Use of absorbents to extend the shelf life of frying oil in food industry

The degradation of the oil due to frying causes the formation of potentially toxic components. There is yet no common European legislation regulating the presence of toxic substances in frying oil. However, a percentage limit for total polar compounds (TPCs) has been established in the legislation of major European countries. This is an imprecise parameter as not only acrylamide but also all other oil degradation products from hydrolysis, oxidation and polymerisation processes are included in this group. The maximum permitted level is around 25% in the majority of EU countries.
Once this limit is reached, the waste oil is converted into a residue, the volume of which is estimated to be around 150 million litres per year in Spain. A part of this large quantity of oil, around 40%, is managed correctly and is usually treated to produce biodiesel. The remaining 60%, of which about 50 million are produced by the industrial sector, is not managed adequately, with the risk of having a significant environmental impact due to its high contaminating power. According to a recent study, the impact of the cost of purifying untreated oil is about 90 million €/year in Spain.
The disposal of oil involves a high cost for the companies, as well as having a significant environmental impact due to the necessary purification treatment. For this reason, it is advantageous to develop methods that will extend the useful life of the frying oil for as long as possible.

Different methods have been used, such as membrane processing techniques or separation of triacylglycerols by supercritical carbon dioxide extraction. However, these methods have not yet been used industrially due to their relatively high costs. More economical and particularly effective techniques are to use filtering or adsorbing materials.
With regard to the use of adsorbing materials, recent studies have shown how to use waste materials from plant production as adsorbents, has led to a significant reduction in the products of oil degradation.
Adsorption is a process where the molecules of a solute (adsorbed) from a liquid phase are concentrated in a solid surface (adsorbent) through the action of intermolecular forces (physical, ionic, affinity) between the solute and the solid. This process has been used for a long time in the food industry for oil and sugar refining processes, or in the clarification of fruit juices; it is also used in other industrial sectors as environmental treatment in the purification of water and gas.
The absorbents considered in this work were bentonite, activated carbon, diatomaceous earth, and silica dioxide. Different studies have already been carried out on these absorbents with regard to their regenerative capacity.
Zhang and Addis reported the removal of FFA from frying oil by diatomaceous earths. As well as activated carbon, it has been shown to improve the quality of rapeseed oil used in combination with silica.
In addition, a recent study has already treated used soybean oils with adsorbing combinations of silicon dioxide (SiO2), activated carbon, active clay, bentonite and citric acids, finding an improvement in the physical-chemical quality of the oil.
The choice to reuse the absorbents already tested in other studies, depends on the fact that in this case the oil taken into consideration is sunflower oil, particularly used in the Spanish food industries, while in previous studies other types of oil have been used.