18 juil. 2025

Homemade Fermented Drinks Kefir Kombucha Ginger Beer

A standard water kefir recipe starts with one litre of water and 30 g of sugar, which is 3 g per 100 ml. What you end up drinking depends entirely on how long you give the microorganisms: after a full fermentation roughly 0.9 g per 100 ml remains, but stop too early and 2.1 g is still there. Per 200 ml glass, that is a spread from 1.8 g to 4.2 g of sugar, or from 6 % to 14 % of the daily added-sugar limit the NHS recommends.

That spread is the whole point of these drinks. Kefir, kombucha and ginger beer are not finished products but processes in motion, and the same recipe gives very different results depending on time, temperature and the moment you move them to the fridge. What follows gives you the markers to steer that, plus an honest measure of what we actually know about their effects.

What fermentation actually does

Yeasts and bacteria consume the sugar in the liquid. The yeasts produce mainly carbon dioxide and ethanol; the acetic and lactic bacteria produce organic acids that lower the pH and give the sour taste. All three arrive together: the more sour and fizzy your drink, the more sugar it has consumed, and the more alcohol it contains.

That relationship explains why you cannot optimise the three separately. Other home transformations rely on comparable microorganisms or enzymes, such as soaking and sprouting legumes. A drink that is both very low in sugar and entirely alcohol-free does not exist in this framework: you pick a compromise, and you should pick it knowingly.

Sugar, before and after

The starting sugar is not a flavour ingredient, it is the culture’s fuel. Without it nothing happens. The useful question is therefore not how to use less at the start, but how much is left at the end.

For the water kefir recipe below, at 30 g of sugar per litre:

Sugar consumed	Left per 100 ml	Per 200 ml glass	Share of the NHS limit
0 % (unfermented)	3.0 g	6.0 g	20 %
30 %	2.1 g	4.2 g	14 %
50 %	1.5 g	3.0 g	10 %
70 %	0.9 g	1.8 g	6 %

The NHS recommends that added sugars stay below 5 % of energy intake, around 30 g a day from age 11, and treats a food as low in sugar below 5 g per 100 g.

The comparison with a soft drink deserves to be made properly. According to the Swiss Food Composition Database, a sugared cola contains 10 g of sugar per 100 ml, or 20 g per 200 ml glass. A fully fermented kefir supplies eleven times less. A kefir stopped after a few hours, still frankly sweet, supplies only three times less. So the healthy alternative to soft drinks line is true or false depending on your patience, which is rarely spelled out. Residual sugar also shapes the glycaemic response, a subject we handle separately in our piece on the glycaemic index for plant-based eaters.

How to tell where you are without a lab: taste it. A drink that is still clearly sweet has fermented little. One that is sharply sour, almost dry, has used up most of the sugar. It is a crude marker, but it is the right one.

Alcohol, and what Swiss law says

Any yeast fermentation produces ethanol. The question is not whether there is some, but how much.

In Switzerland, article 18 of the ordinance on food information requires the alcohol content to be stated for beverages above 1.2 % by volume. Below that, no declaration is required. This is the threshold that shapes the whole market for kombucha sold as a non-alcoholic drink.

A 2026 study in the Journal of Food Science measured nine commercial New Zealand kombuchas using two methods, an automated analyser and gas chromatography. The two agreed very closely, with a correlation of 0.996. The mean sat around 0.56 to 0.68 % ABV, below the thresholds. But 22 % of samples exceeded New Zealand’s 1.15 % ABV limit for drinks labelled non-alcoholic. Interviews with the brewers identified the causes: high initial sugar loads, flavour additions after fermentation, inadequate pH and temperature monitoring, and imprecise measuring tools.

On a commercial product the statement appears or not according to that threshold, which our guide to reading food labels helps decode. Those are exactly the conditions of a home kitchen, where nobody measures anything. A bottle left three days at 25 °C with added fruit juice is fermenting actively, and there is no reliable way to estimate its alcohol by eye. For someone avoiding alcohol on medical, religious or personal grounds, or for a child, that uncertainty matters more than the reassuring average of commercial products.

What the evidence shows, and what it does not

These drinks come with a broad health pitch. The clinical literature is narrower.

Kefir is the best studied of the three. A 2025 meta-analysis covering seven randomised trials and 385 participants found no significant effect on systolic blood pressure (mean difference -1.76 mmHg, 95 % confidence interval -5.21 to 1.69, p = 0.317), on diastolic pressure (-1.19 mmHg, 95 % CI -3.40 to 1.03, p = 0.295), or on C-reactive protein, an inflammation marker (-0.17 mg/L, 95 % CI -0.84 to 0.49, p = 0.609). A subgroup analysis suggests CRP falls beyond eight weeks of consumption, which is a lead rather than a demonstration.

A 2026 review of 28 clinical trials describes possible benefits for the gut microbiota, metabolic parameters and digestive function, but concludes explicitly that the strength of evidence remains limited, given substantial heterogeneity in preparation, microbial composition, dose, duration and study populations. That is the crux: two homemade kefirs do not contain the same microorganisms, and nothing guarantees yours resembles the one in a trial.

