What Is The Difference Between Chemical Refining And Physical Refining?

1. Chemical refining

Degumming
The first step of chemical refining is degumming. Its main purpose is to remove seed particles and impurities, while partially removing phospholipids, carbohydrates, proteins and trace metals.
The crude oil is treated with food-grade processing aids (acids) and/or water to hydrate the main part of the phospholipids, proteins, carbohydrates and trace metals. The concentration of processing aids depends on the quality of the crude vegetable oil. The hydrated gums are removed from the oil by centrifugal force.
 

Neutralization
Alkali neutralization reduces the following components: free fatty acids, oxidation products of free fatty acids, residual proteins, phospholipids, carbohydrates, trace metals and some pigments.
The process involves reaction with an alkaline solution. By this treatment, a second phase (soapstock) is formed, in which undesirable substances are dissolved. This phase is separated and removed by centrifugal force, followed by washing or filtering treatment to eliminate residual phospholipids, soaps and precursor molecules. This process is mainly used for vegetable oils and is not common in tropical oils and fats.
 

Bleaching
The purpose of bleaching is to reduce the content of pigments, such as carotenoids and chlorophyll, as well as residues of phospholipids, soaps, trace metals and oxidation products. These trace components can negatively affect further processing and the quality of the final product.
These substances are removed by adsorption with activated clays and silica under vacuum and at temperatures of about 100°C.
Fresh bleaching earth and filter aids are processing aids used in the refining of vegetable oils and fats.
When selecting bleaching earth, care should be taken to avoid bleaching clays that contain large amounts of chlorine-containing compounds/activated by hydrochloric acid. From the perspective of reducing 3-MPCD, pH-neutral clays can be used. However, on the other hand, acid activation can be preferred with sulfuric acid or citric acid to mitigate GE.
The amount of these adsorbents added should be adjusted to ensure the removal of specific substances.
If the content of polycyclic aromatic hydrocarbons (PAH) reaches a certain level, activated carbon is used to remove them.
The bleaching clay containing all these substances is separated by filtration. If activated carbon is used in the food and feed industry, it should be pre-treated by appropriate means before use.
 

Deodorization
The purpose of deodorization is to reduce the content of free fatty acids and remove odors, off-tastes and other volatile components such as pesticides or light PAHs by using stripping steam. Careful execution of this process will also improve the stability and color of the oil while retaining the nutritional value and retaining or only minimally changing the original triglyceride composition.
The deodorization process is carried out under vacuum (<5 mbar), the temperature is preferably between 180℃ - 230℃, and stripping steam is used. A higher vacuum is recommended to promote the evaporation of volatile compounds.
The specific conditions (temperature, duration, combination of the two) should be adjusted within these ranges as appropriate according to the type of vegetable oil and the type of facility to ensure the removal of specific substances.
For the removal of volatile contaminants, the recommended temperature range is 225℃-230℃. On the other hand, to reduce the formation of heat-altered contaminants such as glycidyl esters or trans fatty acids, a temperature not exceeding 235℃-240℃ should be used. For example, it is recommended to deodorize vegetable oils at 190℃-230℃ to reduce the formation of glycidol.
 

2. Physical refining
A distinction is made between chemical refining and physical refining in the way the fatty acids are removed: in physical refining, the fatty acids are removed by distillation, while in chemical refining, chemicals that bind to free fatty acids are used to facilitate their separation and removal from the oil.
 

Degumming
In physical refining, degumming aims to remove phospholipids, solid impurities and traces of heavy metals.
In the case of physical refining, degumming becomes increasingly important, as phospholipids should be removed to the desired level before bleaching.
Palm oil is usually degummed using two methods using centrifugal separation. For palm oil that usually contains no more than 20 mg/kg Palm oil containing lecithin, with or without dry degumming.
Dry degumming involves mixing acid into the oil and combining it with a bleaching step without washing.
 

Bleaching
The process is the same as described in chemical refining. The same recommendations are valid:
When selecting bleaching earths, care should be taken to avoid clays containing high amounts of chlorine compounds/activated by hydrochloric acid. pH neutral clays are considered from a 3-MCPD mitigation perspective. However, acid activation with sulfuric acid or citric acid may be preferred for GE mitigation.
The amount of these adsorbents added should be adjusted to ensure removal of specific substances.
Bleaching clays containing all of these substances are separated by filtration. If activated carbon is used in the food and feed industry, it should be pretreated by appropriate means before use. These processes are partly carried out under vacuum and at temperatures below 110°C.
 

Deodorization
The purpose of deodorization is to reduce the content of free fatty acids and remove odors, off-tastes and other volatile components such as pesticides and light polycyclic aromatic hydrocarbons by using a stripping medium. Careful execution of this process will also improve the stability and color of the oil while retaining nutritional value.
Deodorization during physical refining consists of two steps: stripping and deodorization.
Stripping allows the removal of free fatty acids and volatile contaminants in a shorter time at higher temperatures (240°C-260°C, below 2 mbar).
The deodorization step is carried out under vacuum, at temperatures between 180°C - 260°C and using a stripping medium. A deeper vacuum is recommended to promote the evaporation of volatile compounds.
The specific conditions (temperature, duration, combination of both) should be adjusted within these ranges as appropriate, depending on the type of vegetable oil/fat and the type of facility, to ensure the removal of specific substances.
 

3. Additional steps that can be included in chemical and physical refining: Dewaxing
Some oils, such as sunflower oil, contain waxes, which crystallize at low temperatures, making the oil look cloudy. Wax treatment is carried out for optical reasons, as waxes do not affect the taste or other quality elements.
To remove waxes, different procedures are applied. They all have low temperatures at which the wax crystallizes (the process is called "winterization").
Waxes can be removed by wet or dry dewaxing processes. Filter aids are often used in dry dewaxing. Filtration can also be performed without filter aids to obtain a semi-solid oil wax product that can be used in the baking industry.
Filter aids can be subdivided into three categories: perlite (a natural volcanic glass based on sodium potassium aluminum silicate), diatomaceous earth (derived from diatomaceous earth) and cellulose and other organic media (produced by sulfite or sulfate processing of hardwood). Filter aids are often used to remove wax and other solid components from crude oil.

4. Additional refining
Depending on the quality of the oil obtained after refining, additional post-refining steps can be applied to reduce the presence of specific substances. This may include additional bleaching and deodorization steps.
If this is done, care should be taken to consider the appropriate parameters (temperature, duration, combination of both). If it is not possible to prevent the formation of certain contaminants in advance or reduce them to the required level, post-treatment can be used.