Starter size question

[InspectorJon]

I was just looking at this thread and it got me thinking .

Let’s say I brew a one gallon batch of beer and pitch the appropriate amount of yeast for a 1 gallon batch. I then take that 1 gallon batch at high krausen and pitch it into an a new 4 gallon batch of the wort with the same composition as the 1 gallon batch. It sounds like that should work fine, like a big starter. So why is it not a good idea to start with 5 gallons and pitch the one gallon yeast pitch? I would be starting with the same amount of yeast and ending up with the same amount or beer.

Everything I know says to pitch an appropriate amount of yeast. I’m just not sure why a smaller amount of yeast would not populate the whole batch. I don’t plan to do this, I am just curious about the dynamics.

 

[Zanatrik]

The short answer from my understanding , assuming healthy conditions, is there is no issue per se. You'll have a longer than average lag phase as the yeast mutiply, but eventually they should still do the job. The problem, again based on my understanding, is that that extra long lag phase will make it much more likely other bacteria can out compete your yeast and derail fermentation. You've basically got 5 gallons of sugary water just waiting to be consumed by something.

 

[JohnDBrewer]

I think you answered your own question, you've made a 1-gal starter. I frequently empty my fermenter (keg it) and pitch another batch right into that fermenter. I consider it a 5-gal starter.

 

[CascadesBrewer]

Everything I know says to pitch an appropriate amount of yeast. I’m just not sure why a smaller amount of yeast would not populate the whole batch. I don’t plan to do this, I am just curious about the dynamics.

I have not seen a really good science focused answer to this. I believe it has a lot to do with the cell healthy and nutrients + oxygen available to the yeast. At the start of fermentation, the yeast cells build up their cell structure and store "resources" needed to grown and ferment. Each time a cell divides, it become weaker. They can support a few division (maybe up to 4?). If you pitch too little yeast, you end up with too few cells in a weak state. They might not be able to ferment more complex sugars or clean up off flavors.

Could you come back in after 24 hours with another dose of oxygen? Maybe add some nutrients?

I do also think lag time and yeast cell count are other issues. Bacteria can reproduce much faster than yeast. You want to give the yeast a big head start to grow, start producing alcohol and lowering the beer pH so they can outcompete any potential contaminants.