Vegetable oil oxidation: Mechanisms, impacts on quality, and approaches to enhance shelf life

Abstract

Vegetable oils (VOs) are essential components of the human diet and serve as the primary source of dietary fats. However, lipids in food are highly sensitive to environmental factors such as exposure to light and high temperatures, which promote the formation of free radicals and accelerate lipid oxidation, leading to an unstable food system. Their susceptibility to oxidation poses a significant challenge to both food quality and human health. In fact, oil oxidation is a major concern for the food industry as it leads to loss of nutritional value, formation of undesirable off-flavors and changes in the functional quality and chemical composition of oils, reducing consumer acceptance. In addition, oxidation produces potentially toxic compounds that pose significant health risks. Considering these concerns, this critical review examines various aspects of oil oxidation, including its impact on oil quality, underlying mechanisms, recent measurement techniques, shelf-life prediction and strategies to prevent and mitigate oxidation. The selection of appropriate methods, adapted to specific food matrices, is crucial for the accurate assessment of lipid oxidation and antioxidant capacity.

Graphical abstract

Highlights

• •Oxidation in VOs has a significant impact on their quality.
• •Oxidation monitoring is performed by measuring quality indices parameters.
• •Real-time and accelerated stability tests are used to rate a product's shelf-life.
• •Various strategies have been employed to increase oil stability.

1. Introduction

Edible vegetable oils (VOs) are considered an essential part of the diet due to their sensory and nutritional properties, as they are a principal source of energy. Edible VOs provide human body with important nutrients as well as essential fatty acids (Chew & Ali, 2021). VOs are obtained from fruit pulp or oilseeds (Gharby, 2022). They consist mainly of diacylglycerides (95–99 %), Other components, such as phosphatides, pigments, sterols, and tocopherols, flavonoids, glycolipids are present in low concentrations (1–5 %) (Vidrih et al., 2010). There is a wide variety of VOs depending on plant matrices from which are extracted and their final end-use. Such VOs are consumed by humans as foods, but they can be used also for other purposes such as cosmetics, pharmaceuticals, etc. The most common VOs are argan, almond, corn, olive, rapeseed, peanut, sunflower, soybean, and sesame (Gharby, 2022; Gharby & Charrouf, 2022; Suryawijaya et al., 2022; Szyczewski et al., 2016; Zhang et al., 2018).

Polyunsaturated fatty acids (PUFAs) are the primary functional components that give VOs their beneficial health effects (Le Priol et al., 2021; Nounah et al., 2021). However, the richness of VOs in PUFAs poses several challenges. Among them, their high susceptibility to oxidative degradation, (Aberkane et al., 2014). The main reason behind the deterioration and loss of quality in VOs is their sensitivity to oxidation (Osanloo et al., 2021). The oxidation is a complex phenomenon that takes place in three radical-generating stages: Initiation, propagation, and termination (Gharby et al., 2022). This leads to the formation of peroxides, intermediate peroxides, and stable oxidation products. In the presence of initiators such as light, metal ions (referring to the essential trace elements e.g., Fe²⁺, Cu²⁺), heavy metals (generally referring to metals with a high atomic weight or density e.g. Pb²⁺, Hg²⁺, and Cd²⁺), temperatures and oxygen stimulates the oxidation reaction (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c). This phenomenon can lead to the formation of hazardous molecules and have a negative effect on the organoleptic qualities of foods (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c; Galanakis et al., 2018; Gharby et al., 2022).

Synthetic antioxidants are frequently used in the oil industry to hinder oxidation reactions (Bijla et al., 2024). Enriching VOs with natural antioxidants is another strategy that can be used to delay the oxidation reaction (Şahin et al., 2017). This review discusses VOs, lipid oxidation, and its prevention.

2. Bibliometric analysis

A bibliometric study is a quantitative statistical analysis of publications and provides an objective, transparent, systematic, quantitative, and reproducible process. It facilitates the identification of connections among publications and research groups, the evaluation of the global impact of a research field, and the ranking of highly cited publications and authors (Blanco et al., 2022).

This study analyzes global publication trends on the oxidation of vegetable oils based on data retrieved from the Scopus Citation Database for the period 2000–2024. A total of 1810 publications related to this topic were identified in the database (Fig. 1a). The paper of Choe and Min (2006b), published in 2006 in the Comprehensive Reviews in Food Science and Food Safety journal, holds the record for being the most cited paper with 1469 citations. The 1810 documents published on the topic “oxidation of vegetable oils” come from 81 different countries (Fig. 1b). China has the best record of publications (224 articles), followed by Spain (103), USA (99), Italy (93), and Brazil (92). WANG X produced the most documents, 27 in total (Fig. 1c). The most frequently referenced paper, of this author, was cited 128 times. Furthermore, keywords occurrence analysis shows that the most frequently used ones are oxidation with 656 occurrences and vegetable oil with 438 occurrences (Fig. 1d).

Fig. 1.

Bibliometric analysis of global research on vegetable oil oxidation (2000–2024) based on the Scopus database: (a) citation trends and publication output, (b) most productive countries, (c) leading authors, (d) term mapping using VOSviewer, and (e) document distribution over time.

The Scopus database's distribution of documents by category is shown in Fig. 1e. It consists primarily of “research article” (83.37 %), then “review article” with 6.19 %, and “conference paper” with 5.86 %. While the book chapter had a small representation of only 3.09 %.

This review analyzes the top ten global journals publishing articles on vegetable oil oxidation based on Scopus data from 2000 to 2024. The Journal of The American Oil Chemists' Society, hosted by SAGE publisher John Wiley and Sons Inc., publishing the most articles (129 according to Scopus). Other journals are European Journal of Lipid Science and Technology (74 papers), Food Chemistry (69), LWT (47 documents), Journal of Agricultural and Food Chemistry (40), Food Research International (37 documents). The final four journals published less than 30 documents.

3. Oxidation of vegetable oils

VOs are subject to oxidation owing to their high unsaturated fatty acids (UFAs) (Aissa et al., 2023; Gharby, Harhar, El Monfalouti, et al., 2012; Gharby, Harhar, Guillaume, et al., 2012). Oils with a higher proportion of unsaturated fatty acids tend to oxidize more rapidly than those with a lower degree of unsaturation. This process contributes to the degradation of VOs quality, their stability, as well as their essential fat content that degrade their organoleptic and nutritional qualities and thus shortens their shelf life. Oxidation is considered the main cause of degradation and it can result in toxic oil during VOs production and storage (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c).

The oxidation reaction starts with the formation of primary products such as hydroperoxides and peroxides (Gagour et al., 2022; Gagour, Oubannin, et al., 2022; Setyaningsih & Siahaan, 2018). These products quickly convert into secondary volatiles oxidation compounds (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c). These volatile compounds include aldehydes, ketones, hydrocarbons, alcohols and esters, responsible for the deterioration in the flavor of vegetable oils, known as “oxidative rancidity”.

VO oxidation is a phenomenon that produces free radicals, which are molecules with one or more single electrons in their outer layer. These free radicals are reported to be harmful to health, and they also enable the formation of hydroperoxides. (Indiarto and Qonit, 2020a, Indiarto and Qonit, 2020b; Vishnoi et al., 2018).

Free radicals are unstable, neutral, or charged chemical substances that seek an electron from their environment to make them more stable. These properties make oxidation reactions very fast and propagate in a cascade. Such phenomenon can result from several reaction pathways (Césaire et al., 2019). In such environments conditions, there kinds of oxidation can be distinguished namely photo oxidation, enzymatic oxidation, and autooxidation (Shahidi & Hossain, 2022).

Oil oxidation is of great interest as regards the palatability, nutritional quality and toxicity of edible oils (Choe & Min, 2006b). Oxidation of edible oils is influenced by several factors such as energy input such as light or heat, types of oxygen, lipid substrate such as composition of fatty acids, and minor compounds such as metals, pigments, phospholipids, free fatty acids, mono- and diacylglycerols, and antioxidants. Extensive efforts have been focused on developing the oxidative properties of oils through systematic studies of the effects of these factors. Nevertheless, measuring lipid oxidation remains a difficult task, as this process is highly complex (Ansorena et al., 2023).

4. Oxidation mechanisms

There are many mechanisms underlying VOs oxidation (Fig. 2). This process occurs through two main mechanisms: direct photolysis and reactions with reactive oxygen species (ROS) generated when solar radiation interacts with photosensitizers. Certain oil components, such as chlorophyll and pigments, absorb light in the visible (400–700 nm) and ultraviolet (UV) regions, leading to photooxidation. UV radiation, a type of non-ionizing radiation emitted by the sun, plays a key role in this process. The UV spectrum is divided into UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (100–280 nm), though UV-C is filtered by the atmosphere. The various oxidation pathways are described and developed in the following paragraphs.

Fig. 2.

Mechanisms of auto-oxidation.

4.1. Photo-oxidation

The photo-oxidation phenomenon is recognized as the most harmful to vegetable oils' oxidative stability. Aliphatic and aromatic oxidized products are produced by photo-oxidation. Photosensitizers absorb visible or near-UV light and transform triplet oxygen into singlet oxygen, a highly reactive agent, non-radical molecules, in the presence of light. Temperature has a moderate impact on singlet oxygen oxidation compared to triplet oxygen, as less energy is required to initiate the reaction with VOs. These undergo photo-oxidation when exposed to sunlight (Ahmed et al., 2016). The triplet state of the sensitizer can take one of two paths type I or II pathways. In the type I pathway, the 3Sen∗ reacts directly with the substrate (RH) and forms radicals by hydrogen transfer or electron transfer. The radicals then react with triplet oxygen or other molecules to produce oxidized products. Another mechanism of mechanism is type II, also known as singlet oxygen oxidation. Oxidation. In this case, 3Sen∗ activates triplet oxygen to singlet oxygen (¹O₂) by triple annihilation (Dridi, 2016).

$${\mathit{Sens}}^3+{}^3{O}_2\to {}^1{O}_2+\mathit{Sens}$$ (1)

Then, the photosensitive molecule, in its excited state, acts as a free radical and draws a hydrogen from the UFAs to form a free radical, which can react with the oxygen molecule in its excited state.

$${\mathit{Sens}}^3+\mathit{RH}\to \mathit{SensH}+R^{•}$$ (2)

VOs containing photosensitizers undergo fast photo-oxidation in the presence of singlet oxygen (Li, Li et al., 2019). The photo-oxidation reaction rate is much faster than that of auto-oxidation.

