what is it about high krausen that is so important? does this allow the yeast to immediately start replicating when it is pitched into the wort? what would happen if it wasn't high krausen? how long does this window of time (high krausen) typically last?
High krausen represents the peak activity of the yeast. It's similar to when a fermenting batch is top cropped and pitched into a new batch at high krausen. The yeast are most active and healthy at this stage.
The timing has been discussed earlier in this thread.
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I suspect there is something a little more to it than that. I believe I have read that early in the fermentation the yeast needs to drop the pH of the wort. I'm under the impression that the fewer yeast cells there are to start with the more difficult this process becomes. but perhaps yeast that are already actively replicating (high krausen?) is a different scenario. some detail on that would be nice.
From Mr Malty aka Zainashef
"Q: At what point do I pitch the starter into the wort? A great deal of discussion rages over this topic. Should the starter be fermented completely, the spent liquid decanted, and the yeast pitched or should the entire starter be pitched when at the height of activity? Most yeast experts say that when propagating yeast, moving at high krausen is optimal. The time of high krauesen can range anywhere from a few hours to twenty-four or more. It depends on the amount of yeast added to the starter wort, yeast health, temperature, and several other factors. Doss says a starter made from an XL pack of yeast into 2 liters of wort will reach its maximum cell density within 12-18 hours. If you're starting with a very small amount of yeast in a large starter, it can take 24 hours or more to reach maximum cell densities. For the average starter, let's just say that the bulk of the yeast growth is done by 12-18 hours. I like to pitch starters while they're still very active and as soon as the bulk of reproduction is finished, usually within 12 to 18 hours. This is really convenient, because I can make a starter the morning of the brew day or the night before and it is ready to go by the time the batch of wort is ready. "
It seems to me that they key controversy is stirplates. Zainashef has said they are the best, Mark says no. I know that stirplates work. I know that oxygenated shook starters with no stirplate work too, but I end up with a far better smelling pitch, and so far the main fermentation appears to take off faster and is more vigorous. But thats from just one trial so far.