How many times can you reuse your cake?

[Grainer]

Being a Geneticist/Microbiologist in a past life I would say 5-6 pitches from the same yeast would hold true if this was a cell line.. but it is not.. it is a yeast! I would say that best practice would to use a primary culture that is uncontaminated and use this as a benchmark for further brewing. but we dont all have the right facilities.. so it makes it a bit more difficult. This would need to be recultured probably fortnightly...ideally frozen down..although be aware that -20dec can cause shearing of the cells and kill them.. even in glycerol....-80 is better... In most cases 1-4 degrees would be best considering resources available. If it was me I would keep a primary culture in glycerol at low temps and then use this primary innoculum as the golden child for further brews. I would then not touch this unless I needed to and subculture maybe every 2-4 months.. but i would need to experiment with this. (I have always had access to -80 or -196 deg cryopreservation for free... Then I would set up a secondary innoculum and use this to set up all other batches. You only need a drop in your starter cultures.. start with 50mls of yeast nutrient solution and then upgrade it to a 100ml-200ml innocculum. and your off and running. best is to do this over 12 - 30 hours at the desired temp on a mixer depending upon the yeast requirements.... any longer than this and you may see a decline in the yeast health due to the introduction of yeast byproducts.

The problem with reusing the yeast for more than approximately 8-10 generations is that it can slightly change the genetics of the yeast and it can also change the efficiency of yeast division/growth and alcohol conversion. It is important to note that all strains of yeast are different and each will have specific requirements.

In addition, it does matter where you collect the yeast.. Early settling yeast may not be good at producing alcohol and same as the last sediment, however the middle yeast tends to be better at alcohol conversion. Skimming of the top or bottom will change the beer characteristics in time.

If it was me I would skim from the middle re-culture this to get a healthy culture and then store this under sterile conditions if you can.

Once again just my opinion.. they key in all of this is to have yeast at its earliest generation with a healthy, young culture for best results.

cheers

 

[Dave70]

I have always had access to -80 or -196 deg cryopreservation for free...

I bet you could produce a cracking Eisebock. And quickly.

 

[verysupple]

Going along with the general consensus of the replies...it depends. Back in the day (before they knew that yeast existed), brewers used 100's of generations of yeast. Things have changed, though (pure cultures etc.). As quite a few other posters, my knowledge comes mainly from "Yeast: the practical guide to beer fermentation" (White and Zainasheff). My understanding is that the point at which you harvest the yeast (so this means not dumping wort onto an entire cake) is critical - because of selective pressure (genetics mumbo jumbo - Google it). I'll assume from the use of the word 'cake' that ur bottom cropping in which case, as many texts have advised, you should take the middle layer of the cake for harvesting. This will limit the deviation in character (both attenuative and flavour-wise) of subsequent generations from the first and maximise the number of generations you can use.

Of course, if you are dumping on the entire cake, there are so many factors that affect the beer it's almost impossible to determine what's affecting the flavour.

 

[verysupple]

I have always had access to -80 or -196 deg cryopreservation for free...

Ha, you got Luke Skywalker trapped in there?

But seriously, other than you, what kind of homebrewer has the ability to handle cultures like that? Even Siberia isn't that cold.