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Review

Comparative Study of the Presence of Heavy Metals in Edible Vegetable Oils

by
Pablo González-Torres
1,
Juan G. Puentes
2,
Alberto J. Moya
1,2 and
M. Dolores La Rubia
1,2,*
1
Department of Chemical, Environmental and Materials Engineering, Faculty of Experimental Sciences, Campus Las Lagunillas, University of Jaén, 23071 Jaén, Spain
2
University Institute of Research on Olive Groves and Olive Oils, GEOLIT Science and Technology Park, University of Jaén, 23620 Mengíbar, Spain
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(5), 3020; https://doi.org/10.3390/app13053020
Submission received: 30 January 2023 / Revised: 19 February 2023 / Accepted: 23 February 2023 / Published: 26 February 2023
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Abstract

:
Currently, the processes of obtaining and producing food as well as consumption habits give rise to new challenges for food safety. The presence of heavy metals in edible vegetable oils has harmful effects for humans. In this study, the scientific literature which reports the presence of heavy metals in different types of vegetable oils was analysed. More than 25 heavy metals are evaluated in 35 different oils from 24 countries. The widely studied metals are Cd, Pb, Cu, and Fe in olive oil, sunflower oil, rapeseed oil, and corn oils, mainly in Brazil, Turkey, China, Iran, and India. Likewise, the presence of Antimony (Sb) as a product of migration from PET containers to edible vegetable oils is a topic of great interest in recent years. Additionally, the different analytical techniques used for this purpose and the standards are analysed. This study highlights the main findings and challenges in this research field.

