G'day , how long after the fermentation has finished does the yeast live ? I am curious because not only is it the cost of the yeast but if you find a particular strain and you like the taste ( providing that all ingredients are constant ) if you could grow it you would be guaranteed the same flavor over and over.
Not to be a pessimist, but if they're not fermenting, then the yeast are continually dying. How fast they die is dependent upon how they are stored.
If you want to just reuse yeast from one batch to the next, then save a quart of yeast slurry from the previous batch and store in the fridge in a sanitized container - a loose fitting lid is ideal, such as foil held down with an elastic band - this lets out any gasses if fermentation hasn't quite finished.
If you use within a couple of weeks then you can pitch directly into the next batch. Longer than that - up to 4 months - and there won't be adequate viable cells remaining, so a starter is needed. Beyond 4 months, the yeast will have mostly died - You can see it visually as it turns from a pale cream to a darker brown peanut-butter color.
Long term storage - around 2 years - is possible by freezing the yeast in a 30% glycerine solution. The amount frozen is usually just a few milliliters, so a starter is always necessary to step up the quantity to proper pitching rates.
To sum up
- short term storage - up to 2 weeks, in the fridge, for direct pitching into the next batch
- medium term storage - up to 4 months, in the fridge, requires a starter before pitching
- long term storage - up to 2 years, in the freezer, requires several step-up starters before pitching
The tl;dr is:
- top cropping from the krausen is better than harvesting from bottom of the fermenter
- harvesting early in the fermentation is better than later
- yeast viability decreases over time, so it's best to re-use the yeast quickly
- you can improve the quality of your yeast by washing it
- harvest yeast from brews with a starting gravity that is low or middling.
- you can re-use a yeast culture for maybe a half-dozen brews
I typically re-use yeast over a number of batches and have never had a problem with under-attenuation or off-flavours from fermentation. I harvest when racking from secondary, so I'm not really doing it right.
Check out this site on harvesting yeast. http://www.wyeastlab.com/com-yeast-harvest.cfm. After primary fermentation, you want to "harvest" the yeast so that you can store it properly. The yeast goes dormant at cooler temperatures (during storage). Then you warm up the yeast, pitch it to a starter (http://www.howtobrew.com/section1/chapter6-5.html) and you can reuse the yeast! You should really only store yeast for about 1 month. You can also, only harvest so many times, usually 3 - 5 generations of daughter cells.
Thanks for all the input and links , I'm just getting started with proper corn worts and just figured that when I get it the way I like it , it would be good to keep that flavor. In regards to yeast only lasting a couple of generations there are to many examples of producers keeping the same genetic strain for over fifty years for example :- jack Daniels , Jim beam , coopers beer and so on. Since posting I have sourced some information , but anymore is always good