4.2. Enzymatic oxidation

Hydrolysis releases fatty acids from the triacylglycerol molecule, resulting in an increase in GLA and a change in flavor. and a change in flavor. Factors influencing hydrolysis include moisture, temperature, enzymes and micro-organisms (Capriotti et al., 2021). Oxidative damage to unsaturated fatty acids (UFAs) involves the action of various enzymes. The process begins with the hydrolysis of acylglycerides by lipases, lipolytic hydrolases, and phospholipases, leading to the release of free polyunsaturated fatty acids (PUFAs). Lipoxygenases then catalyze the oxidation of PUFAs, forming unstable hydroperoxides. Plants contain lipoxygenase enzymes. This enzyme produces the appropriate hydroperoxy derivatives and cis-trans conjugated hydroperoxides when it combines with VOs that include a 1,4-cis, cis-pentadiene system. In the final stage, lyases, isomerases, and dehydrogenases break down these hydroperoxides into a range of volatile and non-volatile compounds, often characterized by strong odors. Similar to those produced through autoxidation, these volatile compounds are also generated by lipoxygenase activity (Madhujith & Sivakanthan, 2019b; Tayeb et al., 2017; T. Wang & Hammond, 2010).

In the case of olive oil, on the other hand, specific enzymatic processes contribute to its characteristic, desirable aroma. Enzymatic oxidation of linoleic and linolenic acids via the lipoxygenase pathway promotes the formation of 13-hydroperoxides, which are subsequently converted to aldehydes and C6 alcohols. These compounds are primarily responsible for olive oil's fresh, green and fruity sensory attributes (Brkić Bubola et al., 2020).

4.3. Auto-oxidation

UFAs in VOs are sensitive to autoxidation, which occurs through the free radical chain mechanism. Speed of the oxidation reaction depends on the number of double bonds in UFAs (Kerrihard et al., 2015). Auto-oxidation is a natural reaction that occurs between VOs and humid air, resulting in a change in their chemical properties, odor, and flavor. It takes place in three phases: The initiation, the propagation, and the termination (Flores et al., 2021; Ghnimi et al., 2017).

The radical mechanism of lipid oxidation is divided into three stages: Initiation (i) (formation of free radicals), propagation (ii) (radical chain reaction, hydroperoxide formation) and termination (iii) (formation of non-radical products). In the presence of heat, light, or metals, the hydrogen atom of the lipid substrate double bond (RH) is extracted while the free radical or alkyl (R^.) is formed. These free radicals react with oxygen to produce a peroxy radical (ROO•) by subtracting the hydrogen atom from another UFAs. This results the formation of primary oxidation products known as hydro-peroxides (ROOH) by following the mechanisms of initiation, propagation, and termination (Ahmed et al., 2016). These mechanisms can occur up to 100 times before two radicals combine and finalize the process (Ayala et al., 2014).

This section describes the main mechanisms of oxidation in vegetable oils (VOs), namely photo-oxidation, enzymatic oxidation and auto-oxidation. During photo-oxidation, photosensitizers in the oil are activated by the absorption of light to produce reactive singlet oxygen, which promotes oxidation. In the case of enzymatic oxidation, which is driven by lipoxygenases, fatty acids are hydrolyzed to form volatile compounds with intense odors. In addition, autoxidation, which occurs via a free radical chain mechanism, leads to the generation of peroxy radicals and hydroperoxides, which reduces the chemical properties and sensory attributes of the oil. Each of these oxidation pathways is influenced by factors such as temperature, light, and the presence of enzymes or photosensitizers, thus affecting the stability and quality of VOs.

5. Oxidation products

5.1. Primary oxidation products

Primary oxidation products (POPs) such as peroxide and hydroperoxide (Gharby, Harhar, Kartah, Chafchauni, et al., 2013; Gharby, Harhar, Kartah, Guillaume, & Charrouf, 2013). These products are rapidly transformed into secondary oxidation compounds (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c). These are formed through a free radical chain reaction involving the abstraction of hydrogen atoms from unsaturated fatty acids, particularly linoleic and linolenic acids, followed by the addition of molecular oxygen (Valgimigli, 2023). The primary products consist largely of conjugated dienes and lipid hydroperoxides such as 13-hydroperoxy-9,11-octadecadienoic acid (13-HPODE) and 9-hydroperoxy-10,12-octadecadienoic acid (9-HPODE), which are commonly derived from linoleic acid (Choe & Min, 2006b).

5.2. Secondary oxidation products

Lipid oxidation may produce hundreds of secondary oxidation products including, 4-hydroxy-trans-hexanal, 4-hydroxy-trans-nonenal, and crotonaldehyde compounds, contribute not only to undesirable flavors but also to various health concerns such as aging, inflammation and cancer. Additionally, these compounds can disrupt cellular signaling pathways, leading to biomolecular damage. Biomarkers associated with lipid oxidation include hydroperoxides, carbonyls, aldehydes, alcohols, furans, keto-cholesterols, epoxy-cholesterols, and oxysterols, which result from both enzymatic and non-enzymatic lipid degradation (Abeyrathne et al., 2021; Q. Hu et al., 2023; Martín-Torres et al., 2022a). Among these, volatile compounds are at the origin of the modification of the odor of the oxidized products (Cuvelier & Maillard, 2012).

6. Impact of oxidation on VOs quality

VOs naturally contain PUFAs, which are easier to digest and more prone to oxidation (Yaseen et al., 2021). VOs oxidation are a complicated phenomenon that can significantly affect VO quality (Matthäus et al., 2010; Zamuz et al., 2020). VOs undergo chemical reactions when they are exposed to oxygen in the air, which leads to degradation of their components. Numerous negative effects, including those on taste, nutrition, health, and the economy, may result from this oxidation (Machado et al., 2023).

6.1. Nutritional impact

Oxidation is an unavoidable process that alters the nutritional quality of VOs. Oxidation can lead to a whole series of negative consequences, such as a reduction in shelf-life and nutritional value deterioration, development of undesirable organoleptic characteristics, and may even lead to the formation of toxic substances (Machado et al., 2023). These undesirable compounds can have harmful effects on the nutritional quality of VOs by reducing the content of UFAs, fat-soluble vitamins (such as vitamins A, D, E, and K), and natural antioxidants (Arab-Tehrany et al., 2012; Matthäus, 2010; Tao, 2015).

The degradation of PUFAs, which are crucial for cardiovascular and neurological health, reduces the nutritional value of oils and contributes to the production of potentially toxic compounds such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) (Valgimigli, 2023; Zheng et al., 2024). Moreover, the loss of natural antioxidants like tocopherols compromises the oil's oxidative stability and diminishes its protective effects against oxidative stress (Collet et al., 2011). Therefore, understanding and controlling oxidation in vegetable oils is critical to preserving their nutritional benefits and ensuring consumer safety.

6.2. Sensory impact

From a sensory standpoint, VOs oxidation can affect the taste, smell, and color of VOs-based foods. VOs with oxidized components may have unpleasant flavors and odors, which lowers their sensory appeal (Let et al., 2005; Matthäus, 2010). The unpleasant flavor notes are contributed by secondary oxidation compounds, which significantly affect edible VOs sensory quality. It is termed as rancidity, this unfavorable product attribute is the main reason why consumers dislike products (Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022a, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022b, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022c).

The breakdown of PUFAs during lipid oxidation results in the formation of volatile aldehydes, ketones, and alcohols, which are primarily responsible for rancid, stale, or paint-like off-flavors (Choe & Min, 2006b). These compounds, even at low concentrations, significantly reduce consumer acceptability and market value of the oils. For instance, hexanal and 2,4-decadienal, commonly formed during oxidation, impart unpleasant grassy or metallic notes (GROSCH, W., 1987). In addition, oxidative degradation of pigments such as chlorophyll and carotenoids can lead to color fading or browning, further affecting the visual appeal (Warner & Frankel, 1987). As a result, the sensory degradation due to oxidation not only compromises the quality perception but also reflects a reduction in freshness and safety of the product. Regular monitoring and antioxidant strategies are therefore essential to preserve the sensory integrity of vegetable oils.

6.3. Health impact

The products of lipid oxidation can cause various biological reactions through dietary intake and in vivo, thereby affecting human health. Due to their high content of UFAs, VOs are sensitive to the oxidation of lipids, which can result in the production of significant amounts of trans fatty acids and other oxidation products with detrimental effects on human health (Ganguly et al., 2016; K. Hu et al., 2020). Nevertheless, the deterioration of oil quality can lead to the formation of reactive and toxic oxidation products that, over time, present health risks, including cancer, atherosclerosis, heart disease, allergic reactions and inflammation (Negash et al., 2019; Redondo-Cuevas et al., 2018). Several authors have conducted systematic reviews on the significance of aldehydes derived from lipid oxidation in regulating key biological processes related to human health and disease. Additionally, lipids oxidation results in the production of some toxic by-products such reactive carbonyl compounds, which may result in advanced lipid peroxidation end products that could be dangerous to human health (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c; Guillén et al., 2005). For example, omega-3 polyunsaturated fatty acids (PUFAs), including DHA and EPA, are vital for human physiology. During food storage and high-temperature processing, on the other hand, they can form aldehydes, such as 4-hydroxyhexenal. These substances are extremely stable and are absorbed from the intestine into the bloodstream (D. Wang et al., 2023).