For fermented foods as a whole, a 2025 review in Advances in Nutrition notes consistent protective associations in observational studies, but stresses the methodological limits and the need for adequately powered trials to establish causality.

What can honestly be said: these drinks are pleasant, low in sugar when properly conducted, and contain live microorganisms. What cannot be said: that they boost immunity, improve digestion or rebalance gut flora in any way demonstrated in humans. If you drink them, drink them because they taste good.

Water kefir

The simplest of the three and the best starting point.

For one litre: 1 litre of unchlorinated water, 30 g of cane sugar, 20 g of water kefir grains, one or two unsulphured dried figs, half an organic lemon. You need a glass jar of at least a litre, a plastic or stainless steel sieve, and a lid set on top without screwing it down.

Dissolve the sugar in the water, add the grains, the fig and the lemon half. Cover without sealing: the CO2 has to escape at this stage. Ferment for 24 to 48 hours between 20 and 25 °C. The fig floating to the surface is the traditional sign that gas is being released properly.

Strain, bottle and refrigerate. The grains go straight into a new batch. If you want fizz, leave the sealed bottle at room temperature for 12 to 24 hours before chilling, after reading the section on pressure.

Three adjustments: chlorinated water hampers the cultures, so filter it or leave it to stand overnight. Below 20 °C fermentation drags; above 28 °C it races and becomes hard to stop at the right moment. Healthy grains are firm and multiply; grains that soften and shrink from batch to batch point to a problem with water, temperature or sugar.

Kombucha

Kombucha needs sweetened tea and a symbiotic culture, the SCOBY, which looks like a gelatinous disc.

Brew 8 g of black or green tea in a litre of water, add 70 g of sugar, and let it cool completely. This is the step people get wrong most often: pouring still-warm liquid onto the SCOBY kills it. Pour into a large jar, add the SCOBY and 100 to 150 ml of already fermented kombucha, which acidifies the medium immediately and protects against contamination. Cover with a fine cloth held by an elastic band.

Fermentation time is the question that always comes up. A 2026 trial in Foods followed green tea kombuchas fermented with four different SCOBYs over 21 days and identified the appropriate bottling point for each: 7 days for two of them, 10 days for the other two, targeting a safe pH, reduced soluble solids and ethanol below 1.2 %. In other words, the right moment depends on the consortium you have, not on a universal duration. Taste from day seven and stop when the acid-sugar balance suits you.

The second fermentation in a sealed bottle, with fruit or ginger, creates the fizz. It also creates all of the pressure and part of the alcohol. Two to three days at room temperature is plenty.

The baby SCOBY that forms on the surface each cycle can be detached and given away. Keep essential oils and concentrated flavourings out of the main jar: they disturb the culture.

Ginger beer

Ginger beer uses a homemade starter, the ginger bug, which plays the SCOBY’s role.

To start it: in a jar, mix 200 ml of unchlorinated water, 2 tablespoons of sugar and 2 tablespoons of grated fresh ginger, skin included if untreated. Cover with a cloth. Every day for 5 to 7 days, add a tablespoon of sugar and one of ginger, and stir. When it bubbles at the surface and you hear a fizz on stirring, the starter is active.

One point recipes leave out: this starter accumulates a lot of sugar. Two tablespoons at the outset plus one a day for seven days amounts to something like 110 g of sugar introduced into 200 ml. Some has been consumed by the microorganisms, but the 100 ml you draw off for a litre of drink carries a non-trivial amount, on top of the recipe’s own sugar. If you want a low-sugar drink, count the starter in the total and cut the added sugar accordingly.

For the drink: heat half a litre of water with 2 tablespoons of sugar and 1 to 2 tablespoons of grated ginger until dissolved. Take it off the heat, add the remaining half litre of cold water to cool it quickly, then the juice of a lemon if you like. Once the mixture is at room temperature, and not before, stir in 100 ml of strained ginger bug. Ferment for 2 to 5 days at room temperature, tasting as you go.

Pressure, and how not to break a bottle

This is the real physical risk in these preparations, and the only one that can injure someone.

The mechanism is simple. While the jar is covered with a cloth, CO2 escapes. The moment you seal a bottle containing residual sugar and an active culture, the gas has nowhere to go. Pressure builds faster the warmer it is and the more sugar is present, and a glass bottle not designed for it can fail.

Four rules cover it. Use bottles rated for pressure, with swing tops, or returnable beer bottles, never a recycled juice bottle. Burp them daily by opening briefly: the hiss tells you where you are, and no hiss means fermentation has finished or the culture is inactive. Keep the second fermentation to two or three days at room temperature. And chill to stop it: between 0 and 5 °C microbial activity slows sharply, which stabilises pressure, sugar and alcohol at once.

If you have to open a bottle you distrust, do it over the sink with a cloth over the top.

Hygiene and when to throw it out

A healthy fermentation protects itself through its own acidity. The real risk sits at the start, before the pH has dropped, which is exactly why you add already fermented liquid as a starter in kombucha.

The principle is the same as for lacto-fermented vegetables, covered in our article on transforming tomatoes. Wash jars, spoons and sieves thoroughly in hot soapy water and rinse well: soap residue hampers the cultures. Avoid aluminium, which acids attack. Use glass, food-grade plastic or stainless steel.

