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The physiological state of a yeast population used for inoculation determines how rapidly the cells adapt to new environmental conditions, begin proliferating and utilising extract. The decision as to whether a yeast culture is suitable for re-pitching should not be based only on viability determinations since this can be misleading. Increased proteolytic activity in a yeast population indicates the onset of senescence. A flow cytometric method has been developed for measuring a wide variety of proteinases in Saccharomyces cerevisiae employing a commercially available casein-dye conjugate. The detection of intracellular proteinase activity gives an early indication of apoptotic events and allows improved assessment of the physiological state of a yeast population. This knowledge will assist the industry to optimize the selection of yeast and its subsequent fermentation performance. Yeast cell autolysis with all its negative consequences for beer quality and stability will thus be minimised.

The interaction of a haze-active protein (gliadin) and a haze-active polyphenol (tannic acid) was studied in a model beer system in order to investigate the principle mechanisms of haze formation at low temperatures. Low concentrations (g/L) of tannic acid, high concentrations of gliadin, and comparatively high temperatures lead to maximum haze values. When considered on a molar basis, the greatest haze levels are displayed at an approximate 1:1 equivalence of polyphenol and protein. The greater part of haze formation was completed within 0.5 h, irrespective of the concentration of gliadin, the concentration of tannic acid, and the temperature of the model solution.

This study focused on the implementation of fluorescence optical methods and laser scanning confocal microscopy for monitoring brewing yeast performance. Physiological parameters and cell compounds in yeast cells (glycogen, neutral lipids, trehalose, bud scars, DNA and intracellular proteinases) have been successfully visualised with the aid of highly specific fluorochromes. The expression and sub cellular localisation of proteinase A during fermentation has been studied employing a Saccharomyces cerevisiae green fluorescent protein clone. This novel approach to monitoring brewing yeast performance provides new insights into physiological events that occur during wort fermentation.