In the wine making process the crushing and de-stemming process releases the “free run” juice from the grape. While this is top notch quality juice there is still quite a bit of juice remaining in the grapes. This is where pressing comes in. After all, more juice means more wine!
Red wines are nearly always made from both free run juice and pressed juice. White wines on the other hand are not always pressed. The very best white wines are made from only the free run juice.
Historically pressing grapes was done by hand or by people stomping grapes. This is not a very sanitary way to go about making wine! Imagine drinking a Zinfandel with an athlete’s foot aftertaste.
Today most wineries use a mechanical press of some sort. These come in two basic varieties, batch presses and continuous presses.
Two Types of Presses
A batch press can handle up to 1-5 metric tonnes per hour with an appropriate team of winemakers on the job. By in large this is the type of press used by amateur wine makers as well as small to medium sized wineries. Grapes are loaded in one “batch” at a time. Different presses can handle different quantities of grapes.
Continuous presses on the other hand are generally motorized and fed a continuous stream of grapes. Because of this automation continuous presses can press up to 100 metric tons per hour. As you might have guessed a continuous press is geared more for a factory winery pressing thousands of tons of grapes at harvest.
For now let’s concentrate on batch presses.
Two Types of Batch Presses
The earliest known mechanical press is the basket press. Still used today at the amateur and professional level this iconic piece of machinery is still a reliable way to go.
A basket press has a ring of vertical staves with gaps between them where the pressed juice pours forth. 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.
When using a basket press wine makers will often add rice hulls to the layers of grapes. These hulls are inert and do not impart any flavor into your wine. What they do is pierce the skins to a) release more tannins and color from the skins and b) provide a path for the pressed juice to flow. If you don’t use the rice hulls the juice will flow very slowly and you’ll leave a lot behind.
Also known as pneumatic presses these are quite common in small to medium sized wineries. These presses have either vertical staves as on the basket press or a cylindrical piece of sheet metal with holes in it for the pressed juice to flow through.
The difference is the pressing mechanism. In the middle of the press is a rubber bladder than is filled with either water or air. As it expands from the center of the press the grape skins are pushed up against the outter ring.
Because air is compressible this is a more gentle way to press. Water, being incompressible, is used to apply more pressure to the skins.
Rice hulls are not used with bladder presses as they are likely to pierce the bladder.
Things to Consider when Pressing
1. How much pressure should you apply?
This wasn’t an issue when grapes were stomped. However, with mechanical presses we have the ability to apply so much pressure that we can rupture the seeds and introduce some harsh tannins and green plant tastes to the wine.
Apply too much pressure and you’ll have funky wine. Apply too little pressure and you’ll be leaving juice behind. This is where experience comes into play. The only way to get experience to to dive in and see what happens.
Here’s a trick. Press your skins up to the point where you feel you’ve extracted most of the juice. Remove that container of juice and get an empty container and slowly keep pressing. This way if you do end up going too far you will not have ruined all of your juice, maybe just the last tail end.
2. How much press juice versus free run juice do you want?
As we’ve talked about already pressed juice will taste different than free run juice. A wine maker might combine 90% free run juice with 10% pressed juice. A this ratio the bulk of your wine is made from premium materials. The 10% pressed juice adds color, tannins, and complexity to the wine.
Some wineries go as far as producing two different “grades” of wine. The free run juice goes into the top shelf wine while the pressed juice is made into a “table” wine. Still others will just combine it all.
This is part of the artistry of wine making. Despite all the science there’s still a lot of art to this process. This is what makes wine making so incredible!
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Photo by: Klearchos Kapoutsis (Flickr: The Wine of Akrotiri) [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons