Fermented food or drink is given its flavour and texture through micro organisms such as bacteria or yeast. Fermentation can improve the nutritional properties of food by breaking down nutrients, making them more 'bio-available' — easier for us to absorb. The bacteria pre-digest the foods, and this can mean we take on more vitamins and minerals than we would in their unfermented state. You might, for example, absorb more nutrients, such as zinc or iron, from fermented soya such as tempeh than you would from the same amount of soya beans.

But the popularity of certain fermented foods really stems from our emerging understanding about how the mix of our gut bacteria affects health — including our immunity, weight and mood. Because of the way they are prepared, some fermented foods contain live 'good' bacteria and yeasts known as probiotics.

The health benefits a probiotic offers when eaten depends on its species and its strain, as well as the number of the micro-organisms.

Some fermented foods — such as kefir, kimchi and sauerkraut — are what's known as synbiotics. This means that as well as containing probiotic 'good' bacteria, they also contain prebiotics. These aren't a type of bacteria themselves, but are carbohydrates we can't digest which act as food for the 'good' bacteria.

However, not all fermented products are equal or healthy. And not all fermented foods are probiotics. Just because a food is labelled fermented does not automatically mean that it contains probiotics. We should thus look for the words 'live' or 'bio' on labels. And there can be a big difference between fermented products produced traditionally and large scale. If a product is pasteurised, this can destroy the live cultures.