Sweetened condensed milk is evaporated milk that has a significant amount of sugar added back to it to make a very thick, sweet product that is often used in ice creams, pies and other creamy desserts. It comes in full fat, low fat and fat free varieties. Most recipes simply call for sweetened condensed milk and are usually designed to work with the full fat product, but since sweetened condensed milk is fairly high in fat and calories, a lot of bakers and cooks wonder about substituting in the low fat or fat free version.
When it comes to baking, there is a difference in using full fat, low fat and skim milk because even a small amount of added fat from the milk can really increase the moistness and tenderness of the finished product. Sugar also does this. Sweetened condensed milk and its lower fat counterparts are very similar in consistency, and all of them still have a very high amount of sugar and that extra-creamy texture. That richness and high sugar content will keep baked goods made even with fat free sweetened condensed milk moist and tender. It is also primarily used in custards or other creamy dishes, rather than in cake and muffin recipes, where the creaminess and sweetness of the milk, rather than the fat content, are the most important elements. Because of these factors, sweetened condensed milk is generally a good candidate for lower fat substitutions in recipes that call for a full fat product, as you’ll still get the same (or darn close to the original) result.
In ice creams, puddings and bar cookie recipes, I’ve always had great results using lower fat sweetened condensed milk. Fat free sweetened condensed milk will thicken when an acid – such as lime juice – is added to it, just as the full-fat variety will, so it works well in fillings for key lime and other pies. I will also use it in Thai iced tea or iced coffee for a slightly lower fat (but still sweet!) drink.