1. Introduction

The concept of food safety (FS) has evolved over time, having to adapt to the consumption needs of human beings. This evolutionary process has led to the emergence of new challenges to be solved within the process of obtaining, preparing, and consuming food.
Currently, enough food is produced for the entire population; however, there are more than 820 million people who continue to suffer from chronic malnutrition. Added to this, is malnutrition, obesity, being overweight, and delayed growth in developing and developed countries. This entails serious consequences for the health of the population, the wealth of nations, and the quality of life of human beings and communities [1].
This problem is accompanied by the current trend towards less land availability, soil degradation and erosion, loss of biodiversity, and the frequent occurrence of extreme weather phenomena. This situation, exacerbated by climate change, has a devastating impact on agriculture and, consequently, on food [2].
Taking into account the general aspects that directly affect FS, it is worth mentioning its influence on food. In this sense, an exhaustive control must be a priority in all the stages of production, transformation, storage, transport, conservation, and domestic preparation of food, so the healthiness is guaranteed [3].
At present, foodborne diseases are being recognized as a real problem for public health due to the burden of mortality and morbidity to which they contribute and the serious economic problems that they are causing to society in general. Food plays a crucial role in the transmission of diseases, and the factors that influence the contamination of food are the soil, water, air, animals, human beings, and the tools they use, as well as the process of production carried out in obtaining them: collection, transport, conservation, storage, processing, and distribution; therefore, food contamination could be defined as the abnormal presence of any extrinsic agent in food that may compromise the safety and health of humans. This presence can occur throughout the entire food chain or as a result of environmental contamination, but always under the premise that it has never been intentionally added to food [4].
At this point, the concept of food intoxication, or food disease, arises. This occurs when there is an incidental, accidental, or intentional ingestion of water and poorly prepared, or prepared food improperly preserved, due to physical, biological, and/or chemical contamination, i.e., toxins generated by bacteria or chemical products that can be found in food naturally. One type of chemical contamination is the contamination by the presence of heavy metals. These are a group of chemical elements that are not biodegradable and are highly toxic to humans, especially when the exposure is prolonged over time. Some of the most harmful are: cadmium, present in the environment due to erosion or industrial and mining activity; mercury, as a consequence of natural volcanic activity, mining activity, or paper industries; lead, a natural contaminant used in fuels, paints, varnishes, or pipes for water; or arsenic, used in pesticides, medicines, or ceramic objects [5].
Traces of heavy metals, such as arsenic, mercury, lead, zinc, nickel, or cadmium, were found in vegetables, meat, fish, or milk, due to mobility and bioaccumulation in water points. Similarly, heavy metals, such as cadmium, were found in molluscs, crustaceans, and shellfish. Depending on the type of heavy metal or the chemical and biochemical reactions in which it is involved, the health conditions are different, ranging from vital organ damage to different types of cancer [5].
In addition, continued exposure to low doses of heavy metals cause accumulation in the skeleton, nervous system, digestive system, cardiovascular system, or kidneys, forming stable metal complexes with proteins, vitamins, nucleic acids, and hormones, leading to pathological changes and a high health risk [6].
In a complementary way, human beings are also exposed to chemical contamination through migration from food packaging. In general, the migration term describes the diffusion process which is influenced by the interaction of the package material and the components of the food [7].
The selected packaging material can affect the nutritional quality and the safety of the food itself. This is why the packaging has to comply with legislation to guarantee safety and avoid possible contamination by transfer or migration of different compounds to food. An example can be the additives and non-polymerized monomers in plastic containers that can migrate to food products [8].
Currently, the polymers used are highly inert and it is very difficult for them to transfer mass to food; however, there are a number of minority components which comprise chemical molecules below 1000 Daltons and which are transferred quite easily to the product already packaged [9].
One of the heavy metals intrinsically related to the migration phenomenon is antimony (Sb). It is a catalyst used in the polymerization reaction for the formation of containers that tends to decompose during this process. Although it appears as a degradation product, it tends to be volatile and is generally found at very low levels [10]. However, more recent studies agree that Sb is the element that most easily migrates among all substances, from polyethylene terephthalate (PET) to water, through leaching [11].
Sb can be found in two oxidation forms: organic, which is the least toxic; and inorganic, which is the most toxic. As previously mentioned, antimony is used as a catalyst in PET synthesis or polymerization processes, as antimony trioxide (Sb2O3). In the chemical reaction, it is in charge of increasing the speed and, at the end of it, the excesses of Sb are coupled to the polymer chain forming antimony–glycan complexes, leaving free traces of Sb2O3 and with no links [12].
Additional to the physical–chemical properties of the container, there are other factors that influence the migration of chemical components to food. These are: the characteristics and nature of the food, i.e., liquids or fatty foods when in contact with the container are most likely to be affected by migration due to their greater solubility power; the temperature, increasing the migration process as it increases; and the contact time, the container–food exposure time is proportional to the migration process [13].
Edible vegetable oils, which are the aims of this review, are a growing sector worldwide, with palm and palm kernel oil leading the consumptions with 36.3 and 4.2%, respectively. They are followed by soybean oil with 27.7%, rapeseed oil with 13.5%, and sunflower oil with 9.5% according to the U.S. Department of Agriculture (USDA, Washington, DC, USA) report of June 2019. The above-mentioned oils account for almost 91% of the total production and consumption of vegetable oils. The remaining 9% are olive, corn, or peanut oils, among others, all of them increasing [14].
This review examines the current status of the presence of heavy metals in edible vegetable oils and analyzes the status of current legislation, from 2015 to 2022, in order to improve the quality of oils and consumer protection. The most studied regions, vegetable oils, heavy metals, analytical techniques, and chemical extracting agents for analytes are also investigated, and the most relevant results of the predominant heavy metals are analyzed (Section 2). Finally, the challenges faced by this line of research are exposed (Section 3) and we present the most outstanding conclusions of this work (Section 4).
Oils from other matrices were not taken into account, such as: oil from algae [15], crude or virgin vegetable oils unfit for consumption [16,17,18,19,20], fish oil [21], or oils from other edible vegetables [22].

