November 05, 2019

Open or Closed Lid for Milk Kefir Fermenting?

Open or Closed Lid for Milk Kefir Fermenting?

Homemade milk kefir can be fermented with either an open lid or a closed lid, or in other words, aerobically or anaerobically. Aerobic basically means a ferment with oxygen, which is usually done with a cloth lid. Anaerobic means without oxygen which is done with a closed lid or airlock.

Traditionally milk kefir was typically fermented aerobically with an open lid, but anaerobic milk kefir fermenting is becoming more popular.

There is really no "correct" way as each has its own advantages and disadvantages. It really comes down to personal preference in the end.

But there are some very distinct differences in the bacteria and yeast and risk of contamination.


Milk kefir consists largely of lactic acid bacteria which does not need oxygen at all. Acetic acid bacteria found in kombucha and water kefir (to a much lesser degree) needs oxygen to properly work, but most milk kefir grains have little to no acetic acid bacteria. The acetic bacteria is what gives kombucha that vinegar flavor. The sourness in milk kefir ferments come more from the lactic acid.

However, there are bacteria strains within milk kefir grains that thrive in oxygen rich environments. Kefiran is one of them. Kefiran is the gooey stuff on the grains that has a whole host of health benefits. Kefiran loves oxygen and does better with an open lid ferment. In one study, kefiran production increased from 36 mg to 44 mg during the aerobic condition.

On the flip side, there are strains that thrive in anaerobic (non-oxygen) environments. One of them is L. kefiranofaciens. This particular dominate strain has been shown to have immunoregulatory, anti-allergic, anti-asthmatic and anti-colitis abilities. However most lactobacillus species have no issue with the lack of oxygen though.


Yeast is mostly aerobic and does best with an open lid. One thing that people notice the most when they switch from an open lid to a closed lid or airlock is that it has a bit more of sour bite due to the bacteria and is noticeably less yeasty. It still tastes like kefir though, it’s just a little different. There is still active yeast, just not as much.

One study demonstrated that S. cerevisiae is significantly more active with an aerobic environment. S. cerevisiae is perhaps the most famous and studied yeast found in a variety of different cultures.

The amount of yeast you like in milk kefir is mostly a matter of personal preference. I would say that the majority of people prefer less yeasty kefir, but there are plenty of people who can’t get enough of that yeasty bread-type flavor and carbonation.  With water kefir, the reverse is true - people usually can’t get enough yeast. The more yeast, the better.

So if you don’t like yeast, try a closed lid or airlock.


One of the biggest benefits of anaerobic ferments is that it prevents airborne yeast or mold contamination. Milk kefir is highly resistant and protective against mold, so that risk is usually quite low. Yeast contamination is a bit more common.  There is plenty of yeast flying around and typically that’s not an issue. Sometimes the grains even pick up the local yeast and use it in the ferment. With milk kefir, you probably wouldn’t even know the difference. Sometimes you will though in the case of kahm yeast. With a closed lid, its much less common to have those issues.

Cross contamination is also something that can occur. If you do multiple cultures within a small area, then you may want to try an anaerobic ferment.  Sourdough and kombucha throw up a lot of airborne yeast. Milk kefir is very resistant to anything foreign, so it probably won’t have too much of an impact. But it’s always a good idea to separate them or use a closed lid.

Instructions on how to do an oxygen rich aerobic ferment:

  • The easiest way is to use a paper towel and elastic band. You can use anything that is porous enough to allow the free exchange of air without being open enough to allow fruit flies (and other bugs) in.
  • We offer a convenient elastic cloth lid that’s easy to wash that makes doing aerobic ferments easier and also make it look beautiful at the same.

How to do an anaerobic / closed lid ferment:

  1. The simplest method is to use lid. The canning lids that come with the jars can and will have a galvanic reaction to the acids from the ferment. Unless you enjoy bits of metal in your kefir, try a plastic lid instead (which is much more inert).

You can do a loose lid or tight lid if you are adventurous. A loose lid is basically just setting the lid on top of the jar without screwing it on. It’s safe because if there is an excessive amount of carbonation it will be able to escape. However, it does not create a seal and is not truly anaerobic. And some people consider that the best of both worlds. But it will still be at-least partially anaerobic and will change the ferment.

A closed tightened lid will create a strong anaerobic environment, however you do risk explosion. Typically it won’t create enough carbonation to break through the jar during the first ferment, but there are rare exceptions. Also there are people who claim that the seal is not strong enough for a true anaerobic environment. For that you need an airlock.

  1. Basic airlockWater-less airlocks are the cheapest and simplest way to get the benefit of an airlock. The air simply vents when needed. It’s not as good as the 3 piece traditional airlock, but it will do the job. We offer these airlocks with a plastic lid bad instead, so you can use that instead of the metal lid bands that come with the jar.


  1. Traditional 3-piece air lock. These are the best and perhaps the only way to get a true anaerobic ferment. If you are looking the the ultimate anaerobic environment, then these air locks are a must have.

Bottom line

Milk kefir can be fermented aerobically (with oxygen / open lid) or anaerobically (without oxygen / closed lid). Both have the advantages and disadvantages and it comes down to personal preference.

Milk kefir aerobic / open lid ferments

  • Oxygen loving bacteria strains like kefiran thrive
  • Low oxygen bacteria like L. kefiranofaciens do NOT thrive
  • Greater yeast content
  • Higher risk of contamination

Milk kefir anaerobic / closed lid ferments

  • Less kefiran production
  • Lower yeast content
  • Lower risk of contamination

What about water kefir? In our follow-up article we discuss the difference between an open and closed lid ferment when it comes to water kefir fermenting.

So, which method do you prefer?