This was was boiled in a lead pot until it had reduced by one-half. Some food sources today see this as a predecessor to Balsamic Vinegar.
For the Romans, it was a way to preserve grape juice, a way to make a sweetener, and a way to make sour wine more useful in the household.
While boiling it, you needed to constantly skim the surface, in order that it would have a better storage life. These skimmings could be added to a batch of Lora being made.
Writers like Columella recommended simmering the Defritum down in lead pots, because they knew that lead would make it even sweeter.
Some versions of defritum apparently could be made from figs. Apicius, writing quite late in the Roman Empire, says Defritum can also be made from figs or quinces (56 : L II – II.8.) Most classical writers, though, refer to it as being made from grapes.
There is much debate still today over the difference between sapa and defrutum. The Roman writer Pliny is more inclined to make a difference; other writers merged the two together in their minds:
- Pliny says Defritum is reduced by one-half; sapa by two-thirds ;
- Columella refers to Defritum as something that is reduced to one-third: ” “Cook sweet-flavoured must down to a third and once cooked, as I’ve said above, it’s called defrutum. Then once it’s cooled, it’s transferred into a vessel and stored away, and used up within an year.”  But he seems to use the words sapa and defrutum interchangeably later in that chapter;
- Varro also says it’s reduced to one-third ;
The order of sweetness of sweeteners in the Roman worlds seems to have been, from greatest to least: honey, sapa, defrutum, caroenum.
You may wish to compare Defritum, Caroenum and Sapa.
 Pliny. Natural History. 14:180
 “Mustum quam dulcissimi saporis decoquetur ad tertias et decoctum, sicut supra dixi, defrutum vocatur; quod cum defrixit, transfertur in vasa et reponitur, ut post annum sit in usu.” Columella. De Re Rustica, Chapter 12, 21,1
 Varro. Nonius Marcellus, De Compendiosa Doctrina, 551:18.
Most people purchase a grape syrup from an ethnic store, or use a grape juice concentrate, such as is sold for winemaking.
Some speculate that defritum, made in a lead pot as it was, may have been responsible for some of the supposed lead-poisoning that inflicted Romans. What the Romans did know was that using copper pots gave a nasty taste to it, and could induce rather immediate vomiting.
Apicius refers to it as “defritum”; Columnella as “defrutum”. In classical Greek, it was called “siraion.”