Does kombucha need to be kept out of sunlight?

Keep Kombucha out of Sunlight. Have you seen the often-given advice that kombucha needs to be kept out of the sunlight for best results? Maybe you saw it in a kombucha recipe in a book, or somewhere on a blog post or forum.

I see the advice to keep kombucha in the dark a lot and hadn’t given it much thought, to be honest. We brew our kombucha in clear glass jars (like the ones I describe in this post), but we keep them on a pretty dark shelf in our kitchen, which doesn’t get any direct sunlight anyway. So I figured we were fine, whether the advice mattered or not!

But then a reader contacted me with this question.

Alejandro asked: “Hi, I have always wondered if Kombucha really dislikes sunlight or if this is another myth?”

Great question!

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So I sat down and started plugging various likely keywords into online research databases to see what I could find.

It turns out that nobody has done a direct study looking at how kombucha grows and behaves under different light conditions – or at least I couldn’t find one – but there are a few other pieces of research around that we can infer results from.

The short answer is that it’s true. Kombucha probably doesn’t like direct sunlight a whole lot.

Direct light disrupts your kombucha brew. It's not the UV light, it's the visible light that's the culprit.

There are three aspects to this that occurred to me as I was reading the research.

The first is about UV light, the second is about visible light, and the third is about heat fluctuations.

TLDR: It’s the visible light that’s probably the main problem for kombucha, with heat fluctuations coming in second place

Kombucha and UV light

You may already know that sunlight is a great sterilizing and bleaching agent. This is largely due to the light rays in the Ultra Violet part of the spectrum. UV light kills microbes and damages cells. It’s the UV rays that damage our own skin cells and cause cancer. It’s UV rays that are used in those handy dandy light-pen water sterilizing things that people use when travelling to places where tap water isn’t drinkable. (Like these things. Super cool.)

UV light kills microbes. Like with this sterilizer pen.
UV light kills microbes. Like with this sterilizer pen.

Now, the glass jar will provide your kombucha some protection from UV rays, as will the windows in your house. But only from UVB rays. UVA rays can pass right through normal glass and still cause damage. That’s why your “window arm” can get burned when driving long distances as, unlike windscreens, most side-windows in cars are not specially treated to block out UV rays (especially in older cars).

In addition to gaining some protection from UV from the glass, it looks like the beneficial bacteria in kombucha get some protection from UV rays from the SCOBY itself.

These three research articles (here, here, and here) all talk about a cellulose biofilm like a SCOBY protecting bacteria from damage from UV rays.

It’s possible that this protection from UV damage is the reason why some bacteria produce cellulose in the first place, which is pretty cool.

But whatever the reason, it looks like UV damage to your kombucha microbes is not actually a big deal. If UV was the only problem with keeping kombucha in sunlight then you’d be fine.

Unfortunately, there’s more …

Visible light can harm kombucha

Visible light seems to be the biggest culprit.

Although the kombucha bacteria don’t seem to care whether the lights are on or off, the same cannot be said for the kombucha yeasts.

This research article, along with this one, show that different yeasts stop growing and/or stop fermenting in the presence of visible light.

In addition, we know from the experience of beer brewers among us that visible light can damage various volatile compounds in beer, leading to ‘off’ flavors, or “skunked” beer – especially damage to the flavor molecules from the hops in beer.

“Skunked kombucha” isn’t a thing because kombucha doesn’t use hops, but it’s possible that other volatile compounds in kombucha are damaged by visible light – especially if you’re using fragrant herbs or spices in your kombucha brew.

Conclusion: Visible light is bad for kombucha.

Now, you might be brewing kombucha on your well-lit bench and are happy with the results. It’s all good. Your kombucha will behave differently from my kombucha since every scoby is different. But give it a go for a few cycles in a dark cupboard and see if it changes anything.

My guess is that your kombucha yeasts will be happier in the dark.

Heat fluctuations not good for kombucha

There’s no direct research article to link to here, but we all know how hot a covered glass container can get while sitting in direct sunlight.

Erratic spikes and drops in temperature will lead to erratic spikes and drops in fermentation and growth of your kombucha. Which in turn leads to unpredictable results for your beverage.

Answer: Keep kombucha out of direct sunlight

It seems to me that the best thing to do for your kombucha is to:

  • Keep kombucha in a cupboard with the door closed if you have a clear glass vessel, or take a look at the ceramic and stainless steel options in this post.
  • Even if you have an opaque vessel, protect it from direct sunlight so you don’t get temperature spikes.

One possible solution to keeping your kombucha out of the sun is to use a Toasted Oak Barrel as your brewing vessel. These are a bit more unusual than your standard glass, ceramic, or stainless steel options, but they have the advantage of wood being quite a good thermal insulator.

This means that even if your brew barrel gets the odd bit of sunlight striking it, it won’t get as hot as some of the other vessel types would in the same situation.

Read about Bryan Bertsch’s experience with oak barrel fermenting over at the Kombucha Kamp website.

Toasted Oak Barrel available from Kombucha Kamp

Great question, Alejandro! Thank you!

If you a kombucha question, contact me through the kombucha research facebook page. I’ll add it to my list of questions and you never know – you might see it pop up in a future blog post 🙂

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should you keep kombucha out of direct sunlight? Yes!

Three reasons why sunlight and kombucha don't mix. UV light, Visible light, and heat fluctuations, and what you can do about it.

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Author: stacey

Stacey lives and works in the South Island of New Zealand with her husband, their 4 children, 4 hens, and a rabbit that they secretly think must have watched Monty Python's Holy Grail movie.