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Yeast Washing


Acid washing is used as a means of eliminating contaminant bacteria from pitching yeast. Such bacteria, if allowed to survive, can spoil beer. Acid washing selectively kills bacteria without harming yeast providing it is done correctly. Acid washing is a highly efficient process. Usually more than 99.9% of contaminant bacteria are killed in a one hour period. It should be noted, however, that this process will not kill wild yeast.


The do’s of acid washing 

  • Wash yeast regularly
  • Use food grade phosphoric acid, diluted 1 in 10
  • Chill the acid and yeast slurry to below 5°C before washing
  • Stir the yeast whilst adding the acid and at regular intervals during the washing period
  • Ensure the pH of the slurry remains between 2.0 and 2.2
  • Wash the yeast for one hour
  • Pitch the yeast immediately after washing


The don’ts of acid washing 

  • Don’t deviate from the temperature, time and pH limits indicated above.
  • Don’t store washing yeast.
  • Don’t wash ‘unhealthy’ yeast (yeast that has been stored for long periods, heavily contaminated yeast, yeast from slow fermentations).
  • Don’t wash yeast from a very high gravity fermentation (over 8% abv)


Preparation is essential. Ensure that you have the yeast slurry and acid fully chilled as the wort nears the end of the boil and start the washing process at this time. The washed yeast will then be ready for pitching as soon as the wort has cooled