3.1 Pressing
Pressing is the most commonly used methodology. The cleaned, peeled and crushed
material is pressed in hydraulic presses. During cold pressing, the according primary
product is milled to mash. This occurs under gently mechanical pressure, at relative
low temperatures between 40 and 60 °C. With this method, a high quality and natural
finish oil occurs, which is the most valuable edible oil. Usually, the process is carried
out in several stages. The oilseed is first pressed without supply of heat. Then, after
cold pressing, the oil seed is heated and pressed for the second time. After pressing,
substances are present in the oil, which affect the colour, smell and taste of the oil
unfavourably [KRIST et al., 2008].
An advantage of hot pressing is that the oil is less viscous, proteins are clotting and
largely mucilages and gums are dropped out. Hot pressing is particularly important for
oils, which are more viscous such as SIO. By heat treatment, oils get less viscous and
thus, they can be separated easily from the solid material. Another advantage is that
the oil yield is increased by five to eight%, comparing with cold pressing [KRIST et al.,
2008].
Cold-pressed oils are not refined after pressing and so valuable materials are
preserved and degradation products, which must be removed again from hot-pressed
oils, cannot be found in cold-pressed oils [KRIST et al., 2008].
Disadvantages of cold pressing are that not fat products and residues of pesticides,
heavy metals and polycyclic hydrocarbons can still be present. These oils are not very
heat-stable because of their high content of polyunsaturated fatty acids and