6.4. Economic impact

VOs oxidation can also have a significant economic impact. Food products containing oxidized VOs may not be appreciated by consumers because of their poor sensory quality, resulting in economic losses for producers and distributors (Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022a, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022b, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022c). For instance, with virgin olive oils, the price of which depends on the quality: “extra”, “fine”, “ordinary” and “lampante”. Extra virgin olive oil is of higher quality and commands a higher price compared to the other categories. “Lampante” virgin olive oil, which cannot be consumed as it sold at low prices for industrial uses, including refining.

This chapter describes oxidation compounds in vegetable oils (VOs), with a focus on primary and secondary oxidation products. The products of primary oxidation, including peroxides and hydroperoxides, are rapidly transformed into secondary oxidation products, among which are aldehydes and volatile compounds that are responsible for unfavorable off-flavors and health concerns, such as inflammatory and carcinogenic effects. Oxidation in VOs has a significant impact on their quality, with implications for their nutritional, sensory and health attributes. From a nutrition perspective, oxidation leads to the loss of vital unsaturated fatty acids, vitamins, and antioxidants, and alters sensory properties such as taste, odor, and color, resulting in rancidity. In addition, oxidation has economic implications as oxidized oils decrease consumer perceptions and decrease the value of oils, particularly virgin olive oils, wherein quality defines price ranges.

7. Methods for measuring oxidative stability of oils

Oxidation is evaluated by simultaneous determination of the peroxide value (PV), refractive index (RI), UV-light absorbance in 232 nm (K₂₃₂), and 270 nm (K₂₇₀), para-anisidine value (p-AV), fatty acids, iodine value (IV), Rancimat test and other tests. PV and K₂₃₂ are very important indicators to determine the primary oxidation products. Such products are quickly transformed into secondary oxidation products. Therefore, they are evaluated by determining the p-AV and K₂₇₀. The phases of oxidation of VOs can also be controlled by other measurements such headspace volatiles (odor), free fatty acids (FFA), thiobarbituric acid number (TBA), IV, Rancimat test, and Shaal oven test (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c; Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022a, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022b, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022c).

7.1. Measurement of primary oxidation compounds

7.1.1. Peroxide Value (PV)

Peroxides are primary oxidation compounds, they are created at the initial stages of lipid oxidation and can be subjected to other oxidation processes to yield secondary compounds (X. Yang & Boyle, 2016). PV is a dynamic equilibrium between two distinct oxidation mechanisms: a peroxide formation step, and a secondary products formation step, as discussed in (Oubannin et al., 2022a).

PV evaluates hydroperoxide and peroxide content formed by the oxidation of UFAs. However, it is a limited measure to the initial phase of the oxidation process because these products are highly unstable at elevated temperatures; they are transformed into secondary oxidation products (Oubannin et al., 2022a). PV is a very important marker for determining the deterioration of UFA products (Flores et al., 2021; Moradi et al., 2023; Randhawa & Mukherjee, 2023). PV determination can be confirmed by measuring the UV absorbance at 232 nm, related to the presence of conjugated diene forms that appear on FA with at least two double bonds (Gharby, Harhar, Kartah, Chafchauni, et al., 2013; Gharby, Harhar, Kartah, Guillaume, & Charrouf, 2013).

In general, a low PV value means that the quality of the oil is better and indicates higher oxidative stability (Drinić et al., 2020; Jafari et al., 2022). The acceptable PV limit for virgin oils like extra virgin olive oil is 20 mEq O₂/Kg (Gagour, Hallouch, Asbbane, Bijla, et al., 2024; Gagour, Hallouch, Asbbane, Laknifli, et al., 2024; Gagour, Ibourki, Antari, Sakar, et al., 2024), extra virgin argan oil is 15 mEq O₂/Kg (Gharby & Charrouf, 2022) and for refined oils is 10 mEq O₂/Kg (Codex Alimentarius, 2023). The PV can be estimated using the method described in ISO 3960:2017, which involves titration with a sodium thiosulfate solution. The PV measures the amount of oxygen chemically bound to an oil or fat in the form of peroxides.

7.1.2. Specific Extinction K₂₃₂

The UV extinction coefficient K₂₃₂ gives additional indications on the formation of primary oxidation products and conjugated dienes (intermediate stage of oxidation). This because the oxidation leads to the formation of hydroperoxides that absorb light at 232 nm (Martín-Torres et al., 2023; Oubannin et al., 2022a). A high K₂₃₂ value implies that the oil is more peroxidized (Elouafy et al., 2022). The K₂₃₂ may be more reliable than the peroxide value (PV) because a spectrophotometer detects a wider range of primary oxidation products, while PV primarily measures the concentration of hydroperoxides.

7.1.3. Measurement of secondary oxidation compounds

The primary oxidation compounds are unstable products, and they are quickly transformed into ketones and aldehydes as secondary oxidation products (Cao et al., 2014; Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c). A wide variety of spectrophotometric methods are available, all of which are based on the measurement of the intensity of the coloration resulting from the complexation of a family of oxidative degradation side products with a specific reagent. Below most of the analytical methods used to assess the quantity of secondary oxidation compounds.

7.1.4. Specific extinction coefficient K₂₇₀

Specific absorption or extinction coefficients at K₂₇₀ indicate the presence of secondary oxidation products such as ketone and aldehyde molecules (Gharby, Guillaume, Elibrahimi, & Charrouf, 2021; Gharby, Guillaume, Nounah, et al., 2021; Harhar et al., 2010; X. Li et al., 2022). The absorbance at 270 nm is not quite equivalent to anisidine analysis since ketones are not detected during such analyses (Aissa et al., 2023; Cong et al., 2020).

7.1.5. P-anisidine value

The content of the secondary oxidation compounds is usually evaluated by measuring the p-anisidine value (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c). The p-AV is a measure to determine the content of products formed in the non-volatile secondary oxidation phase (Gharby, Guillaume, Elibrahimi, & Charrouf, 2021; Gharby, Guillaume, Nounah, et al., 2021). In this phase, the hydroperoxides are transformed into secondary products, many of such secondary products are of a low molecular weight. These are products responsible for a rancid smell in oils and for the deterioration of organoleptic characteristics (Drinić et al., 2020; Gharby, Harhar, Kartah, Chafchauni, et al., 2013; Gharby, Harhar, Kartah, Guillaume, & Charrouf, 2013; Ghohestani et al., 2023; Multari, Marsol-Vall, Heponiemi, Suomela and Yang, 2019a, Multari, Marsol-Vall, Heponiemi, Suomela and Yang, 2019b).

p-AV is higher in oils of strong color and High p-AV indicates advanced oxidation and potential degradation of oil quality, leading to off-flavors and reduced nutritional value (Ismail et al., 2016).

p-AV can be determined according to the method described in ISO 6885, 2016. The analytical procedure involves treating the fatty substance, in an isooctane solution, with the para-anisidine reagent, and the quantity of reaction products is determined spectrophotometrically at 350 nm.

International standards, such as those established by the Codex Alimentarius or the International Organization for Standardization (ISO), typically define acceptable p-AV limits depending on the type of oil, ensuring product safety and consumer protection. Unfortunately, there is no specified p-AV for VOs. However, some authors have reported threshold values that should not be exceeded. For example, Multari et al. classified edible oils as acceptable if the p-AV is below 10 (Multari, Marsol-Vall, Heponiemi, Suomela and Yang, 2019a, Multari, Marsol-Vall, Heponiemi, Suomela and Yang, 2019b). Additionally, Gokoglu et al. suggested that good quality oil should have a para-anisidine value of less than 2 (Gokoglu et al., 2009). For fish oil, the Codex Alimentarius has fixed 20 as a limit value for p-AV (Codex Alimentarius, 2023).

7.1.6. Total oxidation (TOTOX)

To provide a comprehensive picture of the oil's oxidation state, industry professionals recommend using the TOTOX parameter. The TOTOX value estimates the progressive oxidative degradation of oils by combining PV and p-AV. It measures the total oxidation of a given VO (Jalalizand & Goli, 2021). This indicator seems better than PV or (p-AV) separately because hydroperoxides are unstable substances and do not give reliable information on the oxidative stability of VOs (More & Gogate, 2018). TOTOX values take into account primary and secondary products (peroxides and aldehydes) (Alsufiani & Ashour, 2021). It is preferable to use this index over PV or (p-AV) alone, as fatty acid hydroperoxides are unstable and do not provide a reliable measure of the oxidative stability of oils (Oubannin et al., 2022b). In fact, PV indicates the initial stage of oxidation, and over time, this parameter evolves into volatile and non-volatile aldehydic compounds, which can be assessed using another index, the anisidine index (Gagour et al., 2022; Gagour, Oubannin, et al., 2022). In addition, the PV indicates the current level of oxidation (early stage), while the p-AV determines the extent of secondary oxidation. By combining both values, the total oxidation value can be theoretically calculated using the formula: TOTOX value = (2 x PV) + p-AV. This is because a peroxide group contains twice the amount of oxygen compared to the aldehyde group measured by the p-anisidine parameter. The Totox value for high-quality VOs should be less than 10 (Nid Ahmed et al., 2024a), while for fish oil, it should be less than 26, according to the international standard (CODEX STAN 329-2017).

7.1.7. Test thiobarbituric acid

Another parameter for evaluating the quantity of secondary oxidation products is the Thiobarbituric Acid Test (TBA Test). It is considered an indicator of the formation of secondary lipid oxidation products (Yodkaew et al., 2017). TBA is expressed as milligrams of malonaldehyde (MA) equivalents per kg of oil, or micromoles of MA equivalents per gram of oil (Flores et al., 2021), resulting in the formation of a Schiff base of pink color absorbing at 532–535 nm and fluorescent (excitation 515 nm, emission 553 nm) (Schaich, 2016). In general, significant quantities of substances that react with TBA are only formed when they are fatty acids with three or more double bonds (Flores et al., 2021).

7.1.8. Volatile compounds and polyunsaturated fatty acids

Measurement of volatile compounds and PUFAs in vegetable oils is critical for assessing both the nutritional quality and oxidative stability of the oils. Volatile compounds, which are primarily secondary oxidation products, are responsible for off-flavors and are commonly analyzed using gas chromatography coupled with mass spectrometry (GC–MS) or flame ionization detection (GC-FID) (Starowicz, 2021).

