Cultivation and production of vegetable oils

The production of high-quality vegetable oils is a complex process that begins with the careful selection and testing of the seeds used. Since each plant species has a unique chemical composition, processing requires in-depth technological expertise as well as precisely calibrated process parameters.

The following breakdown outlines the standard steps in industrial oil production. This process chain ensures that the specific properties of the raw materials are preserved and that a high-quality end product is produced for the downstream processing industry.

 

HARVEST


The chemical composition and quality of plant oils are largely determined by the timing of the harvest of the seeds and fruits, as well as by the geographical growing conditions. Factors such as specific soil composition, sunlight exposure, and regional cultivation methods directly influence the valuable components of the raw materials.

In professional production, a distinction is made between conventional and organic cultivation methods:

  • Conventional cultivation: Here, proven methods of plant care involving the use of fertilizers and pesticides are employed to ensure stable yields and quality.
  • Organic cultivation: This is subject to stricter criteria. A key feature is the complete traceability of the extracted oil back to its origin. Both during cultivation and in subsequent processing, only approved, EU-certified products are permitted.

To ensure these standards are maintained over the long term, agricultural operations are certified annually. Compliance with recognized organic certification labels guarantees that the plant-based raw materials meet the high requirements of industrial processing.

 

Preparation and Mechanical Processing of Oilseeds

Each type of seed has a specific chemical composition and therefore requires an individually adapted processing method. To achieve maximum oil yield with the desired quality characteristics, the seeds are prepared prior to the actual extraction. This preparation process involves a series of mechanical and thermal steps, which may be carried out sequentially or individually depending on the raw material.

The objective of the preparation stage is to obtain an optimal intermediate product while simultaneously preventing undesirable accompanying substances from entering the final product as a result of unsuitable processing steps.

Drying and Cleaning

The process often begins with drying to precisely reduce the moisture content of the raw materials. Excessive moisture levels in seeds or fruits can accelerate enzymatic degradation and interfere with subsequent processing steps. At the same time, cleaning is performed to thoroughly remove foreign materials such as dust, wild seeds, or weed seeds, thereby ensuring the purity and quality of the oil.

Dehulling and Decortication

Dehulling or decortication removes the outer shell of the seeds. This step serves to reduce fiber content and prevent unwanted substances from the husk from entering the process stream, while also increasing the protein content of the milled material.

Size Reduction and Flaking

Mechanical size reduction increases the surface area of the seeds and partially ruptures the oil cells. This significantly facilitates subsequent extraction or pressing. In certain cases, further processing into flakes is required to optimally adapt the material structure to downstream processing steps.

Cooking and Conditioning

During cooking or conditioning, thermal energy is applied to fully disrupt the oil cells and reduce the viscosity of the oil, thereby promoting efficient oil release. Higher temperature thresholds also allow for the sterilization of the seeds and the elimination of thermolabile components. At this stage, proteins irreversibly coagulate, and the final moisture content can be precisely adjusted.

Downstream Processing

Following preparation, specific oil recovery processes such as mechanical pressing or solvent extraction are applied to efficiently separate the oil from the conditioned intermediate product.

 

Mechanical Processes

The choice of processing method depends on the oil content of the raw material and the desired yield. In general, a clear trend can be observed: oils from seeds with a high oil content are usually obtained mechanically, while seeds with a lower oil content are often directly subjected to extraction after preparation. The previously obtained intermediate product is then further processed.

Pressing

Due to its simple design and low operating and maintenance costs, pressing is one of the most commonly used methods for oil production. Typical machine designs used for pressing include screw presses and wedge presses.

Cold Pressing

Untreated or prepared oilseeds (see intermediate product above) are pressed in the first pressing step without any external heat input. This process yields the classic edible oil referred to as “first pressing.” However, this designation does not necessarily indicate whether the oil has undergone further treatment afterward. Another commonly used term is “native,” which allows only mechanical pre-treatment of the seeds.

These terms are not legally standardized, meaning that differences in interpretation may occur. For example, these guidelines do not apply to olive oil. Another important factor is the pressing temperature itself: the term “cold-pressed” only refers to the absence of external heat input. Due to high pressure, temperatures exceeding 100 °C may still occur during pressing. Additional specific criteria for olive oil classification are described in the section “Olive Oils.”

Hot Pressing

The press cake obtained from the first pressing often still contains a significant amount of oil that could not be removed due to physical limitations, such as high viscosity at low temperatures. By applying thermal energy to the press cake from cold pressing, the viscosity of the remaining oil is reduced, enabling additional oil recovery during subsequent hot pressing (up to 8% higher yield).

Further advantages include the extraction of higher-viscosity components (e.g. waxes) and the deactivation of certain mucilaginous and protein substances. However, these effects can also negatively impact organoleptic and olfactory properties in some cases, as undesirable substances may also be extracted at elevated temperatures. The resulting liquid is subsequently centrifuged.

Olive Oil

As mentioned previously, olive oil is regulated by EU regulations that define analytical limits, approved testing methods, labeling, and sensory evaluation criteria. Although a wide variety of olive oils exist, only four quality grades are relevant for end consumers.

“Virgin olive oils” are produced exclusively by mechanical or other physical processes that do not lead to deterioration of the oil. Blending with other oils and further processing such as refining are not permitted. Only washing, decantation, centrifugation, and filtration are allowed.

To further differentiate chemical composition and sensory quality, virgin olive oils are subdivided into “extra virgin olive oil” and “virgin olive oil.” Extra virgin olive oil has the highest sensory quality and the lowest content of free fatty acids (maximum 0.8 g per 100 g of oil). Virgin olive oil has slightly lower sensory requirements, with a maximum free fatty acid content of 2 g per 100 g of oil.

