The Grape Pressing Process

What comes to mind when you think of grapes being pressed for wine? The scenery is that of rolling hills, planted in orderly rows of uniformly sized vines. A charming European farmhouse is positioned somewhere in the distance. There is a large, shallow, wooden pool, filled to the top with lush grape clusters. A group of laughing women are dancing barefooted atop the berries; their toes are stained red with juice, as they tamp down the fullness of the grapes. Archaic? Very. Unsanitary? Unimaginably.

So what’s the next step? Technology has realized a more advanced status. Though the screw press, or basket press, has been in recorded use since 2 AD, it is still widely used in professional scale vineyards and home winemaking. The next step in winemaking came with the onset of the bladder or membrane presses.

Basket or Screw Press

Before we get to the mechanics of the membrane press, we must first explore the traditional basket press. A deep barrel is filled with grapes, with the sides of the barrel slatted like a basket to allow for juice to seep out and be collected in a vat below.

The bottom of the barrel has spouts to release the juices and there is a huge revolving handle at the top, or 2 separate ones on the sides. Inside, there is a large Archimedes screw, which moves a platform down, crushing and pressing the berries. The juice then pours out of the spigots and pools underneath into a waiting container.

Depending on the grape, this process is either performed before or after fermentation. Maceration of the grapes takes place to allow for free run juices to flow. Here’s where the grape pressing is key. White wines are pressed immediately after maceration. Grape pressing removes all stems, skins and seeds from the juice, which is then fermented, free of interference from tannins leaching into the white of the wine.

The process for red wines is a little different. Maceration occurs, and you are left with the “must”. Must is the pulp of the red grape, and contains the solids that are immediately strained out, or pressed, from white wines. Red wine is left in contact with the must until the end, or nearly the end, of the fermentation process. This gives the wine its color, its body and the tannins also control how dry the wine is. The screw press crushes the grapes, often bruising them in the process of the juice extraction. Most times, multiple presses are required to completely filter out pulp, which is a time consuming process.

Bladder or Membrane Press

Many modern vineyards are employing the use of bladder presses to create what is widely believed to be a more delicate juicing operation in a much more time effective process.

There are two basic kinds of pneumatic presses, with the biggest difference mechanically falling in the placement of the bladder. In some cases, a large vat has a series of strainers on one side and a large empty bladder on the other. Once filled with grapes, the bladder begins taking in air. It inflates and pushes the juice through 180° of straining area.

In other cases, membranes, or bladders, are mounted to a central shaft. The membrane inflates, filling the empty cavity with air and gently pushing the berries through strainers located along all the sides of the drum. For this press, a full 360° of grape pressing is achieved.

Though both of the more modern presses ensure for a shorter grape pressing time, the latter reportedly cuts the process length in half by doubling the straining surface. The air bladder methods are generally regarded as producing the best, most high-quality juice…at least for white wines anyway.

Why is that? Gentle caressing of the grapes by a gradual hardening of the membrane? Juices seem to be filtered more than adequately. An air bladder’s preferential use for white wines is often because of the inclusion of stems, which it cleans out most efficiently.

For red wine grapes with larger berries and thinner skins, many winemakers find that berries sometimes remain wholly intact through pressing with a membrane. The machine sometimes leaves entire pockets of grapes untouched by the operation, leaving vintners frustrated with having to press again. The process is supposed to cut down on grape pressing time, and this is not always the case.

New technology is great, and innovation is what keeps any endeavor interesting beyond a certain point. That being said, winemakers are still, on a large scale, keeping to a process developed over 2000 years ago, for modern day wine production.