Making Food Oil

 

General information about rape
Mildola Process
Raisio Process
	Mechanically pressed and extracted oil	Raw oil purification	Quality of raw materials
	Seed cleaning	Degumming (removal of lecithin)	Interchange esterification
	Seed preheating	Alkali cleaning and mechanical cleaning	Dewaxing
	Flattening	Bleaching	Deodorization (Steam distillation)
	Oil pressing	Hardening
	Pre-treatment of the pressed material
	Extraction
	Separating extraction liquid

General information about rape

Turnip rape (Brassica rapa oleifera) and rape (Brassica napus) are the only oil plants widely grown in Finland today. The oil content of the seeds in both turnip rape and rape is about 40 % and the fatty acid composition of the oil in both plants is the same. That is the reason why we in Finland generally speak about rape seed oil only. Rape demands a longer growing time and is therefore grown only in the southern part of Finland. Turnip rape on the other hand can be grown up to the central part of Finland.

It is possible to get both oil and grain from the rapeseed. Turnip rape is the most significant raw material for margarine in our country and the most common use of cooking oil both in households and catering centres. About one third of the fat used in margarine industry is domestic rape seed oil. The rape grain remaining after the oil pressing of the seeds is used as feeding stuff protein for livestock feeding. Rape seed contains about 22 % protein; the corresponding percentage in grain is about 38 %.

In Finland the use of rape seed as cooking oil started in 1976 when farmers started growing rape species not containing euraca acid. About one third of the oil imported to Finland in 1992 was used for feeding and for technical purposes and the rest for food industry. After threshing rapeseed was delivered either directly from the field or after transit storage to the refineries.

There are two pressing mills in Finland: Raisio and Mildola, they use different kind of techniques and make different kind of products. The processes can be described as follows:

Mildola Process 

Seed is pressed in two steps. Seed is fed into the first pressing; temperature can be altered according to the demands so that the so-called cold pressing can be accomplished. In the second step oil yield is improved so that the final oil content of the pressed material is about 9 %. Rough pressing oil and repressing oil can be treated separately. At first separators are used to remove the seed particles that are repressed. Finally the lecithin content of the oil is reduced so that the oil is as suitable as possible for the final purification at the margarine factory. During the final purification odorant agents, flavouring agents and colouring agents are removed from the oil. The pressed oil can be sold as such e.g. for feed.

After repressing the pressed material is pasteurised to achieve the hygiene desired. After roasting there is Öpex treatment by which the protein of the pressed material is protected to get the optimal pressed material for cattle feeding. Finally the pressed material is dried and cooled.

Raisio Process 

Raisio process is divided into two phases: pressing and extraction. Seed is flocculated before rough pressing. The pressed product is extracted with solvent to increase the oil yield. Then lecithin is removed from oil for the final cleaning. After extraction solvent is removed from the grain and then it is pasteurised to ensure the hygiene. Finally the grain with 3-4 % oil content is dried and cooled.

Raisio process

Mechanically pressed and extracted oil 

Here you can see the simplyfied flowsheet for making oil.

Production stages of making oil by mechanical pressing and extraction.

Production stages of pressed and extracted oil

Seed cleaning 

Rape seed contains small amounts of impurities that must be cleaned before separating oil.

Seeds are cleaned in three steps:

1. Removal of dust and light material by suction
2. Removal of oversized seeds by screening
3. Removal of tiny seeds by re-screening

Seed preheating 

Preheating is the first step before flattening. Seeds are heated to 30-40 ºC. Preheating is necessary to prevent seeds from cracking and crushing at flattening stage. This stage is especially important during the winter months.

Preheating equipment are steam heated rotary equipment – either fluidized bed type or equipped with steam heated jacket. The moisture content of the seeds can be controlled during this stage. The moisture content should be 7.0-9.5 %.

