The first time ever heard about dulce de leche I was in high school. A Mexican classmate was trying to explain it to me, and he described it as something his mother made by boiling an entire, unopened can of sweetened condensed milk in a pot. The whole thing sounded completely crazy to me. It still does, yet the boiled can method remains the World’s most popular dulce de leche-making technique.
How does it work? Well essentially, the can of sweetened condensed is left in a pot of boiling water, usually for several hours, until some of the sugars caramelize, and there you have it. Hang on a second Joe, didn’t you just say in the last post that caramelization only happens at temperatures above 300 degrees — but that boiling water can only get to 212 or so before it turns to steam? What gives?
Well I’ll tell you. It’s true that sugar can’t caramelize at the temperature of boiling water. However it isn’t the water that’s responsible for the heat that’s creating the caramel: it’s the pan. For while the can may be immersed in boiling water, it isn’t completely surrounded by it. The can bottom is resting on the bottom of the pan, which is situated on top of the heat source. As a result the bottom of the can gets hot enough such that caramelization can occur, while the boiling water serves to cool the can and prevent it from exploding. Or at least that’s the theory.
Sweetened condensed milk doesn’t have much water in it (it’s been condensed, after all) though it does have some. Raise the temperature too high and steam pressure builds up, causing the can to explode. Usually explosions are caused when the water level drops and exposes the metal. With the cooling medium gone, heat builds up and boom. Yet explosions aren’t always the result of operator error. Sometimes the cans simply blow up while submerged, and I can tell you, I wouldn’t want to be standing next to the stove when it did.
Some people get around these dangers by poking holes in the tops of the cans to relieve the pressure, and only filling the pot to the lip of the can. That’s probably a better solution, but still not something I’d ever recommend. The best method I know of for converting sweetened condensed milk into caramel is to pour it into a broad pan, put the pan in a water bath, and bake it in a 400+ oven. That way you get good caramelization of the surface while the pan stays cool enough that the caramel won’t burn. Having done it, I can say that it works.
The problem for me is that no matter which method you use for caramelizing sweetened condensed milk, your raw material is still sweetened condensed milk, which produces vastly inferior dulce de leche, at least in my universe.