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 <3.6), whereas a progressive increase in population heterogeneity was determined (increasing the damaged and dead subpopulations) with higher production (41.5 g/L lactate titer) and sugar consumption rates when a pH-controlled strategy at 6.5 was adopted. A segregated kinetic model was additionally developed to better describe the physiological behaviour of microbial heterogeneity, gaining deeper knowledge on the lactic acid-producing ability of each subpopulation under pH-controlled conditions in the mixed sugar co-fermentation. This study provides further understanding on the role of physiological heterogeneity in lactobacilli populations useful to enhance bioprocess performance and thus achieve efficient lactic acid production.