Flattening 

Preheated seed is flattened in one or two steps in smooth surface roll crushers. The purpose for flattening is to crush the cellular walls of the seeds to facilitate oil pressing by simple oil crushers. Remaining oil can be separated from crushed material by extracting with e.g. hexane. Efficient flattening facilitates the further processing of seeds.

Heating 

Flattened material is heated in steam-heated multi-layer boilers or in rotary boilers equipped with steam jacket. Boiling temperatures can vary but generally it is 75-100 ºC. Air can be fed into boilers to control the moisture of the material; appropriate moisture content is 5-7 %. During heating small drops of oil become larger and are then easier to be separated. In addition there is a change in protein composition and therefore oil is easier to be extracted. It is important to control the temperature and heating time to avoid building up of harmful colour and sulphuric compounds and decreasing of protein content.

Oil pressing 

Heated seed is transported to a continuous screw press. The purpose of pressing is to reduce the oil content of the seed from 42 % to 17 %. After this phase the extraction of residual oil from the pressed material is much more efficient and more economical.

The press is a cylindrical jacket with a worm shaft inside. The screw presses seed against the wall of the jacket and oil is released. Pressure and temperature increase inside the jacket and solid is pressed through an adjustable narrow outlet. After this the cake is ready for extraction and crude oil passes on to be re-refined.
 

Pre-treatment of the pressed material 

Some plants pre-treat the pressed material in mechanical aerators to improve the extraction properties. Aerators have a cylinder with a rotating shaft equipped with blades. Steam is blown to aerators. The shaft rotates; the material passes through, the temperature and the pressure inside the jacket increase. The material is discharged through small openings in the nozzle at the end of the cylinder; the loss of pressure causes the swelling of material into porous pressed material easy to be extracted.

Extraction 

After the press or the aerator the material is transported to the extraction equipment. During the transportation the material is cooled from 80-100 ºC by air to avoid the extraction liquid being evaporated during the extraction. Hexane is used as an extraction liquid after it has been cleaned and heated to 50-60 ºC.

Solid material at the temperature of 80 ºC is extracted using counter-current principle. First with hexane containing plenty of oil (micelle), then with hexane containing less oil and finally with pure hexane. The oil content of the solid after extraction is about 1% depending on the equipment and the accuracy during the pre-treatment. The flow of hexane to the extraction equipment is adjusted to get micelle with oil content of 25-30 %. The capacity of the extraction equipment is 600-1200 tons of seeds/day.

Separating the extraction liquid from pressed material and oil 

Hexane is removed from the extraction liquid containing oil in three-phase evaporators. Evaporated hexane steam is condensated and reused.

After extraction the cake is transported to the equipment for removal of extraction liquid e.g. a container equipped with a steam heated plate and rotating blades above them. The cake is heated to about 105 ºC for 30-40 min. Hexane and steam are removed and evaporated. Water and hexane are separated from each other, after that hexane can be reused. Hexane losses are only 1-2 litres/extracted seed ton in case equipment works properly.

The hexane content of the cake that has been cooled in the rotary furnace and dried is 800 mg/kg. After this cake can be ground to form uniform particles or it can be palletised before storage and marketing.

Raw oil purification 

Oil extracted from rape seeds is purified to get mild and well preserving food oil. After purification oil is ready to be used as such or in margarine production. In order to have a proper composition one part of oil for margarine production is hardened.

In theory the refining of raw rape seed oil for eatable products is a simple process. Fig. 3.1 demonstrates the process stages and table 3.1 shows the impurities that are decreased or removed in this stage.

Fig. 3.1 Purification stages of crude oil

Degumming (removal of lecithin) 

The first stage in the purification of crude oil is degumming i.e. precipitation. During the precipitation phosphorous compounds are removed from oil. There are two precipitation procedures: a) use of water b) use of acid and water

Adding water to pre-heated oil makes most water-soluble phosphatides precipitate. The temperature of oil should be 50-80 ºC. If the temperature is too high the solubility of phosphatides in oil increases and on the other hand too low temperature increases the viscosity of oil causing difficulties in separation. The amount of water should be proportional to the amount of phosphatides. Precipitation is not complete in case the amount of water is too small, in case the amount of water is too high forming of emulsion occurs causing losses. The appropriate amount of water is 2 %. Water precipitation is usually accomplished in continuous centrifugal separators.

