How to tell if your kombucha has been stripped of its good bacteria and filled with sugar - and how to find the ones that really will give your gut a boost

Kombucha used to be 'just' the fizzy, vinegary-tasting health drink you could find at farmers' markets. But it's now well and truly mainstream.

The fermented tea — first brewed in China more than 2,000 years ago where it's regarded as the 'elixir of life', and is used for boosting immunity and maintaining the digestive system — is now widely available in pubs, coffee shops and on supermarket shelves.

Stats show the UK consumed 4.4 million litres of kombucha last year — an increase of more than 800,000 litres on the previous year. Its success is credited for our increasing appetite for 'functional health drinks' that quench our thirst and provide health benefits, says Charlotte Foster, a specialist dietitian at Barts Health NHS Trust.

In the case of kombucha, these health benefits are thought to derive largely from the fermentation process the drink goes through.

This produces probiotics — beneficial bacteria — thought to help maintain the gut microbiome, the collection of microorganisms that live in the digestive tract and are important to everything from supporting our immune system to influencing our risk of becoming overweight.

Kombucha - fermented tea first brewed in China more than 2,000 years ago - is used for boosting immunity and maintaining the digestive system

On top of that the tea element 'provides antioxidants, which help with inflammation, and B vitamins which are essential for bodily functions such as the metabolism of carbohydrates and proteins from the diet,' says Charlotte Foster.

'And it's these potential benefits, along with being generally low in calories and sugar, that people buy into when they purchase it,' she adds.

But with more producers jumping on the bandwagon, consumers may not always be buying what they think — as not all kombuchas are equal.

Traditionally, just four ingredients were used to make kombucha, as Dr Amanda Lloyd, a food, health and diet researcher at Aberystwyth University explains: 'Black or green tea infused in hot water, sugar, tea vinegar [also known as starter fluid, basically over-fermented kombucha that's had most, if not all, of its sugar fermented away] and something called a SCOBY.

'This stands for 'symbiotic culture of bacteria and yeast' and is a rather unappealing looking disc-shaped gelatinous blob. It's also known as a kombucha mushroom or kombucha mother.'

This mixture is then left to ferment for ten to 20 days.

Amanda Lloyd adds: 'After this, you can add any further flavourings, such as mint or ginger — then bottle it for another one or two weeks to encourage carbonation. Then, that's it: traditional kombucha full of live probiotics, polyphenols [powerful plant compounds with antioxidant effects] and antioxidants.

'Live cultures will continue to feed on the sugar and produce carbon dixoide, so you need to halt the process by keeping it chilled in the fridge,' she adds.

'Not doing this would not only alter the taste — it would get increasingly acidic — but the bottle might become extremely fizzy. But that's what kombucha is all about — it's living and, as a result, somewhat temperamental.'

Now, however, some brands are making their drinks in a 'more industrialised manner,' says Lou Dillon, a nutritionist and founder of Twisted Kombucha, a brand that makes kombucha in this traditional way.

'These include processes to extend shelf life by up to a year and make the product more stable — even at ambient conditions. Artificial sweeteners may also be used. This may give the consumer a far wider choice — not everyone likes the sweet-yet-vinegary taste of traditional kombucha — but its health benefits might not be the same.'

Statistics show the UK consumed 4.4 million litres of kombucha last year — an increase of more than 800,000 litres on the previous year

Evidence for the health benefits of kombucha are largely anecdotal — partly because such an unstable product is hard to test — but a review of 15 studies in the journal Critical Reviews in Food Science And Nutrition last year concluded there was some 'evidence' that kombucha consumption benefits the gut microbiome as well as obesity-related conditions.

But the studies that have been done have involved traditional 'live' kombucha.

Amanda Lloyd is currently running her own eight-week trial involving 56 volunteers who will drink 330 ml of 'live' kombucha daily for eight weeks — and blood, saliva and urine samples will be tested throughout to watch for changes to markers of stress and inflammation.

