Verjuice (French „verjus“, Italian „agresto“, Persian „ab-ghureh“) is the juice of green, unripe grapes, hence its name „vert jus“ = green juice. It is acidic and astringent and is a perfect substitute for vinegar and lemon juice. Actually, I totally prefer it to vinegar. We even make ceviche with it, instead of lemon juice.
Verjuice was extremely common throughout Medieval Europe, before lemons were available in Europe. Not only for culinary use, though, monasteries produced verjuice as a remedy against infections of all kinds, for soothing the stomach, and against sore throats. The Ancient Romans used it as well, and probably its use goes back to the Ancient Near East. Pliny the Elder (AD 23 – AD 79) mentioned it in his Natural History (book 12, chap. 60) as a remedy.
Ab-ghureh is used as a condiment for salads or as a soup ingredient in Iranian cuisine, where it is also considered a remedy. We take a spoonfull every morning.
We make a few dozens of bottels of agrest every year. As a next step, I will make mustard with verjuice instead of vinegar, as I recently started to make my own mustard. But that will be another post…
For those who read German and are interested in the subject, you can find an extensive article about the history, revival and use of verjuice here: http://www.elmar-lorey.de/Agrest/index.htm