Acid precipitation is used for water insoluble phosphorous compounds. Mineral acids (e.g. hydrochloric acid and nitric acid) have given good results but using them is not practical because of the corrosion problems. Phosphoric acid and citric acid are obviously good precipitation reagents. They form hydrating complexes with unhydrated phosphatides, and then the precipitation with water is possible. There are several precipitation processes but every process is based on that simple requirement that good oil must not contain any hydrating or unhydrating phosphorous compounds. There can be considerable changes in process conditions that can be seen in following precipitation processes:

Total degumming process 
This precipitation can be accomplished for water-precipitated oil and raw oil. Phosphoric acid or citric acid is added into oil and after a certain time some base e.g. lye to get slightly alkaline solution. Neutralisation degree should be low; it is not desirable to get soaps increasing losses. Precipitated products are separated by centrifugation in two steps. The idea is to get as little oil as possible with the precipitate in the first step and to remove remaining precipitation products with a larger amount of oil in the second step. The second precipitation product is returned into oil before the first centrifugation. Then oil is water washed and dried before further treatment.

Superdegumming 
This process is best suitable for raw oil.

Concentrated citric acid is added in oil at 70 ºC. After the reaction time (5-15 min) oil is cooled to 25 ºC and about 2 % water is added. After one hour oil is heated quickly and filtered. Then water wash is done.

In this process cooling is extremely important because at higher temperatures phosphorous compounds remain in oil or move there from water phase.

Figure 3.3 shows the superprecipitation stages

Continuous ultrasonic degumming 

Ultrasonic energy causes phospholipides to take water-soluble form and to be precipitated by adding 2 % water. After the precipitation oil is ready for further treatment. This procedure is still under development.

Special degumming  

In addition to citric acid this precipitation uses weak about 8 % lye to control pH value. This prevents the precipitation complex decomposing and the precipitation itself is accomplished at 60-70 ºC. Also phosphoric acid can be used; then neutralisation is even more important because phosphoric acid is a strong acid.

Fig. 3.4 Stages of special degumming

Alkali cleaning and mechanical cleaning 

Precipitated oil can be refined further in two ways: alkali cleaning or mechanical cleaning. Alkali cleaning (neutralisation) is more common than mechanical cleaning that requires well-precipitated oil containing chlorophyll and free fatty acids.

Alkali cleaning 

Free fatty acids in fat or oil are neutralised with base and the insoluble soaps from fat are separated from fat. Some chlorophyll and phospholipids is removed with soap.

To facilitate the precipitation of phospholipids first 0.05-0.2 % phosphoric acid is added to oil mixing thoroughly. Then 12 % sodium hydroxide-water solution is added, it neutralises free fatty acids and excess phosphoric acid. Mixing times vary from some seconds to 15 minutes and the temperature can be 40-90 ºC.

Before centrifugation oil-soap mixture can be heated to about 90 ºC. Disc centrifuge is generally used for separating the soap phase.

Mechanical cleaning 

Phosphoric acid is added into water-acid precipitated oil. Then oil has to be in touch with acid activated bleaching clay at 90-105 ºC. Clay together with precipitated phosphatides and absorbed chlorophyll is filtered from oil.

These stages together with water-acid precipitation are important in mechanical cleaning. Except for free fatty acids the amount of other materials is as low as alkali cleaned oil has. Besides the amount of chlorophyll has been reduced at the desired level.

Free fatty acids are removed by steam distillation during the deodorisation.

