Toxic aldehydes in cooking vegetable oils: Generation, toxicity and disposal methods

Abstract

The generation of toxic aldehydes in vegetable oils subjected to high-temperature cooking processes, such as frying, poses significant health risks due to their high reactivity and potential to form carcinogenic and mutagenic compounds. This review discusses the mechanisms of aldehydes formation in vegetable oils, focusing on key factors such as oil composition, cooking temperature, and heating time. The major toxic aldehydes identified include acrolein, acetaldehyde, formaldehyde, t,t-2,4-decadienal (t,t-2,4-DDE), 4-hydroxy-2-hexenal (4-HHE), and 4-hydroxynonenal (4-HNE), which have been associated with adverse health effects ranging from respiratory irritation to carcinogenicity. Currently employed air purification methods aimed at mitigating exposure to these toxic compounds in domestic and industrial settings are analyzed. Strategies such as ventilation improvements, activated carbon filters, and emerging technologies like catalytic combustion are evaluated for their effectiveness in reducing aldehyde concentrations. Further research is needed to optimize air purification techniques to reduce air pollution and protect public health from harmful aldehyde exposure.

Highlights

• •Vegetable oils (VOs) are largely employed in food cooking techniques worldwide.
• •During cooking procedures, VOs can generate several by-products such as aldehydes.
• •Aldehydes can be found in both cooking oil fumes (COFs) and VO waste.
• •Most aldehydes are toxic, contributing to environmental pollution and human health.
• •Toxicity mechanisms and innovative purification methods have been recently proposed.

Keywords

Cooking fumes

Cooking oil stability

Deep frying

Harmful byproducts

Lipid peroxidation

Purification methods

Toxic aldehydes

Vegetable oils

1. Introduction

Edible oils are key constituents of our daily diet, being widely used in both household and industrial foods (Brahmi et al., 2020). They comprehend a broad variety of fluids derived from vegetable sources or animal tissues (Przybylski & Eskin, 2011). Vegetable oils (VOs) are obtained through an extraction process employing a range of mechanical (e.g., supercritical fluid extraction, hot or cold pressing), chemical (e.g., hot or cold solvent extraction), and biochemical (e.g., enzyme-assisted extraction) techniques aiming to maximize yield, reduce alterations, and ensure consistent product quality and genuinity (Nde & Foncha, 2020).

The production method varies depending on the oil source such as oleaginous seeds (e.g., soybeans, peanut, rapeseed, and sunflower), oleaginous fruits (e.g., palm and olive), or herbaceous and woody oil crops such as sesame, wheat, and oat (Wang et al., 2017; López et al., 2024). According to the Food and Agriculture Organization Corporate Statistical Database (FAOSTAT, 2023), global VOs production surpasses 200 million tons annually (Murphy, 2024; Wen et al., 2023). Consumers make extensive use of VOs over animal oils due to their renewable nature, higher quality, and absence of cholesterol (Tang et al., 2024). On the other hand, VOs are very susceptible to various factors, including storage conditions such as light, humidity, heating, and cooking techniques such as high cooking temperatures (e.g., deep frying), which may alter oil composition, generating hazardous products for human health (Choe & Min, 2007a; Xue & Ngadi, 2009). Thermo-oxidation process occurs either gradually over extended periods (e.g., storage) or rapidly during the frying cooking in which it is significantly accelerated (Vali Zade et al., 2024). Therefore, high cooking temperatures (180–200 °C) and repeated heating cycles can drastically enhance the occurrence of chemical reactions, leading to harmful changes in oil composition (Songohoutou et al., 2023). In this context, thermal oxidation processes, along with hydrolysis and polymerization reactions, can lead to the formation of toxic compounds such as hydrocarbons, free fatty acids, aldehydes, ketones, alcohols, and lactones (Li et al., 2022; Valle et al., 2024; Yao et al., 2015). Cooking oil fumes (COFs) are also released, representing a major threat to human health and the environment. Actually, they represent the third largest source of urban air pollution after pollutant gases from vehicles and industry, making the development of novel air purification approaches a pressing need (T.-Y. Chen et al., 2020; Q. Wang et al., 2017).

Among the different cooking techniques, deep-frying is particularly relevant due to its widespread use worldwide (Sadawarte & Annapure, 2023). It involves immersing food in oil at temperatures typically ranging from 150 to 190 °C (Waghmare et al., 2018), often for repeated cycles depending on the local cooking traditions. During this process, oil is exposed to intense heat, oxygen, and moisture released from the food, which together can promote changes in the matrix and food components. In particular, physiochemical transformations, including thermo-oxidation, hydrolysis, and polymerization can occur, leading to a quality deterioration and formation of toxic volatile and non-volatile byproducts, such as aldehydes, which can migrate into food or be released into the environment (Choe & Min, 2007b). Therefore, deep-frying represents a critical context for understanding the generation and accumulation of harmful compounds in edible oils.

In this work, we reviewed the main components of the most employed VOs, with a focus on the toxic aldehyde subproducts generated after high-temperature exposure. Also, we reported the mechanisms behind the formation and toxicity of the most abundantly found aldehydes in VOs.

2. Vegetable oils: Composition, dietary benefits, and health risks

In the last decade, according to the US Department of Agriculture, the consumption of VOs significantly rose from 166.76 million metric tons in 2013 to 217.99 in 2023, with an increasing percentage of 23.5 %. In particular, palm, soybean, rapeseed, and sunflower seed oils are the most used in the world (≈ 88 %), with consumption percentages of ca. 36, 28, 15, and 9 %, respectively, as depicted in Fig. 1 (United States Department of Agriculture Foreign Agricultural Service, 2024).

