Use of flow cytometry to follow the physiological states of microorganisms in cider fermentation processes.

The flow cytometry (FC) technique used with certain fluorescent dyes (ChemChrome V6 [CV6], DRAQ5, and PI) has proven useful to label and to detect different physiological states of yeast and malolactic bacterium starters conducting cider fermentation over time (by performing sequential inoculation of microorganisms). First, the technique was tested with pure cultures of both types of microorganisms grown in synthetic media under different induced stress conditions. Metabolically active cells detected by FC and by the standard plate-counting method for both types of microorganisms in fresh overnight pure cultures gave good correlations between the two techniques in samples taken at this stage. Otherwise, combining the results obtained by FC and plating during alcoholic and malolactic fermentation over time in the cider-making process, different subpopulations were detected, showing significant differences between the methods. A small number of studies have applied the FC technique to analyze fermentation processes and mixed cultures over time. The results were used to postulate equations explaining the different physiological states in cell populations taken from fresh, pure overnight cultures under nonstress conditions or cells subjected to stress conditions over time, either under a pure-culture fermentation process (in this work, corresponding to alcoholic fermentation) or under mixed-fermentation conditions (for the malolactic-fermentation phase), that could be useful to improve the control of the processes.