How to use lemon or other fruit acid in Brigadeiro without curdling? Here in Brazil it is customary to make Brigadeiro of varied flavors. The basic recipe is sweetened condensed milk and butter cooked until sticking to the pan.

It is known that, when the acidic fruit juice comes into contact with the milk, it curdles. This is the problem. 

How can I cook 395g of sweetened condensed milk + 15g butter + 25ml lemon juice (or any other acid fruit juice) without it curdling?

PS.: I do not want to use essences or anything with artificial lemon flavor.
As someone pointed out, key lime pie (or "torta de limão com merengue" as we know it) shouldn't curdle, but it is a different type of heat we're talking about, so bear with me for a while.

A few tips:

1) Add less zest at the end of the cooking. If you add everything together, the essential oils evaporate during cooking and you lose the flavor;
2) If you want to use juice, make a white brigadeiro and add the lime juice (our "limão taiti" is actually called lime) after cooking so it doesn't curdle and preserves the flavor