Yeast starter question

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Hey all,
I havent had much luck with repitching yeasts, and im loathe to start so close to christmas, however i have 2 brews that need the same yeast, im thinking of pitching half of my 5litre yeast starter (its alager) and leaving some trub and making a second batch of dme to add to the trub imediatley after pitching, will this work, or is the risk of developing off flavours not worth the savings /time
You could always pitch your 90% of the 5l starter into the first beer and then step up another starter with what you have left from the first one - I have done this a few times. I start with about 500ml to get it going then step up to 1.5l then 3l - it takes a bit of planning, but it gets the job done.

After I spin the 3litres on the stir plate I generally crash it in the fridge and then either step it up again or just pitch the yeast slurry once I have drained the spent starter off. I don't see why it wouldn't work for you.

I have also had good luck with repitching part of a rinsed yeast cake before as long as the first beer isn't wildy different from the first one or some crazy high gravity thing that have given the yeast a workout. I am not a fan of just pitching a new beer onto an old yeast cake - but I know people who do it and they don't seem to have may problems - I just like to start with a clean fermenter and am cautious about ovepitching yeast - that may also be complete crap about overpitching as well - so take it for what it's worth :D
Ive tried reculturing from the old yeast cake in the past, it worked but it just wasnt as fresh, i suspect it was probably oxidisation. I guess i could taste the finished yeast starter and determine if i want to use it or use a seperate vial
I get what you are saying about oxidisation it sort of makes sense when you think about it - say you have a 23 litre batch and then drop a 5 litre starter into it. Depending on how you made the starter wort you would be dropping nearly 22% oxidised starter into your beer.

It really shouldn’t be an issue if you cold crash and decant the spent starter off the yeast - if you go that way.

I have had luck with just taking 300 - 500ml of yeast slurry from a batch, giving it a rinse in a sanitised bottle with some pre boiled and cooled water to separate the trub and gunk and just pitching the top liquid into the next beer.

I found about two weeks in the fridge is about as long as you want to wait if you are just rinsing and repitching. Any longer and I will generally make a starter - decant and pitch.