Actually very easy

Ketchup

The beliefKetchup is assumed to come from a factory and a secret formula, and a home version is expected to end up as plain tomato sauce.

The truthKetchup is tomato with two thirds of its water boiled off and vinegar to sharpen it. What makes the difference is not the recipe but the time.

1 hour

How to make it

What you need

1.5 kg ripe tomatoes
1 onion
120 ml white vinegar
80 g sugar
1 teaspoon of salt
2 cloves, half a cinnamon stick, a pinch of allspice

How the engine read this list (8 lines)

1500 g	Ripe tomatoes	tomato
150 g	Onion	onion
121 g	White vinegar	vinegar
80 g	Sugar	sugar
6 g	Salt	salt
0 g	Cloves	clove
—	Cinnamon	cinnamon
0 g	Allspice	allspice

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.

Roughly chop the tomatoes and onion and boil them uncovered in a wide pan for 35–40 minutes. With the lid off the water leaves; with it on the water stays, and this whole recipe is about the water leaving.
Push it through a sieve and leave the skins and seeds behind. The skins would leave it gritty, and their pectin has already gone into the liquid by now.
Return the purée to the pan with the vinegar, sugar and salt, and drop the spices in tied in a scrap of cloth. Thicken it over a low flame for 20–25 minutes, stirring now and then.
The test: put a spoonful on a flat plate and wait a minute. If water seeps out at the edge, give it five more minutes. If it does not, it is done.
Take the spice bag out and fill hot bottles with the hot ketchup, then close them.

Why it works

Ketchup sits on a spoon because of the tomato’s own pectin, not because of starch or a thickener. About 940 g of a kilo of tomatoes is water; once two thirds of it has gone, the pectin chains are close enough to stop the mixture flowing. That is why a wide pan beats a tall one: evaporation follows surface area. The vinegar does two jobs. The first is taste — acid against the tomato’s sugar stops the tongue reading sweetness alone, and the result feels rounded. The second is safety: below pH 3.7 botulinum spores cannot germinate, which puts ketchup in the small class of home sauces that can honestly be canned. Adding the spice at the end is not a preference either — the volatile oils in clove and cinnamon leave with the steam over forty minutes of boiling.

The rough sum

1858 grams going in

666 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

Fridge, in a closed bottle: 4 weeks
Hot-filled jar with a sealed lid: 6 months unopened
Freezer, portioned in an ice tray: 6 months
Room temperature, once opened: Not advised — a home batch is lower in salt and sugar than the bought one

What gives shop ketchup its shelf life is the vinegar: it takes the pH below 3.7, and at that acidity Clostridium botulinum cannot multiply. Use the same vinegar at home and you get the same protection, but a home batch has less sugar, so the water activity stays higher and mould can still take hold. Wipe the neck of the bottle every time you open it — what spoils ketchup is nearly always the residue drying and moulding around the rim.

When to bin it A white or coloured film on the surface, a hiss of gas when you open it, or a sharp yeasty smell: throw it out. Ketchup is not a fermented product, so if it is fizzing, something unwanted is working in there.

History

Where it comes from

Ketchup began as fish, not tomato. In Southeast Asia *kê-tsiap* was a fermented fish liquid; English sailors carried it to Europe in the seventeenth century, and for two hundred years it was made with mushrooms, walnuts and oysters. A tomato version was written down in Philadelphia in 1812, but the turning point came in 1876, when Henry Heinz raised the proportion of vinegar. Until then ketchup was preserved with sodium benzoate and did not keep long. Heinz’s answer was not to add a preservative but to lower the product’s own pH — which is exactly the answer you use at home today.

The knack

If the colour stays matt rather than turning bright red, the tomatoes were not ripe enough; a spoon of tomato paste supplies both the colour and the missing pectin.
Do not cut the vinegar. If it tastes sharp, add 20 g more sugar: dropping the acid drops the shelf life with it.
A blender is not a substitute for the sieve: the skin stays in the purée and you feel it on the tongue.

Pastes and sauces