Letting selected microbes transform food deliberately, producing acid or alcohol that both flavours it and keeps rivals out.
Technique at a glance
A compact visual model of the lesson. Use the full mechanism and success criteria below when you practice.
Concept
Letting selected microbes transform food deliberately, producing acid or alcohol that both flavours it and keeps rivals out.
What changes
Fermentation is controlled spoilage, and the control comes from making conditions that one organism tolerates and others do not.
What good looks like
Bubbles appear within a day or two and the brine turns cloudy — both are signs it is working, not spoiling.
Practice counts when you cook
Reading this brief does not change your School state. Finish a real practice dish in Cook mode and log the cook; that cook is the evidence that advances the skill.
What to focus on
Mechanism
Fermentation is controlled spoilage, and the control comes from making conditions that one organism tolerates and others do not. In vegetables, salt at roughly 2% by weight is enough to inhibit most spoilage bacteria while lactic acid bacteria — already present on the vegetable — carry on. They convert sugars to lactic acid, the pH falls, and below about 4.6 almost nothing dangerous can grow, so the ferment protects itself as it proceeds. Submersion matters for the same reason: the useful bacteria work without oxygen, and the moulds and yeasts that spoil a crock work only with it, so anything above the brine line is at risk while everything below is safe. In dairy, the same acidification makes casein clump, which is the thickening in yogurt and crema. In bread, wild yeast and lactobacilli produce gas and acid together.
Success criteria
Bubbles appear within a day or two and the brine turns cloudy — both are signs it is working, not spoiling.
The smell is sharply sour and clean. Anything yeasty, cheesy or putrid is a different organism.
Vegetables lose their bright colour and go translucent as the acid works into them.
Taste is the real gauge, and it should be sour enough to make you flinch slightly before it is done.
Failure guards
White film on the surface.
Kahm yeast — harmless, caused by oxygen contact, and skimmed off. Fuzzy or coloured growth is mould and is different.
Soft, mushy vegetables.
Too little salt, too warm, or the vegetable was not fresh. Pectin-degrading enzymes have run ahead of the acidification.
Nothing happened at all.
Too cold, too much salt, or chlorinated water. Chlorine kills the bacteria you are relying on; let tap water stand or use filtered.