If you have done any brewing, it’s very likely you will have noticed the thick layer of sediment, referred to as trub, at the base of the fermenter once fermentation has finished. If you take a good look at the trub, you’ll see that the sediment is stratified. The bottom section of the sediment holds the spent grain and any hops which found their way into the fermenter, in addition to any proteins and fats. The upper section of the sediment holds most of the yeast which fall out of suspension, and you can actually recycle this yeast in other brews. If you do decide to reuse your yeast, the first step is to remove the yeast from the other sediments. This is known as yeast washing.
The method is relatively simple. You disrupt the trub, pour it into a container of water, and then leave it to reset. As the trub begins to fall out of suspension, the yeast will be the final element to fall out. After around an hour, most of the bigger particles have fallen out of suspension, the liquid on top can then be removed, leaving behind the heavier particles on the bottom. This liquid is packed full of yeast, which can then be re-pitched into a different brew. The best part of the process is that it doesn’t require any special apparatus —all you need is two sanitized containers and the trub from your last brew.
A flask which is filled with water and then topped off with the trub from the bottom of a fermenter.
As usual, the first step is to ensure everything is cleaned and sanitized. In this example, a large mason jar with a lid and a 2000 milli-liter Erlenmeyer flask is used; though the flask is not absolutely required, it does make things easier. Once everything has been cleaned, boil around a litre of water for at least 15 minutes, this will ensure that it’s completely sanitized and it won’t contaminate the yeast. In separate pot, boil the two glass jars and at least one lid. It is here where an Erlenmeyer flask is useful, because you can put the flask straight on the gas heat and boil the water in the flask. Again, this ensures that the flask and the water are both sanitized. Once all the equipment has cooled down, fill one of the jars up about halfway with the pre-boiled water.
After around an hour, three separate layers are visible. The middle layer is the yeast that you want to harvest.
The white line at the surface of the sediment is the first yeast beginning to fall out of suspension.
Pick up the fermenter and disrupt the trub by swishing around the small amount of beer which you left behind. If there is no beer remaining, add a small amount of pre-boiled water to the fermenter, this will loosen up the trub.
Pour as much of the trub as you can into the half full jar or flask. Cover the jar with either its lid or a sanitized piece of aluminum foil and mix up the water and trub. Place the jar aside and let it sit for about an hour. During this time, the liquid will start to stratify as the heavier particles begin to fall out of suspension. In the accompanying picture, you can see three distinct layers. The middle layer is full of yeast, while the bottom layer is full of the heavier sediment. The top layer is almost clear of both.
The liquid from the flask should be transferred into the second sterilized jar. After a few days in the refrigerator the yeast will come to rest at the bottom.
After another hour, the bottom layer will become compact. On close inspection, there is a thin layer of lighter-coloured sediment. This is yeast. When you see that this layer has formed, you know that it is time to remove the liquid off the sediment.
It is very important that this step is done slowly and gently. Pour the liquid into the second clean and sanitized jar. Really try to not disrupt the layer of sediment. Put the lid on and place the jar in a refrigerator. Some people claim that the yeast can be stored like this for many months, but in general you should use it within a few weeks.
While the yeast sits in the refrigerator waiting to be used, it will drop out of suspension. Prior to, the liquid can be extracted off the yeast cake. Pour the yeast slurry at the bottom into a starter. However, if this yeast is being used within a week of being obtained, you can pitch it directly into the next brew.