Fig. 1.

The trend of consumption of representative VOs per year in the world. Other oils: palm kernel oil, peanut oil, cottonseed oil, coconut oil, and olive oil.

VOs are rich in fatty acids (88–96 %), including both saturated fatty acids (SFAs) and unsaturated fatty acids (UNFAs). VOs composition is correlated with VOs sources and extraction techniques which, in turn, primarily affect the nutrient contents of the product (Kaur et al., 2014; Lipidomics Standards Initiative Consortium et al., 2019). SFAs are key precursors for steroid hormones and prostaglandins, affect both lipidemia and cholesterol absorption, and also serve as transporters for liposoluble vitamins (Li et al., 2016). VOs rich in lauric acid (C12:0), e.g., coconut oil, increase both low-density lipoprotein (LDL) and high-density lipoprotein (HDL) concentrations. Conversely, myristic (C14:0) and palmitic (C16:0) acids have a minimal effect on cholesterol accumulation, while stearic acid (C18:0) slightly reduces it (Mensink et al., 2003). Among fourteen examined VOs, Orsavova et al. detected palmitic acid (C16:0) as the most abundant SFA, ranging from 4.6 to 20.0 % (Orsavova et al., 2015). UNFAs are categorized according to their number of double bonds: monounsaturated fatty acids (MUFAs) contain a single double bond, while polyunsaturated fatty acids (PUFAs) bear multiple olefinic moieties. Furthermore, UNFAs found in VOs are classified as cis or trans isomers, based on the configuration of their double bonds. Oleic acid (C18:1, ω-9) is the most prevalent MUFA in commercially available edible oils (6.2–71.1 %), while linoleic acid (C18:2, ω-6) represents the most abundant PUFA (1.6–79 %) (Amat Sairin et al., 2022; Orsavova et al., 2015). Among PUFAs, ω-3 and ω-6 stand out for their relevant functional and nutraceutical properties. Both VOs and fish oils are the reference sources for ω-3 intake, especially for those that cannot be synthesized by the human body and must be introduced from the diet, like alpha-linolenic acid (ALA, C18:3) (Demmelmair & Koletzko, 2021). MUFAs and PUFAs play an important role in cardiovascular disease (CVD) prevention by reducing LDLand increasing HDL cholesterol levels. However, the recent meta-analysis STRENGHT published in 2020 questioned the cardiovascular benefits of ω-3 fatty acid diet supplementation. Although ω-3s have certain cardiovascular benefits, this study reports how their impact on preventing serious cardiovascular outcomes remains unclear (Nicholls et al., 2020).

Besides fatty acids, VOs contain additional compounds of nutritional relevance such as phytosterols and phenolic compounds. Therefore, in their entirety, VOs play a crucial role in human health, showing valuable beneficial properties such as antioxidant effects, CVD prevention, anti-inflammatory, anti-obesity, anti-cancer, diabetes treatment, and the kidneys and liver protection (Tian et al., 2023; Zahir et al., 2017). Mediterranean diet is widely recognized for its health benefits, largely due to its emphasis on consuming plenty of vegetables and extra virgin olive oil (EVOO) which is produced using only mechanical and physical methods, with no other refining process preserving its natural nutrient content. (Ramírez-Anaya et al., 2015). This extraction method helps preserve a higher concentration of health-promoting and preserving compounds, such as natural antioxidants. The latter are mainly found in the minor component fraction of EVOO, which includes both lipophilic compounds (fat-soluble) and hydrophilic phenols, contributing to their beneficial effects on consumers health (Prata et al., 2018). For a long time, EVOO was discouraged from cooking due to its relatively low smoke point (around 205 °C) compared to other oils like those from peanut (ca. 225 °C), sunflower (ca. 255 °C), soybean (ca. 242 °C), and palm (ca. 227 °C) (Lozano-Castellón et al., 2022). However, recent studies have challenged the belief that smoke point is a reliable indicator of an oil's performance and stability during cooking. For instance, the chemical and physical changes in oils during heating were demonstrated not to always correlate with their smoke point (Guillaume, 2018). In 2022, a study by K.-M. Chiang et al. investigated the generation of aldehydes during deep frying at 180 °C in three different oils (palm, olive, and soybean). The results showed that olive oil was superior to both soybean and palm oils in terms of toxic emissions during deep-frying (Chiang et al., 2022). This interesting outcome has been attributed to olive oil's high content of MUFAs compared to PUFAs, as well as its significant antioxidant properties, making EVOO one of the best choices for frying (Lozano-Castellón et al., 2022). In this context, the consumption of EVOO in dressings and cooking was found to reduce the incidence of metabolic diseases in subjects at high cardiovascular risk, as assumed in the Prevención con Dieta Mediterránea (PREDIMED) project (Estruch et al., 2018; Salas-Salvadó et al., 2011).

On the other hand, considering their high fat content, a diet based on high consumption of VOs and their derivatives, such as fried foods, may increase the risk of many diseases, including obesity, inflammatory conditions, metabolic diseases, CVD, and cancer (Marrero et al., 2024; Narayanankutty et al., 2017). In the SUN cohort study, the consumption of fried foods in young adults has been associated with a high risk for the development of CVD by increasing central adiposity and high blood pressure (Sayon-Orea et al., 2014).

Although fatty acids in VOs provide numerous health benefits, they are also the primary source of harmful by-products generated from matrix degradation due to improper storage, e.g., sunlight exposure and humidity, or high-temperature cooking, e.g., deep frying.