But a live drink, which is time-consuming to make and unstable out of the fridge, isn't immediately suited to mass production, which is why some manufacturers put their kombucha through pasteurisation (where heat is added to kill off bacteria that could 'spoil' the product) or micro-filtering (which filters out sediment and bacteria).

The problem with that, explains Dr Darshna Yagnik, a senior lecturer and researcher in immunology at Middlesex University, is that while 'pasteurising is effective at removing most of the 'bad' bacteria in a product, it also removes the vast amount of 'friendly' bacteria, too.'

It is these friendly probiotic bacteria which is why many people buy kombucha — so to make up for this shortfall, some producers add their own probiotics. But Dr Yagnik says these added bacteria won't necessarily be as beneficial.

'The live cultures from fermentation would contain a mixture of different probiotic species, which can vary in quantity in every single batch produced,' she says.

'However, any 'added' bacteria are usually limited to a specific strain. A variety of probiotics is far more beneficial as it adds to a healthier defence system in the gut, which supports better immune defence.'

Some brands ferment the product for so long that no sugar remains, which again makes it more stable but means the leftover bacteria have nothing to live on. Being vinegary in this state and not particularly palatable, producers then add sweeteners such as stevia or erythritol.

'Both stevia and erythritol are natural sweeteners — stevia is derived from a plant native to South America and erythritol is naturally found in fruits and vegetables,' says Charlotte Foster (@nutritionalevidence).

'However, there is some evidence that stevia might actually have antimicrobial properties — meaning it could have a negative effect on the probiotic abilities of kombucha itself.' And she adds erythritol 'consumed in large amounts can cause adverse digestive symptoms such as diarrhoea and bloating'.

Other kombuchas may be made with a concentrate or 'instant' mix — essentially, a concentrated tea vinegar, diluted with tea blends, then flavoured with artificial sweeteners with probiotic powder mixed in.

And according to Lou Dillon, a lack of labelling regulation means such products can be hard to spot. 'Producers can get away with noting a product contains live cultures whether they are naturally occurring or simply added after the manufacturing process,' she adds.

So how do consumers get around this minefield?

'If you want to ensure you're buying a traditional kombucha the first thing to do is head for the chiller section — you cannot keep live kombucha at room temperature,' says Charlotte Foster. Next, check the label.

'Just looking at the wording won't help, although the words 'raw', 'pure', 'unpasteurised' and 'unfiltered', are a good sign.

'My suggestion is to look at the back of the drink. If it says the kombucha needs to be kept in the fridge rather than simply be 'chilled on opening', then that's a pretty good pointer to it being traditionally fermented.

'Seeing any sediment at the bottom of the bottle or 'floaty' bits within the drink can be helpful, too — that's bits of the SCOBY and, hence, live culture, although that's not always easy to see in a dark bottle.'

Lou Dillon also notes to look at its best before date — live kombucha will be good for around five to eight months only.

But the best way to ensure you're getting a pure, traditional kombucha is to make it yourself.

There are courses (in-person and online) you can join where they will supply you with SCOBYs — or it's easy to buy your own online (from around £10 for the individual SCOBY — to £80 for a full kit including fermentation vessels and bottles).

'The process isn't necessarily tricky, but you do have to be careful about kitchen hygiene and using sterilised containers,' says Dr Yagnik. 'Not doing this can lead to mould or bad bacteria growing and contaminating the kombucha, making you sick.

'A quick way to sterilise at home is by soaking containers for five minutes in 10 per cent household bleach or 70 per cent iso-propyl alcohol, which you can buy online or at your local pharmacy.

'Next, rinse it in hot water before leaving it to dry completely in sunlight — glass jars can also be dried in the oven at 140c for ten minutes.

'Doing it this way ensures you know what goes into it. Equally, you can make it to suit your own taste — you get to have it as sweet as you like, add natural flavourings you prefer and ferment it as much (or as little) as you like. It's also much cheaper.'