Vegetable oils are produced in several steps after careful selection of the seeds. Depending on the variety, a special know-how is necessary. The following breakdown tries to cover most common steps.
The seeds or fruits are harvested at a specific time depending on the plant species and geographical location. Local conditions such as soil composition, cultivation methods and solar radiation have an influence on the chemical composition of the seeds and fruits.
The cultivation type is either conventional, i.e. with pesticides and fertilizers, or organic. The latter requires special criteria: the obtained oil can be traced back to its origin. Also the further processing of the oil (see below) is only possible with approved agents. Even in cultivation and plant care, only agents that have been tested and approved by the EU may be used. The farmers are certified with different eco-labels in annual intervals.
Each type of seed has a different composition of substances and must be specially prepared. In order to obtain the highest possible oil yield with the desired properties, the seeds are first prepared, depending on the type and desired oil property, and then processed by using the correct method. The following steps can take place one after the other or separately: Cleaning; drying; peeling/ debarking; crushing; cooking/conditioning; processing into flakes; further processing steps (see extraction with solvent and wet pressing). Certain processing steps can also allow undesirable substances from the seeds to enter the oil, which is why some processing methods follow in succession or additional processing steps must be added. As a result of the processing, an intermediate product is created.
Drying leads to a reduction of the water content, which would cause problems in subsequent process steps. If there is too much water in the seed or fruit, enzymatic degradation may occur more quickly.
During the cleaning process, foreign particles such as dust, wild or weed seeds are removed.
By peeling or debarking the seeds, the outer skin is removed to reduce the fibre content. This may be necessary to reduce impurities in the oil (the husk may contain undesirable substances) or to increase the protein content in the ground material.
Crushing the seeds results in a larger surface area and makes it easier to extract or squeeze. In addition, the oil cells are already partially broken up mechanically, which also has a positive effect on the downstream process steps.
During cooking or conditioning, the introduced thermal energy opens up the oil cells and reduces the viscosity of the oil flowing out. If the temperature is further increased, a certain temperature above a threshold the seeds can even be sterilized and unwanted (thermolabile) seed components can be eliminated. These thermolabile substances also include proteins that irreversibly coagulate at a certain temperature. If desired, the heat can also be used to adjust the water content (see drying above).
Under certain circumstances, processing into flakes may be necessary. This step is adapted to the subsequent processing steps and is very similar to shredding.
See wet pressing and extraction with solvent.
Which processing method is used depends on the oil content and the desired yield. In general, however, a tendency can be identified: Oils from seeds with a high content are usually extracted mechanically, while seeds with a low oil content are often extracted directly after processing. ii The intermediate product obtained previously is now processed further:
Due to its simple construction and low maintenance costs, the pressing process is one of the most commonly used methods of oil extraction. Typical machine designs for pressing are screw or wedge presses.
The untreated or processed oilseed (see above intermediate product) is pressed in the first pressing cycle without heat input. This produces the classic edible oil with the designation "first pressing". However, this designation does not indicate whether the oil has undergone further treatment. Another common designation is the so-called "Nativ", which allows the seed to be pre-treated exclusively by mechanical processes.
However, the designations mentioned above are not legal norms, which is why differences may well arise: for example, the guidelines do not apply to olive oil. Another point is the pressing temperature itself: the term "cold pressed" is only used to describe a non-supply of heat. With high pressures however even temperatures of over 100 degrees Celsius can develop. Further special criteria for the designation of olive oils can be found under "olive oils".
The press cake from the first pressing often still contains sufficient oil that could not be removed due to the given physical properties (e.g. too low viscosity due to low temperature). By adding thermal energy to the press cake from cold pressing, the viscosity of the oil still contained in the press cake decreases, which is why an additional yield is achieved in the subsequent hot pressing (up to 8% higher). Further advantages are the extraction of higher viscous substances (e.g. waxes) and the deactivation of some mucilages and proteins. i In certain cases, the above-mentioned successes can also have a negative effect on the organoleptic and olfactory properties, as undesirable substances are also pressed out at these temperatures. The resulting liquid is centrifuged in the next step (see below).
As already mentioned, olive oil is subject to EU regulations that regulate the analytical limits, analytical methods to be applied, designations and olfactory evaluation of the oils. Even though there is a variety of different olive oils, only 4 qualities are important for the final consumer.
"Virgin olive oils" are produced only under mechanical or other physical processes that do not lead to deterioration of the oil. Mixtures with other oils and further post-treatment such as refining etc. are not allowed (only permitted: washing, decantation, centrifugation and filtration). In order to further differentiate the chemical composition and olfactory requirements, a subdivision into "Extra Virgin Olive Oil" and "Virgin Olive Oil" has been created. "Extra virgin olive oil has the highest olfactory requirements and the lowest free fatty acid content (maximum 0.8g per 100g of oil). The "Virgin Olive Oil" has slightly lower claims in both points: smell and taste are slightly lower, with free fatty acids below 2g per 100g of oil.
