Why Homebrew Airlocks Suck

So the other day, against my better judgement, I tried to explain to a newish brewer why it's a bad idea to shake your fermenter in order to rouse the yeast. While this is something that I've seen posted on other homebrew forums, it's a terrible idea for the following reasons. 

  1. You've just introduced a bunch of dead yeast back into your beer. Those yeast had gone to their eternal rest and you've just disturbed it, creating millions of yeast ghosts that will haunt your beer with autolysed flavours.
  2. The yeast that might have been just sleeping are now only marginally more awake. If they've gone through the process of flocculating, they now need to get out of their sleepy time pyjamas which causes diacetyl as they fire up their systems. They're also gonna be real annoyed when they find out there's no new nutrients and just floc out again, this time without cleaning up the diacetyl they just made and they'll autolyse quicker since you made them take off their jammies.
  3. You'll introduce oxygen into your beer causing alcohol to oxidise into acetaldehyde 

It was this last point that this guy took issue with. He was adamant that while the blanket of CO2 might be blown off by rousing the yeast, his high school physics told him that it was impossible for the higher pressure inside the fermenter to be displaced by the lower ambient air pressure. 

So let's go through some fluid dynamics - in which "fluid" also refers to air. 

To start, think back to your air and aerodynamics unit in science class. When fluid moves, it tends to condense. Aircraft take advantage of this by moving air, condensing it, and then diverting it underneath their wings, making it so they're literally riding on a cushion of air. As you do this, the air above the wing ends up being spread thinner, making it less dense and therefore relieving some of the pressure keeping it down.

What does this have to do with your airlock? It doesn't matter how heavy your blanket of CO2 is, if it starts moving, it's gonna create areas that are denser and others that are less dense. These less dense areas are going to start pulling fluid from wherever they can to balance things out because nature hates imbalance. The easiest place to pull it from is the air outside of the fermenter because it's not moving and it's comparatively lighter. 

Commercial brewhouses that oxygenate with air rather than pure O2 use a very simple device using some of these principles. A section of the pipe from the chiller to the fermenter is constricted, causing the fluid to condense. A hole is added which, you would assume, would cause wort to start shooting all over the floor. What actually happens is that the fluid expands on the other side of the constriction causing a low pressure situation and sucks air into the pipe in order to balance it. It's known as the Venturi effect and you can make a venturi tube yourself with almost no equipment. 

So back to the airlock. In theory, an airlock should prevent air from coming into the fermenter in the first place, but that's not what happens. Airlocks only work when there's no other motion happening and they're pretty terrible at even that. Think back to another science experiment that you probably did in which you held a full glass of water upside down over a piece of cardboard. When you yanked the cardboard away, the water stayed in the glass because the air couldn't get above the water to displace it.

Airlocks used in home brewing aren't even at that level.

I set up an experiment with a fermenter and an airlock and then I opened the valve. If airlocks were even remotely capable of preventing oxygen from entering your fermenter, there should be no flow of liquid once I open the valve. Here are the results:

Homebrew airlocks suck.