Grape must is the freshly crushed juice of grapes that still contains the skins, seeds, pulp, and sometimes stems. It is the raw starting material of winemaking, sitting at the stage between whole grapes and finished wine. Ripe grape berries contain roughly 22% to 25% sugar by weight, and that sugar-rich liquid, along with all the solid grape matter suspended in it, is what winemakers call “must.”
What’s Actually in Grape Must
The two dominant sugars in grape must are glucose and fructose, present in nearly equal amounts. In ripe grapes, fructose typically ranges from about 28 to 112 grams per liter, with glucose in a similar range of roughly 32 to 112 grams per liter. These sugars are what yeast later converts into alcohol during fermentation.
The other major players are organic acids, primarily tartaric acid and malic acid. Tartaric acid concentrations generally fall between 5 and 12.5 grams per liter, while malic acid ranges more widely, from about 4.5 to 24 grams per liter depending on grape variety and ripeness. This balance of sugar and acid is what gives must its character and determines the style of wine it can become. Winemakers measure the sugar content in degrees Brix, a scale where each degree corresponds to roughly one gram of sugar per 100 grams of liquid.
How Grape Must Is Made
The process starts immediately after harvest. Grapes are crushed, either by mechanical presses or, in some traditional operations, by foot. Crushing breaks the berry skins and releases the juice, creating that thick mixture of liquid and solids. For red wines, the whole crushed grape, including skins and seeds, stays in the must. This is critical because the skins contain the pigments that give red wine its color and much of its flavor complexity. For white wines, the juice is separated from the skins and seeds early on, so the must is essentially clear juice with minimal solid contact.
The leftover solid material, the skins, seeds, and woody bits, is collectively called pomace. After the juice is pressed out or fermentation is complete, the pomace is separated. Skins are sometimes manually sorted from the seeds and stems for reuse in other food products.
How Must Becomes Wine
Must becomes wine through fermentation, a two-stage process. During primary fermentation, yeast consumes the glucose and fructose in the must and converts them into ethanol and carbon dioxide. This stage generates significant heat and is the fundamental transformation that turns sweet grape liquid into an alcoholic beverage. Without it, must would simply remain juice.
A secondary fermentation often follows, where bacteria convert the sharp-tasting malic acid into softer lactic acid. This step mellows the wine’s acidity and adds complexity to the flavor. Not all wines go through this second stage. Winemakers sometimes skip it intentionally to preserve a crisper, more acidic profile.
To prevent must from fermenting before the winemaker is ready, small amounts of sulfur dioxide are typically added. About 35 milligrams per liter is enough to block the natural enzymes that cause browning and to hold off wild yeast activity until conditions are controlled.
Grape Must vs. Grape Juice
The terms sound interchangeable, but they describe very different products. Commercial grape juice is filtered, pasteurized, and stripped of solids. Grape must retains all the skins, seeds, and pulp, and that physical difference translates into a dramatically different chemical profile.
Must contains far more polyphenols, the plant compounds responsible for color, bitterness, and antioxidant properties. One study comparing red grape juice to red grape must found that must contained nearly twice the total phenolic content (about 2,750 milligrams per liter versus 1,450 for juice). The antioxidant activity gap was even wider: must scored roughly 56% on a standard antioxidant measure compared to just 10% for juice. This makes sense because so many of those beneficial compounds live in the skins and seeds, which are present in must but largely absent from filtered juice.
In chemical analysis, grape must clusters more closely with red wines and pomace than with juice, which groups alongside white wines and white grapes. The solid matter in must fundamentally changes its nutritional and chemical identity.
Culinary Uses Beyond Wine
Must is not exclusively a winemaking ingredient. In Italian cooking, grape must is slowly reduced into thick, sweet syrups. The most famous example is “mosto cotto” (cooked must), a concentrated syrup used to dress meats, cheeses, and desserts. Balsamic vinegar from Modena, Italy, begins its life as cooked grape must that is then aged in wooden barrels for years or even decades.
In parts of the Middle East and Central Asia, grape must is boiled down into a molasses-like sweetener called “pekmez” or “dibs.” Greek and Cypriot cuisines use it in pastries and puddings. In all these traditions, the key step is the same: concentrating the natural sugars through slow cooking until the liquid thickens into something rich and deeply flavored, with caramel and fruit notes that refined sugar cannot replicate.
Fresh, unfermented must is also sometimes consumed as a seasonal drink in wine-producing regions, prized for its intense sweetness and cloudy, rustic texture.