Sourdough Starter Lid On or Off?

Should you have the lid on your sourdough starter on or off? And if it's on, should it be screwed on tight or just sitting on top? These questions are super common when you're making a sourdough starter from scratch - or even reactivating a starter you've purchased.

Ideally, your sourdough starter should be loosely covered with a lid or paper towel to keep out bugs (like fruit flies) and other debris. A loose lid will also stop it from drying out too much. You don't want to screw the lid on tightly because sourdough starters emit gas which can build up in the jar and cause it to explode (and that's a mess you don't want to have to clean up).

While the short answer is that you should loosely cover your sourdough starter, there are times when you can have a tight fitting lid, which I will explain.

A large jar of sourdough starter with a yellow plastic lid. There is a plant and loaf of sourdough in the background. — A plastic lid like this one is perfect because you can remove the seal from the inside and have it just sitting loosely on the top of the jar. This lid is not sealed shut allowing the jar adequate space to "off gas" while the sourdough culture is growing.

How To Cover Sourdough Starter?

The way to cover sourdough starter is:

FIRST TWO WEEKS - a paper towel or light cotton cloth (secured with an elastic band) when first establishing your sourdough starter.
AFTER FIRST TWO WEEKS - a loose fitting lid that is not screwed on tight
WHEN PUTTING IN THE FRIDGE - a tight fitting lid (as long as the jar is large enough) is perfect when storing your sourdough starter in the fridge.

A jar of sourdough starter without a lid - instead it has a piece of paper towel secured with a red elastic band. — This jar of sourdough starter has a piece of paper towel covering it rather than a conventional lid. The elastic band around the actual jar helps to show when the starter has doubled and tripled.

What Is Happening Inside Your Sourdough Starter Jar?

There's a whole lot of things happening inside your sourdough starter jar, even when you feel like there's no bubbles to be seen.

The bacteria and yeast feed on the carbohydrates formed in the flour when water is added. These sugars are consumed by the yeast and bacteria and turned into CO2, acids and ethanol.

All of this activity can be happening right before your very eyes!

But, while there is a living colony inside your jar, your sourdough starter doesn't actually need to breathe in the sense of the word.

A small jar of sourdough starter with the lid on. The starter fills up most of the jar and is being held up for the photo.

Does A Sourdough Starter Need to Breathe?

This questions is certainly one that gets asked often (and one I included in my weirdest questions I get asked about sourdough post).

Sourdough starters do not in fact breathe and therefore do not need to have access to fresh air. However, the organisms in your sourdough starter do emit carbon dioxide, which is a gas that can build up inside the jar.

If there is not adequate space in the jar, or a way for the gas to escape, the jar will eventually shatter and break. If you do not have a back up sourdough starter, this can be a horrible experience! I do not recommend trying to save a sourdough starter from a shattered jar because of the risk of glass shards remaining in your starter.

This is also why it's so important to have the right sized jar for your sourdough starter. Allowing ample head room in the jar ensures that your starter won't build up enough pressure to shatter the jar.

Can I Leave My Sourdough Starter Uncovered?

It's best not to leave your sourdough starter jar uncovered because the starter can attract fruit flies and other bugs. You also don't want to have anything else fall into the jar and ruin your precious bubbly sourdough starter.

If you really don't want to use a lid, then a piece of paper towel, coffee filter or light cotton cloth secured with an elastic band is the best option.

Two small jars of sourdough starter. The jar on the left is dried starter and the jar on the right is liquid starter. Neither jar has a lid on.

Does A Live Sourdough Starter Need Fresh Air When Being Fed?

Your sourdough starter will survive without direct access to oxygen because it is an anaerobic fermentation process, meaning it does not need oxygen to occur.

While your sourdough starter doesn't actually breathe and therefore can have the lid sitting on the jar, it can benefit from fresh air.

Some of the yeast in your sourdough starter do come from the air, so having some access to fresh air is part of colonising a starter. Generally, the air it is exposed to when you feed it is enough, but in my experience, sourdough starters love to be stirred up - even between feedings.

I have had a sourdough starter for a very long time. I've put my sourdough starter through many experiments to find out exactly what it can tolerate and what it can't.

Stirring your starter not only moves the organisms around allowing them to access more of the food in the jar, it also encourages more yeast to enter the jar. This can give your sourdough starter a real boost!

You might hear of die hard sourdough bakers taking their sourdough starter jar on "outings" to different environments, situations or even different countries to increase the different strains of yeast in their starter. It might be a little crazy ... or is it?

Why Is My Sourdough Starter Drying Out?

Sometimes if you are using a paper towel or coffee filter to cover your sourdough starter, it can cause the top layer of starter to dry out. While it's not ideal, there's no harm done. Just scrape the dry top layer off when you discard before feeding.

If it keeps happening, you may want to change to using a conventional lid, just leaving it loosely sitting on the top of the jar.

Does My Sourdough Discard Jar Need a Lid?

You should always have a lid on your sourdough discard jar. Ideally sourdough discard should be stored in the fridge for up to two weeks. During this time, you want to ensure that it is securely covered with a sealed lid so that it does not absorb smells or debris from inside the fridge.

If you have an abundance of sourdough starter discard, you might like to try some of these tried and tested sourdough discard recipes.

Frequently Asked Questions

What does anaerobic fermentation mean?

Anaerobic fermentation is the metabolic process whereby bacteria converts complex carbohydrate into gases and acids without the presence of air. Examples of anaerobic fermentation include lactic acid fermentation like in yogurt and sourdough starter or alcohol fermentation like making beer. This type of fermentation does not need oxygen present for it to occur.

Is sourdough starter bacteria or fungus?

A sourdough starter contains lactic acid bacteria. It is not itself a bacteria or a fungus, but is actually defined as a symbiotic culture of bacteria and yeast, whereby the bacteria and yeast live in harmony with each other to form the sourdough starter.

Can I use a metal lid on my sourdough starter jar?

Yes you can use a metal lid on your sourdough starter jar. Often the metal collars are great when used with paper towel or a coffee filter. I have found that rust is a problem with metal lids however and prefer to use plastic lids for this reason. A metal lid will not harm your sourdough starter in any way.