You must use an airlock when fermenting. Yeast has a complex lifecycle. When exposed to air (aerobic), yeast burns sugar to make more yeast. It is good to prepare a "yeast starter" by placing a little year in a small amount of sugar-water solution and keep it stirred well so that the yeast propagates aerobically thousands of times. This starter solution is then added to your wort. When fermenting, you need to keep the fermentation vessel relatively full of liquid (free of air, but leave some space at the top for foaming) and fitted with an airlock so that CO2 can escape but air can't get in. The yeast will use up the dissolved oxygen in the liquid and then go into its anaerobic mode where it turn sugar into alcohol and CO2. If you don't use an airlock, more oxygen will continue to dissolve and the yeast will use up more sugar to make more yeast and less alcohol.
From: et_couldry <et_couldry@...>
To: new_distillers <firstname.lastname@example.org>
Sent: Thu, Apr 25, 2013 11:40 pm
Subject: [new_distillers] Air locks???
Are air locks required? I read on homedistliers.org
to use them, but I have heard that your also not soposed to use them cause the oxygen lets the yeast run better? If using the air lock. When it stops bubbling does that mean the mash is done?