amount of times you can repitch

So I have recently been washing and reusing my yeast (or pitching the yeast cake) and was wondering why there is a recommended number of repitched for each yeast strain. What is the reason for this? Is it to keep the same flavour or is it because the yeast becomes less active or something else?

Just curious. 

Most people say 3 times because you start to get mutations.  But then I wonder how big breweries work and how yeast producers get "pure" yeast again?  It might be they say that to ensure you buy from them again.  I mean there was once just 1 yeast cell of a particular type right?  And ever since, yeast producers have been propigating it.

That was my thought.... and I have heard that the Granite Brewery use the same yeast over and over and the beer has changed slightly over the years but not in such a way that it is really noticeable.

In general, homebrewers are well advised  to limit the number of times yeast is reused.  Mostly because the practice for harvesting, washing and storing is not all that sanitary.  The main reason breweries reuse yeast is for cost.  On a brewery scale, yeast pitches are expensive.  On our scale, it is pretty cheap considering how often most of us brew.  Cost aside, repitching changes the subsequent beer.

Anecdotal evidence suggests yeast mutations and infiltration have a greater affect in small batches and can be more noticeable.    I find I prefer White Labs WPL001 the 2nd - 5th repitch on my homebrew system.  I have kept in cold in the fridge for 4 weeks and was pleased with the result.  I prefer less than 2 weeks but I don't brew that much any more.

Still somewhat of a newbie in the brewing world, but what I found works for me is to create a starter twice the size I will need for my brew day.  Since 1 vial is good for 5 gal and I do small 2.5 to 3 Gal batches, I make a 2 ltr starter with the whole vial.  For a 5 Gal batch I probably would use 2 vials and 4 liters wort.

When I pitch I make sure the whole starter is evenly mixed with 1/2 of it to the fermentor and the other 1/2 to a sanitized jar to place in the frig for the next brew cycle.  That way I don't have to worry about any washing of yeast and I maintain the strain purity without all the crud and mutated cells being carried into the next brew.  I do this for lagers 6-8 times with excellent results so far.  Works for me..

I've gone as many as five generations without any problems.  Though I don't bother with the washing and all that.  Tried it but didn't have much luck.  What I do is rack and brew on the same day. This allows me to take a scoop of the yeast cake (hop bits, trub and all) and plop it onto the fresh wort. Takes off like a rocket when you do that.  It's probably not the best practice, but it's only a hobby.

I have read that you can wash your yest up to five times, but then after that you can start getting wild strains in the yeast that can cause problems.  One day I might actually try doing this.