2. Analysis of Publications and Trend of Heavy Metals in Edible Vegetable Oils

This section shows a preliminary compilation of relevant data extracted from the different studies analyzed in this research:
Geographical location of the regions with the most studies on the presence of heavy metals in edible vegetable oils.
For this, the sampling point with which the analysis will be carried out is taken as a reference, taking into account that, in most cases, the oils have been acquired in the place of origin or in local markets. Likewise, the countries that carry out the most studies are located in the northern hemisphere, except for Brazil and Argentina. This distribution is not random as there is a direct correlation between said distribution and those countries that consume the most of this type of oil.
Figure 1 shows the percentage of studies in each country. Brazil is the country with the highest percentage with 17.19%, followed by Turkey, China, Iran, and India, with 15.63, 10.94, 9.38, and 6.25%, respectively. Among them are the studies of metals in edible oils in Brazil [23], Turkey [24], China [25], Iran [26], and India [27].
Domestic consumption of vegetable oils expressed in millions of metric tons is also represented, Mill. M.T., in which China is leading with 40.79 Mill. M.T., followed by the European Union, India, Indonesia, and the United States, with 25.13, 23.08, 22.22, and 18.10 Mill. M.T., respectively. In this way, it can be verified that those countries with the most consumers carry out a greater number of studies, as China, India, the United States, or Brazil.
It is necessary to mention some countries in which the consumption of vegetable oils and the percentage of studies have no direct relationship, as is the case of Turkey where consumption is low, with a value of 2.65 Mill. M.T. and the percentage of studies is high (second place), with a value of 15.63% [28,29,30]. In the opposite case, the United States is the fifth most consuming region with 18.10 Mill. M.T. and the percentage of studies is low, with a value of 3.13% [31,32].
Edible vegetable oils studied.
A total of 35 oils were considered, being the most studied olive oil with 14.36%, followed by sunflower oil with 13.81%, and rapeseed oil and corn oil, with 9.94%. In these four oils the majority of the investigations on metals are focused on: iron (Fe), copper (Cu), cadmium (Cu) and lead (Pb). Also, manganese (Mn) in olive oil [33], cadmium (Cd) in sunflower oil [34], nickel (Ni) in rapeseed oil [35], or lead (Pb) in corn oil [36] are importat. There are other oils in a lower percentage such as: avocado oil [37], argan oil [38], linseed oil [30], or peach oil [39].
Heavy metals in edible vegetable oils.
The most analysed heavy metal is Cd, with 14.60% of studies. This is followed by Pb, Cu, and Fe with 12.41, 12.04, and 9.49%, respectively. There are other heavy metals in lower percentage such as: cobalt (Co) in camellia oil (walnut variety) [40], mercury (Hg) in olive oil [41], or titanium (Ti) in coconut oil [42].
Analytical techniques used for determination and quantification.
The most used analytical technique is Induction Coupled Plasma Atomic Emission Spectroscopy (ICP-OES/ICP-AES) with 25% of the studies, followed by Flame Atomic Absorption Spectrometry (FAAS), Mass Spectrometry with Inductive Coupling (ICP-MS), and Graphite Furnace Atomic Absorption Spectrometry (GF-AAS) with 17.65, 16.18, and 14.71%, respectively. There are other analytical techniques to a lesser extent such as: High-Resolution Continuum Source Graphite Furnace Atomic Absorption Spectrometry (HR-CS GFAAS) [43,44], Ion Exchange Chromatography Coupled to Inductively Coupled Plasma Mass Spectrometry (AEC-ICP MS) [45], or Microwave Induced Plasma Optical Emission Spectrometry (MIP OES) [37].
Chemical reagent used for the analytes extraction.
In this regard, the research focuses on studies of heavy metals in olive oils, since they are the most analyzed vegetable oils. At this point, there is a predominant tendency to extract these analytes by acid digestion, using nitric acid (HNO3) as extracting agent, specifically in 37.50% of the studies. Some studies that have used this extracting agent are [46,47,48,49]. There are other types of chemical agents used in lower percentages such as: Dispersive Solid Phase Extraction (DSPE) [50], and Deep Eutectic Solvents (DES) in combination with Tetrahydrofuran [51] or Triethylamine (TEA) [30].
Mainly HNO3, together with other chemical reagents such as hydrochloric acid (HCl) or sulfuric acid (H2SO4), is used regularly for the preparation of emulsions. Its use is due to the fact that the main application of these inorganic acids in emulsification procedures helps prevent the hydrolysis of metal ions and converts the organometallic analytes present in the oils into their inorganic forms [28].

2.1. Comparison between the Most Studied Oils and Metals

Olive, sunflower, rapeseed, and corn oils are the most studied edible vegetable oils. Likewise, Cd, Pb, Cu, and Fe are the most studied heavy metals. Taking these oils and heavy metals as a reference, it was possible to analyse the frequency of study of each metal in these oils (Figure 2).
Cd and Pb are the most studied heavy metals in olive oil, with 31.25 and 27.08%, respectively, while Cu and Fe predominate in rapeseed oil, at 28.57% (Figure 2). In this sense, Cd is once again the predominant metal analyzed for the oils mentioned above, with the exception of rapeseed oil and, in addition, it is the one with the highest percentage of studies.

