You’ve fermented your beer, it’s tasting great, and now it is time to bottle or keg. At the bottom of your fermenter is a whole lot of goop called trub (pronounced “Troob”). The trub consists of protein, hop material and fats, but roughly 40 to 60% of trub in a healthy fermentation are leftover yeast cells that did their job. Can you reuse this yeast? Yes you can! With just a few extra steps you can save money and pitch a healthy second generation of yeast into your next batch of wort. During fermentation the yeast will split 4 to 6 times, meaning if you pitched 200 billion cells you will now have 3-4 trillion cells of yeast in your trub. If you were to just pour your new wort right onto the trub you’d over pitching by a significant margin. The general guidelines for fresh yeast pitch rate is 750,000 cells per ml per °plato of wort for ales, and double that number for lagers. (To find degrees plato just divide your gravity points by 4, 1.060 = 15°plato). For repitching the general rule is 1-2 million cells/ml/°P. So, how do you know how much trub you should pitch? The short answer that I got from Wyeast’s scientists was that a cup to a cup and a half of trub is a good pitch rate for 1.060 wort or lower. But this is very general and there are ways to hone in this number and pitch mainly yeast and none of the other trub material. If you want to get down and dirty, the first thing you need to determine is how much of your trub is yeast. 40 to 60% yeast cells will provide a healthy fermentation. If you find that you have less than 30% you may not want to use that yeast to repitch. The way to determine your yeast percentage (without using a microscope) is to take a small sample of trub using a sanitized spoon and fill an empty nutrient vial. Put this sample in the fridge and allow the yeast to settle out. Generally 40-60% yeast solids (the stuff at the bottom) will correlate to 1.2 billion cells per ml.
Now lets go back to our math. Let’s say we want to pitch 1.5 million cells of harvested yeast/ml/°P.
1.5 million x 18927ml (which is 5 gallons) x 15°P = approx 426 billion cells.
426 divided by 1.2 (our estimated cell count per ml of trub at 40-60% yeast) = 355 ml. or 12 oz or a cup and a half of trub.
What I like do is take about two-thirds of a cup of trub (around 210 billion cells in this example) and pitch that into a 2000ml starter. This allows my yeast to propagate once and get healthy and ready to ferment. If you’re not making a starter you can pitch the proper amount of trub directly into your next batch or store it up to two weeks in a sanitized jar in the fridge. The yeast will reproduce during fermentation and the other material from the trub will just drop out. But you can also separate the yeast from the trub by rinsing it. (Often I hear people refer to this as washing the yeast. Washing the yeast is a different process that requires using phosphoric acid to help kill of any bacteria. I am not going to get into that, homebrewers rarely need to do it). In order to rinse your yeast take your determined amount of trub and add it to a jar of boiled and cooled (i.e. sterilized) water. Refrigerate and allow it to settle. The yeast is lighter than the fats and proteins and will separate from them. After a few hours you will have three layers. The top layer will be mostly water, while the middle layer is yeast which you can decant into boiled and cooled water if you want to rinse it again. You can also decant the middle layer into a sanitized jar to refrigerate for later use, or you can pitch right into your wort. The bottom layer is all of the fat and proteins and stuff that you can just toss. When collecting yeast from your fermenter it is good practice to clear away the top part of the trub and harvest from the middle layer. These will be the medium flocculant yeast. If you want a higher flocculant yeast you can harvest from lower in the fermenter as these are the cells that dropped out first. By doing this you are selecting the yeast that you want to perform a certain way (flocculant vs non-flocculant). It is much easier to harvest from a bucket or conical fermenter than a carboy. I do know of people who harvest from carboys collecting the yeast that blow off during the initial primary phase. I have never tried it so I am not sure about the health or characteristics of the yeast you harvest that way. Yeast can be used up to 10 generations as long as you are careful with your sanitation. It’s important to remember when harvesting your yeast to make sure everything is very clean and sanitary and that you work in a draft free environment (good advice for any kind of cold-side operations). Additionally, if your beer had a very high gravity you shouldn’t harvest from it as there are too many chances for mutations. And of course if your beer tastes bad, don’t use that yeast, they don’t deserve another chance! Have a great harvest!