A wine press works by applying force to crushed grapes, squeezing juice out through small openings while holding back the skins, seeds, and pulp. The basic principle hasn’t changed in thousands of years: contain the grapes, apply pressure, and collect the liquid that flows out. What has changed is how that pressure gets applied, how precisely it’s controlled, and how much it affects the flavor of the finished wine.
The Basic Mechanics
Every wine press has two essential components: a container that holds the grapes and a mechanism that compresses them. The container has gaps or perforations that act as a filter. As pressure builds, juice flows through those openings while the solid grape material (called pomace) stays behind. The juice is collected in a pan or trough underneath and pumped into tanks or barrels for fermentation.
Before any mechanical pressure is applied, a significant amount of juice flows out on its own from the sheer weight of the grapes piled together. This is called “free-run” juice, and winemakers prize it. Free-run juice is softer, rounder, and more concentrated than juice extracted under pressure. It carries less tannin and lower astringency, which makes it especially valuable for crafting smooth, balanced red wines. Press juice, by contrast, tends to be more tannic and rustic, with less body. Winemakers often keep these two fractions separate and decide later whether to blend them or use them for different wines.
Basket Press: The Traditional Design
The oldest style still in use is the basket press, also called a ratchet press. It consists of a ring of vertical hardwood staves with narrow gaps between them, forming an open-sided cylinder or “cage.” You load grapes in through the top, then lower a wooden plate down over them. A ratchet mechanism slowly forces the plate downward, compressing the grapes. Juice flows out through the gaps between the staves and collects in a basin below.
To squeeze out as much juice as possible, wooden blocks are stacked between the ratchet and the plate as the grapes compress further. This lets the plate travel lower without interfering with the tops of the staves. Some winemakers also mix in rice hulls with the grape mass, which create tiny channels for juice to flow through more quickly. Without those channels, juice can get trapped in the compressed pulp and be reabsorbed before it escapes.
Basket presses are slow and labor-intensive. They require manual loading, pressing, breaking apart the compressed cake of pomace, and pressing again. But many premium wineries still prefer them because the gentle, gradual pressure tends to produce cleaner, more refined juice.
Pneumatic Membrane Press: The Modern Standard
Most commercial wineries today use a pneumatic membrane press, which works on an entirely different principle. Instead of pushing grapes downward, it pushes them outward. The press is a horizontal drum with perforated walls. Inside the drum, a thick rubber bladder runs along one side. Grapes are loaded into the space between the bladder and the perforated shell.
When the press cycle begins, compressed air inflates the bladder. As it expands, it pushes the grapes outward against the perforated metal walls. Juice flows through the tiny holes and collects below, while the skins and seeds stay inside. The press follows a programmed routine: it inflates to apply pressure, deflates to rest, then rotates the drum to break up the compressed grape cake. This rotation redistributes the pomace so that trapped pockets of juice get exposed in the next press cycle. The sequence repeats at gradually increasing pressures until the desired amount of juice is extracted.
Pneumatic presses offer several practical advantages over basket presses. They’re easier to fill and empty. The enclosed drum lets winemakers control temperature and limit the juice’s exposure to air, which matters for delicate white wines that can oxidize quickly. Some winemakers even inflate the bladder with air instead of water when pressing grapes for mild or sweet whites, since air is more compressible and exerts softer pressure on the skins. This extracts less tannin, though it also yields less total juice.
How Pressure Affects Flavor
The amount of pressure applied during pressing directly shapes the wine’s character. Grape skins contain roughly 0.5 to 1.0 milligrams of tannin per berry, while seeds contain 3.0 to 5.0 milligrams. Low pressure extracts mainly juice with minimal skin and seed compounds. As pressure increases, more tannins, color pigments, and bitter flavors get pulled into the liquid.
Commercial membrane presses typically max out at about 2 bar (29 psi). For white wines, many winemakers press to just 1 bar (14.5 psi) to keep the juice clean and delicate. For reds, they’ll press in two or three stages up to the full 2 bar, keeping each fraction separate. The first fraction is lighter and more elegant. The last fraction is darker, more tannic, and more intense. The winemaker tastes each one and decides how much of the later, harder pressings to blend back in. Sometimes none of it makes the cut, and the press wine gets used for a less expensive label instead.
White Wine vs. Red Wine Pressing
Pressing happens at completely different stages depending on whether you’re making white or red wine, and this distinction matters more than the type of press used.
For white wine, pressing happens immediately after the grapes are crushed, before fermentation. The goal is to separate the juice from the skins as quickly as possible to avoid extracting color and tannin. The fresh juice, now clear and free of solids, goes straight to a tank where fermentation begins.
For red wine, the process is reversed. Crushed grapes ferment together with their skins for days or even weeks. This extended contact, called maceration, is what gives red wine its color, tannin structure, and depth. Only after fermentation is complete does pressing happen. First, the winemaker drains the tank by gravity, collecting the free-run wine. Then the remaining soggy mass of skins and seeds gets scooped out and loaded into the press. The press wine that comes out is richer in color and tannins than the free-run, and the winemaker blends the two in whatever ratio achieves the style they’re after.
Smart Presses and Automated Cycles
The latest generation of pneumatic presses uses sensors to make real-time decisions during the cycle. Sensors monitor juice flow rate, internal pressure, and tank volume. If juice is flowing freely on its own, the press avoids unnecessary rotation to preserve clarity. If the flow slows, it triggers a targeted rotation to break up juice pockets without excessive agitation.
On some models, the operator simply sets a “quality” level and a “dryness” target on a touchscreen. The press automatically adjusts its pressing intensity and rest intervals, then stops when the pomace reaches the target dryness. This takes much of the guesswork out of a process that previously required experienced operators to monitor by hand. It also makes results more consistent from batch to batch, which matters when a winery processes hundreds of tons of grapes during a harvest that may last only a few weeks.
Cleaning Between Uses
Because a press handles fresh organic material, hygiene is critical. Every press needs to be cleaned immediately after each use. The process starts with removing all visible debris, including skins, seeds, stems, and pulp. A rinse with non-chlorinated water comes first, followed by a cleaning agent to break down residue, then another water rinse. After cleaning, the press is sanitized to kill roughly 99% of remaining microbes. A final rinse with chlorine-free water or a citric acid solution removes the sanitizer itself. Skipping any of these steps risks contaminating the next batch with bacteria or wild yeast that can spoil a wine entirely.