This post was last updated on March 20th, 2018 at 06:42 am
If you’re drinking kombucha mostly as a health tonic you’re most likely drinking it to take advantage of one or other of its two main health benefits. You’re either drinking it because you are wanting to increase the amount and variety of antioxidants in your diet, or you’re wanting a natural and delicious source of probiotics.
Now, naturally you’re wanting to maximise the health benefits of kombucha, while still keeping it tasting great.
The question we’re addressing today is what happens to the probiotics – and especially the Lactic Acid Bacteria – once the kombucha is put into the fridge? Is it still ok? Or is it better to drink it straight after its fermented – without refrigerating? And what might that mean for storing your scoby?
Today’s main research article was recommended to me by a reader in the comments section of my post about Lactic Acid Bacteria. Karl asked for my thoughts on this piece of research, and what it says about how long lactic acid bacteria might last in kombucha when it is put into the refrigerator.
Rather than just responding to Karl in the comments, I decided that this paper was worthy of a post all of its own, so let’s get into it.
So, What’s the TLDR?
The key take-away from today’s paper and the reading I’ve done around this issue is that *many* probiotic foods see a big die-off of lactic acid bacteria once the food is stored in the fridge.
If you are drinking kombucha mostly for its probiotic properties, investigate making it yourself instead of buying it, and drink it fresh.
If you prefer not to drink it at room temperature you can always drop an ice cube into your glass.
If you want to skip the nitty gritty of the paper and just read my discussion and conclusions, scroll down until you hit the “Discussion” heading.
Today’s research paper:
This research was conducted by a research group from Fuzhou, China. As far as I can tell the group does food science research into ‘functional tea beverages’.
What they already knew:
1. If you measure the antioxidant activity in kombucha, you find that it changes over a period of time. It rises to a peak, and then tapers off as you continue to ferment it. (I talk a bit about this in this post)
2. Different recipes for kombucha produce different results. Both the antioxidant activity and the mix of microbes vary when you use different teas, or otherwise change your brew. (I covered this in one of my first posts)
3. If your kombucha has got Lactic Acid Bacteria in it, it will perform better as an antioxidant. (See my earlier post about lactic acid bacteria and kombucha)
4. In a study into Argentinian yoghurt, the Lactic Acid Bacteria levels dropped quickly once the yoghurt was put in the fridge – not staying high enough to meet the existing standards for being called a probiotic food.
5. You can create a kombucha culture using pure yeast and bacterial strains instead of a scoby. In previous research in their laboratory they had isolated a Saccharomyces yeast strain, a Gluconacetobacter bacteria strain, and a Lactobacillus strain. When added to sweet tea, kombucha was produced more quickly than with a traditional scoby. (Similar to how bread made using pure yeast from the supermarket is quicker than using sourdough). I will talk more about this method below, because I think it has some pros and cons to it.
What they wanted to know
So, some basic research had already been done on what happens to the antioxidants in kombucha over time. And some research had been done on which probiotic microbes can be present. But there had been no research yet into what happens to these probiotic microbes over time in kombucha. How and when do those probiotics grow and flourish? How long do they last? And, of particular interest to this group, what happens when you put the kombucha into the fridge? Do you see the same drop off that was seen in the Argentinian yoghurts?
Also to tie in with some of their other research into tea, they wanted to compare kombucha made with three different teas. They used black tea, green tea, and instant powdered tea. I’ve never seen instant powdered tea recommended by anyone to make kombucha, so that makes for interesting reading, too.
What they did
They made three big batches of kombucha using their recipe with the pure cultures of yeast, acetic acid bacteria and lactic acid bacteria instead of a scoby. One batch was made with black tea; one with ‘low-cost green tea’ (as opposed to fancy expensive green tea, I assume); and one with ‘tea powder’ – all purchased from Damin Foodstuffs in Zhangzhou, China.
The kombucha was fermented at 30degC (86degF), with constant shaking, for 90 hours (4 days). (Shaking the cultures is standard lab technique for aerobic microbes).
After the fermentations were finished, they tested each brew for three different types of antioxidant activity, and performed each test three times.
They then put their Green Tea kombucha (which performed most highly in the antioxidant studies) in the fridge. Every second day they took a tiny amount of the kombucha, diluted it out, spread it on agar plates, and grew them 48hrs to count how many microbes were alive in the brew. They did this for 2 weeks.
What they found
Testing the antibiotic properties:
They tested each of the kombucha brews to see how well they neutralized three different free radicals. Unsurprisingly, they found that kombucha made with green tea was the best antioxidant. (Unsurprising because this lines up with previous research).
- When they tested against the Hydroxyl radical they found that green tea was better than tea powder, which was better than black tea.
green tea > tea powder > black tea
- When they tested against DPPH they again found that green tea was better than tea powder, which was better than black tea.
green tea > tea powder > black tea
- When they tested against the Superoxide anion they found that once again green tea was the best, but this time black tea was better than tea powder.
green tea > black tea > tea powder
Checking out the Probiotics
They measured “total bacteria”, “acetic acid bacteria”, “lactic acid bacteria” and “yeast”.
For all except lactic acid bacteria there was a steady decline over time, but good numbers still at end of two weeks.
