Um, if we could just get a word in here…?
You are not talking about three things, you are talking about two things – unsweetened condensed milk and sweetened condensed milk. The important point, though, is that unsweetened condensed milk is called evaporated milk in this country and is sold as evaporated milk.
To resort to mathematics: evaporated milk = unsweetened condensed milk.
Unless there’s some rogue dairy marketer out there in some uncivilized state who’s determined to mess with the system, you cannot buy “unsweetened condensed milk” in this country. It’s just evaporated milk. (Even if you can buy “unsweetened condensed milk” somewhere in this country, it’s still just evaporated milk.)
We have seen the term “unsweetened condensed milk” used in Canadian publications, and perhaps evaporated milk is called unsweetened condensed milk in parts of Canada and elsewhere in the English-speaking world. It’s possible that there are rogue dairy marketers everywhere.
With regard to your historical quest, evaporated milk has been around for centuries. Marco Polo saw the Tartars carrying about a thick milk paste, which could be reconstituted. In the 1850s sugar was added to evaporated milk to act as a preservative. In the 1880s and 1890s, when large-scale canning was becoming practical, it was possible to store sweetened condensed milk and evaporated milk in jars and cans for long periods.
The process for making evaporated and sweetened condensed milk begins the same way – the milk is heated under a powerful vacuum, which lowers the boiling point so that much of the water evaporates. At that point, either sugar is added as a preservative and the sweetened condensed milk is packaged in sterilized cans, or the milk is sterilized with high heat and the evaporated milk is packaged in sterilized cans.
Condensed milk – a term you often see in recipes – is just a shorthand form of sweetened condensed milk.