By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

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 bland flavour, oils are refined. Crude oils normally need refining to render them fit for human consumption.

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

Separate stages of degumming, neutralisation, bleaching and deodorisation or deacidification (steam refining) are normally applied to crude oils before they reach the consumer.

Degumming (desliming):

Some impurities such as proteins, phospholipids, et cetera, 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 either allowing the mixture to settle, followed by draining of the residue, or it may be separated from the oil by centrifugation. Various organic and inorganic acids, for example 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, for example by deacidification. 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 that may be present must be removed to reduce the risk of the development of a 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).

Bleaching:

Bleaching removes colouring matter from the oils or fats. It is essential to bleach oil thoroughly before hydrogenation to improve oxidative stability of the oil. Activated fuller’s earth (0,1 to 3%) is added to the oil or it is sucked into the oil while it is agitated under vacuum (60 to 88°C) for 5 to 60 min. The colourants adhere to the fuller’s earth. The fuller’s earth is removed by filtration over a screen filter or plate-and-frame filter. The whole process is carried out in vertical vessels made of mild steel which hold up to 25 tonnes of oil. Each vessel is fitted with a mechanical stirrer, a means of heating and cooling, valves, sight glass, a vacuum gauge and a thermometer.

Deodorisation:

Deodorisation is one of the final processing steps in the preparation of an oil for edible purposes, such as a salad oil or cooking fat, or in the preparation of an oil stock for margarine. The volatile impurities must be removed by steam distillation under low pressure. The oil is heated to 180 – 280°C and steam is injected into the oil to drive the volatile flavours from the oil and collect them separately. The time of treatment varies according to the design of the plant and the temperature used, from 5 hours in low temperature batch processes to as little as 15 minutes in some continuous systems.

Batch deodorisation vessels consist of vertical steel tanks. A large vapour space is maintained above the liquid level to contain the oil during the violent agitation caused by steam injection. Provision for reducing the entrainment of oil in the escaping steam is made at the top of the deodoriser, where it is connected with the vacuum line. The heating of the oil to deodorisation temperature and its subsequent cooling are carried out under vacuum since it is important that the hot oil should not come into contact with atmospheric oxygen, which would lead to the production of new off-flavours.

A continuous deodorisation vessel consists of a vertical cylindrical shell with compartments, through which the oil flows downward to allow for the heating by steam, agitation, and holding the oil to ensure sufficient removal of all the volatile impurities. The deodorised oil can be filtered after deodorisation, a process known as ‘polishing’.

Physical (steam) refining/deacidification (optional):

In some cases, a crude oil may be of adequate quality and does not need any further processing. Physical or steam refining is a term used for the deacidification of degummed/phosphoric acid treated and/or light bleached oils by high temperature (200°C) steam distillation. It is the refining of oils to remove various undesirable components, while keeping triglycerides intact and with minimum losses of antioxidant tocopherols present in the oils.

The free fatty acids are removed from oil by steam distillation. The oil is treated with hot water (60 – 90°C) in a deodorisation vessel. The oil is heated to a temperature of 163 to 270°C under vacuum and live steam is injected. Fatty acids are thereby volatilised and drawn off. Deacidification may be carried out in a batch, semi-continuous or fully continuous plant. The oil is rapidly cooled down to 30°C within 6 to 8 minutes from the start of the process.

Recovery of oil from oil-cake – medium scale

Since the oil cake contains a relatively high percentage of oil, it can be recovered using a solvent extraction process.

The solvent, usually hexane, is mixed with the oil-cake and kept for 15 to 60 min at a temperature range of 40 to 65°C. Two types of extractors are used, either in combination with each other or in combination with one of the above pressing processes.

The percolation extractor: the solvent is pumped over a bed of the oil-cake and the solvent percolates down, leaving the cylinder through perforated bottom of the cylinder.

Immersion extractor: the oil cake is immersed in a pool of solvent. The cake is propelled by a chain or screw conveyor. The recovered solvent-oil mixture ‘miscella’ is heated to recover the solvent for re-use during future extraction.

Recovery of oil-cake – medium scale

The solvent treated oil cake can be recovered for animal feed by a steam treatment that evaporates the solvent. The oil cake is then dried by hot air blowers, ideally using fluidised-bed air dryers.

Packaging of sunflower oil

Packaging is defined as the containment of a food product in a protective barrier that prepares goods for transport, distribution, storage, retailing and end-use.

Sunflower oil is packed into greaseproof plastic bottles or drums. Retail bottles are closed with snap-on or screw lids with or without special pouring devices. The containers should be hermetically sealed to prevent entry of water, air and oxygen, which would cause spoilage of the oil.

Sunflower oil is an important oil seed product with good flavour stability, coupled with its high linoleic acid content. Sunflower seeds yield between 40 and 60% oil. The oil can be used as salad and cooking oils, shortening, and margarine. Cold-pressed oil is suitable for table use and hot-pressed oil, which is of lesser value, is used for soap-making, candle-making, lubricants, and used for mixing paints. The residue oil cake (press-cake) is used for animal feed.

Legislation

The processor should be familiar with and adhere to the laws and regulations applicable to the products manufactured to ensure the safe and legal marketing of oilseed products.

Next month we shall look at the processing of sunflower oil for small-scale farmers.

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

LITERATURE SOURCES

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