Milk kefir and kombucha are the two easiest live ferments to keep at home. Both are a culture, a food source and time. Both are also alive, which means they need attention and a clear sense of what a healthy batch looks like.
General food information, not medical advice: if you are pregnant, immunocompromised or living with a condition affected by fermented foods or alcohol traces, ask a doctor before adding these to your diet.
Milk kefir
Kefir grains are a rubbery cauliflower-like cluster of bacteria and yeasts. They eat lactose and turn milk into a tart, slightly fizzy drink in about a day.
The routine: put a tablespoon of grains in a jar, add about 500ml of milk, cover with a cloth or a loose lid, leave at room temperature for 12 to 24 hours. When it has thickened and smells tangy, strain through a plastic or stainless sieve, put the grains into fresh milk, and refrigerate the kefir.
What matters: keep the ratio roughly one part grains to twenty parts milk, or it over-ferments and separates into curds and whey. Warm rooms speed everything up, so use less grains in summer. Grains grow; give the extras away or eat them.
A break: grains keep for a couple of weeks in fresh milk in the fridge, fermenting slowly.
Kombucha
Kombucha is sweet tea fermented by a SCOBY, the rubbery disc that forms on the surface. The culture eats most of the sugar and produces acids, which is why finished kombucha is tart rather than sweet.
First fermentation: brew 1 litre of tea with about 60g of sugar, cool it completely, add it to a jar with the SCOBY and a good splash of mature kombucha as starter. Cover with a cloth and an elastic band so it can breathe but nothing can get in. Leave 7 to 14 days at room temperature, tasting from day 5.
Second fermentation: bottle it with a little fruit or juice, seal, and leave 1 to 3 days at room temperature to build fizz. Then refrigerate, which slows everything down.
Non-negotiables: real sugar, because the culture eats it and honey brings its own microbes; plain tea without oils, so no flavoured Earl Grey; cooled tea, because hot liquid kills the culture; and no metal or reactive containers for long contact.
The safety points that actually matter
Exploding bottles. Second fermentation builds real pressure. Use bottles made for carbonation, burp them daily if you are unsure, keep them out of direct sun, and never use decorative bottles that were not designed to hold gas.
Acidity is the protection. Kombucha stays safe because it acidifies quickly, which is what the starter liquid is for. A first ferment without enough starter can sit at an unsafe pH for too long.
Mould versus SCOBY. A new SCOBY looks like a pale, wet, rubbery film, sometimes with brown yeast strands hanging down. Mould is dry, fuzzy and sits on top in circles: white, green, blue or black. Fuzzy means throw it all away, including the liquid, and start again. Do not scrape it off.
Alcohol. Both drinks contain a small amount of alcohol, usually well under 1%, but more if fermented long or warm. Worth knowing if that matters to you.
What healthy looks like
Kefir: thickened, tangy, faintly yeasty, sometimes separating into curds and whey if left too long, which is over-fermented rather than dangerous. Kombucha: sour, slightly vinegary, with sediment and strands at the bottom that are entirely normal.
Trust your nose. Both should smell sharp and clean, like sourdough starter or cider vinegar. Anything that smells rotten, cheesy or off is a batch to bin.
Is it worth it?
A litre a week costs pennies compared with shop-bought, and you control the sugar. It also ties you to a routine: a culture is closer to a pet than a project. If two weeks away would kill it, the fridge pause is your friend, and if the whole idea sounds like a chore, buying a bottle is a perfectly good answer.