Originally Posted by fosgate
I was thinking about this not a little while ago and maybe someone can answer me this. Yeast goes through aerobic respiration when there is O2 present and they basically produce more yeast and store nutrients until you choke them off of O2 (thus the airtight seal). Once you choke off the little bastards they start producing alcohol until they use up the sugar or hit a certain level of alcohol and then flocculation back into a hibernative state. But if the process is interrupted like this when the cap pops off when they are at the fermentation stage, do they go back to aerobic respiration gobbling up sugars but not producing alcohol until you choke them off the second time? Or do they simply continue into flocculation or suspend their cycle completely?
Yeast operate until they reach a limiting resource. An airlock coming off does not really change the make-up of the wort on a scale that will effect the yeast. There may be a little more O2 present at the surface but the amount that can dissolve into the solution will not be much, especially if the yeast are already producing CO2 which is going to create a positive pressure for anything trying to get in.
If there is not a protective CO2 layer, then the surface exposed to open air will start to oxidize, if bacteria are also present then you will have the beginnings of infection. For instance, Acetobacter convert alcohol to CO2 and acetic acid (vinegar). Lactobacillus convert sugars to lactic acid, most do not produce alcohol (homofermentative).
Bacteria have no means of transport on their own so an empty 3-piece airlock is not a big deal for bacterial infection.