Page 393 - Biosystems Engineering

370 Cha pte r T h i r tee n

the oil content of seeds have continued to appear as alternative meth-
3
ods, such as pulsed nuclear magnetic resonance and near-infrared
spectroscopy, but solvent extraction remains the most commonly
used method.

13.2.1 Extracting Oil from Seeds
The oil seed is first freed from impurities by screening and winnow-
ing. Once completed the following processes are adopted.

Decortications
In this process, the seed kernels are separated from the hulls. Decorti-
cations are done by passing the seeds between rollers with sharp edges
or grooves to break the shells without damaging the kernel inside.

Milling
To break down the oil-containing cell, the kernel is crushed. The
crushing machine consists of a number of chilled iron rolls placed one
above the other. Seeds pass through successive rollers under increased
pressure, crushing the kernel.
Cooking
The crushed kernel is placed in a steam-jacketed pan fitted with a
stirrer and heated to about 80°C by live steam to coagulate the albu-
minous matter and to lower the viscosity of the oil.
Extraction
Oil is extracted by the following processes.
Pressing There are two types of pressing machines, the open press
and the enclosed or cage press.
In the open-type press, oil-seed meal is fed into bags of filter cloth
resting between horizontal plates; the oil escapes through the hori-
zontal plates. However, a cage press consists of a barrel-shaped box
with perforated walls where the meal is fed directly. Pressure is
applied in two stages by a hydraulic press. The oil content of the cake
is reduced to 5 to 6 percent. The oil escapes through the perforation,
whereas solid matter is retained in the box.

Expelling The oil screw press carries out expeller extraction of oil
continuously. The press consists of a warm shaft in a perforated cyl-
inder. The oil meal thus receives a steadily increasing pressure as the
revolution of the shaft pushes it forward against the small outlet at
the end of the cylinder. Oil is discharged through the perforations and
the cake through the cylinder outlet. It can be used for any type of
seed but is mostly used for groundnut and tung seeds. Two types of
expellers—a low-capacity hand-operated oil expeller and a high-
capacity expeller—are shown in Figs. 13.1 and 13.2.