Lactic acid from mixed food waste fermentation using an adapted inoculum: Influence of pH and temperature regulation on yield and product spectrum

Environmental conditions (pH and temperature) are expected to influence microbial community composition and product spectrum in mixed-culture food waste (FW) fermentation. However, some conditions may favour growth of multiple organisms that compete for common substrates or consume target metabolites. The inoculum plays an integral role in mixed-culture fermentation, but it is currently unknown how an adapted inoculum, known to selectively produce the target metabolite, would influence fermentation, and how environmental conditions could control fermentation outcomes. Therefore, this study assessed the effects of pH (uncontrolled vs. controlled pH 4.0–6.0) and temperature (35–60 °C) on lactic acid (LA) from synthetic mixed FW batch fermentation (80 gVS·L−1) utilising an adapted fermentation inoculum known to produce significant LA (10% inoculum volume). Concentrations of LA and competing organic acids were measured. Uncontrolled pH encouraged Lactobacillus growth but resulted in a low LA yield due to inhibitory conditions. Controlled pH 6.0 improved LA production but introduced LA consumption and competitive butyrate production. Observed butyrate production was dependent on pH and temperature and correlated with the growth of Clostridium Sensu Stricto 12. At pH 6.0 and 50 °C, observable LA consumption was eliminated, and the LA yield was maximised at 0.55 gLA·gVS−1 (39 gLA·L−1) while Lactobacillus remained dominant. The adapted inoculum effectively promoted LA production, while pH and temperature regulation were effective control levers to target LA.