"Refined oils" are divided into two subgroups like the "Virgin Olive Oils": "Olive oils consisting of refined olive oils and virgin olive oils" and "Olive Pomace Oil". The former is a mixture of refined and virgin olives in any proportions, with a maximum free fatty acid content not exceeding 1 g per 100 g of oil. Olive-pomace oil" is a mixture of refined olive-pomace oil and virgin olive oil in any proportion, with a maximum free acidity content of 1 gram per 100 grams of oil.
The difference to pressing is the raw material used: it is the fresh fruit, whereby the desired oil is usually found in the fruit flesh (e.g. palm fruits - not palm kernels). In this type of pressing, the complete fruits are first sterilized with heat and autoclaved, whereby the fruit flesh can now also be removed more easily from the core. After pressing, the resulting mixture is passed through a screw press and finally separated into its components by a centrifuge.
After the various pressing processes, many undesirable accompanying components such as organic solids or water are still present. In order to separate these substances, the centrifugal forces and the different densities of the individual substances are used. These forces separate the (crude) oil phase from the aqueous phase in a centrifuge, leaving solid residue particles in the aqueous phase. Typical machine types are self-cleaning bowl or disc centrifuges and high-performance decanter centrifuges.
Due to its high fiber and carbohydrate content, the resulting press cake is used as a raw material for various products such as animal feed or for energy production in biogas plants.
The aim of this so-called pre-pressing is to increase the permeability of the oil cake during the subsequent extraction. The seeds are first prepared, pre-pressed and then passed on to the extraction step.
After pre-pressing, the prepared seeds are placed in an extraction plant (usually on a percolation or immersion basis). Using the solvent hexane, a mixture of n-hexane and methyl pentanes (boiling points between 65-75 degrees Celsius), the oil is extracted from the pre-pressed seeds. Advantages of the extraction process are the very high oil yield compared to the pressing process (less than 2% residual oil content compared to the pressing process with approx. 9%) and the reusability of the solvent. However, it has two decisive disadvantages: first, the hexane mixture used is explosive and second, the purification of the hexane oil mixture is very difficult. For this reason, another extraction process has been established, which uses liquefied and supercritical gases such as CO2 as solvent. iv These gases evaporate without problems after extraction.
The purpose of additional treatment is to remove impurities and contamination from the oil. Leavening these substances in the oil can lead to unwanted taste and smell sensations.
The refining with alkalis follows the following principle: The required degumming is followed by neutralization with alkalis and a final washing and drying process. The background is the reduction of the following substances: free fatty acids (lead to rapid spoilage of the oil); phosphatides and phosphorus compounds with emulsifying properties (change the organoleptic perception negatively); coloring substances (e.g. chlorophyll) and metals.
In the first step, the so-called degumming, water and/or phosphoric acid and/or sodium chloride is added to the mixture to precipitate phosphorus compounds and metals. The complexes formed flocculate and can be removed with the aqueous phase. Known substances such as chlorophyll and lecithin can be removed in this way, for example. The oil thus has a longer shelf life and shows better chemical properties.
After degumming, the actual neutralization with alkalis takes place. By adding bases, the free fatty acids are transformed into fat-insoluble soaps and can be removed again with the aqueous phase. The resulting soaps adsorb further unwanted substances such as phosphatides, oxidation products, colorants and slimy substances.
A subsequent washing process with hot water removes excess soap or alkalis from the previous steps. Finally, the remaining water is removed under vacuum to prevent enzymatic reactions.
Hot steam (over 240 degrees Celsius) can be used to extract further (harmful) substances from the oil due to different boiling points. Substances such as polcyclic hydrocarbons, mycotoxins, polychlorinated cyclic hydrocarbons and heavy metals can thus be reduced to trace amounts. For this purpose, however, the oil must be low in phosphatides and metals and insensitive to heat.
Certain substances, for example carotenoids and chlorophyll, are still present in small quantities and can be removed from the oil by an adsorption process. With the help of the surface activity of bleaching earth, coal or synthetic adsorbents based on silicate, adsorption to the substances mentioned takes place (reaction temperature is at approx. 90 degrees Celsius). The particles are removed by filtration.
With the help of vacuum, dry steam and high temperatures, the oil is treated over a longer period of time. The process thus removes odours, volatile substances and possible residues of extraction agents. Since some substances are destroyed at these temperatures (over 150 degrees Celsius), the oil is often discolored.
These so-called "refinement steps" change the physical and chemical properties of the oil in order to adapt it exactly to the respective requirement ranges.
Winterization, also known as fractionation, removes solids and waxes from the oil by filtration at low temperatures. This process allows the appearance to be precisely adjusted to physical properties such as melting point and color.
The aim of curing is to adjust the rheological properties of the oils. For this purpose, the unsaturated fatty acid chains are converted into saturated fatty acid chains with the aid of a catalyst. At overpressure and temperatures around 100 degrees Celsius, catalysts such as nickel or platinum (which are later filtered out) and hydrogen are added to the oil to start the chemical reaction. The oil thus changes from a liquid to a spreadable state. This can be also termed as "hardened oils".
Special requirements are made, for example, in the production of drugs. The oil is purified again with the help of activated earths, whereby predominantly highly polar molecules are removed.