasive, and minimal-signal-processing sensing platform for lactic detection and spatially resolved analysis, with potential for future integration into in-line monitoring systems in bioreactors.