Acetic fermentation

The reaction, and why it needs air

Acetic acid bacteria — mostly Acetobacter and its relatives — oxidise ethanol. That word is doing real work: oxygen is a reactant, not an incidental condition, and without it the reaction cannot proceed at all.

This is what distinguishes acetic fermentation from the other three fermentations in the kitchen. Yeast fermentation and lactic fermentation both happen best with air excluded; acetic fermentation demands it. That single difference explains almost every practical rule about it.

It also means acetic fermentation is a second step. Something has to make the alcohol first. Vinegar is always the product of two fermentations in sequence: sugar to alcohol by yeast, then alcohol to acid by bacteria.

What follows from needing air

• Vinegar is made in a shallow, wide, covered-but-breathing vessel, not a sealed one. Cloth over the top, never an airlock.
• Wine and beer are protected from air, and the reason is precisely to keep these bacteria out. Airlocks, full bottles and topped-up barrels are all defences against acetic fermentation happening when it is not wanted.
• An open bottle of wine goes to vinegar given long enough. That is not the wine "going off" in some vague sense — it is a specific, named, predictable reaction.
• Kombucha needs a cloth cover during its main ferment, for the same reason vinegar does. The sealed second fermentation is for carbonation, and it is a different stage.
• A "mother" is a cellulose mat the bacteria build at the air-liquid surface, where they can reach both the ethanol and the oxygen. It looks alarming and is a sign the process is working.
• Fermenting vegetables under brine cannot go acetic, because the brine excludes the air. That is one of several reasons submersion matters.

The vinegars, and what they started as

Every vinegar is named after the alcohol it was made from.

Wine vinegar

Wine. Red or white, and the character of the wine carries through.

Cider vinegar

Cider, from apples. Fruitier and softer than wine vinegar.

Malt vinegar

Ale, brewed from barley. Distinctively British and distinctively malty.

Rice vinegar

Fermented rice, and generally the mildest and least aggressive of the common vinegars.

Balsamic

Grape must, cooked and aged — the traditional kind for many years in a series of casks. Sweet as well as sour, and the cheap supermarket version is a different product with colouring and sweetener added.

Distilled or spirit vinegar

Made from a neutral alcohol, so it tastes of acid and nothing else. Useful for pickling where you want the vegetable to taste of itself.

Acid as preservation

Acetic acid preserves food, and it preserves it in a different way from a lactic ferment even though both end up acidic.

In a lactic ferment, the food makes its own acid slowly from its own sugars, and the microbial community shifts as it goes. In an acetic pickle, acid is poured over the food from outside and the environment is hostile from minute one. That is why a vinegar pickle is stable almost immediately and a ferment takes days, and it is why the two taste so different — one has had a microbial community living in it and the other has not.

Both routes depend on getting the acidity right, and how acidic is acidic enough is a safety question rather than a flavour one. FoodHQ does not publish salt percentages, pH figures or keeping times for home preserving. Those are safety numbers, they are set by a food safety authority rather than by a cookery writer, and the right ones depend on what you are fermenting and where you are. In the UK the Food Standards Agency is the authority — use a recipe from them or from another body with the same standing, and treat this page as the explanation of why that recipe is shaped the way it is.

Common questions