Bleaching 

After alkali cleaning oil has to be bleached. In bleaching bleaching-clay or activated carbon bind oxidation products containing fat, dyes, phospholipides and soap. Those components adhere to activated bleaching clay that is filtered from fat after the treatment. Bleaching is made more effective with citric acid addition facilitating soap removal and binding metals e.g. iron and copper, which accelerate oxidisation of fats. Activated carbon binds both dyes and poly-aromatic compounds (PAH compounds) and insecticides. Combustion gas is an example of poly-aromatic compounds; it has been proved to be carcinogenic.

Hardening 

Hardening changes the melting properties of fat and improves the hardness of oxidation. During hardening hydrogen atoms are added into fat, then double bonds are decreased and melting point is increased. Nickel is used as a catalyst.

Components in hardening reaction:

Solid catalyst
Unsaturated double bond of fatty acid
Hydrogen gas

Many mass transfer mechanisms occur in hardening process depending on dominant conditions.

Reaction phases

1. Hydrogen gas passes into liquid fatty phase in which some hydrogen is dissolved. Then dissolved hydrogen and fat molecules pass into the outer surface of the catalyst. High temperature, hydrogen pressure and efficient mixing increase the solubility of hydrogen. The size of fat molecule effects its reaction speed: one single fatty acid passes more quickly than triglyseride, large amount of double bonds in the mixture makes the reaction quicker.

1. Binding nickel with an extremely porous carrier substance e.g. ciliceous earth makes the catalyst. Catalyst has therefore a large surface area. In the next phase hydrogen and double bond of fat molecule are absorbed on the surface of the catalyst where they reach active nickel.

2. In the third phase both hydrogen and the double bond of fatty molecule are absorbed on the surface of the catalyst. Water molecule moves to the atom level and forms NiH and NiH₂ bonds with nickel atom. The distance between nickel atoms is approximately equal to the distance between two double-bonded carbons; therefore double-bonded carbon atoms can combine with neighbouring free Ni atoms. The group containing several double bonds is absorbed easier than the group containing one double bond.

3. There are several reactions taking place on the surface of the catalyst causing the saturation and the isomerization of the fatty molecule. All details about the reactions are not known yet. The reaction is exothermic and heat is released from the catalyst to the fat around it.

4. When the reaction is completed reaction product leaves the pore into the fat around it.

Quality of raw materials 

Both the fat to be hardened and hydrogen can contain impurities preventing the catalyst from functioning. Impurities cause quality changes in hardening products and raise the process costs.

Impurities can be:

1. Toxic substances are elements in group V, VI and VII that make the catalyst entirely passive by bonding themselves with nickel.

2. Catalyst inhibitors block nickel pores mechanically, then the active surface area gets smaller. These inhibitors are e.g. waxes, lecithin, soap, and oxidised fat and decomposition products.

The removal of the catalyst is accomplished by adding filter aids to oil and by filtering it in two phases.

Interchange esterification 

Interchange esterification is another procedure to change the melting and crystallisation characteristics of oils and fats. This procedure is commonly used in Europe but not in North America.

In natural fats fatty acids are decomposed in glycerol in a manner characteristic to each acid. According to this decomposition they are either solid or fluid and their use is limited in food industry. By means of interchange esterification the natural order of fatty acids in glycerol stem can be changed to obtain fat with new melting and crystallisation properties.

In the chemically catalysed interchange esterification fatty acid composition is not changed as in hardening but the positions of fatty acids in glycerol/among glycerols.

Dewaxing 

Rape seed is oil remaining clear and liquid in refrigerator temperature without cold resistance treatment. Oil can contain a small amount of compounds, which can be precipitated later. To keep oil clear and nice these waxes can be removed by cooling oil in continuous heat exchangers to about 5 ºCv and by filtering them with an additive.

Deodorization (Steam distillation) 

Generally steam distillation is the last cleaning stage for food fat and food oil before filtering.

In deodorization evaporating substances causing taste and smell are removed.

Steam distillation principle: Direct blow steam is blown through oil at 225-260 ºC and under pressure 0.3-0.6 kPa. Components to be distilled are removed from oil. They are collected from the reactor shell and from the gases escaping through vacuum equipment.

Now oil is ready for packaging.