What is the purpose of step starters?

[MannyEdwards]

Can I just pour a vial into 2 liters of wort and get twice as much yeast as if I had used 1 liter? Is the reason for stepping your starters only because of the size limitation of the flask?

 

[bruteforce]

Yeast will only reproduce so much in a given wort. After that they settle in and just make beer. Doing a step starter gives them multiple times to reproduce with more oxygen, no alcohol and fresh materials.

 

[passedpawn]

PROBLEM: If you start with a small amount of yeast in a large (e.g., 5L) starter, it can take a while for the alcohol levels to rise in that starter. During that time, the wort has no protection from infection.

SOLUTION: If you start with a 1L starter (or smaller), the ABV of the starter will rise more quickly, thus protecting it from infection. Once fermented, the 1L starter can be chilled, decanted, then dumped into a 5L starter. With all the new, active yeast, the 5L starter will ferment quickly.

This was my thoughts anyway. I used to grow yeast from small frozen samples using 1L and 5L flasks. I doubt that stepping up with larger amounts of good yeast (i.e., Wyeast packs or White Labs tubes) warrant doing this.

 

[thekraken]

There is also a sweet spot in pitch rate for optimal growth. Or so I've been lead to believe. For example "For best growth, you want to keep your inoculation rate between 25 and 100 million yeast cells per ml." http://www.brewunited.com/yeast_calculator.php

 

[MannyEdwards]

That all makes good sense -- thanks. Very helpful!