Verjus or verjuice

Verjus is the name given to the juice taken from unripened white grapes. At the time, there were three different sorts of grapes: one for wine, one for the table, and one for verjus. Verjus is acidic and was thus used as an alternative to lemon juice. In 19th century, a change in tastes meant that acidity was considered common and vulgar. As a result, the verjus, whose taste was particularly strong, was gradually replaced by citrus fruits, initially orange, then lemon.
The fresh grapes would be peeled and depipped. They were then pressed into a syrup, into crystalised verjus or indeed verjus conserves, the latter of which were greatly sought after and very expensive due to the need for just as much sugar as verjus to make them. The juice was kept in bottles and used as a base for sauces, serving as an alternative to vinegar.
It was said that a small glass of verjus was an excellent “pick-me-up” after a fall.

Period Recipe

Recipe for making verjus jelly to be used in cooking. "Take the unripened grape, remove the seeds one by one, put it in a pot, add two glasses of water and stir it as it comes to the boil until it is cooked enough to be ready for squeezing. Next it should be pressed through a cloth until the essence has been extracted, but one should take care not to lose neither pips nor the skin. The liquid is then put in a container on the furnace and heated on a low heat until it turns to syrup or jelly. One can test to see if it is cooked enough by taking a spoonful of it and letting it cool: once it starts to run, care should be taken not to let it cook too much.  Anonymous, beginning of 19th century, original French text quoted by Madeleine Ferrières in Histoires de cuisines et trésors de fourneaux, Larousse, 2008.

Type of Recipe