Deep frying is a globally popular cooking method, highly valued for enhancing the sensory properties of foods, e.g., golden colour, aspect, flavor, and cherished texture, which are highly reliant on oil type, fatty acid composition, and food. Fried food characteristics are influenced by the generation of volatile and non-volatile compounds produced by oxidative degradation of the frying media (Ananey-Obiri et al., 2018; Whitfield & Mottram, 1992).

However, when exposed to high temperatures (>180 °C), commonly used in deep frying, VOs can undergo transformations that can degrade their natural beneficial components and result in harmful by-products (Fig. 2). Specifically, at elevated temperatures, VOs react rapidly with atmospheric oxygen, leading to a range of chemical reactions, including hydrolysis, cis/trans isomerization, polymerization, and lipids oxidation (e.g., peroxidation) (Zhuang et al., 2022). These processes can produce pote potentially toxic compounds such as aldehydes, ketones, and other toxic by-products, which can pose health risks when consumed in excess (Islam et al., 2019). Thus, while high-temperature exposure can sometimes enhance the flavor and appeal of foods, it often leads to the formation of harmful by-products in vegetable oils (VOs), such as polyaromatic hydrocarbons (PAHs), aldehydes, sulfites, and non-volatile compounds like acids, alcohols, aldehydes, and carbohydrates (Bhattacharya et al., 2008) (Fig. 2). Among these, aldehydes, PAHs, and acrylamide are particularly concerning, since are commonly produced during the frying of foods containing carbohydrates and proteins (Binello et al., 2021).

Fig. 2.

Chemical classes of compounds present in VOs (green panel) and hazardous by-products generated during high-temperature heating (orange panel). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

PAHs are globally recognized as carcinogenic and endocrine-disrupting compounds, with approximately 16 compounds of this class classified as priority pollutants by the U.S. Environmental Protection Agency (EPA). In the food processing industry, several high-temperature cooking methods, including frying, grilling, smoking, drying, and baking, are significant contributors to PAH formation (Singh et al., 2016).

Therefore, monitoring the composition and quality of VOs during the cooking process is crucial to prevent hazardous changes and reduce the formation of harmful chemicals, including aldehydes (T.-Y. Chen et al., 2020). Several analytical approaches are used to detect oil adulterations and hazardous chemicals in COFs. VOCs are typically analyzed using gas chromatography–mass spectrometry (GC/MS), high-performance liquid chromatography (HPLC), Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NMR) spectroscopy (Cordella et al., 2012; Kawai et al., 2006; Sudhakar et al., 2023; Wang, Chen, et al., 2024).

3. Aldehydes generation and toxicity in cooking VOs

During deep frying, the concentration of harmful by-products is strongly affected by different parameters, including the initial composition and quality of frying oil, the decline in its oxidative stability, the amount of employed oil, the replenishment of fresh oil, and the frying conditions.

In this context, N. Kalogeropoulos et al. investigated the formation of oxidated fatty acids in five different types of VOs - cotton-seed oil, sunflower oil, vegetable shortening, palm oil, and virgin olive oil - through eight successive pan-frying (175 ± 5 °C) or deep-frying (170 ± 5 °C) sessions of pre-fried potatoes. By means of GC/MS technique, the authors measured the content of epoxides and polymerized triacylglycerols in potatoes and fried oils highlighting a noteworthy trend: pan-frying consistently produced more epoxides than deep-frying. Interestingly, when compared to the other tested oils, EVOO showed the greatest overall concentration of oxidized fatty acids after eight cycles of pan-frying (Kalogeropoulos et al., 2007).

Likewise, C.-Y. Peng et al. studied the emissions of aldehyde in VO fumes during household frying with palm, rapeseed, sunflower, and soybean oils through three frying methods, in particular, stir-frying (143–151 °C), pan frying (149–169 °C), and deep frying (170–182 °C), and using two foods matrices (potato and pork loin). COFs were quantified by means of HPLC-UV, and deep-frying was found to generate the largest amount of aldehydes in most cooking combinations, whereas pan-frying and stir-frying produced only low quantities (Peng et al., 2017).

Aldehydes represent one of the main by-products of heated VOs and, due to their known toxicity, several studies have been conducted to identify and characterize them (Katragadda et al., 2010; Katsuta et al., 2008). Aliphatic linear and branched aldehydes are formed through specific reactions, such as lipid oxidation (Kim et al., 2022). This process is the main cause of degradation in VOs, leading to both nutritional and sensory losses (Echegaray et al., 2022). In VOs, oxidation can occur via autoxidation and photosensitized oxidation, during which oxygen species react with oil components, especially fatty acids. Lipid oxidation involves a radical-mediated process based on three key steps: initiation, propagation, and termination. In this context, these two oxidation pathways mainly differ in their initiation process resulting in the formation of distinct peroxyl radical intermediates and, subsequently, hydroperoxides. Indeed, in photosensitized oxidation, ¹O₂ attack on linoleic acid generates four hydroperoxides, two non-conjugated (10-OOH, 12-OOH) and two conjugated (9-OOH, 13-OOH); conversely, free radical attack in autooxidation generates mainly 9-OOH and 13-OOH (Fig. 3). Briefly, photosensitized oxidation occurs via singlet oxygen that is highly electrophilic, due to its electron distribution, thereby it directly reacts with electron-rich regions such as PUFAs double bonds (Verduin, 2020). On the other hand, the autooxidation process requires a radical form of fatty acid, which is rapidly generated by heating, ultraviolet, metals, and visible light, acting as catalysts. Firstly, a hydrogen radical (H‧) is removed from the weakest C—H bond of the unsaturated fatty acid (e.g., the C11 position of linolenic acid), generating a highly reactive alkyl radical, which is stabilized by delocalization on the carbon chain, leading to the rearrangement and formation of a double bond (McClements et al., 2007). Subsequently, the highly reactive alkyl radical initiates a cascade reaction by rapidly reacting with oxygen to form a high-energy peroxyl radical (ROO‧). This peroxyl radical promotes the subtraction of a hydrogen atom from another UNFA, forming a hydroperoxide and another peroxyl radical, responsible for the following propagation step. However, these hydroperoxides are very unstable and break down into alkoxy radicals through homolytic cleavage of the R–OOH group. The alkoxy radicals then undergo β-scission, breaking the C—C bond and forming volatile compounds such as aldehydes, alkenes, alkanes, alcohols, and carboxylic acids (Yadav et al., 2018). For instance, 9-linoleic acid hydroperoxide, the corresponding 9-diene hydroperoxide resulting from the early oxidation of linoleic acid, undergoes a second degradation forming the titled toxic compounds 4-hydroxynonenal (4-HNE) and t,t-2,4-decadienal (t,t-2,4-DDE), which in turn can be oxidized to trans-4,5-epoxy-(E)-2-decenal (Kim et al., 2022) (Fig. 3). Moreover, early peroxidation of 9-linoleic acid also produces the hydroperoxide isomer at position C13, which undergoes several reactions to form carbonyl and carboxylic by-products (Sayre et al., 2006).