“Refined oils” are also divided into two subcategories: “olive oils composed of refined olive oils and virgin olive oils” and “olive pomace oil.” The former is a blend of refined and virgin olive oils in any proportion, with a maximum free fatty acid content of 1 g per 100 g of oil. Olive pomace oil consists of a blend of refined olive pomace oil and virgin olive oil, also with a maximum free fatty acid content of 1 g per 100 g.

Wet Pressing

Unlike conventional pressing, wet pressing uses fresh fruits as the raw material, with the desired oil typically located in the fruit pulp (e.g. palm fruits, not palm kernels). In this process, whole fruits are first sterilized and autoclaved using heat, which also facilitates separation of the pulp from the kernel.

After pressing, the resulting mixture is conveyed through a screw press and finally separated into its individual components by centrifugation.

Centrifugation

Following the various pressing processes, the oil still contains undesired accompanying substances such as organic solids and water. These components are separated by exploiting centrifugal forces and differences in density. In a centrifuge, the crude oil phase is separated from the aqueous phase, while solid residues remain in the aqueous fraction.

Typical equipment used includes self-cleaning bowl or disc centrifuges and high-performance decanter centrifuges.

Press Cake

Due to its high fiber and carbohydrate content, the resulting press cake is used as a raw material for various secondary products, such as animal feed or for energy generation in biogas plants.

Pre-Pressing

The purpose of pre-pressing is to increase the permeability of the oil cake for subsequent extraction. The seeds are first prepared, then pre-pressed, and finally transferred to the extraction step.

Solvent Extraction

After pre-pressing, the prepared seeds are transferred to an extraction system, typically based on either percolation or immersion processes. The oil is extracted from the pre-pressed seeds using the solvent hexane, a mixture of n-hexane and methylpentanes with boiling points ranging between 65 and 75 °C.

The main advantages of solvent extraction are the very high oil yield compared to mechanical pressing (residual oil content below 2% versus approximately 9% in pressing processes) and the reusability of the solvent. However, this method has two significant drawbacks. First, the hexane mixture used is highly flammable and poses an explosion risk. Second, the purification and separation of the hexane–oil mixture is technically demanding.

For these reasons, alternative extraction methods have been established that use liquefied or supercritical gases, such as carbon dioxide (CO₂), as solvents. These gases can be easily removed by evaporation after extraction, resulting in a cleaner and safer process.

 

Refining of Crude Oil

The purpose of additional refining is to remove impurities and contaminants from the oil. If these undesirable substances remain in the oil, they can lead to unwanted flavors and odors and negatively affect product quality.

Alkali Refining

Alkali refining follows a defined sequence of processing steps. After an initial degumming step, the oil undergoes neutralization with alkalis, followed by washing and drying. The main objective is to reduce the following components: free fatty acids (which accelerate oil spoilage), phosphatides and phosphorus-containing compounds with emulsifying properties (which negatively affect sensory characteristics), coloring substances (e.g. chlorophyll), and metals.

In the first step, known as degumming, water and/or phosphoric acid and/or sodium chloride are added to the oil to precipitate phosphorus compounds and metals. The resulting complexes flocculate and can be removed together with the aqueous phase. Substances such as chlorophyll and lecithin can be eliminated in this way. As a result, the oil shows improved chemical properties and extended shelf life.

After degumming, the actual neutralization with alkalis takes place. By adding alkaline substances, free fatty acids are converted into fat-insoluble soaps, which are then removed with the aqueous phase. These soaps additionally adsorb other undesirable components such as phosphatides, oxidation products, color pigments, and mucilaginous substances.

A subsequent washing step with hot water removes residual soaps or alkalis from the previous processing steps. Finally, the remaining water is removed under vacuum to prevent enzymatic reactions.

Physical Refining

By applying hot steam at temperatures above 240 °C, additional undesirable substances can be removed from the oil based on differences in boiling points. Compounds such as polycyclic hydrocarbons, mycotoxins, polychlorinated cyclic hydrocarbons, and heavy metals can be reduced to trace levels. However, this process requires the oil to be low in phosphatides and metals and to be resistant to heat.

Bleaching

Certain compounds, such as carotenoids and chlorophyll, may still be present in small amounts and can be removed using adsorption processes. Adsorption occurs through the surface activity of bleaching earths, activated carbon, or synthetic silicate-based adsorbents at reaction temperatures of approximately 90 °C. The adsorbent particles are then removed by filtration.

Deodorization

Deodorization involves treating the oil for an extended period under vacuum, using dry steam and high temperatures. This process removes odors, volatile compounds, and possible residual extraction solvents. Since some substances degrade at these elevated temperatures (above 150 °C), partial decolorization of the oil often occurs.

 

Modification

These so-called “finishing steps” modify the physical and chemical properties of the oil in order to precisely adapt it to specific application requirements.

Winterization / Fractionation

During winterization, also referred to as fractionation, solids and waxes are removed from the oil by filtration at low temperatures. This process allows the appearance as well as physical properties such as melting point and color to be precisely adjusted.

Hardening / Hydrogenation

The aim of hardening is to adjust the rheological properties of oils. For this purpose, unsaturated fatty acid chains are converted into saturated fatty acid chains with the aid of a catalyst. Under elevated pressure and temperatures of approximately 100 °C, catalysts such as nickel or platinum (which are subsequently filtered out) and hydrogen are added to the oil to initiate the chemical reaction. As a result, the oil transitions from a liquid to a spreadable state. These products are referred to as “hardened oils.”

Chromatographic Purification

Certain applications, such as pharmaceutical manufacturing, require particularly high purity standards. In these cases, the oil is further purified using activated earths, with a primary focus on removing highly polar molecules.