Must (from the Latin vinum mustum, “young wine”) is freshly pressed fruit juice (usually grape juice) that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace; it typically makes up 7%–23% of the total weight of the must. The length of time that the pomace stays in the juice is critical for the final character of the wine.
When the winemaker judges the time to be right, the juice is drained off the pomace which is then pressed to extract the juice retained by the matrix. Yeast is added to the juice to begin the fermentation, while the pomace is often returned to the vineyard or orchard to be used as fertilizer. A portion of selected unfermented must may be kept as Süssreserve, in order to be added prior to bottling as a sweetening component.
Some winemakers create a second batch of wine from the used pomace by adding a quantity of water equivalent to the juice removed, letting the mixture sit for 24 hours, and draining off the liquid. This wine may be used as a drink for the employees of the winemaker or as a basis for grappa.
Must was commonly used as a cooking ingredient in ancient Rome. It was boiled down in lead or bronze kettles into a milder concentrate called defrutum or a stronger concentrate called sapa. It was often used as a souring agent and preservative, especially in fruit dishes. Geochemist Jerome Nriagu published an article in the New England Journal of Medicine in 1983 hypothesizing that defrutum and sapa may have contained enough lead acetate to be of danger to those who consumed it regularly.
This teaching goes back at least to Pope Julius I (337–352), who is quoted in Thomas Aquinas's Summa Theologica as having declared that in case of necessity, but only then, juice pressed from a grape could be used. Aquinas himself declared that it is forbidden to offer fresh must in the chalice, because this is unbecoming owing to the impurity of the must; but he added that in case of necessity it may be done.