Processing of oil seeds – Part 18: Extraction, clarification and refining of sunflower oil

By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

Oil is extracted from the seed by means of a pressing action or by the use of solvents or a combination of both. As a rule, solvent extraction only becomes cost-effective when the processing capacity increases above 200 tonnes of raw materials per day.

We thank the ARC Agricultural Engineering in South Africa who made the information available to the readers of ProAgri Zambia.

Pressing:

Cold pressed oil is of superior quality. Mills or screw presses are used to break down the cell walls and set the oil free. The pressure is applied for five to nine minutes. The oils from the seeds are squeezed by physical pressure using (1) a hydraulic press or (2) a screw press.

The hydraulic press consists of a perforated vertical metal cylinder. A pressure plate is lowered onto the product and the pressure is gradually increased. The liquid expelled flows through the perforations and is collected. The residual oil content in the press cake is approximately 3%.

The screw press consists of a horizontal metal cylinder into which a stainless-steel helical screw is fitted. The pitch of the screw flights decreases towards the discharge end of the cylinder, increasing the pressure as the product moves through the cylinder. The final section of the cylinder is perforated to allow the expelled liquid to be collected. The product is fed into the barrel at the wide end and is expelled without the oil at the other end. The pressure is approximately 300 atm. The cylinder may be water-cooled to ensure that no damage occurs to the expressed oil as a result of heat developed during the movement of the product through the cylinder. The residual oil content in the pressed cake is approximately 3%.

Solvent extraction:

The oil can also be extracted by dissolving it in a solvent. The oil is present in vacuoles in the cell walls of the seeds, and the cell walls must be softened for optimum recovery of the oil. Solvent extraction requires the seeds to be rolled between rollers to break cells and set the oil free. The solvent, usually hexane, is added at a ratio of 1:1 to 1:3. Extraction conditions vary, but typically times of 15 to 60 min and temperatures of 40 to 65 °C are used. Two types of extractors are used, either in combination with each other or in combination with one of the above pressing processes.

Percolation extractor: the solvent is pumped over a bed of the seeds and the solvent percolates down, leaving the cylinder through a perforated bottom of the cylinder.

Immersion extractor: the crushed seeds move through a pool of solvent. The seeds are propelled by a chain conveyer or a screw conveyor. The recovered solvent-oil mixture, ‘miscella’, is heated to recover the solvent for re-use during future extraction.

The extracted seeds / flakes are called “spent flake” or “cake”. The residual oil content in the cake is approximately 1%.

Clarification of sunflower oil

In the case of pressed oil, an oil settling and filtering process is required to remove suspended debris. The crude oil is pumped into settling tanks and allowed to stand for at least 48 hours for most of the impurities to settle. The clear oil is then drawn off and the remaining suspended solids in the oil may be removed by either filtration or centrifugation.

Refining of sunflower oil

Crude oils and fats, especially vegetable oils, contain various kinds of extraneous matter such as dirt, moisture, gums, waxes, carbohydrates, proteinaceous material, pigments, flavouring substances, trace metals, antioxidants (commonly called tocopherols or vitamin E), free fatty acids and other unwanted components. To remove most of these undesirable components and to achieve a pleasing colour and flavour, oils are refined. Crude oils normally need refining to render them fit for human consumption. Separate stages of degumming, neutralisation, bleaching and deodorisation or de-acidification (steam refining) is normally applied to crude oils before they reach the consumer.

Degumming (Desliming):

Some impurities such as proteins and phospholipids must be removed. The oil is heated to just below 100 °C and hot water (3 to 5%) or a weak salt/alkali solution (0,02 to 0,5%) is added. The oil mixture is agitated and all the impurities flocculate (sediment) after 1 to 60 min. The sediment is removed by allowing the mixture to settle and then draining the residue, or it may be separated from the oil by centrifugation.

Various organic and inorganic acids, such as aqueous citric and phosphoric acids, may be used to improve the efficiency of the degumming process. The phospholipids may react in the seed or in the oil to form calcium salts which do not hydrate, and which remain dissolved in the oil. They will interfere with processing such as de-acidification. An advantage of phosphoric or citric acid degumming is to ensure that these non-hydratable salts are rendered hydratable and can be removed together with other phospholipids.

Neutralisation:

The free fatty acids which may be present must be removed to reduce the risk of the development of rancid taste, due to the reaction of free fatty acids with oxygen. The oil is treated with an aqueous alkali (caustic soda or sodium carbonate [soda ash]). The aqueous alkali is added as a fine spray from above (sparging) and heated (75 to 98 °C). The fine droplets of aqueous alkali fall through the oil, reacts with the free fatty acids, and form soaps soluble in warm water. The soaps are separated from the oil by centrifugation or settling, where the oil mixture is washed with warm water to remove the soaps before drying under vacuum (optional).

The equipment used for neutralisation consists of mild steel vessels holding up to 25 tonnes of crude oil. The vessels have conical bottoms, mechanical stirrers, heating coils and means for spraying alkali into the oil. The bleaching process may be done in the same vessel; therefore, it is capable of being closed and evacuated for drying and bleaching. A continuous plant can also be used, which is based on continuous alkali and wash water addition, in conjunction with centrifugal separation. In continuous neutralisation, the time of contact of the oil with alkali is considerably shorter than is the case with batch processing.

Next month we shall look at bleaching, deodorisation, hydrogenation, winterisation and packaging of sunflower oil.

Published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.