THE BASICS

Fermenting works because the conditions allow good bacteria to thrive and push out bad, food poisoning-causing bugs. Two things are on your side: salt and acidity. Most savoury ferments start with a salt solution of about two per cent, to keep the bad bugs at bay. Kombucha relies on enough acidity in the base liquid to make an unfriendly environment for pathogens, while in kefir, the culture inhabits the milk with healthy bacteria, preventing it from going “off ”.

So how do we make the right environment so food ferments rather than rots? Start with a spotless kitchen and scrubbed hands. Next, look at your equipment: many recipes warn you to avoid metal. This is true up to a point: reactive metal such as cast iron or old-fashioned aluminium will taint the ferment and may even kill some of the microbes. Stainless steel is fine: it’s what all large-scale producers use. That said, some cheap stainless steel is poor quality, and sieves in particular can lose their coating. You may feel it’s safer to stick to ceramic or glass. Equally, plastic containers work but are often slightly porous, so may transfer flavours.

Use good-quality vegetables. “It may seem OK to use a bruised cabbage, but in fact as soon as it is damaged it starts oxidising, and that will spoil the ferment,” explains Pat Bingley of Eaten Alive, a fermented foods business.

One more warning: most ferments produce carbon dioxide. If they are kept in sealed containers at room temperature, they can explode if they’re not opened occasionally (every couple of days is best) to release the pressure. Putting them in the fridge slows the fermentation so much that this ceases to be a problem, which is why live ferments are always refrigerated in shops.