Throw it out without hesitation if you see dry, fuzzy mould, green, black or blue, on the surface. A smooth whitish film on kombucha is usually a forming SCOBY, not mould; raised fuzz is mould. Discard too if the smell is putrid or cheesy, as opposed to the normal sour, vinegary smell. Do not salvage the layer underneath on the theory that it is clean.

For storage, the Food Standards Agency reference points apply: fridge between 0 and 5 °C. A finished drink keeps for several weeks there, slowly continuing to acidify.

Who should abstain or ask a doctor

These drinks contain live yeasts and bacteria in uncontrolled quantities, which is not trivial for everyone.

The yeasts in these cultures include the genus Saccharomyces. A hospital series published in Mycopathologia in 2026 analysed 196 patients from whom Saccharomyces cerevisiae had been isolated: 30 of them, or 15 %, had a sterile-site infection, with 30-day mortality of 27 %. Prior probiotic exposure was found in 30 % of cases, and intensive care admission was an independent predictor of mortality. The authors conclude in favour of cautious probiotic use. These infections remain rare and concern fragile patients, but they justify anyone immunocompromised, carrying a central line or in intensive care asking for medical advice before introducing non-standardised live cultures.

A case published in 2023 reports massive hepatic necrosis associated with kombucha consumption in a woman with no medical history. A single case establishes no causality and says nothing about population-level risk, but it is a reminder that these drinks are not inert and that a very large intake is not better than a moderate one.

Three further situations warrant caution: pregnancy, because of unquantifiable alcohol and uncontrolled cultures; children, for the same alcohol reason; and anyone avoiding alcohol, given that homemade kombucha is not guaranteed below 1.2 % ABV.

Finally, if you tolerate FODMAPs poorly or have irritable bowel syndrome, introduce these drinks in small amounts. The bloating often comes from the CO2 itself as much as from the microorganisms.

Five common mistakes

Sealing the jar from the start. The first fermentation needs gas exchange. The jar is covered, not closed.

Pouring hot liquid onto the culture. Beyond about forty degrees you kill what you just bought. Wait for it to cool completely, no exceptions.

Trusting a duration rather than the taste. Seven days at 18 °C and seven days at 27 °C are not the same drink. Temperature leads, duration follows.

Forgetting a bottle in second fermentation. This is how accidents happen. Burp daily or refrigerate.

Rinsing kefir grains under chlorinated tap water. Chlorine is designed to kill microorganisms and it does the job very well.

Depending on your situation

You are starting out. Begin with water kefir: a 24 to 48 hour cycle, a robust culture, cheap mistakes. You will understand the mechanism before investing in a SCOBY.

You want to cut back on soft drinks. Ferment to the end and accept a properly sour drink: that is where the gap with a cola becomes real, eleven times less sugar rather than three. Our flavoured drink ideas without sugar round out the approach.

You already ferment. The equipment and the reflexes overlap heavily with lacto-fermenting vegetables and with easy homemade ferments.

You avoid alcohol. A short water kefir, stopped at 24 hours and refrigerated, is the most predictable of the three. Kombucha in second fermentation is the least predictable.

You want to cut waste. The cultures multiply and get passed on, and the drink saves buying bottles. The logic matches that of zero-waste cooking.

Frequently asked questions

How much sugar is left in my kefir? Between 0.9 and 2.1 g per 100 ml depending on progress, starting from 30 g per litre. You cannot measure it without equipment, but taste locates you: the more sour, the less sugar remains.

Does homemade kombucha contain alcohol? Yes, always some. Commercial products measured average around 0.6 % ABV, but 22 % of a New Zealand sample exceeded 1.15 % ABV. An uncontrolled homemade batch can go higher.

Can you use honey instead of sugar? For water kefir yes, bearing in mind that honey is antibacterial and can slow the culture. For kombucha, white cane sugar remains the most reliable.

Do kefir grains need rinsing every batch? No. An occasional rinse in unchlorinated water is enough, and routine rinsing depletes the culture.

Are these drinks probiotics? They contain live microorganisms, which is not the same thing. The term probiotic implies identified strains at doses linked to a demonstrated benefit, which a homemade batch cannot establish.

My kombucha has brown strands hanging down. Those are yeasts, normal and a good sign. Strain if the look bothers you.

How long do these drinks keep? Several weeks between 0 and 5 °C, growing steadily more acidic. They do not become dangerous, they become vinegary.

Can children drink them? The unquantifiable alcohol calls for caution. If you do, favour a short water kefir, chilled quickly and without a second fermentation.

What this page cannot settle

The residual sugar figures are calculated from the recipe given here and from representative consumption rates, not measured on your batch. A real measurement would need a refractometer or laboratory analysis.

The ethanol measurements come from commercial New Zealand products, and the SCOBY trial covered four consortia under laboratory conditions. This work gives solid orders of magnitude, not a prediction for your jar.

Finally, the clinical trials cited overwhelmingly concern milk kefir. Water kefir, kombucha and ginger beer are far less studied in humans, and transposing results from one to another is not legitimate.