Actually very easy

Apple cider vinegar

The beliefHome vinegar is assumed to replace shop vinegar for every job.

The truthHome vinegar is good for salads and sauces, but because its acidity is unmeasured, the safety of a pickle or a canned food is never trusted to it.

6–8 weeks

How to make it

What you need

1 kg apples, or apple peels and cores
1.5 litres of water, boiled and cooled
100 g sugar
A 3-litre wide-mouthed glass jar, muslin and a rubber band

How the engine read this list (3 lines)

1000 g	Apples, or peels and cores	apple
1500 g	Boiled and cooled water	water
100 g	Sugar	sugar

An empty weight means that line could not be turned into grams — either the amount is open ("to taste") or the ingredient has no record here. Neither enters the sum as zero; they do not enter it at all.

Wash the apples and chop them roughly; peels and cores can go in too. They should fill about half the jar.
Dissolve the sugar in the water and pour it over the apples to 3–4 cm above them. Keep the apples under with a small plate.
Cover with muslin and leave it at 20–25 °C in a dark place for two weeks, stirring daily. Foam and a light smell of alcohol are normal.
Strain and discard the apples. Pour the liquid into a clean jar, cover it with muslin again and leave it undisturbed for 4–6 weeks.
A thin, jelly-like layer — the mother — will form on top. When it tastes sharply sour, strain it and bottle it.

Why it works

Vinegar is two fermentations one after the other. In the first, yeasts turn the apple’s sugar into alcohol and carbon dioxide; this needs no oxygen and shows as foam in the jar. In the second, Acetobacter bacteria combine the alcohol with oxygen and turn it into acetic acid; this needs oxygen, which is why the jar is covered with cloth rather than a lid. The first stage takes 1–2 weeks at 20–25 °C, the second 3–6 weeks. Sugar decides the final acidity: roughly, each unit of alcohol gives about one unit of acetic acid, so as much acid as the sugar made alcohol. But at home neither the sugar all becomes alcohol nor the alcohol all becomes acid, and the acidity of home vinegar varies from batch to batch, and nobody knows it without measuring. The safety of pickles and canned food rests on getting below pH 4.6, and that needs vinegar whose acidity is printed on the label and is at least 4%. That is why home vinegar is not used for those jobs.

The rough sum

2600 grams going in

907 kcal in total

100% of the mass has figures behind it. This is the sum of what GOES IN rather than of what comes out. Wherever something evaporates, drains or ferments the two differ: a kilo of milk is 150 g of cheese.

Diets it qualifies for

Gluten-free Vegan Vegetarian Lactose-free

Storage & shelf life

How long it keeps

Cool dark cupboard, in a closed bottle: 1 year
Fridge: Not needed

Vinegar is kept by its own acid and is almost impossible to spoil. The cloudy, jelly-like layer that forms in the bottle over time is the “mother” — the acetic acid bacteria themselves. It is harmless and can start the next batch; you can also strain it out.

When to bin it Fuzzy, coloured mould on the surface (the mother is smooth and slippery, mould is fuzzy) or a rotten smell: throw it out.

History

Where it comes from

The Turkish word “sirke” comes from the Persian “serke”; the French “vinaigre” means “sour wine”: vinegar was discovered when wine soured by itself. In Babylon it was made from dates, in Rome from grapes, and the “posca” mixed into soldiers’ water was a vinegar drink. In Anatolia many households made their own from apples, grapes, hawthorn or mulberries; even leftover fruit peel was not wasted.

The knack

Add the mother, or 100 ml of this vinegar, to the next batch: it starts with live bacteria, so the second stage gets going sooner.
Fruit flies come for the smell of alcohol: fasten the muslin tightly with a band.

Fermentation