Fig. 3.

Generation of toxic aldehydes from linoleic acid via peroxidation mechanisms: autoxidation (purple panel) and photosensitized oxidation (green panel); 4-HHE: 4-hydroxy-2-hexenal; 4-HNE: 4-hydroxynonenal; t,t-2,4-DDE: t,t-2,4-decadienal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fatty acid stability is closely related to the number of unsaturated bonds, with more unsaturated fats being prone to oxidation and hydroperoxide formation (Orsavova et al., 2015). The strength of the C—H bond explains the oxidation rate of various fatty acids: stearic acid (100 Kcal/mol), oleic acid (75 Kcal/mol), linoleic acid (50 Kcal/mol), and linolenic acid (25 Kcal/mol) (Min & Boff, 2002). Therefore, VOs characterized by a balanced ratio of SFAs+MUFAs and PUFAs (e.g., EVOO, sesame, rice bran, rapeseed, and peanut oils) are generally more stable against oxidation compared to those with abundant levels of PUFAs as linoleic acid and/or linolenic acid (e.g., safflower, sunflower, grape seed, wheat germ, and pumpkin seed oils) (Redondo-Cuevas et al., 2018). Noteworthy, the stability and composition of fatty acids in EVOOs have been widely studied showing that the MUFA/PUFA and oleic/linoleic acid ratios provide information on the oxidative stability and rancidity of oils. Indeed, oils with higher values of ratio are generally associated with greater oxidative stability and reduced rancidity in VOs (Hernández et al., 2021).

In this context, Kim et al. studied four frying VOs (soybean, corn, canola, and palm oils) at three temperatures (140, 165, and 190 °C) analyzing volatile components generated by Welsh onion frying. The study, as previous ones, confirmed that during frying, aldehydes are the main chemical class generated, and the content of certain aldehydes, such as hexanal, t,t-DDE, 3-methylbutanal, and 2-phenylacetaldehyde, increase significantly with the increase in temperature. Additionally, repeated use of VOs further diminishes their beneficial properties (Kim et al., 2022). Reheating oils after deep frying is a widespread but dangerous practice in both industrial and domestic cooking processes. Nonetheless, besides SFAs being more resistant to oxidative degradation, SFA-rich oils are neither healthier products nor more advisable choices for culinary use. Indeed, dietary SFAs intake needs to be moderated because these are correlated with CVD risks raising non-high-density lipoprotein (HDL) and cholesterol (Teasdale et al., 2022). To summarize, when selecting VOs, it is essential to consider not only the fatty acid composition but also the content of minor components content, such as phenolic compounds and other antioxidants, which help protect fatty acids and vitamins from thermal degradation, resulting in lower degradation during cooking (Montaño et al., 2016). Moreover, the presence of these bioactive compounds is highly dependent on the extraction process and refining, in fact, these are particularly abundant in not-refined oils (e.g., EVOO) (Olmo-Cunillera et al., 2024).

The genotoxicity and cytotoxicity of harmful lipid peroxidation products (e.g., PAHs and aldehydes) are well documented and associated with cancer and Alzheimer's or Parkinson's diseases (Ganesan & Xu, 2020; Sjaastad et al., 2010). Small carbon chain aldehydes, such as acetaldehyde and formaldehyde, are recognized as probable or known carcinogenic agents (Sun et al., 2007). Conversely, aldehydes with a longer methylene length, e.g., t,t-2,4-nonadienal and t,t-2,4-DDE are known mutagens with tumor-promoting properties (Sjaastad & Svendsen, 2008; Wu et al., 2001). Moreover, many known mutagenic and tumorigenic aldehydes have been found in reheating cooking oils (RCOs) and COFs, including alkenals like acrolein, t,t-2,4-DDE, t,t-2,4-nonadienal, and alkanals like formaldehyde and acetaldehyde (Sjaastad & Svendsen, 2008; Yadav et al., 2018).

