YEAST VITALITY DURING CIDER FERMENTATION: TWO APPROACHES TO THE MEASUREMENT OF MEMBRANE POTENTIAL

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Abstract
Simulated cider fermentations were carried out using a monoculture of a cider strain of Saccharomyces cerevisiae in a sterile apple juice‐based medium. The membrane potential of the yeast was assessed in two different ways: firstly by using the fluorescent dyes oxonol and rhodamine 123 and visualising the yeast with flow cytometry and fluorescent microscopy, and secondly by using the acidification power test. These techniques were used on yeast at various stages of a simulated cider fermentation 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 2‐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 mitochondrial membrane potential showed a drastic decline during the first 3 days. The total acidification power index remained almost constant with fermentation age, but the individual components of the index showed considerable changes.