Verjuice is one of the things that Medieval recipes sort of take for granted. It's an acidic, tart juice made from unripe grapes, unripe or tart apples, gooseberries or whatever else was available. We sort of take it for granted that it was used instead of vinegar, but a lot of recipes (see, for example, Le Menagier de Paris) call for verjuice along with vinegar or wine.
I usually make my own by just squeezing unripe grapes, which are pretty easy to get. It holds well in the refrigerator.
You can also find it bottled but it's super-expensive. However, if you happen to have a Middle Eastern shop nearby, ask around for something called husrum.
This is the recipe that I use. I will quote it here in the interest of accessibility and in case it ever goes down, but it's not mine!
- For about 800ml of verjuice, wash 1 kg of grapes. Pluck each grape from the pedicel. Put some music or a podcast on. You are going to be here a while.
- Place the grapes in a food processor and add 1/2 tsp powdered ascorbic acid to temper oxidation, then pulse just enough to rupture the skins and cut through the fruit but not to disturb the integrity of the seeds, which would add an unwanted astringency.
- Prepare a bowl with a sturdy piece of muslin or other filtering cloth that can accommodate a good quantity of pulp. Dump some of the pulp in, gather the cloth around it and squeeze, twisting a bit. The grapes will be very yielding. Squeeze until the dripping has stopped.
- Here you have a couple options. You can either pour this juice into an ice cube tray to be used as needed, or you can filter it again and place in the refrigerator. It will slowly age this way.