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How is a starter different than what's going on in my 5gal fermentation vessel? From what I understand, a starter is you just making a mini beer, allowing yeast cells to multiply... But, why doesn't the same thing happen (yeast multiplying) if you pitch a small amount of yeast into 5 gallons of wort? There's plenty of sugars in my wort - why isn't the yeast multiplying?

I've also heard of people taking the yeast after fermentation and using it for another beer... That seems like it's active and usable still.

The reason I ask - I'm in the middle of a homebrew where I pitched a liquid yeast into a 1.057, did NOT use a starter, and when I measured SG at the three week point, it only came down to 1.027. A homebrew friend of mine suggested I underpitched, so I took another liquid yeast in fridge, created a starter, ... and pitched that today into the wort. Airlock activity has absolutely increased - I'll measure gravity in a couple days.

But as I did this, I'm just so confused - why does 100G of DME per liter allow yeast to multiply but the yeast I pitched into 5 gal of 1.057 not multiply (enough)

(Yes, I'm a noob)

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  • Have you tried "rousing" the yeast by gently swirling the vessel? Or has the ferment been cold? Cold conditions can cause apparent stalling. I would swirl the settled-out yeast back into suspension, and if the temperature is below ~15C warm the fermenter up somehow. – Kingsley Oct 15 at 4:24
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    It's not that the yeast does not multiply, it does. But during the growth phase, yeast creates lots of flavourful esters. So if you under-pitch, you may get too many esters, leading to unwanted flavours. Obviously if you over-pitch, you get less esters, possibly making your beverage bland. It's a balancing act. As @Czernia answers, a very sugary environment is hard on the yeast, and many of then die. In this case you pitch much more yeast to mitigate this. But 1.057 is not that high, so I doubt this is the issue here. If your yeast was mis-handled, maybe it was not healthy at start. – Kingsley Oct 15 at 4:31
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The point of a starter is to obtain a higher and healthier yeast count before you pitch into the wort. This is accomplished because the 100g of DME/Liter create enough sugar for the yeast to reproduce while also not inducing an osmotic shock from the change in environment. The same thing happens if you pitch yeast from the package into the wort, except for the fact that your yeast count will be lower. My two cents is that there wasn't enough healthy yeast in the package pitched, and that caused a stressed or stalled ferment. As for reusing yeast, there are methods, but I never bother with it, so I can't help you there.

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  • Thx @czernina ... So ELI5, the density of sugars in the starter is higher than the sugar in my wort? The yeast has more sugars to chew through? – James Boutcher Oct 14 at 23:24
  • I'll just add to this that the yeast tend to exhaust themselves going from package into a full batch. They have to go trough growth phase and then do fermentation. With a bigger healthier pitch from a starter (or super fresh packs or mulitple packs) the yeast have less work to do. Its seems counter intuitive but that's the biology/biochemistry of it. Yeast are not machines that just run because there is fuel (sugar). They are creating waste products that slow them down and micronutrients needed for growth are also needed for fermentation. When they are spent they are spent. – brewchez Oct 16 at 13:17

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