repropagating yeast cultures

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repropagating yeast cultures

Postby Lobby » Tue Jun 17, 2003 4:18 am

I have had some experience with culturing from liquid yeast. I have a procedure of bottling 5-300ml jars of the yeast slurry and fermenting wort to keep bacteria away. My question is...I have had some in a refreigerator for 4 months and wanted to repropagate. So, I pured off the beer and topped up with new wort. Is this another generation for the liquid yeast? And should I store these with distillied water for longer/healthier maintence of the yeast solids?
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Actually, "Repitching Yeast Slurry"....

Postby Mesa Maltworks » Thu Jun 19, 2003 7:38 pm

If I read your post correctly, what you are doing is to save some dregs from the bottom of a fermenter, trub and all, and storing it. This is referred to as a re-pitch, not yeast culturing. The problem with this method is that since there is trub matter in the slurry, you are carrying an infection risk into subsequent batches. This potential multiplies the longer you store the slurry before re-use.

A way to minimize the risk of a carry-over infection is to collect the yeast from the secondary, not the primary. As long as you are transferring completely fermented beer and leaving all the trub behind from the primary, the sediment that accumulates at the bottom of the secondary will contain very little trub and the most viable yeast cells.

After collecting this slurry, the handling depends on how you want to use it. If you are going to re-pitch within 2 weeks, leave it under beer and keep it as close to 32 deg. as possible. To start it back up, let the slurry come to room temperatue naturally, and add some 3.9 deg. P wort (1.015) and aereate well. Pitch at krausen. If you are storing it longer than 2 weeks before use, the beer whould be decanted off the top of the yeast and then mixed with STERILE distilled water and kept as close to 32 deg. as possible. The latter method will give a shelf life of 4 months or greater. But... to re-use this preparation does require yeast culturing. This means preparing agar plates and growing colonies of the yeast to select the best performers from which to isolate a single colony of cells. A portion of these cells are then grown up to pitching volume in successive steps in a wort solution. This method when done correctly gives the best and most consistent results.

Now, regarding your 4 month old fridge slurry... you can use it, but I recommend that you perform yeast washing at a minimum or acid washing for the best protection. Neither will guarantee or increase the yeast's viability or vitality. I can pretty much guarantee you are below the 90% level of viability and the cell vitality is probably pretty low. The minimum viability you want to see in a pitch is 95%. The yeast vitality will not return until subsequent re-pitches, but will probably have suffered mutations the will forever affect the flavor profile of the yeast.

The best thing to do with this stored slurry would be do a yeast wash, plate out some cells and follow the process as if you are culturing from the distilled water suspension. By doing so, you will re-select the best among the population to start over with. Otherwise, get a new smack pack or RTP and feel secure !

Eric
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