Yeast washing is a means of cleaning yeast to separate the viable yeast cells from unwanted partials.
To wash yeast, The yeast is harvested from a fermentation vessel and combined with water. It is allowed to settle until the water has separated to the top and the yeast to the bottom. The water and the top of the yeast is then poured into several jars. The yeast and debris that has settled to the bottom of the first container is then discarded.
Normally I don't wash my yeast, I just pour the cake into three or four quart size mason jars, but I have been curious as to what benefit may be achieved from yeast washing. My understanding is that the intent is to separates the hops and dead yeast cells from the live yeast cells. In the last few days I have been running experiments on this technique to find out what it does, and if it is worth the extra time.
Yeast washing is beneficial, but not for the reasons that I had anticipated.
After the yeast has settled into the container it divides into roughly three sections. Common brewing wisdom indicates that the top portion is mostly water, the light colored middle section contains viable yeast, and the darker bottom contains dead yeast, hops, and other debris. However, it seems that this in not the case.
The viability throughout the container is roughly the same.
In four test cases the viability was not statistically different in these layers. Tests were run with three slurries with 10%, 50% and 90% viability. In all tests the viability of the yeast in each section did not vary more than one standard deviation.
What was interesting was that the bacteriological content was much higher in the top portion of the yeast containers than in the lower parts. There was about 100 times more bacteria per live yeast cell in the top "liquid" section.
Another strange finding was that the concentration of non-yeast debris followed the cell density. While the cell concentration at the bottom of the container was twice what it was in the middle, the viability was the same, and the concentration of non-yeast material per yeast cell was virtual identical. The hops and other partials did not separate from the yeast. So when you are throwing out the junk, it takes just about as much viable yeast with it as it takes debris.
The above experiments were done in test tubes. To make sure this was repeatable, I tried yeast washing with a full sized batch. The results were the same. The viability of the part that would have been thrown away was the same as that of the part that would have been kept. The viable cell density was also very similar.
For further information on washing a a full sized batch see the new blog post: Yeast Washing Revisited.
The tests were conducted with WLP566, WLP004, EC-1118 and S-04.
When washing yeast, discard the liquid when washing yeast to remove bacteria. Keep the thick slurry and add clean water on top of it for storage and to wash out additional bacteria.
I would be happy to talk through the numbers with anyone interested.