There are some ferments that need to be exposed to as little air as possible, and others that actually require air exposure for things to work right. This can definitely be confusing for new fermenters, but the key is to understand that there are different types of fermentation: bacterial and yeast, cultured and uncultured, aerobic and anaerobic, so each food needs to be made a little differently.


Think of the way you would manage the cooking of any other food. If you prepare lettuce the same way you prepare steak, you’re going to be in for a soggy mess of unidentifiable goop! Although from a biologist’s perspective all fermentation would be anaerobic (airless), things get complicated in food fermentation. So here’s your cheat sheet to common ferments and their need for (or aversion to) oxygen.

No Air Necessary

Vegetables: Pickles, Kimchi, Sauerkraut, etc.  Vegetable lactic acid ferments are best kept away from the air. An easy method for doing this is applying a weight to the top of the vegetables in your jar or crock so that a thin layer of brine covers the vegetables. Loosely cover with a lid or secure a kitchen napkin or coffee filter with a rubber band to keep out dust and debris. 

Alcohol: Wine and Beer. After the very first days of fermentation, alcohol must be kept away from air exposure. Generally, an airlock is your best bet to make sure that the bacterium responsible for turning alcohol into vinegar (acetobacter) is not not getting in there.

Oxygen Required

Kombucha. Kombucha is an aerobic ferment, so make sure your SCOBY can breathe. Your best bet is to fill your kombucha-making vessel no more that 3/4 full and cover securely with breathable material, such as a cloth napkin, paper towel, coffee filter or a tight-woven cheesecloth. This allows air to pass through, but keeps out household dust or any beasties that may try to fly in there.

Vinegar. Vinegar is an aerobic ferment and needs air to culture properly. I ferment vinegar exactly as I ferment kombucha (above) and have great results. Once vinegar is fully fermented, make sure to seal it up though! Air is friendly during fermentation, but will continue to transform vinegar even after its done.

Optional Air

Bread Cultures: Sourdough Starter, etc. Yeast need air to reproduce in early fermentation, and since these are all short ferments or cultures that require regular feeding, the risk of mold is very low. Cover with a cloth that allows air exposure. You are unlikely to have any trouble fermenting sourdough starter in a tightly covered container, though. 

Yogurt and Other Cultured Dairy.  Most cultured dairy involves Lactic Acid Bacteria, which are anaerobic bacteria. However, the short fermentation time means no/low mold risk, so there’s no need to get crazy about making sure they’re not exposed to air. Most people ferment yogurt in closed jars, which does keep air exposure to a minimum, but you can certainly cover it with a cloth, depending on your incubation method. There will be a small amount of carbonation built up in a closed jar yogurt ferment, so be prepared for a pffft of escaping CO2 when fermenting this way, and make sure you don’t forget about it while it sits in its warm environment!

Kefir: Milk or Water Kefir. It actually doesn’t matter a whole lot! Both of these are mixed yeast and bacterial cultures, and although Lactic Acid Bacteria (LAB) are not big fans of air exposure, they do just fine when a little air enters the equation, and with the short fermentation time required to make these beverages (1 to 2 days), there is very little risk of any surface yeasts developing from air exposure. Some people choose to cover with a cloth, while others close these beverages in a mason jar or European-style gasket and clamp jar. Be aware that both fermentation processes produce CO2, so you should attach a lid loosely to allow any carbonation to escape. 

Amanda Feifer writes about food fermentation of all kinds on her blog, Phickle.com and penned the forthcoming cookbook Ferment Your Vegetables (Fair Winds Press, October 2015). When she’s not concocting crazy vats of bubbly things in the kitchen or ranting against the use of anti-bacterial soaps, she’s doing what she loves most: teaching people to make their own fermented foods, from kefir and kombucha to kimchi and koji. She lives in South Philly with her husband, Jake, and their rambunctious pit bull pup, Laika.