Household ferments such as pickled vegetables and soured milk are made without buying a culture. The organisms are already present, and the cook's job is to select which ones thrive.
The organisms arrive with the ingredients
Lactic acid bacteria live on the surfaces of vegetables and in the air of a kitchen that ferments regularly. Nothing needs to be introduced.
What a traditional recipe controls is conditions. The cook creates an environment in which one group of organisms outcompetes the rest.
This is why the same recipe made in two households produces recognisably different results. The starting population differs.
Salt does the selecting
Salt draws water out of plant tissue and raises the concentration in the surrounding liquid. Many spoilage organisms cannot tolerate that concentration.
Lactic acid bacteria tolerate it comparatively well, so raising salt tilts the competition in their favour before any acid has been produced.
Too little salt and the ferment goes soft or off. Too much and it stalls, because even the intended organisms slow down.
Acid is a by-product that becomes the preservative
The bacteria consume sugars in the vegetable and release lactic acid. They are not preserving anything deliberately; the acid is waste.
As acidity rises, the environment becomes hostile to almost everything else, including eventually the bacteria that produced it.
The ferment therefore stabilises on its own. A jar that has reached full sourness is largely self-protecting, which is why these methods predate refrigeration.
Temperature sets the pace and the flavour
Warmth speeds fermentation. A jar left in a warm kitchen sours in days; the same jar in a cool store takes weeks.
Speed changes the outcome, not just the timing. Different organisms dominate at different temperatures and produce different flavour compounds.
Traditional practice usually starts a ferment warm to establish activity, then moves it somewhere cool to develop slowly.
Why the recipes give no fixed times
Older recipes specify salt ratios precisely and timings loosely, often instructing the cook to taste rather than to wait a set number of days.
That is a sensible response to variables the recipe cannot control: ambient temperature, sugar content of the vegetable, and the population it arrived with.
Judging by taste and smell is not vagueness. It is the only reliable measurement available in a kitchen for a process whose rate is genuinely unpredictable.