Lactic acid bacteria, however, declined markedly at 2 – 4 days. By 10 days the numbers of Lactic acid bacteria were barely detectable.
Note, though, that the graph is a bit misleading at first glance. The lactic acid bacteria numbers are actually two orders of magnitude lower than the others to begin with, but they have mooshed the data together to fit nicely on the same graph. The Lactic Acid Bacteria use the axis on the right hand side of the graph, while all the others use the axis on the left hand side.
So, although it looks like there are lots of LAB to start with and then a dramatic drop to almost zero, the reality is that the numbers of LAB to begin with barely show up if you plot all the numbers using the same scale. Like this:
Having said that, the numbers of Lactic Acid Bacteria do definitely show a massive drop off from their original numbers. The yeast and acetic acid bacteria have a much more gradual decline while the lactic acid bacteria show a sharp decline to practically zero by about day 8.
And that difference may be the difference between getting enough lactic acid bacteria in your gut and getting none at all.
There are a few points of interest brought up by this paper that I want to pick up on here.
I found it intriguing that tea powder was, on balance, better than black tea. I’ve had a look through the Damin Foodstuffs store to see if I can work out which product it is, but there are a number of different tea powders and instant teas, and I have no idea which one they used – it is likely that the range available has changed since this paper was published in 2013. Still, worth keeping in mind. And great to know that if you’re in some bizarre situation where you don’t have your normal tea but do have an instant tea powder, then your kombucha will be fine!
Using pure strains instead of a SCOBY
This research group used three pure strains of yeast and bacteria to make their kombucha, instead of a SCOBY.
This isn’t really helpful to us as home brewers, as we have no way of isolating or growing such pure strains, but it is good information for commercial brewers. This is similar to how beer yeasts are carefully cultivated and kept pure to get very particular flavors and effects. It also gives the researchers highly predictable effects.
As we know from previous papers, no two SCOBYs are the same which can make scientific research into kombucha quite unpredictable. It is hard to know what is the result of the experiment, and what is due to the changing mix of microbes in the SCOBY. If every research group used this mix of three specific microbes in place of a SCOBY then the baseline behavior of the kombucha would be highly predictable.
On the other hand, by using these three microbes instead of a SCOBY, the research becomes less useful to those of us brewing kombucha the normal way. And they also miss out on capturing data about the complex interactions of the whole wider community of microbes that live in a SCOBY and the kombucha brew.
Which brings me to my main criticism about this paper. We don’t actually know if this lactic acid bacteria die-off really does occur in homebrewed kombucha. We can say that it is likely, given that it is also seen in various yoghurts and other commercial probiotics, but we don’t know for sure.
In this paper the researchers do say that the presence of yeast often has a protective effect on the viability of lactic acid bacteria – just not at the low temperatures found in a refrigerator. It is possible that the natural mix of microbes found in a SCOBY survive in the refrigerator better than the pure strains do.
But we just don’t know.
What makes a probiotic?
We actually don’t know how many live bacteria are needed in a food in order to reliably deliver a dosage of good bacteria to our guts.
Despite finding some draft probiotic guidelines (pdf), I couldn’t find a definitive answer as to what amount of lactobacillus is needed in a drink or food for there to be a good transfer of live bugs to the gut. It seems that each type of probiotic organism, and each strain of each type will have a different number that it will need to hit before there’s a near-guaranteed transfer.
If you do come across this information anywhere, share it with us in the comments!
My own gut feeling (pun totally intended) is that if we’re eating or drinking a variety of live probiotic foods on a daily or near daily basis then we’re likely to be getting a low-level continual inoculation of good microbes into our system – even if the numbers are variable, even if they are not always in high enough doses to guarantee the transfer, even if it’s not consistent enough for a manufacturer to put a label on it.
But it’s great to know that the lactic acid bacteria prefer room temperature. It means that the old-fashioned lacto-fermenting of sauerkraut, for example, of just leaving it in the cupboard and eating it up quickly once it gets to the stage that you like it (rather then putting it in the fridge to stop the fermenting) is likely to give the best probiotic boost to your body.
There are two main take-aways that I have from this research:
Drinking kombucha for its probiotic powers
More research is needed, especially using growth conditions that mimic the way most of use brew – in batches, at room temperature, from a scoby, and without shaking. But for the home brewer, if you’re drinking kombucha to maximise your probiotic intake, it now seems that it’s better to drink it freshly-brewed, rather than after refrigeration. If you like it cold, drink it with ice.
Storing your SCOBY
Until now, I have recommended storing your spare SCOBYs in the fridge, because they’re nicely stable there and don’t need feeding and caring for because they mostly go kind-of to sleep.
But if you’re wanting to keep the lactobacillus happy, I would now change that recommendation to storing it at room temperature in a scoby hotel and feeding it fresh sweet tea every week or two. (Read my post on how to make a scoby hotel).
Alternately, you can keep it in the fridge and plan on reintroducing lactobacillus to your brew when you bring it out of storage. To be honest, that’s what I think I’ll do, because I tend to have an ‘out of sight out of mind’ problem with my kitchen, and I just know I’ll forget to feed my poor stored back-up SCOBYs, and then they’ll run out of food, and then I’ll still get massive die-off of my probiotics and no live SCOBY for if I have a disaster of some sort. Which would be bad!
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