Passum

One of the ingredients the Romans used as a sweetener was something called passum, or raisin wine. This was wine made from dried grapes that were then mixed with must and pressed, i.e. made from raisins before being fermented and so extra sweet (and again, I am totally cribbing this from Grocock and Grainger). There are plenty of raisin wines (also called straw wines) still made in the Mediterranean area but they can be hard to come by, as well as expensive stuff to use just as a flavoring. So I cheated, and made a pseudo-passum by steeping raisins in already-made wine.

It's said to be sweet, and I assume red, so I used a port (again, I am not a drinker and know next to nothing about wines. Seriously I had to look this stuff up on Wikipedia). The bottle was 750 ml, so I used most of a medium sized (twelve ounce) box of raisins; I put them in some Tupperware, poured the wine over it and let it all soak in the fridge for a few days.

When that was done I strained it back into the bottle, then took a spoonful at a time of the soaked raisins and squashed them up in the mortar, then strained them. But that got pretty tedious pretty quick, so I ended up going all 21st century and throwing them in a food processor with a bit of the wine to make a slurry, then straining that back into the bottle.

When I tasted it it was definitely sweeter, with a strong raisin flavor to it. Hopefully that will be an acceptable substitute.