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Open or Closed Lid for Milk Kefir Fermenting?

by Nathan Pujol November 05, 2019 6 Comments

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?

Nathan Pujol
Nathan Pujol


Co-founder and author of Graduate degree in clinical psychology. Researcher with emphasis on the gut microbiome, fermentations and their connection to mental and physical well-being. He has 15 years of experience with making, sharing and teaching about traditional ferments.

6 Responses

Yemoos Nourishing Cultures
Yemoos Nourishing Cultures

February 20, 2020

Hi Mo – That’s a very interesting question. Basically I would say that there’s nothing to worry about with a cold. The grains and kefir are highly resistant to anything foreign. It’s not like a petri dish that will grow whatever bacteria gets near it. Kefir is a very hostile place to foreign mold, bacteria, viruses, etc and will neutralize those threats. I would still use common sense and use clean hands and not directly sneeze into it and it should be just fine.

Mo Adamski
Mo Adamski

February 20, 2020

I should be receiving my kefir grains in the next few days and have just come down with a cold. Will the bacteria from my cold affect the kefir? How can I protect it?

Yemoos Nourishing Cultures
Yemoos Nourishing Cultures

December 01, 2019

Hi Cheryl – That’s a great question. I personally would not have sourdough near a open lid kefir ferment. Sourdough is very active and there is likely going to be at least some kind of yeast transference. Whether that constitutes contamination is debatable as there are plenty of airborne yeasts that naturally get mixed in with kefir on a daily basis. However, the problem is that its significantly more concentrated with the sourdough and that may cause temporary imbalances or yeasty kefir. I would recommend separating them or putting a lid on the milk kefir.


December 01, 2019

Hi, Thanks for this highly informative blog! I’ve recently returned to keeping a kefir culture and I’ve learned a lot here about taking proper care of it. I’ve also just started my own sourdough culture. I’ve been keeping it beside my kefir culture so I remember to take care of them. Both are covered with cloth. Is this a bad idea to keep them together? Could it cause contamination? Do you have any experience based advice about this?

Yemoos Nourishing Cultures
Yemoos Nourishing Cultures

November 14, 2019

Hi Judy – If you haven’t recently changed milk or didn’t go through a significant temperature change, there may be some kind of yeast contamination. I would give them a good rinse and see if that helps. Also trust your senses. If it smells and tastes right to you, it should be fine.

judy van den Heever
judy van den Heever

November 13, 2019

HI Nathan, what a beautiful clean and uncluttered site! I have loved my brewing and culturing these past few year. However I have recently moved and am experiencing differences in my milk kefir. The last batch, once separated from the grains and in the fridge has gone very slimy? I also notice a thick white film on the top of the jar that I am storing my grains in?

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