Wine must is the freshly crushed grape mixture that exists before fermentation turns it into wine. It includes the juice, pulp fragments, skins, and seeds produced when grapes are crushed, and it serves as the raw material that yeast will eventually convert into alcohol. The International Organisation of Vine and Wine formally defines it as the liquid product obtained from fresh grapes through physical processes like crushing, stemming, draining, and pressing.
What’s Actually in Must
Must is not simply grape juice. It’s a heterogeneous mixture containing liquid juice, solid pulp fragments, grape skins, and seeds, all suspended together after the grapes have been crushed but before fermentation begins. The skins and seeds are especially important because they contain pigments, tannins, and flavor compounds that shape the final wine. The solid portions that aren’t extracted during winemaking eventually become pomace, the dry leftover material sometimes used for making grappa or compost.
The sugar content of must determines how much alcohol the finished wine will have. White grapes are typically harvested between 20 and 24 degrees Brix (a measurement of sugar concentration), yielding wines of roughly 11 to 13.3% alcohol. Red wine grapes tend to come in a bit sweeter, between 22 and 26 Brix, producing wines in the 12.1 to 14.3% range. A quick rule of thumb: multiply the Brix reading by 0.55 to estimate the final alcohol percentage if the wine ferments completely dry.
Must also contains organic acids, primarily tartaric and malic acid, which give wine its characteristic tartness. The pH of finished wine typically falls between 3.3 and 3.8, making it quite acidic. Nitrogen compounds in the must are critical too, because yeast needs them to grow and ferment properly. White grape must generally needs around 150 milligrams per liter of usable nitrogen for a healthy fermentation, while red must needs at least 100 mg/L. When nitrogen levels are too low, fermentation can stall or produce off-putting sulfur smells. Higher nitrogen levels, around 250 to 350 mg/L in whites, tend to produce fruitier, cleaner-tasting wines.
How Must Differs for Red and White Wine
Most grape juice is actually clear, regardless of whether the grapes are red or green. The color difference between red and white wine comes down to how long the juice stays in contact with the grape skins. For red wines, the must ferments together with the skins, sometimes for days or weeks. This extended contact pulls color pigments called anthocyanins from the skins into the wine, along with tannins that create that dry, gripping mouthfeel.
For white wines, the approach is nearly opposite. Winemakers press the grapes and separate the juice from the skins quickly, then ferment the clarified juice on its own. Rosé falls in between: the juice spends a short time on red grape skins, picking up just enough color to turn pink before being separated.
Free-Run vs. Press Fractions
Not all must is created equal. When crushed grapes sit in a tank, some juice flows out freely under its own weight. This “free-run” juice is generally softer, more aromatic, and more delicate. As winemakers then apply increasing pressure to extract more liquid, the resulting “press fraction” picks up additional tannins and phenolic compounds from the skins and seeds. Press juice tends to be more structured and full-bodied but less elegant on its own.
Many winemakers blend the two fractions to balance the wine. Free-run juice alone can be too delicate to age well, while press juice alone can taste coarse. The phenolics from the press fraction build body and structure, while the free-run portion contributes finesse and pure fruit expression.
Preparing Must for Fermentation
Fresh must rarely goes straight into fermentation without some adjustment. One of the most common additions is sulfur dioxide, which acts as an antiseptic, antioxidant, and antimicrobial agent. It’s been used in winemaking for centuries to prevent spoilage organisms from taking hold before the desired yeast can do its work. European regulations cap total sulfur dioxide at 160 mg/L in red wines and 210 mg/L in whites and rosés.
Winemakers may also adjust sugar levels in cooler climates where grapes struggle to ripen fully, a practice called chaptalization. In warmer regions, acidity might need boosting if the grapes got too ripe and lost their natural tartness. If nitrogen levels are low, winemakers add nitrogen supplements to keep yeast healthy and avoid stuck fermentations.
White wine must typically goes through a clarification step before fermentation. The most traditional method is cold settling: the must is chilled in a tank, solids gradually sink to the bottom over several hours, and the clear juice is racked off the top into a clean vessel. Winemakers often add enzymes that break down pectin to reduce the juice’s viscosity, helping solids settle faster. A faster alternative called flotation uses tiny bubbles to carry solids to the surface instead of waiting for them to sink.
Why Skin Contact Matters
The skins in grape must are a concentrated source of anthocyanins, the pigments responsible for red, purple, and blue hues in wine. These compounds are also potent antioxidants with well-documented anti-inflammatory properties. In winemaking, they’re extracted naturally as the fermenting juice sits on the skins, with alcohol acting as a solvent that pulls the pigments out more efficiently as fermentation progresses.
Tannins come primarily from skins and seeds, and their extraction increases with longer skin contact and higher temperatures. This is why red wines, which ferment on their skins, have noticeably more tannin than whites. It’s also why winemakers carefully manage how long the must stays in contact with its solids and how aggressively they mix or punch down the floating cap of skins during fermentation. Too little contact produces a thin, pale wine. Too much can make it bitter and astringent.
The must stage, in short, is where all the raw potential of the grape gets set up for transformation. Every decision a winemaker makes at this point, from how hard to press, to how much skin contact to allow, to what adjustments to make, shapes what the wine will eventually become in the glass.