Carbon dioxide is the protector of beer, especially its flavor. That's right CO2 plays a large role in keeping your beer fresh from the moment fermentation starts to the moment it is dispensed from a keg or bottle. Some things to think about...
- Why does a keg of beer go bad when you use a picnic tap to pump air into it?
- Why put airlocks on our fermenters?
- Why evacuate the air from a bottle or keg before packaging?
- What does oxygen do to a finished beer?
What would beer be like if we didn't have CO2? It wouldn't retain its fresh flavor for very long. Here's why. Beer and oxygen don't get along very well. When beer comes into contact with air the oxygen changes the flavor of the beer. CO2 can help protect your beer from air.
CO2 and beer are inseparable. Any time brewer's yeast is involved, CO2 follows. When fermentation starts and CO2 is produced it is released into the atmosphere. Where does it go? That largely depends on where it can go. Imagine a dinner plate full of beer fermenting. I know it seems kinda silly, but, bear with me. CO2 is heavier than air so think of it running off of the plate of fermenting beer. Now, let's consider one other item that helps CO2 do its job, a vessel such as a cup, bowl, fermenter, bottle, anything that can hold the fermenting beer. Now imagine the fermenting beer filling three quarters of the vessel. What occupies the remaining headspace during active fermentation. CO2 of course. It is that headspace filled with CO2 which preserves the beer even in the early stages of fermentation. Comparing the plate full of beer and a deeper vessel we see that the deeper vessels have an advantage. They have a place to hold a blanket of CO2 over the beer.
Once fermentation is complete and the beer is finished we proceed through all other activities such as racking, bottling and kegging risking oxidation at every turn. This is why we should do our best to eliminate air from our packaging process. I presume bottle conditioning helps this by creating a closed environment for fermentation where the oxygen is used by the yeast.
I'm rambling a bit here so let's wrap it up. Yeast eats the sugars. Carbon dioxide is released into the beer. CO2 floats to the top of the fermenting beer and lays on top of the beer filling the vessel's headspace and displacing the air. This blanket of carbon dioxide protects the beer. Beer fermented without this CO2 blanket wouldn't taste the same. It makes you wonder how important earthen vessels were to the earliest brewers.
So, is Carbon Dioxide the most important thing about brewing?
Note: Edited for clarity and readability 7/10/09. I'm sure I'll be yet again.