Specifically, the mutagenicity of oil degradation products such as formaldehyde, glutaraldehyde, t,t-2,4-DDE, t,t-2,4-nonadienal, t-2-decenal, and t-2-undecenal has been assessed in vitro by histidine reverse mutation mechanism via Ames test on mutant strains of Salmonella typhimurium (i.e., TA98 and TA100) models (Wu et al., 2001). Furthermore, it has been reported that cookers exposed to these aldehydes have a greater risk of suffering from lung cancer (Wu et al., 2019). International Agency of Research on Cancer (IARC) classified long exposure to deep frying fumes as group A2 cancer risk factor (IARC Working Group on the Evaluation of Carcinogenic Risks to Humans, 2010; Lee et al., 2022). COFs emit a variety of fine airborne particulate matter (PM), which are substantial contributors to human health problems (Sjaastad et al., 2008). In particular, aldehydes are extremely reactive molecules and their toxicity is primarily related to their electrophilic nature, enabling them to form covalent bonds with nucleophilic functional groups in proteins, nucleic acids, ion channels, and lipids (LoPachin & Gavin, 2014). According to hard-soft and acid-base definitions, formaldehyde, acetaldehyde, and longer-chain saturated alkanals are relatively hard electrophiles. Due to the localized electron deficiency, the carbonyl carbon atom in these molecules is a hard electrophilic site, and it reacts preferentially with a hard nucleophile. Therefore, the toxicity of these aldehydes is associate to their ability to undergo 1,2-addition reactions with amines, forming Schiff base adducts. For instance, acetaldehyde can cause DNA damage at the replication fork and activate cell cycle arrest, and DNA repair mechanisms (Noguchi et al., 2017). Whereas, α,β-unsaturated aldehydes, such as acrolein or crotonaldehyde, are soft electrophiles. In these aldehydes, the high electronegative carbonyl oxygen atom can withdraw electron density from the C—C double bond, thereby creating an electrophilic center at the β‑carbon atom. As a consequence of this polarization, acrolein can react either with the harder carbonyl carbon atom (direct or 1,2-addition) or the softer β‑carbon atom (conjugate or 1,4-addition) (LoPachin & Gavin, 2014).

Thus, the β‑carbon of unsaturated aldehydes emerges as a primary site for interaction with soft electrophiles bearing thiol or hydroxyl groups of amino acids such as histidine, lysine, and cysteine, consequently increasing the intracellular pool of reactive oxygen species (ROS) in biological systems (C.-H. Chen et al., 2014). Additionally, aldehydes may cross-link and damage DNA leading to nucleic acid fragmentation and subsequent cell death (Miriyala et al., 2016). The major toxic aldehydes by-products generated by VO PUFAs oxidation are acrolein, t,t-2,4-DDE, 4-HNE, and 4-HHE (Guillén & Uriarte, 2012; Jamal et al., 2012). These 2-alkenal products, as electrophilic compounds, rapidly react with biological nucleophile residues, generating Michael and Schiff base adducts with many cellular components such as DNA, membrane channels, and proteins. This disrupts cellular homeostasis and increases oxidative stress and pro-inflammatory responses, ultimately leading to cell death and necrosis (Hellenthal et al., 2021) (Fig. 4).

Fig. 4.

Cellular toxicity induced by electrophilic aldehydes generated from VOs degradation processes. Toxic aldehydes produced by fatty acid peroxidation induce several cellular damages such as conformational changes in enzymes and ion channels, protein adducts formation, ROS production, and activation of pro-inflammatory pathways which increase oxidative stress, DNA fragmentation, and necrosis.

4. Representative examples of toxic aldehydes from cooking VOs

Toxic aldehydes are harmful compounds formed during the cooking of VOs. These aldehydes can pose significant health risks when consumed in large quantities or after exposure over prolonged periods (Yen et al., 2023). The most studied toxic aldehydes produced during cooking VOs include acrolein, formaldehyde, t,t-2,4-DDE, 4-HHE, and 4-HNE. Each of these compounds has distinct toxicological properties and mechanisms of action, contributing to various adverse health effects ranging from respiratory irritation to potential carcinogenicity (Grootveld et al., 2020; N. Lee et al., 2020; Xu et al., 2020). Understanding the formation and impact of these aldehydes is crucial for evaluating the safety and health implications of using different vegetable oils in cooking.

4.1. Trans,trans-2,4-decadienal (t,t-2,4-DDE)

t,t-2,4-DDE is formed during VOs frying process and is often found in fried foods. Detected in both frying oils and the fumes released during the process, this aldehyde has cytotoxic and genotoxic properties and promotes LDL oxidation (L. W. Chang et al., 2005a). Boskou et al. reported that t,t-2,4-DDE concentration is higher in deep frying fumes and food, compared to pan and stir-frying, with the highest levels observed in sunflower oil, which has a high PUFAs content, and the lowest in olive oil, which is rich in MUFAs (e.g., oleic acid) (Boskou et al., 2006). Furthermore, t,t-2,4-DDE production in COFs is a major threat to human health (Pan et al., 2014). Several in vitro studies correlate t,t-2,4-DDE with impaired cellular mechanisms and increased cellular oxidative stress, which leads to enhanced cell proliferation, DNA synthesis, and a rise in pro-inflammatory cytokines (e.g., tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1 β). For instance, exposure to COFs such as t,t-2,4-DDE has been shown to induce ROS production and accumulation, which have been proven to enhance tumor cell proliferation and cytokine secretion, which in turn cause lung inflammation reaction and tumorigenesis (L. W. Chang et al., 2005b; Lin et al., 2014; Pan et al., 2014; Young et al., 2010). Therefore, Hu et al. investigated the in vivo effects of t,t-2,4-DDE on Sprague-Dawley rats. After intraperitoneal administration of various doses of t,t-2,4-DDE over a 28-day period, the rats exhibited oxidative stress, endothelial disruption, and cell apoptosis, which led to blood pressure disorders (Hu et al., 2020). Moreover, like other toxic aldehydes, t,t-2,4-DDE is likewise implicated in cancer progression by promoting cancer cell proliferation and suppressing antioxidant enzyme function (Y.-C. Chang & Lin, 2008; Hung et al., 2007).

