Olive presses has been evolving for over two thousand years, and arguments have raged for centuries about which crushers are best, which stirrers are best, and which separators are best. But there is no room for argument about the three stages of pressing.
Good pressing requires finesse as well as technique. It is not just a matter of removing leaves and olives, or keeping the temperature right. It is also a matter of judging when the olives are optimally malaxed, how much pressure to put the paste under, and how much oil to draw out of the paste.
The process of drawing the oil out of olives is all in the pressing. Pressing olives involves three pressing processes; crushing, stirring, and separating.
Crush: the olives must be crushed. For centuries olives were crushed under giant stone rollers, and today’s they are mostly ground in stainless steel hammer mills. Crushing makes it possible to draw out the oil while the olives are whole.
Malaxing: the crushed olives must be stirred, a process called ‘malaxing’. For centuries, the olives were malaxed by letting them slosh in the pit with the grindstones rolling around. Today’s way is to stir the olive paste is with rotating stainless steel blades in stainless steel tubs. Malaxing is an essential part of pressing, because most of the oil in olives is held in microscopic sacks called vacuoles, and the oil cannot get free until the skins of the vacuoles have been breached. The temperature should be in the mid-twenties Celsius – not so cold that the vacuoles cannot release the oil, not so hot that the oil loses its quality.
Pressure to Separate: the crushed and stirred olive paste must be put under pressure to separate the oil from the rest of the olive. For centuries, pressure was applied by squeezing the paste between two wooden plates, and some processors still regard this as the best method, although the plates are now steel instead of wood. The more common method today is to press out the oil is with centrifugal force. Whatever the method, pressure is an essential to let the oil run free.