Additionally, chromatography techniques can be used to measure the formation of secondary compounds such as aldehydes, alcohols, hydrocarbons, ketones, and short carboxylic acids, as well as the degradation or consumption of PUFAs (Domínguez, Pateiro, et al., 2019; Domínguez, Purriños, et al., 2019).

These compounds formed through the breakdown of lipid hydroperoxides, especially those derived from PUFA oxidation (Guillen & Goicoechea, 2008). On the other hand, the profiling of PUFAs, such as linoleic acid (C18:2) and α-linolenic acid (C18:3), is typically performed by gas chromatography after derivatization to fatty acid methyl esters (FAMEs). This process involves transesterification using methanolic potassium hydroxide or sulfuric acid, followed by GC-FID analysis for quantitative assessment (Ichihara & Fukubayashi, 2010). The presence of high PUFA content in oils contributes to nutritional value but also makes the oils more susceptible to oxidation, which is why simultaneous monitoring of both PUFA levels and oxidation volatiles provides comprehensive insight into oil quality during processing and storage. Therefore, combining these analytical approaches is essential for characterizing oil freshness, predicting shelf-life, and developing strategies to enhance oxidative stability (Guillén & Cabo, 2002).

7.2. Accelerated oxidation tests

The oxidative stability of oils can be determined using a variety of standard techniques. Accelerated oxidation tests are widely used to evaluate the oxidative stability of VOs, which is a critical factor determining their shelf life and quality. This accelerates the oxidation process, allowing researchers to predict the oil's stability over time under normal storage conditions (Gagour, et al., 2022; Gagour, Oubannin, et al., 2022). The data obtained from these tests can help in determining the oil's resistance to oxidative degradation (Aissa et al., 2023). Several rapid techniques have been devised to evaluate the oxidation resistance of edible fats and oils. These methods typically employ elevated temperatures, as it is well-established that the rate of oxidation increases exponentially with temperature. In this section, we present the most commonly used techniques in quality control and scientific research.

7.2.1. Swift test or Active Oxygen Method (AOM)

Active oxygen method is a technique used to measure the oxidative stability of fats and oils by accelerating the oxidation process with active oxygen. This test is widely used to assess the oxidative stability of VOs and has been used for six decades, undergoing several modifications by some authors. The fundamental principle of this test revolves around subjecting the oil to a consistent high temperature within a glass tube, while simultaneously passing dry air through it at a specific rhythm. The purpose of such a procedure is to significantly expedite the aging and development of rancidity in the oil. By employing this method, researchers are able to evaluate the oil's susceptibility to oxidation under controlled conditions (Flores et al., 2021).

7.2.2. Oven method or Shaal test

The Schaal Oven Test assesses the oxidative stability of oils by subjecting them to accelerated oxidation in a monitored and controlled environment. The test involves heating the oil in a heated oven at 60–65 °C for a specified period of time, promoting oxidation and simulating long-term storage conditions. The close to the real conditions of storage and the simplicity on use and its standardization among the methods referenced AOCS (Botosoa & Karoui, 2022; Flores et al., 2021; Lkrik et al., 2015; Y. Yang et al., 2016). Monitoring of oxidation during the test is typically performed by measuring key parameters such as peroxide value (PV), p-anisidine value (p-AV), and conjugated dienes (K232, K268), which indicate the extent of primary and secondary oxidation. Oven test is one of the most reliable tests for determining oxidative stability in VOs (J. Chen et al., 2020).

7.2.3. Rancimat test

The Rancimat method involves measuring the conductivity of volatile compounds generated during oxidation. First developed in 1974 by Hador and Zuecher to determine the induction period (IP) of fats and fatty acid esters 100, the Rancimat method was designed and developed as an alternative to the AOM complex system for determining the induction time of fats and fatty acid esters. (Hadorn & Zurcher, 1974). and has a positive correlation with particular advanced oxidation measurements, like differential scanning calorimetry (Tan et al., 2002). The oxidative stability of VOs is evaluated using this technique, based on conductometric measurement of volatile degradation products produced by thermal oxidation of VOs (Redondo-Cuevas et al., 2018). The induction period, a numerical value used to evaluate the relative stability of oils and fats, is what determines the end point of Rancimat (Pawar et al., 2014).

The sample is held in a sealed reaction tube at a constant temperature, the majority of tests are performed at 110 °C - 120 °C while a continuous flow of air (10–20 L/h) is passed through it. Some studies use different temperatures ranging from 100 to 150 °C to estimate the shelf life of oils under normal conditions (Aissa et al., 2023; Farhoosh, 2007a; Farhoosh & Hoseini-Yazdi, 2014; Gagour et al., 2022; Gagour, Oubannin, et al., 2022). Rancimat technique is one of the methods traditionally used to evaluate the oxidative stability of VOs.

It has several advantages (good reproducibility, easy and fast) (Le Priol et al., 2021). The UFAs in the sample are oxidized to peroxides (primary oxidation products). After some time, these primary oxidation products decompose completely, forming secondary oxidation products (Gharby & Charrouf, 2022). In addition to volatile compounds, these secondary products include low-molecular-weight organic acids. An air flow transports them to a measuring vessel containing deionized water (70 mL) as the absorption solution. The conductivity of this solution is recorded continuously; as soon as volatile compounds are formed in the sample; this is indicated by an increase in conductivity. The time until secondary reaction products are detected is known as induction time. It characterizes the oxidation stability of fats and oils (Table 1). The resulting curves were evaluated automatically by the software of the Rancimat (Hajib et al., 2021). (See Table 2.)

Table 1.

Oxidative stability of VOs using Rancimat test.

Sample	Temperature	Flow	PV (mEq O2/kg)		Reference	COX	Reference
Amolnd oil	110 °C	20 L/min	13.12 ± 0.66		(Benbouriche et al., 2022)	–	–
Cactus oil	120 °C	20 L/min	2.20–3.90		(Nounah et al., 2024)	6.70	(Asbbane et al., 2024)
Cosmetic Argan oil	110 °C	20 L/min	12–14		(Oubannin et al., 2023)	–	–
Food argan oil	110 °C	20 L/min	24–31		(Gharby et al., 2011a, Gharby et al., 2011b)	3.85	(Asbbane et al., 2024)
Nigella oil	110 °C	20 L/min	15–19		(Gharby et al., 2017)	6.26	(Ait Bouzid et al., 2024)
Olive oil	110 °C	20 L/min	11.90–26.10		(Gharby, Harhar, El Monfalouti, et al., 2012; Gharby, Harhar, Guillaume, et al., 2012)	2.75	(Gagour, Hallouch, Asbbane, Bijla, et al., 2024; Gagour, Hallouch, Asbbane, Laknifli, et al., 2024; Gagour, Ibourki, Antari, Sakar, et al., 2024)
Peanut	110 °C	20 L/min	9.00 ± 0.18		(Benbouriche et al., 2022)	4.63	(T. Xu et al., 2015)
Picholine	110 °C	20 L/min	17–24		(Gharby, Harhar, El Monfalouti, et al., 2012; Gharby, Harhar, Guillaume, et al., 2012)	2.74	(Gagour, Hallouch, Asbbane, Laknifli, et al., 2024)
Pomegranate oil	110 °C	20 L/min	3.60 ± 0.93		(Hajib et al., 2021)	–	–
Rapeseed	110 °C	20 L/h	8.00 ± 0.10		(Symoniuk, Ksibi, et al., 2022; Symoniuk, Wroniak, et al., 2022)	4.36	(Wroniak et al., 2021)
Sesame seed oil	110 °C	20 L/min	28–29		(Gharby et al., 2017)	4.52	(Ait Bouzid et al., 2024)
Soybean oil	120 °C	20 L/h	8.00		(Li, Yang, et al., 2019)	7.69	(Bachari-Saleh et al., 2018)
Sunflower	100 °C	20 L/h	12.90		(Velasco et al., 2004)	5.69	(Nid Ahmed et al., 2024b)
	PV: Peroxide value. COX: Theoretical Oxidizability Index of Oils

Table 2.

Shelf life for some vegetable oils.

VOs	Brief Description	Shelf life	Reference
Mechanically extracted food argan oil	Storage at 5 °C, 25 °C (protected or exposed to sunlight), and 40 °C for two years	24 months	(Gharby et al., 2011a, Gharby et al., 2011b)
Traditional food argan		18 months	(Gharby et al., 2011a, Gharby et al., 2011b)
Cometic argan oil	The shelf life of oil was determined by using Rancimat test	11 months	(Aissa et al., 2023)
		196–435 days	(Matthäus & Brühl, 2015)
Refined argan	The shelf life of oil was determined by using Rancimat test	6 months	(Aissa et al., 2023)
Virgin olive “arbequina	The shelf life of oil was determined by using Rancimat test	9 months	(Gagour et al., 2022; Gagour, Oubannin, et al., 2022)
Virgin olive “Moroccan Picholine”	The shelf life of oil was determined by using Rancimat test	17 months
Flaxseed		1614 days	(Hashim et al., 2023)
Soybean oil	The shelf life of oil was determined by using accelerated method, temperature (35, 45 and 55 °C) in acceleration/environmental chamber (model VC3 4060/VC3 7060)	36.953 (week)	(Alemayhu et al., 2019)
Peanut oil		42.115 (week)	Alemayhu et al., 2019)
Cottonseed oil		37.816 (week)	Alemayhu et al., 2019)

7.2.4. Oxitest

The Oxidation Test Reactor (OXITEST) is an accelerated oxidation test that measures the induction period (Wy et al., 2023) and the kinetics of oxidation processes. OXITEST can analyze various sample types, including liquids, solids, and powders (Wy et al., 2023). Furthermore, the interior components of the equipment, including the oxidation chambers, sample holders, and covers, are constructed from titanium to resist chemical corrosion (Wy et al., 2023). This instrument monitors the oxygen (O₂) uptake of reactive components in the oil to evaluate oxidative stability under accelerated conditions. The reactor temperature can be regulated from room temperature to 110 °C, and it operates under high O₂ pressure, up to 8 Bar (Comandini et al., 2009). Indeed, the OXITEST subjects the sample to a high-oxidative-stress environment (oxygen overpressure and high temperature) to assess, in a short period of time, the resistance of the oil to oxidation and renders to predict the shelf-life. For greater efficiency of this method he interior components of the equipment, including the oxidation chambers, sample holders, and covers, are constructed from titanium to resist chemical corrosion Several studies have reported its use for determining the oxidation stability of oils (Comandini et al., 2009; Tinello et al., 2018), and the American Oil Chemists' Society (AOCS) has developed an Official Methods (Cd 12c-16) based on this instrument.

7.2.5. Theoretical oxidizability index of oils

The oxidizability value is a crucial factor in evaluating the stability of oils and fats. Indeed, the degree of unsaturation in the fatty acid composition significantly influences the susceptibility of oil to oxidation (Nosratpour et al., 2017). The oxidizability of different edible oils depends on the relative oxidative stability of their constituent fatty acids. After determination of fatty acids profiles using chromatography technique, The Cox can be determined by the percentage of unsaturated C18 fatty acids, following the formula:

• COX = (1 [C18:1 %] + 10.3 [C18:2 %] + 21.6 [C18:3 %])/100 (T. Xu et al., 2015)

Table 1 shows the COX of some VOs.

7.2.6. Oxidation Induction Time (OIT) evaluated by DSC

Oxidation Induction Time (OIT) is a standard test procedure that characterizes the stability of oils against oxidation (Islam et al., 2023). Generally, a shorter OIT indicates reduced oxidative stability, suggesting a higher susceptibility to oxidation. The OIT values were obtained using isothermal differential scanning calorimetry (DSC). The oxidation induction time is defined as the period from when the atmosphere is replaced until the rise of the exothermic peak caused by oxygen absorption. OIT is the time that elapses between the initial exposure to oxygen (t0) and the onset of oxidation.

DSC also serves as an accelerated test of oxidative resistance by exposing samples to elevated temperatures in the presence of excess oxygen (Tan et al., 2002). Both liquids and solids are suitable for DSC analysis. DSC method is widely used as an analytical, diagnostic and research tool from which relevant information and provides structural indications starting from stability up to structural changes (e.g. glass transition, crystallization, etc.) (Costa-Fernandez et al., 2022). This method is a thermal analysis technique used specifically to determine phase transition temperatures, such as glass transition (Tg) and melting temperature (Tm), and the heat capacity (Cp) of a sample. The sample is placed in a container of known dimensions, which can be hermetically sealed if required, and subjected to a controlled heating rate (°C/min). The variation of the heat flow through the sample is continuously recorded and plotted as a function of temperature (Mahmoud & Tan, 2018). Additionally, the DSC technique is used to study the oxidation process of oil samples in an oxygen-flow cell (Różańska et al., 2019). Oxidation curves may show an increase or decrease, depending on the DSC measurement approach - heat flow DSC or power compensation DSC. Nevertheless, oxidation remains an exothermic reaction, independent of the measurement method. The oxidation temperatures and kinetic parameters determined by DSC analysis provide a basis for classifying lipids according to their oxidation stability (Saldaña et al., 2013).

The oxidative stability of oils is evaluated by different methods, among them the determination of primary oxidation compounds such as peroxide value (PV) and specific extinction coefficients (K232), as well as secondary oxidation products such as p-anisidine value (p-AV), K270 and total oxidation (TOTOX). These measures are useful in measuring the extent of oxidation, whereby PV and K232 highlight early stages of oxidation, while p-AV and K270 indicate progressive oxidation. Rapid tests, such as the Rancimat test, the Schaal oven test, and the Active Oxygen Method (AOM), are employed to determine the oil's stability to oxidation under monitored conditions, thus offering useful insights into the shelf life and stability of the oil.

8. Determination and Prediction of Shelf Life of VOs

8.1. Shelf-life of vegetable oils

Shelf-life is defined as the duration, usually indicated on the label as “best before” dates under defined environmental and distributing conditions”. The duration for which food can be stored while retaining its safety, chemical, physical, sensory and biological properties, as well as compliance with all label information, is known as the shelf-life. In other words, is the duration for which a product may be stored before it is considered unsuitable for use or consumption (Manzocco et al., 2010; Piergiovanni & Limbo, 2019; Young & O'Sullivan, 2011). Shelf-life therefore generally has no bearing on the safety of the food product in question, since foods that have exceeded their recommended shelf life are not considered immediately unsafe for human consumption, but they fail to meet a given set of quality parameters (Young & O'Sullivan, 2011).

The longevity of oils is mainly determined by their oxidative stability, i.e. their ability to resist oxidation. This stability refers either to the time it takes to reach a critical oxidation point, which can be revealed by changes in taste or smell, or by a brutal increase in the rate of oxidation (Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022a, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022b, Martín-Torres, Tello-Jiménez, López-Blanco, González-Casado and Cuadros-Rodríguez, 2022c). As described earlier in the present work, various factors such as light exposure or heat treatment or even occurrence of microorganisms affects the oxidative stability of oils, leading to both oxidative and hydrolytic rancidity and then, limited shelf-life stability (El Maouardi et al., 2023; Syed, 2016). This knowledge is crucial as the appetence, nutritional value and, in some cases, toxicity. Food shelf-life estimation is not only a fascinating methodological feat for food researchers, yet a critical challenge for food manufacturers seeking to sustain their market reputation in the marketplace (Manzocco et al., 2016).

Two stability testing methods are commonly used to estimate a product's shelf-life: Real-time stability testing, and accelerated stability testing. The first one involves keeping the product under suitable storage conditions and monitoring it until it no longer meets the specified criteria. In contrast, the second exposes the product to high-stress conditions, such as oxygen, humidity, and temperature, to predict its degradation under normal storage conditions, based on established relation between degradation rate and acceleration factor (Haouet et al., 2019; Magari, 2003). A better scientific understanding of deterioration processes, reaction rates and their environmental dependencies allows mathematical models to be applied to evaluate various alternative scenarios or improve distribution systems, thus improving quality at the end of the oil's shelf-life (Corradini, 2018). Estimating product shelf-life requires modeling the deterioration process as outlined by (Corradini, 2018). To achieve this, it is necessary to have a thorough understanding and description of two key relationships, namely (a) how quality attributes and safety markers change according to time, and (b) the impact of such conditions and associated changes and their associated rates as discussed herein.

Developing a cost-effective trial design for assessing the shelf-life of foods subjected to oxidation reactions requires a systematic approach to applying. This entails defining an acceptability limit that differentiates those which products are acceptable versus those that are no longer acceptable. Following the selection of an acceptable oxidation indicator, shelf-life tests must be conducted to anticipate the product's lifespan using kinetic modeling. Based on the deterioration in product quality over time, shelf-life tests are conducted either under real storage settings or under accelerated storage conditions. While testing perishable items under real storage settings utilizing direct measurements is possible. After choosing the most suitable acceleration factor for oxidation kinetics, stability testing under conditions that hasten quality degradation is necessary for long-term shelf-life prediction (Manzocco, 2016; Realini & Marcos, 2014).

8.2. Real-time shelf-life testing

In real-time stability testing, tests can be carried out under recommended conditions simulating that provide a reasonable simulation of the expected conditions on the market and monitored until it fails the specification (Haouet et al., 2019). The basic rule for performing a reliable real time shelf-life test is that the storage environment conditions (light, oxygen, temperature, humidity, etc) must be maintained at the same level as the normal storage conditions of the VO. Therefore, it is necessary to maintain a measurement system operating in real time, complemented by predictive models. A persistent question in monitoring lipid oxidation is to determine which oxidation products are most appropriate to be monitored. Nevertheless, EVAO shelf-life can be assessed by means of variations in fatty acid composition, volatile compounds, and peroxide value. SL is also monitored using UV absorption coefficients. When measured at 270 and 232 nm, these are typical of the conjugated trienes and dienes produced when PUFAs undergo oxidation respectively (Kharbach et al., 2021), total phenol content identification of tocopherol and pigment profiles, pyropheophytin evolution.

8.3. Accelerated stability tests

8.3.1. Use the Rancimat test for Shelf-Life Prediction

To ensure rapid induction of lipid oxidation, this test requires exposing an oil sample to a uniformly high temperature (50–200 °C) and a specific air flow rate (between 1 and 25 L/h), thus ensuring adequate oxygen input (Félix-Palomares & Donis-González, 2021). The acids generated are rapidly dissolved in distilled water in a separate vessel, while the conductivity of the solution is controlled at room temperature. The final step in the rancimat test, which measures the relative stability of fats, is established by the induction period (h) depending on temperature (110 °C, 120 °C…). The oxidative stability index (OSI) exhibits a strong correlation with lipid oxidation stability under different conditions, as well as with data collected through separate sensory and analytical approaches (J. Wang et al., 2016).

Temperature plays a crucial role as an accelerating factor in the Accelerated shelf-life testing (ASLT) due to its significant impact on oxidative kinetics and the opportunity provided by a mathematical equation that captures the temperature dependency and responsiveness of reaction rates. The temperature coefficients for fat samples are represented by the slope of the oxidation rate curves. A commonly used metric, known as Q₁₀ number, predicts oxidation rate increases with a 10 °C increase in temperature. This is particularly relevant for oils and fats as their degradation rate can be characterized using the Arrhenius equation (Farhoosh, 2007b; Simoes Grilo et al., 2020). The formula known as the Arrhenius equation is used extensively to determine how reaction rates depend on temperature, making it a crucial tool for calculating both the rate of chemical reactions and the energy of activation which is widely used in different VOs (Gagour et al., 2022; Gagour, Oubannin, et al., 2022).

$$\mathit{Ln}\left(k\right)=\mathit{Ln}\left(k_0\right)-\left(\frac{E_a}{R}\right)\times \left(\frac{1}{T}\right)$$ (I)

$$k=k_0\times e^{-\frac{E_a}{\mathit{RT}}}$$ (II)

The Arrhenius Eqs. (I), (II) are a mathematical model that incorporates several variables, including the reaction rate constant (k), the molar gas constant (R) (8.31 J/Kmol), the absolute temperature (T) measured in Kelvin, the apparent activation energy (Ea) in J/mol (representing the minimum energy required to initiate a chemical reaction), and the pre-exponential factor (k_o). By conducting oxidation rate measurements at a minimum of three different temperatures, it becomes feasible to predict the reaction rate at a desired target temperature using this equation (Calligaris et al., 2004; Gagour et al., 2022; Gagour, Oubannin, et al., 2022).

$$Q_{10}=e^{10T_c}$$ (III)

The factor of temperature acceleration at 10 °C higher temperature is a quick way of determining the increase in reaction rate/decrease in shelf life due to an increase in temperature. T_C represents the temperature coefficient, indicating the sensitivity of the oxidation rate to temperature. This factor is known as the Q₁₀ value, as mentioned in the Eq. (III) (Mizrahi, 2004).

Prior to using the Arrhenius pattern to estimate shelf-life, it is crucial to identify the activation energy and the temperature range in which the pattern is suitable. With this preliminary approach, the maximum temperature can be selected at which testing can be carried out in order to achieve the estimated shelf life within a short period of time. Nevertheless, it should be pointed out that for either the Arrhenius model or the Q₁₀ approach to be successfully applied, foods must not suffer any adverse consequences except oxidation when subjected to higher temperatures (Farhoosh, 2007b). The activation energy (Ea, kJ/mol) and frequency factor (A, h ⁻¹) were estimated using the slopes and intercepts of the Arrhenius Eq. (IV).

$$\mathit{Ln}\left(k\right)=\mathit{Ln}\left(A\right)-\frac{E_a}{\mathit{RT}}$$ (IV)

The Eq. (V) derived from the activated complex theory involves the reaction rate constant (k), which is the reciprocal of the observed stability index (OSI) or induction period (IP) measured in units of hours (h⁻¹). The molar gas constant (R), with a value of 8.3143 J/mol K, is also used in the equation. By conducting a regression analysis, the enthalpies (ΔH⁺⁺) and entropies (ΔS⁺⁺) of activation can be plotted by relating the logarithm of k divided by temperature (T, measured in Kelvin) to the inverse of the temperature (1/T). This analysis allows for the determination of the thermodynamic parameters associated with the activation process:

$$\mathit{ln}\left(\frac{k}{T}\right)=\mathit{ln}\left(\frac{k_b}{h}\right)+\left(\frac{∆S^{++}}{R}\right)-\left(\frac{∆H^{++}}{\mathit{RT}}\right)$$ (V)

ΔH⁺⁺ and ΔS⁺⁺ were calculated using the slope and ordinate of the lines, where the value of kB, which is Boltzmann's constant (1.380658 $\times$10⁻²³ J/K) representing the proportion of R to the Avogadro number (6.022 $\times$10²³ mol⁻¹), as well as h as the Planck's constant (6.6260755 $\times$10⁻³⁴ Js), are taken into consideration.

Assessing the shelf-life of VOs prior to marketing is crucial. To this end, the oxidative stability of oils and fats is examined with the accelerated Rancimat test method, offering a simple but effective means of determining their shelf life. Measurements of induction time at high temperatures (generally 100, 110, 120 °C) within a matter of hours, then tracing the logarithm of the data as a function of temperature, allow us to estimate the shelf life of oils at room temperature (25 °C) (Gagour et al., 2022; Gagour, Oubannin, et al., 2022; Kochhar & Henry, 2009). The calculation approach used for the shelf-life factor is analogous to that employed for the Q₁₀ index, as it gives a valuable indication of shelf life decrease attributable to an increase in temperature.

Shelf life is the length of time a product maintains its safety and preserves its quality under the conditions of storage. In the case of vegetable oils, shelf life is primarily determined by oxidative stability, which is affected by factors such as temperature, light and microorganisms. Two major testing methods are used: real-time stability testing, in which oils are kept until they cease to meet specifications, and accelerated stability testing, in which oils are exposed to harsh conditions to determine their longevity. The Rancimat test is often used for accelerated testing, and mathematical models such as the Arrhenius equation are used to predict shelf life on the basis of oxidation rates at different temperatures.

9. Factors influencing the oxidation of vegetable oils

Oxidation of VOs can be catalyzed through many factors such as fatty acid composition, exposure to light, heat, concentration and type of oxygen, transitional metals and processing (Kozłowska & Gruczyńska, 2018).

9.1. Fatty acid composition

Fatty acid composition is a quantitative indicator of the oxidative stability of VOs. This stability depends on all on the nature of the dominant fatty acids. (Redondo-Cuevas et al., 2018) reported that the type of fatty acid is of paramount importance for the oxidative stability of oils and fats and that UFA and PUFA are negatively linked to VOs oxidative stability (Gharby et al., 2011b; Gharby, Harhar, El Monfalouti, et al., 2012; Gharby, Harhar, Guillaume, et al., 2012). Highly UFAs oils show a faster oxidation reaction than their low UFAs counterparts (Gharby, Harhar, El Monfalouti, et al., 2012; Gharby, Harhar, Guillaume, et al., 2012; Gharby, Harhar, Kartah, Chafchauni, et al., 2013; Gharby, Harhar, Kartah, Guillaume, & Charrouf, 2013; Madhujith and Sivakanthan, 2019a, Madhujith and Sivakanthan, 2019b), which generates aromas and deleterious products such as peroxides, dienes and ketones (Pattnaik & Mishra, 2020). Moreover, PUFAs have an unstable chemical structure and are sensitive to oxidation (Jurić et al., 2022). Other authors noted that the PUFAs undergo more rapid oxidation processes than MUFAs (Dodoo et al., 2022; Symoniuk, Ksibi, et al., 2022; Symoniuk, Wroniak, et al., 2022). In this context, it is indicated that sunflower oil with high oleic acid content is more stable to oxidation than oil rich in linoleic acid (Fan & Eskin, 2015). While more saturated VOs (such as coconut, palm, and babassu) are better in terms of oxidation stability (Giakoumis, 2018). MUFAs and PUFAs are beneficial fats that have many health benefits (Pattnaik & Mishra, 2020). Animal fats contain more saturated fatty acids (SFA) than VOs (Okrouhlá et al., 2018).

Gas chromatography remains the most frequently adopted analytical method for determining the fatty acid content of various fats and oils (Bialek et al., 2017). Structurally, the impact of fatty acids on oxidation is directly related to their chain length, their number of unsaturation and the configuration of these unsaturation. It was documented that as the length of the fatty acid chain increases, the greater the oxidation of lipids. A higher degree of unsaturation is directly associated with lower oxidative stability (Qi et al., 2016; Rodríguez et al., 2020). The reactive sites in a fatty acid's structure are one or more double bonds in the chain and the acid group (Enferadi Kerenkan & Béland, 2015). UFAs are particularly susceptible to oxidation because their double bonds contain hydrogen atoms that are more easily removed, leading to molecular rearrangement and the formation of oxidation products (Ismail et al., 2016). Both carbon atoms directly attached to the double bond can be in the cis or trans configuration. However, most UFAs naturally have a cis configuration (Enferadi Kerenkan & Béland, 2015). Yet, the stability of the trans isomers is much greater than that of the cis isomers (Domínguez, Pateiro, et al., 2019; Domínguez, Purriños, et al., 2019; Ghnimi et al., 2017). For example, elaidic acid (18:1n-9trans) is significantly more stable than its cis isomer, oleic acid (Ghnimi et al., 2017).

Additionally, the position of the fatty acids on the glycerol molecule affects the rate of lipid oxidation (Toorani et al., 2019). On glycerol backbone, the fatty acid linked to the sn-2 position is less prone to oxidation than the fatty acid linked to the sn-1 and sn-3 positions (Kato et al., 2018). Oxidation resistance could be important when the UFAs have been attached to the sn-2 position (Toorani et al., 2019).

All fatty acids do not have the same sensitivity to hydroperoxide production (Montaño et al., 2016). According to Table 3, conversion of saturated stearic acid to monounsaturated oleic acid by introducing a double bond into the fatty acid chain results in a 10-fold increase in the relative oxidation rate of stearic fatty acid (Talbot, 2016). Thus, the oxidation rate of a fatty acid structure with two double bonds can be 10 to 40 times faster than with a single double bond (Kozłowska & Gruczyńska, 2018). Linoleic acid and linolenic acid are more unstable than palmitic acid and oleic acid (J. Li et al., 2018). Abd Razak et al. (2021) studied the stability of canola, palm olein, soybean, and sunflower oils under frying conditions. They reported that oils with high UFAs content (canola, soybean, and sunflower) were more susceptible to oxidation, while palm olein presented better frying performance given that it contains a balanced proportion of SFA and UFAs (Abd Razak et al., 2021). Similarly, Li et al. (2018) studied the oxidative stability of four types of VOs (linseed, palm, rapeseed and sunflower oils) and revealed that their order of stability is as follows: Linseed oil $<\text{sunflower oil}<\text{rapeseed oil}<$palm oil. This finding is explained by the lower amount of UFAs in palm oil and rapeseed oil (J. Li et al., 2018).

Table 3.

Impact of level of unsaturation on the relative rates of oxidation of C18 fatty acids at 100 °C (Talbot, 2016).

Fatty Acid	Formula	Number of Unsaturation	Relative Rates of Oxidation
Stearic	C18H36O2	0	1
Oleic	C18H34O2	1	10
Linoleic	C18H32O2	2	100
Linolenic	C18H30O2	3	150

9.2. Influence of light

Exposure to sunlight induces photo-oxidation reactions in both crude and refined oils (Ahmed et al., 2016). In fact, under the effect of light, chlorophyll acts on oxygen in its fundamental state by producing singlet oxygen (¹O₂) that reacts with UFAs to form hydroperoxide (Caipo et al., 2021). The triplet oxygen in the atmosphere (³O₂) can be converted into the singlet state (1O₂) by exposure to light through photosensitizers such as porphyrins, riboflavins and chlorophyll (Mozuraityte et al., 2016).

