Supplementation with electrically conductive materials unlocks lactic acid conversion into volatile fatty acids during cheese whey fermentation

Cheese whey (CW), a by-product of the dairy industry, poses environmental challenges due to its high organic load and substantial production volumes. Dark fermentation (DF) offers a promising biological approach to valorizing CW by converting its carbohydrate-rich organic matter into valuable products such as organic acids, alcohols, and hydrogen. This study investigated the application of electrically conductive materials (ECMs) - specifically magnetite, biochar, and graphite - to enhance CW fermentation and increase the production of high-value volatile fatty acids (VFAs). Batch fermentation experiments revealed that incorporating ECMs significantly influenced the DF process. Notably, VFA production, particularly propionic acid, was markedly enhanced. In unamended control microcosms, CW fermentation led to an almost complete conversion of carbohydrates into lactic acid. Among the ECMs tested, magnetite had the greatest impact, increasing total VFA concentrations to 45.3 ± 5.9 g COD/L - a 22.5-fold improvement over the control. The addition of ECMs promoted the growth and enrichment of microorganisms capable of lactic acid reduction into propionic acid, such as Clostridiaceae and Propionibacteraceae, while also altering the microbial community and electron flow dynamics. This resulted in a significant increase in acetic acid production, which was over five times higher in ECM-amended treatments compared to controls. ECMs likely facilitated the disposal of excess reducing power, possibly via direct interspecies electron transfer (DIET), which further enhanced lactic acid conversion to propionic acid. From an environmental perspective, this study offers a sustainable solution for managing CW, reducing its environmental impact by converting it into valuable biochemicals. From an industrial standpoint, the enhanced production of VFAs, particularly propionic and acetic acids, presents a pathway to generate precursors for bio-based polymers, food additives, and other high-value applications.