The Science of Fizz: Why Ferments Make Gas
At its heart, fermentation is a biological process where microorganisms like bacteria and yeast convert sugars into other substances. For lacto-fermentation, the kind used for vegetables like sauerkraut and kimchi, bacteria break down the natural sugars in the produce.
A primary byproduct of this activity is carbon dioxide (CO2) gas. Think of it as the microbes eating a meal and exhaling. In the first few days of a ferment, this activity is vigorous, producing a surprising amount of gas. A single five-gallon batch of beer, for example, can produce over 400 litres of CO2. While your quart jar of pickles won't produce that much, it's more than enough to create a problem if it has no place to go.
The Danger of a Sealed Jar
When you seal a fermenting mixture in an airtight container like a standard mason jar with a tight lid, you are creating a potential pressure bomb. The CO2 gas builds up, but with nowhere to escape, the internal pressure can rise dramatically. The initial signs are often a bulging or domed lid. If ignored, the pressure can become strong enough to force the brine up and out, causing leaks and a sticky mess. In more extreme, though not uncommon, cases, the pressure can overcome the strength of the glass itself, causing the jar to crack or even shatter violently. This not only wastes your ferment but also poses a serious risk of injury from flying glass.
The Best Solution: Using an Airlock
The most reliable and hands-off way to manage this pressure is by using a fermentation airlock. An airlock is a simple, one-way valve that fits into the lid of your fermentation vessel. It allows CO2 to bubble out while preventing oxygen and contaminants like mold spores from getting in. This creates the ideal anaerobic (oxygen-free) environment that beneficial bacteria need to thrive. Airlocks come in several forms, including the common S-shaped or three-piece models which use a small amount of water to create the seal. There are also specialty lids for mason jars with built-in silicone valves that achieve the same purpose. Using an airlock largely turns fermentation into a set-it-and-forget-it process.
The Manual Method: How to 'Burp' Your Jars
If you don't have an airlock, you can still ferment safely using a standard sealed jar, but it requires daily attention. This method is called "burping." To burp your ferment, you simply unscrew the lid slightly until you hear a hiss of escaping gas, and then immediately tighten it again. You do not need to open the jar fully. This should be done at least once a day, and sometimes twice a day during the first, most active phase of fermentation (typically the first 3-5 days). Signs that your jar needs burping include a domed lid or brine rising towards the top. While effective, burping does require consistency and momentarily allows oxygen to re-enter the jar, which is why an airlock is often preferred for a more foolproof result.
