Classification of presses
Presses can be classified in several ways. The most common criteria are:
- The position of the cylinder or screen that holds the grapes or pomace to be pressed
- The type of energy applied to the pressing element in contact with the grapes
- The type of pressing element in contact with the grapes
- The mode of operation
Batch presses, the most widely used today, work in repeated cycles that can be summarized as follows:
- Loading the press
- Pressing
- Unloading the press
- Cleaning
Continuous presses do not follow this cycle, since while they are running they are loading, pressing and unloading all at once.
Batch presses
Vertical basket presses
These presses, used since ancient times and still used today by small wineries, are operated manually or hydraulically and produce high-quality pressings. As the photo alongside shows, they consist of a set of wooden staves or stainless steel profiles, thick enough to withstand the working pressures, arranged in a circle with gaps between them and held together by metal hoops. The crushed or whole grapes are placed inside this circle, and then the lid or piston, made of wood or metal, is lowered by hand with the help of a screw, pressing the grapes against the walls and releasing their juice.
These presses work at pressures that usually do not exceed 3 bar, and the liquid flows out perpendicular to the direction of the pressure applied. One of the important tasks when using this equipment is breaking up the press cake. Between each pressing, the mass being pressed is broken up and rearranged, by hand or with tools. This allows almost all of the liquid to be extracted from the pressed mass without having to apply excessive pressure.
When the press cake is not broken up, the outer layer of skins in direct contact with the solid parts of the press forms a barrier that liquid cannot pass through, forcing higher pressures to be applied. This produces cleaner liquid, but quality suffers from the excessive pressure, which can even damage some plant tissues and release substances with unpleasant sensory qualities into the must or wine.
Advantages of vertical presses
- The must or wine is usually forced through a thick layer of skins, which filters the liquid passing through it and produces clearer juice or wine.
- The working pressure used when the cake is broken up yields good-quality musts or wines, although they are cloudier because breaking up the cake disrupts the filtering layer of skins. This does not necessarily mean lower quality, since it can be solved by gravity settling or with fining agents.
Disadvantages of vertical presses
- They work in batches, so pressing cycles tend to be quite long. This must be taken into account when the quantities of grapes or pomace to be processed far exceed the capacity of the winery’s presses. One way to solve this problem without losing the benefits of this type of equipment is to install a large enough battery of presses.
Horizontal pneumatic presses
This type of press consists of a rotating horizontal drum with an air bladder made of high-strength rubber running along its entire length inside. The grapes or pomace are loaded through an opening in the middle of the cylinder. Once the cylinder is loaded, compressed air is pumped into the bladder, which expands and presses the grapes against the walls of the cylinder, extracting the juice or wine from the solids. After a set period, the air is released and the bladder deflates, freeing the grapes. The drum is then rotated with the grapes inside so that they rearrange themselves, and pressing starts again at progressively higher air pressures. A full pressing cycle in this type of press takes about 3 hours.
Continuous presses
Unlike the previous types, these presses have the advantage of working continuously: feeding, pressing and unloading all happen at the same time, making them much more effective for pressing large quantities of grapes. However, their disadvantage is that they treat the grapes far too harshly, badly damaging the plant tissue and releasing a range of undesirable substances into the wine.