Pressing means expelling oil from an oily material by mechanical pressure. Obtaining oil is affected by several factors acting on the relationship of oil with the solid particle of the oilseeds, mainly the water content, the composition of oilseeds, and the oilseed treatment process before pressing. Treatment depends on the type of seed and the pressing method used. Pressing can be low-pressure or high-pressure. The most common method is use of a screw press.
- In low-pressure pressing /pre-press/ the degree of grinding is increased and the level of conditioning is decreased. Oilcakes from the pre-press contain 16-20% fat and pressing is carried out at a pressure of 5-16 MPa.
- In high-pressure pressing /additional press/ the fat content of the cakes can go as low as 5-7% and pressing takes place at a pressure of 40 MPa.
- In direct one-step compression, the pressure is increased to 100 MPa. Oilcakes then contain 13-14% fat.
Pressing can be divided into two basic part according to the seed pre-treatment method, as follows: