The microbial cocktail in kombucha can go a little wild.
At this time of the year, many of us resolve to drink less. Could kombucha aid our health while still letting us have a glass of fun?
Maybe. The recall of a kombucha brand from shop shelves last month because of excessive alcohol content is a reminder that it can contain more fun than intended – or spoil things if you unknowingly drink a boozy version while meaning to avoid alcohol for health or driving reasons.
The recall followed testing by New Zealand Food Safety (NZFS), which showed the brand’s kombucha contained about 3% alcohol. Drinks can be sold as non-alcoholic if they contain up to 1.15%.
The basics of kombucha-making are this: make sweet tea, let it cool and add a piece of symbiotic culture of bacteria and yeast (Scoby), a jelly-like disc produced during a previous kombucha fermentation. The microorganisms in the Scoby ferment the tea into kombucha over two weeks or longer. Yeasts gobble the sugar, producing ethanol (alcohol) and other byproducts. The bacteria, lagging behind the yeast, consume the alcohol and other byproducts to produce acids that give kombucha its tang. Its fizz comes from the carbon dioxide the microorganisms produce.
Kombucha is often consumed for its live microorganisms, which are thought to act as probiotics. The probiotic effect hasn’t been well studied in clinical trials, but lab studies show kombucha has more antioxidants and polyphenols than the tea it’s fermented from. It seems to enhance the liver’s ability to remove substances such as heavy metals and drugs, and to have anti-diabetic effects in rodents at least.
The diversity of kombucha’s microorganisms makes the brewing complex, says Graham Eyres, a senior lecturer at the University of Otago who specialises in food, flavour and brewing chemistry. “With a simple ferment like beer, you have a sugar source (the malt) and usually one strain of yeast, so you know that for every gram of sugar you add, you produce a certain amount of ethanol. But with kombucha, you have a cocktail of organisms and multiple reactions, and ethanol is an intermediate product, so it goes up then comes back down.”
Brewing vagaries can change the outcome, he says. “Variation creeps in with temperature changes, the size of the brewing vessel, the composition of the Scoby or its size relative to the ferment, and whether the Scoby sinks.” He says the bacteria need oxygen, and a sunken Scoby is deprived of that.
But the extra alcohol probably snuck in after bottling, says Pat Silcock, a food scientist also at Otago. “The balance of acidity and sourness with sweetness is important from a flavour perspective. So, some manufacturers bottle their kombucha when some of the original sugar remains, or they include fruit juice.”
After bottling, any sugar can be transformed by yeasts into alcohol. But the bacteria that could turn the alcohol into acid are stifled by the airlessness inside the bottle.
There are ways to prevent this process. The beverage doesn’t need sugar or juice present at bottling, though some palates prefer it and it results in more fizz. Or it can be pasteurised, which kills yeasts but also probiotic microorganisms. Or it can be constantly chilled below four degrees.
“That’s easier said than done,” says Eyres. “Manufacturers can control only their part of the process. And live kombucha can have a cocktail of yeast strains, some of which might tolerate fridge conditions reasonably well.”
NZFS deputy director-general Vincent Arbuckle agrees the recalled kombucha’s excess alcohol most likely arose from fermentation in the bottle. “The product coming off the production line was inside the allowed level of alcohol. But we tested samples at the retail point and found alcohol levels much higher than you’d expect.”
He says, “Consumers need to understand there is some volatility in kombucha. If it’s kept inside the temperature range, it should maintain the alcohol level. But part of its interest to people is its wildness. If you’re concerned, reconsider whether you’ll drink it.”
