Answer: Lead Salts

To a modern reader, the idea of sweetening wine let alone anything else with lead salts seems like an absurdly unsafe thing to do (and, to be certain, it is). To a Roman who wanted sweeter wine and had no concept of the dangers of lead poisoning, however, it was a cheap and easy way to sweeten things in an area of the world where natural and safe sweeteners were few and far between.

In ancient Rome, wine makers would boil must, the juice of freshly pressed grapes in lead pots. The acidity of the juice would leach lead out of the pots and create lead acetate salt in the liquid. Lead acetate salt, unlike most compounds toxic to humans, has a sweet flavor instead of a bitter one. They would reduce the liquid into a syrup called defrutum, which would then be reduced again into a very sweet and very concentrated syrup called sapa. The sapa would then be added to wine to sweeten it, lacing it with lead in the process.

For centuries afterwards, lead was used as a sweetener in wine and other foods. Even after the dangers of lead were recognized, it was difficult to detect lead salt in foodstuffs, so the practice continued. The advent of modern chemistry and the ease in which lead can now be detected in trace amounts, however, led to a rapid decline in the use of lead salts.