( 3 ) 
It is advisable therefore, to describe these two 
classes separately, since the methods of extraction 
and the commercial uses are so distinct. 
Fixed Oils. Occurrence and Extraction . — Fixed 
oils are found chiefly in the seeds of 
plants where, together with starch and proteins, 
they are available as food which would be required 
by the seed if allowed to germinate. The prepara- 
tion of the oil therefore consists in an extraction 
from the seed, and is carried out by one of two 
methods, either by a mechanical process of sub- 
mitting the seed to high pressure in a press or by 
treating the seed with chemical solvents which will 
dissolve only the oil. 
When pressing oil seeds or nuts, the general method 
at present in use is as follows: the seeds or nuts 
are cleaned and decorticated if necessary, and the 
kernels converted into a pulpy meal by being passed 
between a series of rolls. 
If the oil is to be used for edible purposes, the meal 
is then expressed in the cold, such oil dissolving 
the smallest amount of extraneous impurities from 
the seed. By pressing in the cold, only a portion 
.of .the oil can be recovered, so the cakes are dis- 
integrated and the meal heated and pressed again. 
If the material is particularly rich in oil, or if the 
oil is viscous, this process may have to be repeated 
and a third expression carried out. 
It will be seen that it is only possible with certain 
seeds, for example castor oil, groundnut and gin- 
gelly, to employ the cold pressing method; this 
would not be possible where the oil is solid at the 
ordinary temperature, as for example with palm 
03 
- 4 *> / £>1 
