Generally speaking 50% of the yeast growth occurs in the first 24 hours and 90% of the yeast growth is achieved by 48 hours. (see: http://www.woodlandbrew.com/2014/05/fermentation-model.html) Warm temperatures (70-80°F) and plenty of available oxygen (stir plate) typically double the growth rate.
If the starter is allowed to run to completion (typically 5-7 days) then the yeast will prepare for dormancy, building their glycogen reserves, and absorbing compounds from the broth that they may have expelled during fermentation. When the yeast are transferred to a new environment glycogen and these compounds can aid in their adaption.
In my experience, allowing the starter to run to completion before cold crashing leads to a faster, more vigorous and complete fermentation that pitching yeast directly from an active starter. (see: http://www.woodlandbrew.com/2013/02/side-by-side-starters-3-of-4.html)
Decanted yeast? Should I siphon off the liquid, then add new cooled DME on top of yeast?
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