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! John