4.2. 4-Hydroxy-2-nonenal (4-HNE) and 4-hydroxy-2-hexenal (4-HHE)

4-HNE is a harmful 4-hydroxy-2-alkenal produced during the degradation of ω-6 PUFAs, such as linoleic acid (C18:2, ω-6) and arachidonic acid (C20:4, ω-6). 4-HNE has been extensively studied due to its propensity to modify and crosslink proteins, thus showing its toxic effects. 4-HHE is a secondary lipid peroxidation product derived from the oxidation of docosahexaenoic (C22:6, ω-3) and eicosapentaenoic acids (C22:5, ω-3). Although 4-HHE is produced in smaller amounts than 4-HNE, they appear to have comparable toxicity (Ma et al., 2020). In 2011, Gerde, Hammond, and White reported that the 4-HNE/4-HHE formation ratio is not only correlated to PUFAs concentration but also antioxidant, as tocopherol, content in the oils. (Gerde et al., 2011). In this context, several studies have reported the importance of the minor components for their beneficial properties and VOs stability (e,g., antioxidant), likewise, synthetic antioxidants (e.g., butylated hydroxyl anisole (BHA), butylated hydroxyl toluene (BHT)) in VOs have been widely employed as food additives to slow down the oxidation process both during cooking and storage. Although the minor components in VOs usually constitute less than 2 % of the total oil composition, the safety of synthetic antioxidants remains the subject of a controversial discussion because of their possible toxic effects during long-term intake (Taghvaei & Jafari, 2015). Noteworthy, EVOO, compared to other VOs, contains abundant minor lipophilic compounds (e.g., tocopherols, squalene) and hydrophilic phenols (e.g., secoiridoids and flavonoids) providing it with greater oxidative stability and a lower production of toxic by-products during cooking (Lozano-Castellón et al., 2020). In particular, R.P. Córdoba et al., analyzed oils extracted from various olive cultivars and subjected to different cooking techniques. Their study showed that EVOOs with higher phenolic content, such as those from the Picual and Arbequina cultivars, exhibited greater resistance to culinary treatments, whereas other cultivars, including Hojiblanca and Cornicabra, proved to be more sensitive and more significantly affected by the same processes. Indeed, VOs containing a higher amount of antioxidants, such as tocopherols, showed a lower production of toxic products such as aldehydes during high cooking temperature (Córdoba et al., 2023).

In humans, the aldehyde dehydrogenase (ALDH) enzyme family plays a protective role by participating in the metabolism of 4-HNE which is converted into its non-toxic form, 4-hydroxy-2-nonenoic acid. In general, ALDH enzymes have a role in detoxifying cells by scavenging both endogenous and exogenous toxic and non-toxic aldehydes. In fact, mechanistically, ALDHs oxidize aldehydes into carboxylic acids by using NAD(P)⁺ as a cofactor (Magrassi et al., 2024). In this context, the deletion or low expression of ALDHs was found to be associated with various diseases, including stroke (Wenzel et al., 2008), diabetes (Giebułtowicz et al., 2014; Kim-Muller et al., 2016), and CVD (Zhang et al., 2023), due to the accumulation of toxic aldehydes and their detrimental effect to the cell. On the other hand, their overexpression was found in cancer stem cells and other cancer cells, making them less susceptible to chemo- and radio-therapies. (Boumya et al., 2023; Magrassi et al., 2024; Mao et al., 2013; Quattrini, Gelardi, Coviello, et al., 2020; Quattrini, Gelardi, Petrarolo, et al., 2020; Quattrini, Sadiq, Petrarolo, et al., 2020).

Another enzyme involved in the cell detoxification function is aldose reductase (AKR1B1) which is involved in the first reaction step of the polyol pathway, converting glucose to sorbitol (Bernardoni et al., 2024). This NADH-dependent enzyme also reduces toxic aldehydes as HNE to its harmless derivative 1,4-dihydroxynonene (Ramana et al., 2006). In the end, glutathione S-transferases (GSTs) also play a role in removing aldehydes from the cytoplasm by conjugation with glutathione, which promotes their excretion (La Motta et al., 2007; Quattrini & La Motta, 2019; Siems & Grune, 2003).

However, the cellular concentration of 4-HNE varies depending on the cell type. Notably, the degradation of 4-HNE occurs more rapidly in liver cells compared to brain and lung cells. This observation aligns with the pathogenic and epidemiological factors associated with lung cancer risk and it is correlated with different metabolic activity within tissues (Zheng et al., 2014).

