Prolonged, steady heat is responsible for some really spectacular culinary alchemy, not the least of which is the glossy, coffee-colored milk jam made throughout Latin America. In Argentina, Uruguay, and Paraguay, where it’s referred to as dulce de leche (similar preparations go by different names in other countries), cooks reduce sweetened milk (and often baking soda) until the dairy’s proteins and sugars react and transform the mixture into a gooey, butterscotch-y preserve that finds its way into almost every kitchen pantry.
“It really is everywhere,” said Josephine Caminos Oría, Argentine American author of Dulce de Leche: Recipes, Stories, and Sweet Traditions (2017). These days, tubs of commercial dulce de leche line supermarket shelves throughout Latin America like peanut butter does in this country—but the milk jam is arguably more pervasive.
“If you’re diabetic in Argentina,” she said, “you can get a prescription for a dulce de leche with stevia that your insurance company will cover at the pharmacy.”
The Gooey Science Behind How Milk Turns to Jam
Cream-colored, simply sweetened condensed milk will develop a deep butterscotch tone and complexity when it undergoes caramelization as well as the Maillard reaction, the heat-induced interplay between sugars and proteins that causes foods to brown and take on rich, toasty flavor. But the dramatic shift in its consistency—from treacly fluid to jammy goo—is due to a separate change to the milk’s proteins. Given ample heat (starting around 170 degrees) and time (an hour or more), they denature and form a gel that gives dulce de leche its semisolid body.
Surprisingly, evaporation plays a minimal role here; most of the milk’s water is removed during the condensing process, and much of what’s left is bound up in the mixture’s high concentration (about 50 percent by weight) of sugar. In fact, the thickest dulce de leche is made when there’s no evaporation at all: in a multicooker’s pressurized chamber, where the milk reaches 240 degrees and forms an exceptionally stiff gel. In the oven, water in the milk can’t rise above its natural boiling point, so its proteins link up less thoroughly, and the jam is a little looser.
It’s important that these reactions occur steadily, lest the milk brown unevenly or curdle. (I omitted the dash of baking soda that many recipes call for, since the alkaline agent raises the pH of the milk, catalyzing the browning reactions at a rate that caused my dulce de leche to easily overcook.) The steam-filled multicooker heats evenly, while using a water bath in the oven steadies the milk’s temperature so that it cooks consistently.
There are myriad styles, too. Dulce de leche tradicional is a schmear for toast, a sweetener for everything from coffee to vinaigrette, and a topping for pancakes; thicker repostero is a confectioners’ product used to fill pastries and cookies such as alfajores; and heladero is a dark-amber base for ice cream.
Caminos Oría spoke lyrically about the lush flavor and consistency of from-scratch versions made by meditatively stirring a pot of fresh milk and sugar for hours. But she also acknowledged that most modern cooks who make the preserve usually start with sweetened condensed milk—which is essentially parcooked dulce de leche. Heated further, the viscous, ivory liquid thickens enough to sit up on a spoon, tans deeply, and takes on toffee-like depth.
No Can Do
Never heat sweetened condensed milk in the sealed can. The vessel can explode if the water bath evaporates too much or if it’s opened before cooling completely. (Some manufacturers even warn against this on their product labels.)
Cooking the canned milk is simple—the only real effort is time. But it must be done carefully, the milk heated in a water bath so that it cooks gently and evenly. Each of my methods transforms the runny, sweetened milk into a luscious, complex-tasting preserve.