A good yeast sample can be retrieved, stored & reused regularly across brews. Is there a limit to the number of generations a sample can last?
I've heard that after so many generations a strain can mutate into something different, is there a rule of thumb for how many uses this is? Ignoring mutation, can yeast otherwise last forever if it's properly stored & handled?
In theory, baring any outside influence in a completely 100% sterile environment, a yeast strain would be good to use over and over again. The issue arises with sterility and outside contamination. I personally don't feel that there is an exact number as to how many times it can be done simply due to the fact that every situation is different and every time you harvest you are going to risk outside contamination.
Generally you will have few commercial brewers that will reuse it more than 5 or 6 times with out re-culturing; and re-scaling up a starter. Ignoring mutation it could theoretically last for ever, but we cannot ignore this.
For ale yeasts you would ideally top crop at high krausen and repitch within 24/48 hours. For a lager yeast you would bottom cop and again reuse quickly. If bottom cropping for ale or lager as is common these days due to tall cylindrical fermenters then taking neither the first or last to flocculate is a must as otherwise you will put selective pressure for early or late floccuation.
Reasons you would not want to reuse ad infinitum are (in no particular order):
If the culture gets contaminated with bacteria or wild yeast it is time to ditch it and start again. Unless you fancy trying an experimental wild batch, but 99.9% of the time ditch it.
How crucial is it that you make an identical beer every time? For a brewer making 1600HL batches of Budwieser it is critical, for a craft brewer it may be as important or not; it is a decision for the individual.
The rate of mutation, determines how quickly your yeast will drift form the desired profile. The rate that yeast mutates is affected again by many stress factors: the optimal temperature range for the strain, the strength of the brew higher alcohol level tend to cause more rapid drift, the number of generations more generations leads to more drift, under or over pitching can stress yeast and there by increase the rate of mutation, under aeration of the wort is another factor.
Once you have got to the point where your style drift or contamination is too much you have a few options.
1) Reorder from your stockist, very easy if you use a off the shelf yeast. 2) Reculture from a slant, hopefully you have kept some slants of your yeast in a -80C freezer. 3) Reculture from your last slurry, plate it out, pick a colony that meets your expected morphology.
Finally as Dustin Rasener suggested in the earlier comments get a copy of Yeast (http://amzn.to/iiDdtA), it is an excellent book.
This answer from memory only - I hope I have been accurate:
(See comments for strikethrough reasons)
Assuming that the yeast that is re-used comes from the conditioned beer not the fermenting bucket, and (like the answer before me @Bullet86) also that there are no contaminants, there is no reason why the strain shouldn't continue to be used.
Commercial brewers invest in their yeast as it is an extremely important aspect of their beer and flavour - it is stored in yeast banks, should they have a disaster they can 'withdraw' a sample.
I have witnessed a bath tub full of live yeast being used in a commercial brewery. I also had the luck to use some of this living yeast and my subsequent brew took on some of the characteristics of the commercial product. Their strain had been used over and over for years as it imparts its own flavour to the finished beer.
Keep it clean and healthy and it will be fine to re-use, but remember to re-use yeast from as late in the process as possible as this will be the best yeast for the job.
The actual number of harvests depends on strain too. Some strains (german hefe specially) are known for fast mutations - their ability to produce desired amount of esters degrade over each harvest. Others are more immune, like scottish or irish strains.