4.3. Acrolein

Acrolein is the most reactive aldehyde generated during the lipid oxidation of PUFAs in food, particularly during thermal processing and storage. Its formation occurs mainly through the initial oxidation step, with α-linolenic acid 13-hydroperoxide serving as the crucial intermediate (Ewert et al., 2014; Stevens & Maier, 2008). Ewert et al. demonstrated that in VOs rich in linoleic acid (e.g., linseed, rapeseed, and perilla oils) the acrolein content is directly correlated with the temperature and time of exposure. In particular, heating oil for 24 h at 180 °C, a typical frying temperature, yields 207.4 mg of acrolein per kg of oil, compared to 174.4 mg/Kg after 24 h at 100 °C and 94.1 mg/Kg after just 2 h at 180 °C (Bastos et al., 2017; Ewert et al., 2011). Moreover, acrolein has been detected also in COFs in a range of 26.4–64.5 μg/m³ (Seaman et al., 2009). As an electrophile, this small aldehyde exhibits high reactivity towards cellular nucleophilic components, affecting many functionally critical proteins, including redox-regulating proteins like thioredoxin (Trx) and thioredoxin reductase (TrxR), cytoskeletal proteins such as α-tubulin and vimentin, and transcription factors like nuclear factor κB (NF-κB) (Seiner et al., 2007; Uemura et al., 2019; Zhu et al., 2011). Furthermore, acrolein can attack the C1 and N2 positions of deoxyguanosine, leading to the formation of mutagenic cyclic adducts, α- and γ-hydroxy-1,N2-propanodeoxyguanosine (α- and γ-hydroxy-PdG). The predominant γ-hydroxy-PdG adduct may cause interstrand DNA crosslinks or DNA-protein crosslinks and has been detected in various human tissues, including brain, lung, liver, serum, and saliva (Liu et al., 2010). Additionally to the harmful potential of acrolein, this small aldehyde can undergo further reaction forming acrylamide which has been identified in most heat-treated foods and classified as a type 2 A carcinogen from IARC. Acrylamide is primarily formed during food heating through the Maillard reaction, a chemical process between amino groups, such as those belonging to amino acids like asparagine, and reducing sugars (e.g., glucose or fructose) that occur at high temperatures (above 120 °C). Subsequently, the unstable Schiff base formed undergoes Strecker degradation to form Strecker aldehydes (e.g., acrolein), which favor acrylamide formation (Govindaraju et al., 2024). Additionally, another minor pathway, involving acrolein oxidation produces acrylic acid, which in presence of ammonia produced during Strecker degradation, forms acrylamide (Friedman, 2003).

5. COFs purification methods

The components of COFs, including aldehydes, pose a significant threat to human health (Sinharoy et al., 2019). Therefore, techniques and systems aimed at improving air purification are urgently needed to hamper the continuous increase of environmental burden and human health hazard. COFs containing PM and VOCs are directly released into the atmosphere via an exhaust pipe at the building roof after the simple treatment of the range hood, both in restaurants and domestic kitchens (Xu et al., 2018). Individuals frequently exposed to COFs, such as chefs, face an elevated risk of developing health issues associated with fume exposure (Svedahl et al., 2009). In fact, COFs can affect mucous membranes and the respiratory tract, potentially leading to chronic illnesses, nausea, headaches, and other adverse reactions or chronic diseases and cancer. Thus, several technologies are employed today for the purification of kitchen fumes, to preserve both human health and the environment. Recently, Tao et al. published an extensive comparative review of COF purification methods (Tao et al., 2023a). COFs from professional kitchens are better monitored and refined compared to street food stalls, which do not use any air-filtration system. The latter is classified into physical capture, chemical degradation, a combination of the two systems, and catalytic methods (Liang et al., 2022). Among the physical methods, those working through mechanical separation are based on a cheap and less efficient technique that is mainly employed for small quantities of air pollution, whereas filtration-adsorption systems consist of porous material (e.g., molecular sieves) that regularly requires maintenance and may produce secondary pollution, alike washing absorption method. Also, electrostatic deposition is one of the most used methods and involves the ionization of COFs through a high-voltage field (Gallego et al., 2013; Tamaddoni et al., 2014). However, one of its significant drawbacks is its association with fire hazards after long-term use because of the oil film deposition on the electrode surface (Cheng et al., 2019). On the other hand, chemical decomposition methods are suitable for VOCs purification and preferred to physical methods since rely on oxidizing and breaking VOCs into non-toxic small molecules such as H₂O and CO₂. Chemical purification methods are photolysis, thermal oxygen incineration, catalytic combustion, low-temperature plasma, and biodegradation. The photolytic method uses UV light to decompose COFs and, recently, specific catalysts were reported to improve its purification efficacy. However, it can only be used for low concentrations of oil fume exhaust gases due to limited light energy consumption and unstable purifying results (Shayegan et al., 2019). While thermal oxygen incineration can completely oxidize VOCs, it is very expensive in terms of energy consumption and costs. Furthermore, catalytic combustion achieves similar results at lower temperatures, and, thus, reduced energy costs (Wang, Wu, & Chen, 2024). This method uses gas-solid phase catalytic reaction, where VOCs undergo flameless combustion at low light-off temperatures generating H₂O and CO₂ (S. Wang et al., 2023). Low-temperature plasma method uses high-frequency discharge to decompose VOCs into harmless molecules or generate high-energy electrons and reactive radicals which react with VOCs. However, this method is highly expensive and the possibility of explosive reaction with waste gases significantly limits its application (Z. Chang et al., 2020). As a biochemical method, biodegradation is a recent environmentally beneficial approach which involves microorganisms able to digest VOCs into small harmless compounds. Despite its low secondary pollution and inexpensive cost, this approach has not been extensively employed due to its unstable working conditions and excessive time consumption (Liang et al., 2022; Sarkar et al., 2022). Overall, physical methods compared to chemical ones are more efficient in removing PM than VOCs from the air. Otherwise, catalytic combustion is probably the most promising due to its high efficacy (90 % purification), limited costs, and low secondary pollution (Tao et al., 2023a).

In Table 1, the most reported purification methods with their main characteristics are summarized (Tao et al., 2023b).

Table 1.

Summary of purification methods within their mechanism, benefits, and drawbacks.

