Reusing liquid yeast

Mark Cupples

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Trial Member
Jul 29, 2013
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Part 1

After i collect a container of yeast sludge .... How does one calculate how many yeast cells per ounce / gram of sludge?

Part 2 - how does one decline this with age?

I have scanned the site and found a lot on dry yeast and then on liquid yeast (fresh from the packet) but cannot see the reused calc.
The only real way to know the cell density is with a microscope. Most estimates are between 1 and 5 billion cells per ml.

I would use the slurry as soon as possible, days are better than weeks. If you use it in under a couple weeks, you will be okay. One time I pushed it several months, and the resulting beer was sub-par. Note that this is not a matter of pitch rate, it is a question of overall yeast health.

Overall yeast health also depends on the previous fermentation. Was it in the right temperature range? Does everything smell and taste as it should? Did it flocculate as expected? Was the previous beer a normal beer or a 'strong' beer? Re-pitching from a high alcohol beer is more risky because it stresses the yeast. Another issue is how long the primary phase lasted. A week to 10 days is okay, but if it was much longer, that yeast would be too far gone for me to want to re-pitch.
I would use about 40 ml of the slurry to make a new starter, which should also help to rejuvenate the yeast. Theory is by using a small amount of the slurry to make a starter you are pitching younger healthier yeast. However chances of an infection does go up.
I've reused my wyeast 3787 a few time now and I wash the yeast cake and get a small amount from it by the time I wash it a few times. So far I have taken this amount and tossed it into a 1L starter. So far the reused yeast has out performed the smack pack. The last one I used it on got 85% attenuation.

The picture is a small mason jar that I use for cleaning yeast. I start with the big carboy to a 1 gallon jug and down to a small mason jar. I don't know how much yeast would be in here or after making a starter with it but it works!
I usually don't end up with the amount of slurry I want or think I should have. I do what people here say and make a small starter and go to a bigger starter then pitch. I don't like taking extra steps because of the risk of air born infection, which I got a couple of years ago. I am better now in my process and haven't had a yeast infection ( laugh now ) since then. I tossed out 4 vials of a lager yeast 2 years ago and that forced me to buckle down and do things right.