Actually very easy

Milk jam (dulce de leche)

The beliefMilk jam is assumed to need a tin of condensed milk, and to be impossible from scratch.

The truthMilk jam is sweetened milk cooked down to a third of its volume. Bicarbonate speeds the colour; the rest is patience.

2 hours 30 minutes

How to make it

What you need

1 litre whole milk
250 g sugar
A quarter teaspoon of bicarbonate of soda
1 teaspoon of vanilla extract, optional

How the engine read this list (4 lines)

1030 g	Whole milk	milk
250 g	Sugar	sugar
1 g	Bicarbonate of soda	baking-soda
2 g	Vanilla extract optional	vanilla

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.

Bring the milk and sugar to the boil in a wide, heavy pan, stirring.
Take it off the heat and add the bicarbonate. It will foam up, so the pan should be at least half empty.
Return it to a low flame and simmer it uncovered for 2–2.5 hours. Stir every ten minutes for the first hour, then every five.
Stop when it turns light caramel and a finger drawn across the back of the spoon leaves a line that does not close. It should be about 350 ml.
Stir in the vanilla and pour it hot into a clean jar. It thickens as it cools.

Why it works

The colour of milk jam does not come from caramelisation but from the Maillard reaction. Caramelisation is sugar breaking down on its own above 160 °C, and this pan never passes 100 °C. What happens here is that amino acids in the milk proteins bind with the lactose and the added sugar and produce brown, nutty compounds. Maillard is slow at low temperatures but speeds up as the pH rises; a quarter teaspoon of bicarbonate nudges the milk up from pH 6.7 and speeds the colour. Its second job is to stop the lactose crystallising and the milk curdling. A litre of milk with 250 g of sugar comes down to about 350 ml in 2–2.5 hours; at a third of its volume it coats the back of a spoon and holds a line.

The rough sum

1281 grams going in

1596 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 Vegetarian

Storage & shelf life

How long it keeps

Fridge, in a closed jar: 3 weeks
Freezer: 3 months
Room temperature, once opened: Not advised

Milk jam is protected by its sugar: as the water leaves and the sugar concentrates, the water activity falls and bacteria struggle. But nobody measures the water activity of a home jar, so you cannot know it is low enough for the shelf. Keep it in the fridge and use a clean spoon every time.

When to bin it Mould on the surface, a sour or yeasty smell, or gas when the lid comes off: throw it out. If the sugar has crystallised into grains it has not spoiled — stir it in a warm water bath and it smooths out.

History

Where it comes from

Argentina and Uruguay both claim dulce de leche, and an Argentine story ties it to a servant who forgot a pan of sweetened milk on the stove in 1829. It is a good story with no document behind it; similar products were known elsewhere too, in France (confiture de lait) and in India (basundi, khoa). The shared logic is the same: in a world without refrigeration, thicken milk with sugar and it keeps for weeks.

The knack

The last half hour is when it catches most easily: stir with a wooden spoon, scraping the bottom.
If it has gone lumpy, give it ten seconds with a stick blender.

Milk and culture