In the beginning, people did not use presses to extract the must from grapes that had just been crushed underfoot. This means that the only must they could ferment was the must that drained off by gravity after the grapes were trodden.
It was the Egyptians, among other peoples, who invented the first press to increase the amount of liquid that could be separated from the solid parts of the grape. This first press was very simple: it consisted of a linen cloth that was filled with the crushed grapes, closed, and then twisted from one or both ends.
This pressing system changed very little over the following centuries and was also used, with some variations, by the Greeks and Romans. These devices then evolved toward the use of levers to increase the force per unit of area, that is, the pressure. This is how beam presses and vertical wedge presses came about.
Presses work on the basis of Pascal’s principle: “Pressure applied at any point of a confined liquid is transmitted with equal intensity in all directions.” From the basic formula that defines pressure:
P=F/A
and based on Pascal’s principle, it follows that:
F1/A1=F2/A2
General aims of pressing
The main aim of pressing is to separate the solid parts, made up of the grape skins, seeds, leaves and pieces of stem, from the liquid. It is used both for pressing white grapes before alcoholic fermentation and for pressing the pomace from tanks or vessels of already fermented grapes, as in the case of red wines.
With white varieties, the grapes can be whole-cluster pressed, or they can be destemmed and/or crushed first, depending on the winemaking approach of the person in charge and the type of wine to be made.
When the grapes have been crushed beforehand, the press can be used much more efficiently: since the berries are broken, they begin to release the must they contain, which drains out of the press and makes room for more grapes. In some types of press, such as batch presses, the volume of free-run juice released by gravity while the press is being loaded can reach about 50% to 60% of the total liquid in the grapes when pressing fresh fruit. The actual figure depends on the grape variety, the degree of crushing, the use of enzymes, the ripeness of the grapes, and so on.
How the berry responds to pressure
According to a study carried out by Humeau in 1975, a grape berry can burst under a pressure of 0.8 kg/cm2, and the main components of the grape can be extracted at pressures of up to 6 kg/cm2. At pressures between 6 and 10 kg/cm2, components from the skin cells begin to be extracted, and at pressures above 10 kg/cm2, water begins to be extracted from the stem tissue if stems are present during pressing.