It was reported that increased exposure to light accelerates the oxidation of lipids (Liu et al., 2019). It has an important effect on the generation of primary oxidation products: it accelerates photo-oxidation (Bodoira et al., 2017). A short wavelength (< 300 nm) (higher energy) light exposure induces higher rates of oil oxidation than higher wavelengths (> 300 nm) (Loganathan et al., 2022; Shankar et al., 2015). However, UFAs cannot absorb light if it has a wavelength below 220 nm (Ahmed et al., 2016; Madhujith and Sivakanthan, 2019a, Madhujith and Sivakanthan, 2019b). Experimentally, it has been proven that the oxidation stability of VOs is better preserved in the lack of light (Lashko et al., 2019). Lashko et al. (2019) revealed that lighting conditions influence the organoleptic, physical and chemical parameters of crude and unrefined pressed sunflower oil and linseed oil (Lashko et al., 2019). Similarly, Dodoo et al. (2022), studied the oxidation of plant oils exposed to direct sunlight and kept in the dark for six weeks. They revealed that photosensitized oxidation increased in the order: coconut oil < palm kernel oil < soybean oil < sunflower oil (Dodoo et al., 2022). Almeida et al. (2018) showed that the oxidation reactions of crude and refined palm oil are intensified when stored between 26 and 32 °C and exposed to light (Almeida et al., 2018).

9.3. Influence of temperature

Temperature stands out as one of the most important factors affecting lipid oxidation (Liu et al., 2019). Higher levels of oxidation compounds were observed in olive oils stored at higher temperatures than in those stored at lower temperatures (Caipo et al., 2021). Furthermore, there is an exponential relationship between oxidation rates and temperature, resulting in a logarithmic decrease in the shelf-life of a food lipid with increasing temperature (De Alzaa et al., 2018). Elevated temperature exposure causes a change in the fatty acids that contain two or three double bonds (Nayak et al., 2016). The oxidation rate of oils is generally doubled for each 10 °C increase in temperature (Talbot, 2016). In the high temperature range of 150 °C and above, the decomposition of the hydroperoxides is faster, so they remain only slightly or lacking completely (Madhujith and Sivakanthan, 2019a, Madhujith and Sivakanthan, 2019b). Moreover, the carbon‑hydrogen bond on carbon 11 of linoleic acid must be broken with 50 kcal/mol of energy in order for free radicals to occur (Aladedunye, 2015).

Thermal oxidation can occur in oil at temperatures starting from 100 to 120 °C, depending on the type of oil, with the process accelerating at higher temperatures (Grabowski & Szwarczyńska, 2024). As well, due to the effect of high temperature (heating/frying), VOs are subject to oxidation, polymerization, isomerization and hydrolysis (Rubalya Valantina, 2021). (Tsuzuki et al., 2010) showed a significant increase in the amounts of trans isomers formed when canola oil was heated from 180 to 200 °C. Mohanan et al., 2018 found that higher temperatures result in significantly higher rates of peroxide formation in linseed oil. An excellent way to control lipid oxidation is to lower its storage temperature (Johnson & Decker, 2015). (Cheng et al., 2018) revealed that a temperature higher than 120 °C can induce an evident cis/trans isomerization of oleic acid (Mohanan et al., 2018). It has been shown that storage of peanuts at 15, 25, and 35 °C results in different degrees of oxidation of their lipids and that storage at 15 °C and short-term storage at 25 °C provide adequate preservation (Liu et al., 2019).

9.4. Influence of oxygen

Oxidation of lipids occurs as oxygen combines with unsaturated lipids (Lee et al., 2023). In fact, the highly UFAs in edible VOs oxidize when exposed to oxygen in the air (Barden et al., 2015; Lee et al., 2023). Oxygen is an essential factor in the oxidative degradation process (Jimenez-Lopez et al., 2020). The oil oxidation is influenced by the type and concentration of oxygen (Madhujith and Sivakanthan, 2019a, Madhujith and Sivakanthan, 2019b). High level of oxygen accelerates lipid oxidation (Liu et al., 2019). The double bonds of the fatty acids in these lipids are converted into epoxy acids in the presence of oxygen (Talbot, 2016). Yet, a small amount of O₂ is enough to damage sensitive food products (Linke et al., 2020). However, without oxygen, oxidation cannot take place (Talbot, 2016). Oxygen is a critical reactant to the autoxidation process (Lee et al., 2023) and is essential for the lipoxygenase cascade reactions (Jimenez-Lopez et al., 2020).

UFAs contain pentadiene structures that have easily extractable hydrogen atoms that can react with oxygen (Ismail et al., 2016). Both oxidation and the formation of primary oxidation products from UFAs require oxygen (Partanen et al., 2008). In nature, the triplet of oxygen is the most stable and abundant form of oxygen (Choe & Min, 2006a). It is the existing ground state of the oxygen molecule which can then be transformed into two excited singlet states after its activation (Y.-Z. Chen et al., 2017). The singlet oxygen can directly bind to the double bond of fatty acids forming lipid peroxide (Mozuraityte et al., 2016). The rate of lipid reactivity with singlet oxygen is significantly higher than with triplet oxygen (Choe & Min, 2006a). For example, the reaction rate of triplet oxygen and singlet oxygen with linoleic acid is reported to be 8.9 × 10¹/M/s and 1.3 × 10⁵/M/s, respectively (Choe & Min, 2006a).

As the main catalyst for lipid oxidation, oxygen is 3 to 10 times more soluble in VOs than in water (Budilarto & Kamal-Eldin, 2015). Oil oxidation rate increases in proportion to the amount of dissolved oxygen it contains (Aladedunye, 2015). That amount of oxygen depends on the partial pressure of oxygen in the headspace of the oil. Furthermore, higher the oxygen content in the headspace, increased the amount of dissolved oxygen in the oil, leading to increased oxidation of VOs (Kaleem et al., 2015). In this sense, it has been proven that, during storage, the sensory quality of the soybean oil is improved by rinsing with nitrogen to remove oxygen in the headspace of tanks and bottles (Arruda et al., 2006). As a result, reducing the initial oxygen content from >15 to 3 % can extend shelf-life from 60 to 180 days. To increase the shelf-life of food goods, it is frequently recommended to reduce the oxygen content (Linke et al., 2020).

9.5. Influence of water

Water is well known to act as an oxidation catalyst (Kasimoglu et al., 2018). It accelerates the deterioration of VO frying (Nayak et al., 2016). Water affects VO quality, with more water VO quality tends to deteriorate due to oxidation (Mariana et al., 2020). In a recent study, Sakar et al., (2023) observed a positive correlation (r = 0.323) between humidity (water content) and K232 as an indicator of primary oxidation products. Triglycerides decompose in the presence of water and steam in the oils, and the extent of hydrolysis depends on the amount of these two elements (Nayak et al., 2016). Moreover, the presence of water in the fat or oil as well as the enzymatic activity can lead to hydrolysis that causes rancidity (Mariana et al., 2020). Kim et al. (2015) found that the α-tocopherol content of stripped corn oil decreased with increasing moisture and that lower moisture promoted better oxidative stability of this oil (J. Y. Kim et al., 2015). However, the role of water in lipid oxidation processes is potentially linked to the formation of association colloids in edible oils at the initiation phase, when both amphiphilic molecules and hydroperoxides initiate aggregation (Modugno et al., 2019).

9.6. Oil processing

VOs oxidative stability can be impacted by processing. Non-triglyceride substances are present in all crude oils that have been extracted from their source. Non-triglycerides cause undesired processing side effects such blackening, foam formation, precipitates, and the development of disagreeable flavors. Therefore, crude oils need to be processed to remove unwanted components. Although the goal of refining is to remove impurities as much as possible, processing unfortunately leads to the degradation of naturally occurring bioactive molecules like phenols, sterols and tocopherols. The severity of these losses depends on the processing parameters and VO type. Effect, refined VOs might not be as stable against oxidation as their unprocessed counterparts (Ayyildiz et al., 2015; Madhujith and Sivakanthan, 2019a, Madhujith and Sivakanthan, 2019b).

9.7. Influence of metals

Heavy metals refer to metals and metalloids with an atomic density greater than or equal to 4000 kg/m³ (Edelstein & Ben-Hur, 2018; Vardhan et al., 2019). The quality of the oils depends directly on the concentration of trace metals (Mendil et al., 2009). Soil contaminated by industrial pollution is the main cause of heavy metal contamination of VOs (Ghane et al., 2022). In fact, contamination of oils by various heavy metals such as As, Cd, Cr (VI), Cu, Hg, Mn, Ni, Pb and Zn can occur throughout their growth (Xue et al., 2019; Zhou et al., 2020). Soil type, fertilization, plant species and hydrological conditions (irrigation) are among the factors that can influence the concentration of heavy metals in oilseed plants (Mendil et al., 2009). More recently, Gagour, Hallouch, Asbbane, Bijla, et al. (2024), Gagour, Hallouch, Asbbane, Laknifli, et al. (2024) and Gagour, Ibourki, Antari, Sakar, et al. (2024) have outlined positive correlations among some heavy metals in soil and plant leaves as evidenced in the case Cu in olive tree (Gagour, Hallouch, Asbbane, Bijla, et al., 2024; Gagour, Hallouch, Asbbane, Laknifli, et al., 2024; Gagour, Ibourki, Antari, Sakar, et al., 2024). In VOs, heavy metals exist in trace amounts, however they have an effect on VO oxidation rate (Huang & Jiang, 2001). (Mohanan et al. (2018) showed that metals cause a high sensitivity to the oxidation of PUFAs present in flaxseed oil (Mohanan et al., 2018).

