Boiling the wort drives out the oxygen so aeration of some sort is needed
prior to pitching the yeast. There are several methods the homebrewer can use.
While this method works and you get beer. It might not taste like you want
it to. Yeast need oxygen to reproduce.
This is about as effective as doing nothing. It has to do with surface area
and pressure…
This works pretty good. Most of us can’t do it with out the risk of back injury
or smashing our carboy to bits on the floor. Also you are limited to the air in the
empty space of the carboy to get O2 from.
This is the best way for most beers. Cheap and no O2 tanks to buy. And there
is no risk of over saturation of O2 which is not a good thing.
This is the most expensive way and you run the possiblilty of over saturation of
O2 which leads to oxidation of the yeast and that is not good for them.