Why is a yeast starter necessary? How is it different than standard fermentation?

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)

Topic yeast-starters homebrew

Category Mac

1
answered at

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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