If you're supposed to pitch the starter at the height of cell production then what is the point of cooling the starter to decant, seems like that would put the hard working yeast to sleep.
I was under the impression that the core purpose of creating a starter is to create more yeast and to prepare it to ferment wort, hence why all yeast calculators are based on final cell count.
Its confusing, but I'll try to explain.
There are two approaches to pitching starters. Either you pitch at the height of activity or after activity has declined when the yeast cells are preparing for dormancy.
The advantages of pitching at high activity are reduced lag time, because the yeast are at full metabolic activity when pitched. The disadvantage is that this can potentially stress your yeast. If the starter is a different compositions of sugars and density to the main wort then the yeast has to adapt. Yeast are lazy and stop making enzymes that consume certain sugars if they are not constaintly exposed to them. Also the temp at which your starter is at if warmer than your wort which normally it will be then this will cause further stress. Normally you want to keep your starter at 20-23C but you want your pitch temp lower than your starting fermentation conditions, let say 16C. If you pitched to a wort at 23C your potential get funky flavours or you will have to cool it down. Yeast generally don't like being cooled down as this tiggers expectations of dormancy, which is not good to have at the begining of fermentation.
So IMO although pitching at high krausen is good for lag times you need to have a starter very similar to your main wort and the pitching temperature needs to be similar to your wort. This rules out most beers especially high gravity beers and ale and largers at cool fermentation temperatures. So most beers imo.
If you let the yeast starter reach high activity and then give it a further 18 hours what happens is during that time the yeast prepare for dormancy. They make a series of carbohydrates for a food storage and to fortify their cell wall, this is because they are anticipating that food will run out and that the environment that will follow will be different. Futher more they are readying themselves for a changes in temp, avalible sugars and osmotic pressure.
If you put the starter in the fridge 18 hours after full krausen, so basically once flocculation has mostly completed, approx. 30-36 hours after making the stater the yeast will be the most tolerant to a change in conditions once pitched. You will be pitching from a low temperature to a higher one meaning the yeast will be switched on by a favourable temp change as opposed to off by a negative one. The production of those carbohydrates i mentioned mean the yeast is less stressed by the change in osmostic pressure and are switched on to make any enzymes required to consume any types of sugars they physically can. The start will be slower because you have to wait for the yeast and wort to reach fermentation temp but yeast health should in theory be better.
I use the second method and I normally get strong airlock activity with 12 hours, probably less but I pitch over night so I can't be sure exactly when. So the additional lag imo is a not an issue.
You also have the advantage that you can decant off the rubish starter beer, which is important for pale beers in particular.
