Fermentation has become a trend in recent years, but it’s a preservation technique that’s been used around the world for centuries. Chances are, you’ve been consuming more fermented products than you think: bread, chocolate, cheese and beer, for example, all use fermentation processes during production. In recent years, products such as kimchi, kefir and kombucha have become popular outside their countries of origin, becoming front and centre of a movement that’s as much a hit with professional chefs as it is with home cooks.

Lacto-fermenting vegetables is fun, easy and produces interesting, flavourful results. It’s also a great way to use up any leftover odds and ends (or preserve a glut), plus it has the added bonus of bringing healthy gut bacteria into your diet. Follow a few simple steps to get started and you’ll be enjoying your own fermentation journey in no time.

Lacto-fermentation refers to the good bacteria lactobacillus, which is naturally present on the skins of vegetables. When we chop or grate the vegetables and add salt, their natural juices are drawn out and a brine is formed. This brine submerges the vegetables, preventing them coming into contact with oxygen and spoiling. The salt also acts to ward off any nasty bacteria while the fermentation process gets underway. It’s a rather magical process that works with nature to change the characteristics of vegetables and fruits, which can then be consumed alone or used as an ingredient.

Fermented vegetables are generally salty from the brining process, and develop a pleasantly sour flavour, which can also be quite ‘funky’ depending on the type of vegetable used. Kimchi is a great example of a sour, well-developed fermentation with a pronounced depth of flavour, while something like a cherry tomato produces a lighter result that could be described as slightly ‘fizzy’.

There are two methods for salting vegetables ready for fermentation: brining and dry salting. Brining is used when the vegetable pieces are larger and have a lower water content, so they need some extra liquid to ensure they are fully submerged during fermentation. This method is used for larger pieces of vegetables cut into batons, e.g. carrots, for chillies to make hot sauces, or for lacto-fermented pickles.

Dry salting/brining is used when the vegetables are cut very finely, pureed, or have a naturally high water content. This method is used for sauerkraut, kimchi and fermented salsas. The vegetables are salted then weighed down to extract their juices.

The amount of salt used to make a brine for vegetables is generally between 2-5% of the total weight of vegetables used. It’s important to weigh your vegetables because if you use too little salt, there will not be enough to inhibit the growth of pathogenic bacteria, and if you use too much, the fermentation process may not get started.

The amount of salt you use between 2-5% is a matter of personal preference, although it’s easier and more foolproof to use a larger amount of salt when you first start fermenting. The key to fermentation success is experimentation, so have some fun with it, and be prepared to encounter a few unexpected turns along the way.

The lacto-fermentation process takes around 5 days at room temperature. After this time, the fermentation process slows down but is still occurring, and the vegetables will start to take on some more of those ‘funkier’ flavours. Once you’re happy with the flavour of your ferment, move it to the fridge to stop the process.

Almost every type of vegetable (and some fruits) can be fermented. However, some are popular choices for good reason, especially cabbages, cucumbers and radishes. Try adding spices, herbs and flavourings to your ferments, too. For example cumin works well with beetroot, coriander seeds with chillies and fresh herbs with cherry tomatoes.

Stone fruits and citrus fruits are generally the best options for fermenting. Peaches, plums, cherries, apricots, oranges and lemons are all good choices.

‘Burping’ simply refers to opening the jar to allow the gases that have built up as a result of the fermentation process to escape. It’s best to do this daily while your vegetables are fermenting at room temperature, otherwise they can build up to a sufficient level that they will force the lid open, and you will have a mess on your hands (and all over your kitchen).

Fermented vegetables make a fantastic accompaniment to richer dishes or fried foods. Try kimchi with fried eggs for example, or a fermented tomato salsa with grilled meats or fish. We love to use fermented vegetables and fruits in salads, and fermented garlic is a great ingredient to have on hand for whizzing into mayonnaise or eating with barbecued kebabs.

What can you do if your ferments look funny? Don't panic - you might be able to save them! 

Soft, slimy fermented pickles like cucumbers have spoiled and shouldn’t be consumed. Make sure you ferment at room temperature but not in a place that’s too warm, and check the salt level of your brine - it’s possible that you didn’t use enough salt.

While mould formation on fermented vegetables is not ideal, it doesn’t necessarily mean the whole batch is ruined. If the mould is flat, white and a thin film on the surface, it’s probably not mould at all but kahm yeast - skim this off and remove any vegetables it's growing on. Give the batch a sniff and a taste - if it seems fine then there’s no need to discard it. If it smells or tastes ‘off’, it’s time to start again.

Fuzzy moulds and slime however are bad news - they’ll ruin the batch and it will need to be discarded.

When it comes to the question of why moulds form, it could be due to a number of factors. It may be that the amount of salt used was insufficient to ward off unwanted bacteria; the batch may have been fermented in an environment that was too warm; or the vegetables may have been exposed to oxygen.