Lacto-fermentation is the process of using natural bacteria to ferment foods, usually vegetables and fruits. The bacteria involved are called lactobacilli, meaning they create lactic acid as a byproduct of their metabolic process. This acid helps to preserve the fermented food. Lacto-fermentation is used for hot sauce, kimchi, pickled vegetables, sauerkraut, etc. It is also a component of sourdough but that topic is covered separately.

The principle of lacto-fermentation is well covered by other people. Therefore this article only gives a basic introduction, then covers some of the particularities not well covered elsewhere.

Fruits and vegetables containing sugars will spontaneously ferment if the bacteria can get access to the sugars. This happens when the food is over-ripe, but can also be induced before that point by breaking down the food. A smooth end product like hot sauce can be finely chopped. Cabbages can be sliced and pounded to break down cell walls. Other foods can be coarsely chopped or otherwise have their flesh exposed.

Salt is an important aspect of lacto-fermentation. It plays two roles (beyond flavour): it draws moisture from and therefore breaks down the cells, enabling bacteria to access the sugars, and it makes an environment that is inhospitable to spoilage organisms. The active lactobacilli are "halotolerant", which means they can operate in the salty environment, while most other micro-organisms cannot function in that environment.

Salt may be added in two ways:

  • Added directly to the food, usually at 2% of the weight of the food;
  • In brine, usually in a concentration of 5% salt, to cover the food.

Either way, the salt draws moisture from the food and creates a brine. Food is usually weighted down to ensure it remains under the natural or added brine. This helps protect it from spoilage during fermentation and facilitates free movement of the sugars and bacteria so fermentation can continue easily.

The great enemy of lacto-fermentation is mould that can grow on top. The mould is not sufficiently inhibited by the salt, but does require oxygen, so only grows on the top of the ferment. If allowed to persist, it can adversely flavour the brine, and can also grow tendrils into the food making it very hard to remove. While some people simply scrape off mould as it forms, it is much better to avoid the problem. Because mould requires oxygen, exclusion of oxygen protects the ferment. Fermenting food produces carbon dioxide that bubbles up from it and drives off oxygen, so all that is required is to prevent new ambient oxygen from reaching it.

Home lacto-fermentation mainly requires the ability to keep the ferment under the brine, and to exclude oxygen. But vessels dedicated to this purpose tend to be expensive, and jury-rigging weights and airlocks can be difficult. A good solution is to use jars with a rubber gasket and a clamp or screw band lid. With the lid firmly fastened, carbon dioxide pressure builds up inside the jar. At a certain pressure, the gasket allows the gas to escape, so the pressure does not build up to a dangerous level. Oxygen in the jar when it was closed is driven off and the space is replaced by carbon dioxide, so mould cannot form. Because there is a seal that doesn't allow incidental gas transfer, and there is positive pressure inside the jar, oxygen from outside never enters the jar. Therefore the ferment can proceed and the food is preserved as long as the jar is not opened.

In this system, weighting of food isn't always necessary. Some foods that would float high in the brine, like pickles, do need to be weighted down so they will ferment evenly, but they don't require heavy weights, and simple inserts like plastic container parts cut to fit usually work. But other foods do not require weighting at all even when the recipe recommends it. Because of the absence of oxygen, the food is not at risk of mould. The carbon dioxide pressure in the headspace of the jar offsets the pressure from carbon dioxide being generated in the ferment, so the food does not rise up very much. The food does have less contact with its brine and therefore may ferment more slowly, but it does ferment effectively.

Once a jar is opened, the protective layer of carbon dioxide escapes. Although the salt and acid help to preserve the food, there usually is not enough to fully protect it from spoilage organisms when oxygen is present and temperatures are warm. Therefore, an opened jar should be transferred to the refrigerator. In that environment it will keep weeks to months. It is helpful to ferment food in smaller jars that can be opened and refrigerated individually, rather than ferment the food in larger jars that keep the entire batch together but won't be shelf stable after the initial opening. It is also possible to can and heat process fermented food, but this destroys some of the nutritional benefits of a live and raw lacto-ferment.

Unopened, lacto-ferments can last at least a year. They are not at risk of spoilage, though the food does break down slowly and eventually this goes beyond palatability. Purées like hot sauce last the longest because there is less need to preserve structure. After the initial fast fermentation lasting up to a month, fermentation continues slowly and the food continues to become more sour. This is part of the character of lacto-ferments and it remains edible.