Processing of oil seeds – Part 19: Refinement, hydrogenation, winterisation and packaging of sunflower oil

By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

The refinement of sunflower oil is a very complex process that must be carefully continued up to the end before a clear and appealing product appears. This month we look at the final steps – neutralisation, winterisation, hydrogenation and packaging.

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

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, and reacts with the free fatty acids to 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.

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 minutes. The colourants adhere to the Fuller’s earth. The Fuller’s earth is removed by filtration over a screen filter or a 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, a sight glass, a vacuum gauge and a thermometer.

After bleaching, oils and fats may either be deodorised for immediate use, or they may be hydrogenated. If the oil is hydrogenated, a second bleaching treatment is performed on the hydrogenated product before it is deodorised, mainly in order to remove traces of hydrogenation catalyst.

Deodorisation

Deodorisation is one of the final processing steps in the preparation of oil for edible purposes, such as salad oils 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 between 180 and 280°C, steam is injected into the oil, and the volatile flavours are driven from the oil to be collected 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 holding up to 25 tonnes of oil. 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 performed 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 of the oil to ensure sufficient removal of all the volatile impurities. The deodorised oil can be filtered after deodorisation in 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 refinement of oils to remove various undesirable components, while keeping triglycerides intact and with minimum loss 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 to 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 batch, semi-continuous or fully continuous plants. The oil is rapidly cooled down to 30°C within 6 to 8 minutes from the start of the process.

Hydrogenation of sunflower oil (optional)

Hydrogenation, or hardening, increases the melting point of fats through the addition of hydrogen to some, if not all, of the double bonds present in the fatty acids of the triglycerides, in the presence of a catalyst, usually nickel (Ni). It also stabilises the fat against oxidation, flavour deterioration and often has a bleaching effect.

Hydrogenation is used for the preparation of dual purpose cooking and salad oils. These oils are given a light hydrogenation treatment to ensure that the final product remains a liquid. The oils used for margarine are given a greater degree of hydrogenation. Hydrogenation is followed by winterisation to help reduce the level of unsaturation of the salad oils. Hydrogenation is done in special pressure vessels, with an operating pressure of between 1 to 8 bar, and a temperature of 100 to 180°C. A catalyst in the form of nickel is required.

A catalyst is a material which, when added in minute quantities to a mixture of substances capable of reacting with each other, greatly increases the rate of the reaction. After the catalyst is added, hydrogen gas is pumped into the oil that is vigorously stirred. Hydrogenation is an exothermic reaction and the vessel must be cooled to counteract the heat produced by the reaction.

The quantity of catalyst used in hydrogenation is 0,1% of nickel on the weight of the oil for a fresh catalyst, but rising to between 1 and 3% for a poisoned or spent catalyst. Catalysts can normally be reused for a number of batches, with artificially poisoned or sulphured catalysts lasting much longer than fresh catalysts, as in the latter case impurities in the oil gradually poison the catalyst and lower its activity.

Hydrogenation is a usually a batch process, but continuous processes are available where the exit oil preheats the oil entering the process. It usually consists of five to six stirred tank reactors in series, and the catalyst is supported in heated columns. The hydrogenation process can be controlled by measuring the refractive index, iodine value or melting point of samples withdrawn at intervals. Hydrogenated fats are filtered to remove the hydrogenation catalyst, subjected to a light earth bleach and deodorised before they can be used for edible purposes.

Winterisation of sunflower oil (optional)

Winterisation is a process of fractional crystallisation of oils in which the higher melting glycerides are removed, giving the oil a clear, bright appearance even after refrigerated storage. Winterisation is used for the dewaxing of sunflower oils to prevent clouding of the oil. The oil is required to stay clear for 5 to 6 hours at 0°C and this is particularly important for salad oils.

It is done in chilled brine tanks under carefully controlled conditions. The higher melting glycerides will solidify and are then separated from the oil. The oil is cooled from 21 to 28°C to 13°C within 6 to 12 hours. The cooling rate is then reduced for the next phase, where the oil is cooled to 7°C within 12 to 18 hours. It is then held at a low temperature for a further 10 to 20 hours to allow complete solidification of the glycerides. It is then filtered across frame-and-plate presses to remove solid particles. The yield is between 75 and 80%.

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. Next month we shall look at the production of margarine or sunflower oil fat spread.

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

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