Manufacture of edible oil: Screw press method

Extraction of fat from different sources like oilseeds, nuts, oil containing fruits, animal fat are different and use different techniques.

  1. Animal fat is extracted by rendering technique that uses heating (moist/dry) fatty tissue.
  2. Oil from oilseeds and nuts is extracted by using mechanical expellers (screw press) that can extract 90 – 95 % oil in seed.
  3. Oil from oil cakes and bran is extracted by solvent extraction using hexane as solvent.
  4. Oil from flesh of fruits is extracted by using hot water extraction technique where flesh of fruit is macerated in hot water and extracted oil can be recovered by settling or centrifugation method.

Extraction of oil by screw press method:

This process is suitable for oilseed with more than 25 % oil content.  The oilseed undergoes cleaning (removal of dust, dirt, metal pieces, and foreign particles) and decortication (removal of seed coat) and size reduction (for large seeds like soybeans) and then processed for conditioning. During conditioning, oil seeds are treated with steam to rupture the oil bearing cells. Thus prepared material is fed from the hoper in screw press. As the worm rotates, oil is pressed and conveyed simultaneously. The oil comes out through the screen of cage while cake leaves through narrow concentric opening at the end of worm shaft. Crude oil is rested for days to settle the mucilage and moisture and finally strained with muslin cloth or filter press.

During pressing, greater range of oil is extracted in plugging section as there is greater pressure and heat and oil seed granules have become fine. Extraction of seed oil is less in ramming section and least in feeding section. In general, we can say that as feed traverse through screws in screw press, the pressing advances and oil extraction becomes more. Eventually, after major of the oil are extracted, cakes are used as feed in solvent extraction process. Screw press cycle is carried out for 4 – 5 times until oil remained in cake is less than 5 %.

Material containing low amount of oil such as oilseed cake (meal) and rice bran are suitable for solvent extraction. Hexane is most widely used solvent and this process can extract oil form feed until oil present in feed is reduced to 0.5 % or less.

Perforated basket attached in conveyor belts holds the feed. Fresh warm solvent is sprayed from the top. Hexane is used as solvent whose boiling point is nearly 60°C. The basket moves up carrying the feed which is extracted through screen at the bottom of basket. The residue is treated with fresh solvent and finally defatted cake is obtained. The half miscella is recirculated through pump and again comes in contact with solvent. The full miscella is evaporated, refined, bleached and deodorized to obtain refined bleached and deodorized (RBD) oil.


About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.