Washed yeast life expectency

How long can washed yeast live and what is a good temperature to store them at

Last year, I successfully made 2 different starters out of pilsener yeast that was about a year old. I had it in the refrigerator the whole time. I built up significant starters with them, and had good fermentations with both.

Ok amazing news thanks

Viability decreases rapidly from what I have read. I would use it within the 2 months.

Go to Mr Malty and plug it into the calculator, select viability "by date" and see what it says. That's about as good a guess as you'll find.

What is that? Mr malty?

If you make a starter, then the age of a yeast isn't really an issue. When you make a starter, and grow it, you're replicating more yeast to make up for any loss. You're making new, fresh yeast.

Bobby M did a test on year old stored yeast here; https://www.homebrewtalk.com/f13/testing-limits-yeast-viability-126707/

And my LHBS cells outdated tubes and packs of yeast dirt cheap 2-3 dollars each and I usually grab a couple tubes of belgian or other interesting yeast when I am there and shove it in my fridge. and I have never had a problem with one of those tubes.

I usually make a starter but I once pitched a year old tube of Belgian High Gravity yeast directly into a 2.5 gallon batch of a Belgian Dark Strong, and after about 4 days it took off beautifully.
The purpose of a starter is to reproduce any viable cells in a batch of yeast....that;s how we can grow a starter form the dregs in a bottle of beer incrementally...and that beer may be months old.

Even if you have a few still living cells, you can grow them....That's how we can harvest a huge starter (incrementally) from the dregs in a bottle of some commercial beers. You take those few living cells and grow them into more.

A follow up question that I have been wondering about. Brewers talk about using a batch of yeast for some number of generations (like 5 to 10). I assume that involves using the entire yeast cake immediately after racking off the prior fermentation.

When you wash and store yeast and then build it up in a starter is that the beginning of a new "generation", or is that new batch of yeast (generation zero)? What constitutes a new generation? 5% of the pitching qty, 50% of the pitching qty? i.e. if I take 100 ml of washed slurry and build it up to 200 ml to pitch, is that a new batch or an additional generation?

Thanks

Generation 1 is the first batch you brew with a new yeast.

Generation 2 is when you use part of the cake, either straight or washed and a new starter. Any re-use is a new generation.

Reason: The yeast manufacturers keep pure yeast strains, and grow each batch from the original strain under lab conditions. Each time you use it, it mutates. Everything you do with the yeast; you alter it's environment, or alter the fermentation temperature, alter the beer gravity, the make-up of the wort, when you harvest the yeast ....... it all makes for selecting different characteristics of the yeast, and the yeast evolves to match it's environment. Therefore the more times you use the yeast, the more it changes from it's original characteristics.

This is not always all-bad. You are essentially creating your own strain, but it will be different from the original.

Sometimes, the way we handle yeast we get bad characteristics. Some people harvest from the secondary (because it is cleaner), but in doing so, you are harvesting the yeast that did not flocculate quickly. So you are starting to select particular traits of the yeast.

So would that result in a cleaner or a slower fermentation