In "APICIUS Cookery and Dining in Imperial Rome" translated by Joseph Dommers Vehling, it only mentioned passum in passing as a raisin wine.
However, in "APICIUS The Roman Cookery Book" translated by Barbara Flower and Elisabeth Rosenbaum, there is a much more detailed description: "Another specially prepared cooking wine is the passum. Like defrutum, it was used to sweeten sauces. It is not only sweeter than defrutum, but has a different flavour. Palladius (XI, xix) even says that one can use it like honey." It goes on to give two recipes for passum. These recipes appeared to be similar to the making of wine, except that raisins were used instead.
The recipe below is my attempt to make a reasonable substitute for Passum, without the time commitment involved with the wine making process. In the Flower and Rosenbaum translation: "Instead of passum we have used very sweet Spanish wine, being aware, of course, that this wine provides only the sweetness required, but not the original flavour. " Since I wanted to more closely approximate the original flavor my solution was as follows:
Combine wine and raisins in a jar with a secured lid. Let set for 6-8 weeks. Remove raisins from the wine, the resulting liquid is used a substitute for Passum.
For a quick version, combine wine and raisins in sauce and place on low heat; reduce liquid by one half.
The red port wine was used because it is a very sweet wine to begin with. The infusing or cooking the raisins into the wine should impart their unique flavors into it.
The Roman Cookery Book translated by Barbara Flowers
and Elisabeth Rosenbaum, Peter Nevill Limited, 1958.
Cookery and Dining in Imperial Rome, edited and translated
by Joseph Dommers Vehling, Dover, 1936.