This is the most important step in the extraction of oil by expeller method. The cracked seeds are properly cooked with open steam. Following are the main objects of cooking:-

• To regulate moisture content of the seed meal.
• To coagulate the proteins in the walls of the fat containing cells.
• To make the cell walls permeable to the flow of oil.

The oil droplets are almost ultramicroscopic in size and are distributed throughout the seed. One effect of cooking is to cause these very small droplets to unite into drops large enough to flow from the seed. Second effect of cooking is drying of the seed to give the seed mass the proper plasticity for efficient pressing. Third effect of cooking is to decrease the affinity of the oil for the solid surface of the seed, so that the best possible yield of oil may be obtained when the seeds are subsequently pressed. Other important effect of cooking are insolubilization of phosphatides and other undesirable impurities, destruction of molds and bacteria, increase of fluidity of the oil through increase in temperature and in case of cotton seed, detoxification of gossypol or related substances. Very dry seeds cannot be expressed efficiently. Cooking operation regulates the moisture. The optimum moisture of cooked seeds varies widely accordingly to the variety of the seed and the method to be used for expression.

During the process, the cracked seed is cooked in steam kettles made right over the expeller machine. The temperature of the seed is raised and the moisture of the seed is raised from 3-5% to 14-15%. Thereafter the moisture is again be brought down to about 5% drying the cooked seed in a steam jacketed type kettle. Normally the real retention time in expeller kettle for cooking/drying operations is considered 40-45 minutes for groundnuts. However, this may vary little from seed to seed. There are, however no hard and fast rules for the treatment of these seeds, and the operation is purely a matter of experience. The theory involved in the process of cooking is that the combined heat and moisture break up the oil cells, soften their gelatinous coatings, increase the celerity of the oil and coagulate the  lbuminous sediment forming practical’s in the seed. Thus making it readily, possible for the oil to flow out easily when subjected to pressure.