YEAST VITALITY DURING CIDER FERMENTATION - 2 APPROACHES TO THE MEASUREMENT OF MEMBRANE-POTENTIAL

Simulated cider fermentations were carried out using a monoculture of a cider strain of Saccharomyces cerevisiae in a sterile apple juice-ba sed medium. The membrane potential of the yeast was assessed in two di fferent ways: firstly by using the fluorescent dyes oxonol and rhodami ne 123 and visualising the yeast with flow cytometry and fluorescent m icroscopy, and secondly by using the acidification power test. These t echniques were used on yeast at various stages of a simulated cider fe rmentation and also on freshly grown yeast (3 days after inoculation) after the addition of an alcohol mixture similar to that accumulating in mature cider (10% ethanol, 200 ppm propan-1-ol, 32 ppm butan-1-ol, 70 ppm butan-2-ol, 190 ppm amyl alcohol, 17 ppm hexan-1-ol and 260 ppm S-phenyl ethanol). Little change was observed to the plasma membrane transmembrane potential as indicated by oxonol exclusion over a 4 week fermentation period, but rhodamine 123 staining which indicates mitoc hondrial membrane potential showed a drastic decline during the first 3 days. The total acidification power index remained almost constant w ith fermentation age, but the individual components of the index showe d considerable changes.