Every homebrewer can remember a particular change in their brewing process that fundamentally improved the quality of their beer. For me it was yeast. I used dry yeast for many years, and always assumed that the variable results that I enjoyed were due to my less than perfect sanitisation regime. I switched to using liquid yeast when my homebrew store ran out of dry packets of yeast. The results that I got from liquid yeast ensured that I have never gone back to using dry yeast.
But using liquid yeast brings along challenges of its own. It cost me around R100 ($15.00 Australian) to import or buy a liquid yeast, meaning that it made economic sense to reuse yeast from batch to batch. This can be a relatively hazardous exercise, as your yeast can become infected quite easily. Yeast washing made reusing yeast a pleasure. This is what I want to discuss and how to do it.
Below I will set out the method that I use to wash my yeast. I must first warn you that I did not take biology in high school. So if I can do this, anyone can. How I wash yeast does work for me and will work for you.
Why Acid Wash?
Yeast taken from the slurry of a recent fermentation has a couple of features that are less than ideal to reuse again:
- Cells are coated with trub constituents. In effect the coating may
slow down how the performs because it 'clogs' up the cell wall, just
like a filter will clog up over time.
- Cells are clumped together. If cells are clumped together only the
outside cells are exposed.
- The slurry is often infected. No matter how clean you are there will
always be a small amount of infection in your beer. Usually this level
is so low it doesn't matter, but reusing yeast increases the risk that
it will become a problem. Acid washing can only kill bacteria. A wild
yeast infection cannot be removed.
- There is a lot of dead cells and trub. The slurry doesn't just contain living yeast. There is lots of other stuff there that can give off flavours to the beer. The acid washing process set out below addresses these problems, breaking up and cleaning the yeast, removing a lot of rubbish and killing most types of infection
Yeast cells before washing.
How to Acid Wash
Put yeast in a solution with pH of 2 for about 2 hours. My method is as follows: On brew day, once the boil has started, I rack the beer off my yeast starter, and transfer the yeast slurry across to a smaller vessel. I strain the slurry through a stainless steel sieve, to break up the clumps. I then use phosphoric acid to drop the pH to somewhere between 2.5 and 2. (I use 2ml of phosphoric acid per 500ml slurry). I measure the acid in a syringe and then add a drop at a time.
After washing.
It is important to add the acid slowly and stir a lot as:
1. It is easy to add too much (check the pH after each addition the first time you try.)
2. You can damage the yeast if you allow sub pH 2 spots to develop. I then put a bubbler on the yeast vessel and leave it at room temps for about 1,5 hours (typical duration of the boil).
While it is generally better to use phosphoric or lactic acid as they are a natural flavour in beer, you can use citric and tartaric acid for yeast washing. I have in the past, but that being said, phosphoric acid has the advantage that you battle to detect any trace of it in your beer. I ran into problems with citric acid when my tap water pH was high, and I had to use a fair amount of the stuff. My light lager tasted vaguely reminiscent of Fanta Orange. Tartaric gives a wine flavour, although this is more subtle than that of citric acid.
At the end of the boil I flash chill a litre of wort and add this to the yeast, so that by the time I am ready to pitch (30 minutes later), it has already started fermenting. You should decant off any excess liquid and trub material before you add the wort. Measuring pH
Now how do know when you get to a ph of 2 to 2.5. I would suggest you read the pH article also available on this web site and use some of the sources suggested there to get an idea how much acid you need to add.
Happy Brewing,
Ant Hayes Gauteng; South Africa