Originally Posted by beds
You are safe with 10 if you don't introduce much air and if you put your wort right on top of the old stuff. After 10 generations it would mutate even under the best conditions. If you're worried about the volume of yeast sediment, just throw a bunch of it away. You don't need that much, as you know.
I don't know. That is roughly the commercial rule of thumb and:
1. They have microbiological skills
2. They pitch viable yeast, not pack on to cakes.
3. They observe the characteristics of their yeast and discontinue use when they change. Someone to lazy to wash yeast is too lazy to do this.
4. They are not leaving finished beer on yeast for weeks after fermentation is complete.
5. They are putting the yeast into a container that was sanitized between batches.