when reusing a yeast cake, can I stir it up or do I just rack on top of it? My aeriation method thus far is to hold the brew pot about 4 feet above the fermentor and pour it in. I generally have a huge head of foam...can I do this on top of a yeast cake or not? Also what is the time frame I am working with? Do I need to put the new wort in immediately or do I have some time in between?