10. Prevention oxidation of vegetable oils

Antioxidants assist guard against the oxidation phenomenon already explained. Free radicals are unstable chemicals that can be harmful to cells. Antioxidants scavenge and neutralize free radicals to stop them from harming the body and suppress the harmful effects of oxygen generation (Saad et al., 2007). They can be further classified into synthetic and natural antioxidants.

10.1. Use of synthetic antioxidants

Synthetic antioxidants have a very important capacity to prevent the oxidation of vegetable oils (Mohanan et al., 2018). Tert-butyl hydroquinone (TBHQ), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and propyl gallate (PG) are examples of synthetic antioxidants (primary antioxidants) that control oxidation and prolong the shelf life of oils by delaying or inhibiting lipid deterioration (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c; Morsy et al., 2022). Citric acid, EDTA, tocopherols, polyphenols, lignans, and ascorbic acid are examples of the second class of antioxidants that can slow the rate of oxidation (Madhujith and Sivakanthan, 2019a, Madhujith and Sivakanthan, 2019b). It is frequently claimed that adding mixtures of primary and secondary antioxidants has a synergistic impact, meaning that their combined activity is more effective at delaying lipid oxidation than the sum of their individual effects and lengthens the induction period (Haj Hamdo et al., 2014; Marinova et al., 2008). Currently, phenolic antioxidants are the most widely utilized synthetic antioxidants (Rodil et al., 2012). These synthetic phenolic antioxidants can prevent food from oxidizing, increasing food stability, and extend food shelf-life, among other benefits, but their improper or excessive use could be harmful to people's health (J.-M. Kim et al., 2016).

They have been used for many years with great success and safety (Ghosh et al., 2019). However, excessive use of antioxidants can be cytotoxic and carcinogenic to the human health (Indiarto and Qonit, 2020a, Indiarto and Qonit, 2020b). Based on potential endocrine disruptions and carcinogenic consequences, the European Food Safety Authority (EFSA) concluded that the daily allowable intake of BHA (ADI) for adults and children is 1 ppm bw/day (EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS), 2011).

As a result, governments strictly control the use of synthetic antioxidants in food products, and the permitted levels of use frequently differ from one nation to the next. For instance, TBHQ is permitted in the USA but prohibited in the nations of the European Union (X. Xu et al., 2021). TBHQ can only be added to food products up to 0.02 %, as it may have adverse health consequences (Osanloo et al., 2021).

10.2. Enrichment of vegetable oils with aromatic and medicinal plants

Vegetable oil producers and the food processing industry are looking for natural antioxidants to replace synthetic antioxidants to inhibit oil oxidation and rancidity, also to improve the quality of VOs (Osanloo et al., 2021). Consumers are drawn to antioxidant substances because of their health benefits and natural origins (Fadda et al., 2022a, Fadda et al., 2022b, Fadda et al., 2022c). Natural antioxidants are abundant in aromatics and medicinal plants extracts such as marjoram, oregano, rosemary, sage, and thyme (Kozłowska & Gruczyńska, 2018). The selection of naturally occurring antioxidants for the prevention of oxidation of VOs depends on a number of variables, including VO fatty acid content, minor components, and the structure and usefulness of the antioxidants themselves (Mohanan et al., 2018). The benefits of adding natural antioxidants to VOs to prevent its oxidation have been demonstrated in recent years (Y. Yang et al., 2016).

The beneficial effects of these natural antioxidants have been amply demonstrated in numerous studies, including the study by Gülmez and Şahin where natural ingredients have been found to be superior to synthetic ones for stabilizing hazelnut oil (Gülmez & Şahin, 2019). Also, in another study of Zhang et al. (2010) for the oxidative stability of sunflower oil, more efficiency was seen at 200 ppm of rosemary extract than at BHA and BHT (Zhang et al., 2010). An extended in the oxidative stability from 7.52 to 13.5 h in (Thermal oxidation conditions (180 °C)) of refined sunflower oil using 1000 ppm rosemary extract (Ramalho & Jorge, 2008). Saffron (Crocus sativus L.) stigmata powder and extracts, thyme (Thymus vulgaris L.), extracts from agrifood by-products (like spent coffee grounds) were used to improve oxidative stability VOs including argan oil, soybean, and sunflower (Nid Ahmed et al., 2024a, Nid Ahmed et al., 2024b; Bijla et al., 2024; Oubannin et al., 2023; Samira et al., 2023).

(Freitas et al., 2022) added Thymus mastichina L to olive oil (Olea europaea L). They found that the induction period goes from 10.3 h for the oil control to 16.6 h for the enriched oil. And from an acidity of 0.20 g/100 g for the oil control 0.17 g/100 g for the enriched oil.

Interesting results have also been obtained using Oregano (Origanum vulgare L.) for the stabilization of olive oil, in fact, the enriched oils showed six times lower PVs at all concentrations compared with the control, in addition to improved sensory characteristics. Regarding sensory attributes, oils flavored at concentrations of 20 and 40 g/L were rated more positively at the end of storage than the control and the oil flavored at the lowest concentration (Gambacorta et al., 2007). Rosemary (Rosmarinus officinalis L.) has also been widely used for its antioxidant effects (Sakar et al., 2023). Moreover, enhancement of oxidative stability by adding an extract of 20 mg/100 g of rosemary the peroxide value rose from 105.93 ± 0.12 mEqO₂/Kg to 98.70 ± 0.50 mEqO₂/Kg for enriched hemp oil (Moczkowska et al., 2020).

10.3. Modification of the Fatty Acid Composition

Regulating the fatty acid composition by controlling the concentration of UFAs to improve the oxidative stability of VOs (Indiarto and Qonit, 2020a, Indiarto and Qonit, 2020b). Ethyl methanesulfonate and gamma radiation have both been used as mutagens to genetically modify fatty acids. The result is oil with a low linolenic acid and a high oleic acid contents, which increases oxidation stability (Patil et al., 2007).

10.4. Blending of different oils

Blending is a widely recognized and preferred approach for enhancing the physicochemical properties, nutritional value, modification of fatty acid composition and oxidative stability of VOs without affecting their chemical composition (Table 4). This method is favored due to its simplicity, cost-effectiveness, non-destructiveness, and avoidance of chemical treatments. Two commonly employed strategies for formulating blends involve combining VOs with contrasting fatty acid compositions or blending omega-3 fatty acids and antioxidant-rich minor oils with major oils. (Madhujith & Sivakanthan, 2019a; Sharma et al., 2022).

Table 4.

Blended vegetable oils.

Blended oil	Ratio	Mains results	References
Canola + Olive	80:20, 60:40, 50:50, 40:60, and 20:80	Canola: olive blend at ratio of 80:20 had the best physicochemical properties	(Roiaini et al., 2015)
Sunflower + flaxseed	65:35	Increased antioxidant Conten	(Umesha & Naidu, 2015)
Olive + sesame + linseed	65:30:5; 60:30:10 and 55:30:15	Balanced ω6/ω3 Composition	(Hashempour-Baltork et al., 2018)
Groundnut + Mustard + Soybean + Sun flower	85:5:5:5; 70:10:10:10; 55:15:15:15 and 40:20:20:20	Decreased peroxide value	(Kumar et al., 2018)

Blending oils with high stability and good nutritional values is an effective method to reduce the rate of oxidation (Hashempour-Baltork et al., 2016).

This chapter describes techniques for protecting vegetable oils from oxidation, with an emphasis on the role of antioxidants. Synthetic antioxidants such as TBHQ, BHA, BHT, and PG are typically used to extend the shelf life of oils, yet the concerns about their health effects has resulted in rigorous regulations. Natural antioxidants from aromatic and medicinal plants such as rosemary, oregano and thyme have shown promising results in improving oxidative stability. Other strategies include modifying fatty acid composition through genetic engineering, blending oils with complementary properties, and incorporating antioxidant-rich oils. All of these strategies can help maintain oil quality, improve stability, and meet consumer preferences for natural alternatives.

11. Conclusions

Lipid oxidation is a significant quality issue in many processed foods, leading to a series of reactions that follow various pathways, including photooxidation, autooxidation, and enzymatic oxidation. These processes break down fatty acids into carbonyl compounds, unsaturated aldehydes, ketones, and other by-products, triggered by factors such as oxygen, light, and photosensitizers, resulting in an unstable food system. It leads to a significant loss of essential fatty acids and vitamins, reducing nutritional value, and causes sensory changes such as altered color and texture, and the development of rancid odors and flavors - factors that impact consumer acceptance. More importantly, lipid oxidation produces potentially toxic compounds linked to serious health issues, including cancer, inflammation, and aging. Therefore, Monitoring lipid oxidation is essential for maintaining food quality and ensuring consumer acceptance. To evaluate oxidation levels in foods and biological systems, a range of analytical methods have been developed, each based on specific indicators or mechanisms. To enhance the oxidative stability of oils, diverse strategies have been explored, such as incorporating natural or synthetic antioxidants, optimizing storage conditions, using modified atmosphere packaging, modifying fatty acid profiles, or blending with more stable oils.

CRediT authorship contribution statement

Saïd Gharby: Writing – review & editing, Writing – original draft, Project administration, Methodology, Investigation. Abderrahim Asbbane: Writing – review & editing, Writing – original draft, Validation, Software, Data curation. Moussa Nid Ahmed: Writing – review & editing, Writing – original draft, Resources, Methodology, Investigation, Formal analysis. Jamila Gagour: Writing – review & editing, Writing – original draft, Resources, Investigation. Otmane Hallouch: Writing – review & editing, Writing – original draft, Resources, Investigation. Samira Oubannin: Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation. Laila Bijla: Writing – review & editing, Software, Resources, Methodology, Investigation. Khan Wen Goh: Writing – review & editing, Validation, Investigation, Funding acquisition, Data curation. Abdelhakim Bouyahya: Writing – review & editing, Methodology, Investigation, Formal analysis. Mohamed Ibourki: Writing – review & editing, Validation, Supervision, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary material

Data availability

Data will be made available on request.