... I went and saw a mate who only uses kits and the dried yeast they come with and he's been recycling the yeast cake for the last few months and it has dramatically improved his beer! Still not the greatest but it's definatly improved. A mutation for the better!...
Time and temperature will greatly reduce the viability of yeast, and I'm
certain kit-yeasts have not been stored in anywhere near optimal conditions.
In addition the kit-yeast sachets are 5g or 7g - so you're already underpitching if even if you
did have optimal viability.
As a consequence, my response to the observation that subsequent brews have "
definatly improved" - even assuming that it
is actually
only the yeast responsible - is that now a more optimal amount of live yeast is being pitched, resulting in a cleaner, healthy ferment rather than any yeast "
mutation for the better" being the cause of the improvement.
In addition, while it may be possible to successfully mutate a yeast strain as at home on such a small scale, with barely adequate sanitation it - IMHO - poses is a
much greater risk of infection, unwanted bacterial growth, wild yeast infestation and a host of other adverse consequences compared to any possible positive outcome.