How to acid wash yeast

[greenxpaddy]

LAST UPDATED 5th June 2013

I've decided to compile a post for acid washing yeast. This is not because I claim acid washing yeast is an optimum way to treat yeast. Far from it. But because in searching for information on this I have found very little available on the forums.

Acid washing yeast is distinctly different from washing yeast, where distilled or boiled water is used to attempt to separate trub from healthy yeast.

Acid washing of yeast is used where a batch of yeast is used over and over again because of the quantity used and to save the need to keep stepping up yeast from healthy single cells which is time consuming. Commercial breweries acid wash yeast, maybe not with phosphoric acid it is true, but perform it regularly to ensure they maintain low bacteria levels in their re-used yeast.

Why am I acid washing yeast in the first place?

To clean yeast I want to use more times than a homebrewer would, for economical purposes and maximum beer production.

I will be using Brewpaks Phosphoric Acid as sold at many places including thehomebrewshop. This is supplied at 75% phosphoric acid concentration.

http://www.the-home-brew-shop.co.uk/aca ... IKhLa5NvNI

IMPORTANT: It is highly recommended to wear protective clothing especially goggles whilst handling phosphoric acid especially at 75% concentration. You do not want that stuff on your skin, let alone your face, let alone in your eyes.

ph guidelines

Yeast cake at ph2.8 Needs 12-24hrs to kill most bacteria
At ph 2.0 - 2.4 = 1-1.5 hrs

All acid washing needs to be at a temperature below 5 degrees C. So your yeast and the acid needs to be refrigerated before you mix them.

Also note the timescale for the completion of your acid wash. This is because it is important to pitch the yeast as soon as possible after the acid wash is complete to limit cell damage to the bulk of the yeast cells. Never store the yeast within the acid.

Preparing the yeast cake.

To save on the amount of acid you need to use, you will want as little liquid in the yeast cake as possible, but not so thick as to make stirring in difficult. 30-50% liquid is about right. Add the acid solution and stir the yeast cake with a sanitised implement to homogenise the yeast into the acid.

Some people dilute the acid before adding to the yeast but I have found that it is possible to add directly to the yeast provided you add slowly and stir in very quickly.

As a rough guide I have found it takes about 5 ml of acid into a 250ml slurry to get to a ph of 2.1

[greenxpaddy]

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[greenxpaddy]

Note acid washing does not get rid of the build up of wild yeast.

[greenxpaddy]

This guide has been rewritten. The chart previously posted was substantially incorrect and misleading. My calculations were wrong. I am satisfied that the acid ratios now are completely accurate & checked against info on wikipedia.

[greenxpaddy]

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[greenxpaddy]

I accidentally added my yeast too much and too quick on Sunday and the ph dropped as low as 1.6. It took a few minutes to get the yeast ph back up to 1.95ish.

Hardy little buggers they are - it seemed the yeast did hardly suffer, the pitching has resulted in a an apparently normal fermentation. Time will tell whether there are any stressed yeast notes in the beer