Yeast Washing
TroubleShooting
Michael Miller - Poway, California asks,
I have always wondered about the desire of many brewers to yeast wash. It seems to me that if you are trying to save yeast that you should simply make more yeast starter than you need and save the excess starter for the next batch. This seems easier and more likely to maintain the integrity of the strain. What am I missing?
I too question the desire of many brewers to acid wash their yeast. But before diving into why yeast washing is used I want to touch on another topic raised in your question. The idea of growing a large starter, using what is needed for today’s brew and saving the remainder is an appealing idea. The problem with doing this, however, is that the viability of a yeast slurry declines over time and it is really advisable to use fresh slurry within one to two weeks following propagation, assuming that the slurry is stored right around 32 °F (0 °C). Most commercial breweries with a yeast propagation program would rather not use yeast that is stored more than a few days. The bottom line is that when yeast viability drops during storage the likelihood of fermentation problems increases.
And this is why most breweries harvest yeast from a batch that just completed primary fermentation, store the yeast for as short a time period as possible and pitch it into a fresh batch of wort. It is this method that can benefit from washing. Since yeast from a previous fermentation can become contaminated with bacteria and wild yeast, the practice of re-pitching can be problematic. I think this problem is not as common today as in the past because of improvements in process control, especially tank cooling, process technology, specifically closed fermentation vessels and hygienically designed pipe and valve systems and overall improvements in cleaning methods. But not all breweries have the latest and greatest technology, especially older ones. So yeast washing is alive and well today.
Yeast washers come in two basic camps. One camp sees yeast washing as something that can be done in a pinch. These brewers monitor yeast and beer in microbiological labs looking for potential problems and/or simply wash after a certain number of generations that coincides with problems (this is brewery specific and requires sufficient data to establish a routine that makes sense). If they pick up certain bacteria (usually lactics are the beasts of interest) on their test media and really need to use the yeast related to the sample, acid washing can be used as a form of chemotherapy to reduce the lactic acid bacteria population without harming the yeast too much.
Make no mistake, reducing the pH of yeast slurry to 2.2 to 2.4 and holding it for about two hours is not super friendly on the yeast. If the temperature warms up above about 40 °F (4 °C), the pH is errantly lower than 2.2 or the time extended beyond two hours, damage will occur more than the method already permits.
There are other washing methods used that are not as harsh, but the phosphoric acid method is still common among brewers who like to acid wash. Chlorine dioxide is a much gentler washing solution, has gained popularity in recent years and is equally, if not more, effective in reducing bacterial populations than phosphoric acid (note that these methods are not effective at reducing the population of wild yeast strains).
The other camp always acid washes yeast after cropping. These brewers have a pragmatic and fairly logical thought process. Since it takes time to determine if there is a problem, always acid washing cuts out the lab time that may simply indicate that a problem exits. Washing the yeast every time is a way of being safe rather than sorry. The other part of this camp’s philosophy is that acid washing and chlorine dioxide washing reduces yeast viability, and reduced viability will change the way yeast behaves compared to yeast that has not been acid washed, so in order to have predictable yeast and predictable fermentation performances the yeast should always be acid washed.
This is a pretty big topic and I only scratched the surface. Yeast can also be water washed, although I don’t know of any breweries still practicing this method. Yeast can also be harvested and passed through a tight screen to remove trub. And there are as many ideas on how and when to crop yeast from the fermenter as methods to screen, store and wash. Most craft brewers that I communicate with crop, store and re-pitch without any screening or washing steps. I hope this gives some insight into your question.