Method	Mechanism	Benefits	Drawbacks	Efficiency in removal of
				PM	VOCs
Mechanical Separation	(Physical capture) Inertial, gravity, or centrifugal forces applied to separate PM	Simple, low cost, good for home kitchens	Low efficiency, cleaning cycles frequently required
Filter-Adsorption	(Physical capture) Adsorption onto porous materials, e.g., activated carbon	Simple, low cost, easy maintenance, effective at room temperature, good for home kitchens	Easily clogged, limited lifespan, frequent replacement required
Washing Absorption	(Physical capture) Gas-liquid contact dissolved pollutants into absorbent solution	Easy management, stable management, good for home kitchens	Secondary pollution: high absorbing liquid consumption	especially water-soluble PM
Electrostatic Deposition	(Physical capture) High-voltage ionization charges particles (ionization) that are then collected on electrodes	Compact, efficient, wide applicability, good for home and restaurant kitchens	Fire hazard risk, reduced efficiency over time
Thermal Oxygen Incineration	(Chemical decomposition) Complete oxidation at high temperature	High efficiency, effective deodorization, good for industrial kitchens	High energy consumption and high cost
Catalytic Combustion	(Chemical decomposition) Complete oxidation at low temperature, via catalyst	Low secondary pollution, effective deodorization, good for industrial kitchens	Catalyst cost and periodic replacement required
Photolysis Oxidation	(Chemical decomposition) Oxidation induced by UV or visible light in presence of a catalyst	Simple, long life, deep purification, deodorization, good for low-flow systems	Low utilization of light energy, effective only at low VOC concentrations, results can be inconsistent
Low-Temperature Plasma	(Chemical decomposition) Reactive radicals and high-energy electrons decompose VOCs	Compact, safe, effective, possesses germicidal action, highly efficient, good for specific kitchens	Expensive, explosion risk with flammable waste gases, catalyst periodic replacement required
Biodegradation or biofiltration	(Chemical decomposition) Microbial metabolism converts VOCs into safe compounds (use of enzymes)	Green, low-cost, highly selective, and low secondary pollution	Long processing times (slow kinetics), low stability, unstable performance
Combination Methods (1–3)	Integrates multiple technologies	High overall efficiency, good for industrial kitchens	Complexity, cost
1. Mechanical + Electrostatic		High efficiency
2. Plasma + Photocatalysis		High efficiency	Highly expensive
3. Biotrickling Filtration + Photocatalysis		High efficiency	Complex setup, unstable performance

As highlighted by Table 1, different methods have been reported to date and the selection of the more appropriate one is highly dependent on the target pollutants (PM or VOCs), operational context (domestic vs industrial), and cost-effectiveness. Physical methods are generally more suitable for removing PM and are often used in home kitchens due to their simplicity and lower cost. However, their efficiency in VOC removal is limited. Chemical methods, especially catalytic combustion, offer high VOC removal rates (often above 90 %) and minimal secondary pollution, making them more appropriate for industrial and restaurant settings. Combination methods represent a promising approach by leveraging the advantages of multiple technologies, particularly where both PM and VOC removal are essential.

Given the higher emission concentrations and complexity of pollutants in industrial kitchens, catalytic combustion systems or combination methods (e.g., electrostatic deposition and photocatalysis) are recommended. In contrast, home kitchens may benefit from lower-cost systems such as electrostatic deposition or activated carbon adsorption, provided they are properly maintained to avoid fire hazards or performance drops.

Anyway, the incidence of illness due to COFs poisoning remains alarming. This is particularly prevalent in Asian countries where frying methods are extensively used, and regulations often fail to address this issue adequately. In response, European regulation has recently been introduced to address harmful substances produced by deep frying mediums and establish mitigation measures to reduce hazardous materials. Besides, the EU has recently set limits on trans-fatty acid content in food (EU 2019/649) and acrylamide levels in carbohydrate-based food (EU 2017/2158) (Conte et al., 2020).

Similarly, the Landfill Directive 1999/31 EC mandates the prohibition of certain liquid wastes, including RCOs, from being disposed of in landfills. This directive reflects the EU's efforts to promote sustainable waste management practices and reduce environmental pollution.

6. Conclusions

VOs are the worldwide preferred dietary fat due to their well-known beneficial properties. However, used at high temperatures in cooking methods, they generate fumes containing several hazardous compounds that pose risks to both human health and the environment. Aldehydes are the second most present and toxic class of compounds in COFs, which can promote acute (e.g., inflammation) and chronic (e.g., cancer) diseases. They have been established to represent the third largest air pollutant after vehicles and industry, thereby, making air purification urgently needed in order to mitigate the increasing environmental burden and health risk. Despite RCOs by-products being widely recognized, a lack of public awareness and legislation addressing this issue is clear. Consequently, this practice persists not only in household kitchens but also in various small-scale industrial sectors. Furthermore, despite the well-documented hazardous features of COFs and the availability of several methods for air purification, the exposure of people is still high, both in professional kitchens and on the streets where cooking hoods release COFs. To summarize, in addition to regulatory efforts by governments to better recognize the risks of COFs to human health, the development of novel approaches and techniques for COFs reduction and purification is urgently needed and remains a challenging topic for researchers.

Authors contributions

The manuscript was written through the contribution of all authors. All authors have approved the final version of the manuscript.

CRediT authorship contribution statement

Fabio Scianò: Writing – original draft, Investigation. Bianca Laura Bernardoni: Writing – review & editing. Ilaria D'Agostino: Writing – review & editing. Giulia Ferrara: Investigation. Andrea Tafi: Writing – review & editing. Silvia Garavaglia: Writing – review & editing. Concettina La Motta: Writing – review & editing, 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.

Data availability

No data was used for the research described in the article.