Condensed milk and dulce de leche

Why

Sweetened condensed milk is milk concentrated by evaporation and sweetened with sugar, so it holds a lot of sugar and milk protein; during long heating the milk sugar lactose and the proteins react in the Maillard reaction, and the mass browns, thickens and takes on a caramel flavour without the sugar having to be burnt as in caramel (Caramel card). The thick, sweet mass circulates poorly in the pan, heat builds up at the bottom, and without stirring or over high heat the milk scorches there before the rest has browned. Dulce de leche thickens further as it cools, so it is thinner while hot; graininess that appears in storage is caused by lactose crystals, not by curdled protein. Boiling an unopened can in water is a widespread habit, but the condensed milk maker Carnation does not recommend it because the can may burst: the contents heat up and expand inside the sealed metal, and the risk grows when the water boils away and the metal overheats or when the can is opened while still hot.

How to do it

• Pour the condensed milk into a heavy-based or non-stick pan and heat it over low heat, stirring constantly, including the bottom and corners.
• A water bath is gentler: a bowl over a pan of barely simmering water, or a covered baking dish set in a water bath in the oven; stir now and then.
• Take it off the heat a shade earlier than you want: it darkens a little more and thickens as it cools, so cook it longer for filling and spreading than for a sauce.
• Do not boil an unopened can in water or heat it in the oven or microwave; hot dulce de leche sticks to skin like caramel and burns deep.
• For brigadeiros or toffee, cook condensed milk with the other ingredients only until thick, stirring constantly; when the bottom starts to catch, turn the heat down.
• Move leftover opened condensed milk into a sealed container in the fridge and use it like fresh milk; the manufacturer says within 3 days.

If it goes wrong

Do not scrape up a scorched bottom: pour the unscorched part into a clean container and taste it; if all of it tastes burnt, it cannot be saved. Let thin dulce de leche cool, and if it stays thin, cook it further; thin an overly thick one with a spoonful of hot milk or cream. Warm a grainy one with a little milk, stirring until smooth, and use it soon, as graininess can return. If a can is already in boiling water, keep it fully submerged to the end, topping up the water, then let it cool completely and open it wrapped in a towel, pointing away from you. For burns, follow the Caramel card.