One of the frequent causes of difficulty in extracting oil is the excess of moisture in the seed. It should always be borne in mind that moisture and heat are both necessary in the cooking kettle. The dam-ping of the material is best effected by adding hot moisture or open steam by means of damping sprays fitted in the heating kettle with suitable regulating devices, so that in the same could be controlled according to the needs.
If the seeds initially contain high percentage of moisture, the process of cooking will not be good. Therefore care should be taken to see that the seeds pressed do not contain moisture above a certain limit, if best results are to be obtained. With seeds having high moisture contents it will not be possible to add the necessary cooking and tempering moisture, in this cases the cooking suffers and all oil can-not be extracted. Excess of moisture in the cooked meal reacts very unfavorably in case of expellers, so even if the meal is dry when fed into the kettle, too great a quantity of moisture should not be added.
Moisture in oil seed is not pressed with the oil in the expelling and contrary to expectations. It is the moisture and not the oil which forms the lubricant for the material sliding through the expeller pressing cage. It is the ability of the material to resist being pushed through the expeller pressing cage on account of friction with the lining, which can be applied to it and it will be seen that an excess of moisture will so reduce the resistance that the material will slide too freely through the pressing cage and thus reduce the pressure which can be applied, resulting too much oil being left in the cake.
On the other hand, if the cooked material is too dry when fed to the pressing cage, the friction with the lining maybe so great that the material any even refuse to slide along the cage, in this cage the cage and pressing worms will choke up with hard pressed meal, causing machine to stop. Breaking of the pushing portions of the knife bars may result from running the expeller on cooked meal which is too dry.
The moisture in the cooked material must be closely regulated and the material should contain just sufficient moisture for lubrication purposes, which will allow it to be pressed through the cage and at the same time maintain friction with the lining of the cage.
Cooked meal which is too dry will not form a proper cake but break into small pieces on leaving the discharge end of the expeller Excessive moisture on the other hand produces a soft cake, even through the cake is the, and if the cake leaves the discharge cone in a stringy torn condition, or shows a tendency to twist with the cone, it indicates that the cooked meal is too high in moisture. When the meal, however, is in correct condition for pressing as far as moisture is concerned, the cake should emerge all round the cone in large pieces and should be tough enough, when first made to resist being pulled apart very easily.
Sometimes due to not proper cooking the oil is in emulsion form or hazy in nature due to presence of some phosphatides, fine foots and some surface active agents. It has also been found by experience that damaged seeds give lower yield of oil than undamaged seeds of equivalent oil content.
