Cooking science

Lactic fermentation

Yoghurt, sauerkraut, kimchi, sourdough, salami and most cheese all rely on the same thing: bacteria converting sugars into lactic acid. The acid is what gives the sourness, changes the texture, and makes the environment hostile to the organisms that would otherwise spoil the food.

What the bacteria do

Lactic acid bacteria consume sugars and excrete lactic acid. As the acid accumulates the pH falls, which does three useful things at once: it tastes sour, it changes the texture of proteins, and it suppresses most other microorganisms, which cannot tolerate the acidity.

The texture change is often the most striking part. Acid makes milk proteins clump, which is how milk becomes yoghurt without anything being added but bacteria. In vegetables the acid firms pectin and preserves crunch. In dough it strengthens gluten and gives sourdough its characteristic chew.

These bacteria are ubiquitous — on vegetables, in milk, in flour, on skin. Most traditional ferments are not inoculated with anything; the conditions are simply arranged so that the lactic bacteria out-compete everything else.

The familiar ferments

Yoghurt

Milk held warm with a live culture. The acid clots the casein into a set gel.

Sauerkraut and kimchi

Cabbage salted and pressed under its own brine. Salt selects for lactic bacteria and excludes most spoilage organisms.

Sourdough

A flour-and-water culture of wild yeasts and lactic bacteria. Yeast supplies the gas, bacteria the acidity and flavour.

Cheese

Lactic acid drops the pH, which helps rennet set the curd and determines how the finished cheese behaves.

Salami and cured sausage

Acid plus salt plus drying. This class genuinely depends on food-safety controls and is not something to improvise.

Pickles in brine

Fermented, and quite different from vinegar pickles, which are simply acidified.

What makes a ferment go the right way

• Salt at the right concentration. Enough to inhibit spoilage organisms, not so much that the lactic bacteria are suppressed too. Vegetable ferments are usually around 2% of the vegetable weight.
• Keep everything under the liquid. Exposed to air, vegetables grow mould and yeast. Weigh them down.
• Temperature. Warmer is faster and more sour; cooler is slower and more complex. Most vegetable ferments do well at ordinary room temperature.
• Time, and tasting. A ferment is finished when it tastes right, not on a schedule.
• Clean, not sterile. Scrupulously clean equipment, but there is no need to sterilise — you are cultivating organisms, not excluding them.

Fermentation and spoilage

The honest distinction is that a ferment is a controlled microbial process producing something you want, and spoilage is an uncontrolled one producing something you do not. The organisms overlap considerably and the difference is whether the conditions favour the ones you are after.

A vegetable ferment that smells sour, tangy and clean is working. One that smells putrid, or that has fuzzy coloured mould rather than a flat white film, has gone another way and should be discarded.

Beyond that general guidance, fermentation for preservation is an area where the consequences of getting it wrong can be serious — particularly for anything low in acid, anything involving meat or fish, and anything canned or sealed. FoodHQ does not publish preservation procedures or safety thresholds. For those, consult the Food Standards Agency or an equivalent authority in your jurisdiction, and follow a tested recipe from a reputable preserving source.

Common questions