Physiological heterogeneity in Lactobacillus casei fermentations on residual yoghurt whey

•Efficient lactic acid production by Lactobacillus casei in mixed sugar fermentation.•Monitorization of Lactobacillus casei physiological states through flow cytometry.•Lactobacillus casei cells remained metabolically active under uncontrolled-pH conditions.•Increase in physiological heterogeneity of Lactobacillus casei under pH-controlled conditions.•A segregated kinetic model derived from flow cytometry data was developed. Lactobacillus casei is a well-known lactic acid-producer with substantial industrial interest. Currently, inexpensive lactic acid substrates such as residual yoghurt whey are being increasingly employed as revalorization strategies for such polluting food industry wastes. However, the influence of different bioprocessing conditions on the cellular functionality and physiological status of L. casei at single cell level has barely been evaluated to date. In the present study, monitoring the different physiological states of L. casei through multiparametric flow cytometry during lactic acid production from residual yoghurt whey showed that the majority of L. casei cells remained in healthy, metabolically active state (∼70%) under uncontrolled-pH conditions (pH