THE RESPONSE OF BREWING YEASTS TO ACID WASHING

Brewing yeasts are inherently resistant to acid washing treatments but, under some conditions this resistance is diminished. Sixteen yeast strains, including both ale and lager strains, have been successfully washed using phosphoric acid (pH 2.1) and, in some cases, sulphuric acid (pH 2.0) or acidified 0.75% w/v ammonium persulphate (pH 2.8). Cell viability, fermentation performance and flocculation/fining behaviour were unaffected. However, changes could be observed in the yeast resulting from the washing treatment. These included alterations of the cell surface, as shown by scanning electron microscopy and leakage of adenosine triphosphate from the cells during the wash. Acid washing is successful provided that, i) food grade acid is used, ii) the acid is chilled before use, iii) the yeast and acid are well mixed, iv) the temperature of the yeast does not exceed 5°C during washing and, v) the yeast is pitched immediately after washing. ‘Unhealthy’ yeast (yeast which has been stored for long periods, heavily contaminated yeast, or yeast from slow fermentations) may respond poorly to acid washing and a shorter washing time, or higher wash pH value should be employed.