Weather Conditions for Expelling
Role of Moisture
One of the frequent causes of difficulty in extraction 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 damping 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 devise, so that 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 case the cooing suffers and all oil cannot be extracted. Excess of moisture in the cooked meal reacts very unfavourably 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. It is ability of the material to resist being pushed through the expeller pressing cage on account of friction with the lining, which to a large extent determines the amount of pressure 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 may even refuse to slide along the cage, in this case 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 from a proper cake but break in to 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 thin, 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 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 phosphatised, 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.
Role of Temperature
Excessive heat sometimes has the effect of causing oil and cake to become dark in colour. There are many other factors which affects the above change and cooking temperature alone may not be taken as the sole cause for it. Until the behaviour of the different oil seeds have been noted it is till then, not advisable to deduce the cooking temperatures at the expense of proper and sufficient cooking of the material. Since different oil seeds behave differently towards heating conditions, it is not advisable to find out by experience the most suitable temperature at which to press the meal to produce oil and cake with suitable colour.
As a general rule it may be taken that proper yields cannot be in any case expected with cooked meal temperature below 75°C, whereas for most of the oil seeds the temperature of the meal should be between 90°C-95°C.
The thoroughness of the heating can be quite closely estimated by taking a sample of the seed meal from the cooking kettle in the bare hand. The temperature should be as high as to prohibit more than a momentary handling. When firmly compressed in the hand. The meal should from a compact ball like mass not crumbling and oil should ooze out freely between the fingers.
The cooking of oil seeds is generally done in heating kettles or tempering trough. The heating kettles are normally round shaped cylindrical vessels, steam jacketed either at its base and sides or at its base only. Various types of kettles are available in the market. These kettles are equipped with agitators and open steam admission arrangements. The kettles may be from one high to five high depending on the seed and design. Another device employed for cooking the meal is called ‘Tempering Trough’ and consists of a steam jacketed conveyor fitted with a small feed container tank. The conveyor may be either single staged or double staged, in the latter case two conveyors are placed one above the other, with suitable discharge ends, and driving arrangements. In this the meal takes longer time to travel and is thus cooked better than in a single staged kettle.
Open steam is supplied to the meal at the entrance by means of a steam nozzle. Thus atomised steam get well mixed up with the meal to be heated for giving the requisite moisture content .The slow forward movement of the meal effect a thorough cooking.
If the seed is cooked carefully at critical temperature and critical moisture required for the seed, the efficiency of the expeller can increase by 50% or so.
Importance of Fibrous Materials
Apart from heat and moisture, there is yet another very important factor, which contributes towards a high percentage of oil being left in the cake and that is fibrous material.
Seeds or meals, deficient in fibre, prove very soft and smooth for pressing cage. They offer very little resistance and readily slip through the cage. Thus it may not be possible to exert sufficient pressure up on them as to secure the best yield of oil. Such meals have a tendency to readily escape through the drainage spaces between the lining bars of the cage and the pressure as soon as it is exerted is lost. On account of the meal being soft and smooth, the pressing worms are unable to get a popper grip upon such materials, and the meal is often worked up to a paste which will not move along the cage and no oil can be obtained from them.
It is necessary that the materials which are to be pressed in the expeller should contain a proportion of fibrous matter or ‘binder’ which will get up resistance to pressing and which will bind the material together to enable it to be pressed
The skin of such seed as linseed, rape sesame undecorticated cotton seed etc. From a good binder. But when such seeds like cotton seeds are decorticated and the meats are very cleanly separated from the husk, those meals become too soft to set up any resistance in the expeller and the come too soft to set up any resistance in the expeller and the best yield of oil are difficult to be secured. When processing of such decorticated cotton seeds it is decidedly advantageous to allow a portion of the husk to go with the meals to the expeller From 5-8% of husk with meats will have negligible effects or the quality of the oil or cake, but the expression of the oil will greatly improve, since the presence of the husk impart to the meal good binding properties.