Mon Jan 10, 2011 6:26 am
There are several events that show you that a healthy fermentation is underway. The first is not visible - it is a substantial, say 0.3 to 0.8, drop in pH. The comfort detection of this brings is responsible for half the value of a pH meter IMO. The second is the appearance of tiny scintillations at the surface of the wort when a flashlight is shone upon it at a shallow angle. These are caused by tiny bubble breaking the surface. The third is the appearance of little islands of foam usually in the center (in a cylindroconical) or at the edges. The fourth is expansion of the foam. I guess bubbling of an airlock would probably start around this point. Finally is the rocky head and vigorous bubbling of the airlock. IMO it's a good fermentation if all this takes less than 24 hours or so. A pH drop should be apparent after a few hours. That's why it is such a valuable test.
Oxygenation has a profound effect on lag time and vigor of ferment. I was (wrongly) cursing Wyeast for turning out bad slap packs because three starters I made with their Budvar yeast wouldn't start (took a couple of days). After the third one (from three different lot numbers) I decided something else must be responsible and finally figured out that my O2 flow meter was leaking most of the oxygen going through it into the atmosphere with little going into the wort.
As to how much oxygen is enough - that depends on the yeast strain. Even if you knew the answer for a particular strain you would have no way to control it without purchasing a relatively expensive DO meter. I strongly encourage people to buy pH meters because they are inexpensive and can be used for several things. DO meter - not so. The effects of too little oxygen are dramatic. The effects of too much less so. If you pass enough O2 through a sintered stone so that bubbles just break the surface and do that for a couple of minutes you should have sufficient but not too much oxygen to make most strains happy. FWIW I generally do about 20 mg/L.
