Pressing 101

In the winemaking process, juice is typically extracted from the grape in two stages. The first is during crushing and destemming, which is called “Free Fun” juice (see the Two Birds article, also in the month’s newsletter). Free run juice contains significantly less tannin, and therefore lower acid levels, than pressed juice. While it is considered to be the higher quality of the two, it leaves a substantial amount of juice in the grape to still be released. This is where pressing comes in.
Because the juice that is extracted through pressing has more tannin, it’s often used in red wines (in conjunction with the free run juice) to add complexity and flavor. There are a number of white wines, though, that do not contain any pressed-juice.
For about 7,000 years, wine was made from grapes that were pressed in large vats by bare feet. Thankfully, sometime during the first century A.D., the Romans got tired of their wine tasting like whatever they walked through in their open-toe Birkenstocks and thus, invented the ratchet press, which has remained relatively unchanged for almost 2,000 years.
There are two types of presses used in modern winemaking- Continuous and Batch Presses. Continuous Presses are motorized and are generally reserved for large-scale wine production, where thousands of tons of grapes are continually fed into the press. Batch Presses work, just as their name suggests, by pressing one batch at a time, and range in capacity from less than 1 Gallon to more than 100 Gallons. The two types of Batch Presses most commonly used are Ratchet Presses and Bladder Presses (Mini Presses, which have capacities of 3 Gallons or less, are typically used to press pre-crushed and/or pressed, soft fruits (NO NUTS!) in small batches yielding a couple gallons or less).
The earliest known mechanical press is the Ratchet Press (also called a “Basket Press”). Developed over 2,000 years ago, this iconic piece of machinery has remained relatively unchanged and is still used by many home winemakers and professionals due to its reliability and functionality. A ratchet press has a ring of vertical hardwood staves with gaps between them, known as the “cage”. Grapes are loaded in the top, then a wooden plate is lowered down over the grapes and a ratchet is used to slowly apply pressure to the grapes, forcing the grape juice to flow from the gaps between the staves. In order to expel as much juice as possible from the grapes, wooden blocks are placed between the ratchet and the plate, enabling it to be forced as low as possible without the tops of the staves with the ratchet.
When using a basket press winemakers will often add rice hulls to the layers of grapes. These hulls are inert and do not impart any flavor into your wine. The rice hulls pierce the skins of the grapes, which causes them to release more color and tannins into the wine. The rice hulls also provide a path for the pressed juice to run, enabling it to flow quickly from the pulp, preventing it from being re-absorbed and left behind in the remaining mass.
The Bladder Press (also called a “Hydro Press” or “Pneumatic Press”) are usually found in small to medium sized wineries. Unlike the ratchet press, where the grapes are pressed down and the juice flows through the individual slats from the bottom, the bladder press forces the grapes out against a cylindrical piece of sheet metal full of tiny holes (the “cage”) for juice to flow out of. At the center of the press is a thick rubber bladders that is secured and sealed at the top and bottom of the press. The bladder is filled with water, typically through a spigot on the bottom with an independent shut-off valve, forcing it to expand and press the grapes to edges of the press against the perforated sheet metal walls. When pressing grapes for a mild or sweet white wine, to reduce the amount of tannins released from the skins, winemakers will sometimes fill the bladder with air instead of water, as it is more compressible and softer on the grapes (but yields less juice).
The logic is sound- more pressing means more juice, more juice means more wine, and more wine means more happiness!... However, it actually is possible to “overpress” your grapes. Apply too much pressure and you run the risk of rupturing the grape’s seeds, adding undesirably harsh, pungent tannins that will cause your wine to taste “leafy”. Apply too little pressure, and you end up leaving juice behind. Knowing when you’ve pressed enough is something that will come with experience. Until you get to that point, the trick is to press the skins to the point where you think you’ve extracted the majority of the juice, remove the container of juice, then grab another empty container and slowly continue pressing. This way, you’ll avoid ruining the bulk of your juice if you end up pressing too much.
When it comes to how much of your wine should be made from Free Run juice and how much should be made from Pressed juice, there’s really no right or wrong ratio. Some winemakers will start with their Free Run, then add Pressed juice until they reach the desired color and complexity. Others will produce multiple “grades” of wine where the highest quality “top shelf” wine is made from 100% Free Run, the lowest quality “table wine” is made from 100% pressed, and the different grades in between are dependent upon the ratio of the two juices. And many will just combine the juices and just call it good.
When you break it down, winemaking really is a simple process: Crush Fruit → Ferment Juice → Drink Wine. But even the most basic of details, such as pressing grapes, inspires the most scientific minded and artistically gifted of us to turn this basic process into something unique and beautiful. This is the true intrigue of winemaking.