Many guides consider a clean, well-acidified homemade batch low risk for most healthy adults, but it is not right for everyone. The main risks are mould and contamination, a batch that is not acidic enough, containers that can leach metal, and a small amount of alcohol that can vary more at home than in commercial kombucha. Good hygiene, enough starter and checking for warning signs cover most of these.
This article is general information, not medical advice. If you are unsure whether kombucha is right for you, speak to a doctor or other health professional.
The Real Risks
- Contamination and mould. Harmful moulds and bacteria can grow in a batch if the setup is not clean, the culture is weak, or the brew is not acidic enough early on. Mould is easy to spot and should always be taken seriously.
- Low acidity. The culture and starter make the brew acidic, which helps stop other microbes from taking hold. Starting with enough starter and keeping things clean are the main protections. Many home-brewing guides suggest the mixed brew should test below about pH 4.5 at the start, and use about pH 4.2 or lower as a target for finished kombucha.
- Unsuitable containers. Lead-glazed ceramics, some crystal, and certain metals can leach into an acidic liquid over time. Glass or food-grade stainless steel is the safer choice.
- Alcohol. Fermentation makes a small amount of alcohol. Home batches can vary more than commercial ones, and the amount is hard to measure at home.
- Personal health. Some people react badly to live cultures, to alcohol or to acidic drinks. Kombucha is not suited to everyone.
Reports of illness linked to homemade kombucha do exist. Many involve contamination, unsuitable containers or unusual brewing practices. Following the steps below reduces these risks, but it cannot remove them completely.
Choosing Safe Containers and Tools
- Brewing jar. Glass is the safest default.
- Larger vessels and utensils. Food-grade stainless steel, such as grade 304 or 316, is widely used.
- Avoid. Lead-glazed or unknown ceramics, crystal, unlined copper, aluminium, galvanised metal and chipped enamel.
- Bottles. Plastic bottles made for fizzy drinks are common. Many brewers prefer glass or stainless for the fermenting stage itself.
- Cleaning. Wash in hot water with plain dish soap, then rinse well. Avoid bleach and strong scented cleaners, which can leave residue. Wash your hands before touching the SCOBY.
- Insects. A tightly woven cloth or a coffee filter keeps out fruit flies and dust, while still letting the brew breathe.
Keeping the Brew Acidic
- Use enough starter. A common guide is about 10 to 15% of the total batch as starter liquid from a finished batch.
- Test if you can. pH test strips are inexpensive. Check that the brew is below about pH 4.5 at the start.
- Keep the temperature steady. A room of roughly 20 to 27 °C is the usual range.
- Taste and look. A sour, vinegary smell is normal. A rotten smell is not.
Normal, Kahm Yeast or Mould?
| What you see | What it usually means | What to do |
|---|---|---|
| Brown or black stringy bits, bubbles, a new cloudy layer | Normal yeast and a forming culture | Carry on |
| A flat, wrinkled white film that is not fuzzy | Kahm yeast, often a sign of a weak culture | Many guides advise discarding and starting again |
| Fuzzy, dry or raised spots in white, green, blue, black or pink | Mould | Discard the whole batch, including the SCOBY and liquid. Don’t skim it off. |
| A rotten, putrid or chemical smell | Likely contamination or spoilage | Discard |
| A sharp, vinegary smell and sour taste | Over-fermentation | Usually fine to drink if mould-free. See why kombucha turns too vinegary. |
How Much Alcohol Is in Kombucha
Fermentation makes some alcohol in every batch, commercial kombucha included. Commercial brands usually state a low alcohol content on the label. Home batches can vary more, and the amount is difficult to measure with basic equipment. Extra sugar in a sealed bottle can add a little more alcohol during the second ferment.
If you avoid alcohol for any reason, or you are giving kombucha to a child, treat home batches with more caution, or avoid them. How to brew non-alcoholic kombucha explains what you can and can’t control.
Who Should Check With a Doctor First
Many health guides advise the following groups to avoid home-brewed kombucha, or to speak to a doctor before drinking it:
- People who are pregnant or breastfeeding
- Children and babies
- People with a weakened immune system, whether from illness or treatment
- People with liver or kidney conditions
- People who avoid alcohol or are managing alcohol dependence
- People with histamine intolerance or a known sensitivity to fermented foods
- People who take regular medicines and are unsure how they might interact with kombucha. A doctor or pharmacist can advise.
If you feel unwell after drinking kombucha, stop drinking it and speak to a doctor. Seek urgent medical help if symptoms are severe.
Starting Slowly
If you are new to kombucha, start with a small glass, around 100 to 125 ml, and see how your body responds. Many guides suggest building up gradually over a few weeks. Bloating and gas are common reasons people cut back.
Bottling and Pressure
Pressure can build quickly in sealed bottles, and it can cause bottles to burst. To reduce the risk:
- Use bottles designed for fizzy drinks, and throw out any that are chipped or cracked.
- Don’t bottle very sweet batches with lots of fruit or juice added, as they can build pressure fast.
- Check the bottles daily, and loosen the cap briefly to release gas if needed.
- Refrigerate once the bottles are fizzy enough.
- Open cold bottles slowly, over the sink, with the bottle pointing away from your face. Keep bottles away from children.
When to Throw a Batch Out
- There is any mould.
- The batch smells rotten or putrid.
- The surface looks unusual, with strange colours or a foul, slimy layer.
- A bottle is cracked, leaking or bulging.
- You are not sure about a batch. If in doubt, throw it out.
For more on sour batches that are still fine to drink, see why kombucha turns too vinegary. For the basics of what the drink is, see kombucha explained.
A Note on Medical Advice
This article is for general information only. It is not a substitute for advice from a doctor, pharmacist or other health professional who knows your health history.