2.2. Comparison between the Oils and the Most Used Techniques

According to the above, ICP-OES, FAAS, ICP-MS, and GF-AAS are the most used analytical techniques; furthermore, these techniques are the most used in olive oil, sunflower oil, rapeseed oil, and corn oil. Table 1 shows the values of the detection limits (LD), the techniques for the determination of heavy metals, and the most studied vegetable oils.
For the elaboration of Table 1, in addition to the filters initially established with vegetable oils and heavy metals most analysed, those articles that do not refer to the LD of the equipment used were eliminated and the most sensitive techniques of each most analysed heavy metal were selected (Cd, Pb, Cu, and Fe).
In this sense, it is also complex to establish criteria that allow comparing the sensitivities of the techniques used due to the multiple factors that must be taken into account, such as: sample preparation, selection of the analyte extraction agent, choice of coupling of the technique in question, etc. However, from this research it has been extracted that the greatest sensitivity to Cd has been obtained using GF-AAS, with an LD of 2.00 × 10-6 mg/kg in corn, olive, and sunflower oils [52]; for Pb, with an LD of 2.00 × 10−5 mg/kg in olive, sunflower, and corn oils [25,52,53], and for Cu, with an LD of 5.00 × 10−4 mg/kg in olive [54]. However, for Fe, the highest sensitivity found is in ICP-OES, with an LD of 9.00 × 10−4 mg/kg in rapeseed oil [25]. However, although the predominant technique for Fe analysis is ICP-OES, the sensitivity values found are lower than GF-AAS for any of the other predominant heavy metals analysed.
Table
Table 1. Detection limits (LD) of the most widely used techniques for the determination of heavy metals and most studied vegetable oils, expressed in mg/kg.
Table 1. Detection limits (LD) of the most widely used techniques for the determination of heavy metals and most studied vegetable oils, expressed in mg/kg.
Types of OilsHeavy MetalAnalytical TechniquesLimit of Detection (LD)Reference
Rapeseed oilCdICP-OES6.00 × 10−5[53]
Olive oilCdGF-AAS2.00 × 10−6[52]
Sunflower oilCdGF-AAS2.00 × 10−6[52]
Corn oilCdGF-AAS2.00 × 10−6[52]
Rapeseed oilPbFAAS3.06 × 10−4[25]
Olive oilPbGF-AAS2.00 × 10−5[53]
Sunflower oilPbGF-AAS2.00 × 10−5[25]
Corn oilPbGF-AAS2.00 × 10−5[52]
Rapeseed oilCuICP-OES1.20 × 10−4[53]
Sunflower oilCuFAAS3.50 × 10−4[55]
Corn oilCuGF-AAS4.34 × 10−4[56]
Olive oilCuGF-AAS5.00 × 10−4[54]
Olive oilFeICP-OES1.00 × 10−3[52]
Sunflower oilFeICP-OES1.00 × 10−3[52]
Corn oilFeICP-OES1.00 × 10−3[52]
Rapeseed oilFeICP-OES9.00 × 10−4[25]
The ICP-OES technique is the most used, although is not the most sensitive as Table 1 shows. Therefore, Atomic Absorption Spectrometry (AAS) technique in combination with other chromatographic techniques—such as the Graphite Furnace (GF-ASS)—is more sensitive than Mass Spectrometry (MS)—combined with other techniques, such as Induction Coupled Plasma (ICP-OES)—for the determination of heavy metals in edible vegetable oils.
There are no clear criteria among the literature for the use of AAS or EM. Although it seems that there is a predominance in the use of ICP-OES/ICP-AES in the years 2017 and 2020, with a percentage of 17.65 and 29.41%, respectively. The ICP-OES/ICP-AES technique is surpassed in the year 2021 by ICP-MS with 5.88 compared to 17.65%. In addition, the use of the FAAS technique is equal to ICP-OES/ICP-AES with 11.76% and the use of the GF-AAS technique exceeds ICP-OES/ICP-AES with 17.65% in 2018 (Figure 3).
On the one hand, the use of AAS in combination with another chromatographic technique, such as GF-AAS, is considered an adequate analytical technique for the quantification of metals due to its low sensitivity; however, the quantification of some metal is a challenge due to its low concentration in this type of matrices [57]. In this sense, the FAAS technique is one of the most widely used for the detection of heavy metals due to its low cost and easy handling, although it presents low nebulization efficiency and short residence time of the excited atoms in the region of the flame [58]. Another drawback of the use of AAS techniques is its expensive calibration due to the high price of organometallic standards [59].
On the other hand, the use of EM in combination with another chromatographic technique, such as ICP-OES, presents a high performance with a low flow rate of solvent in mobile phase and with the ability to suppress the problem that AAS has for matrices dense matrices and their difficulty in extracting analytes from such matrices [59]; also, ICP-MS is the most powerful technique for the determination of trace metals in different matrices, including olive oils, with the advantages of its high sensitivity, its good linearity, and precision, and as disadvantages its high cost and reduced availability in laboratories [60].

2.3. Results of the Analysis of Heavy Metals in Edible Vegetable Oils

To find out how the presence of heavy metals in edible vegetable oils is currently being managed, the following criteria were selected.
Elimination of the virgin, crude seed oils, and no edible oils before refining;
Review of current national and international legislation to compare with the results of heavy metals in edible vegetable oils (Table 2);
Selection of the studies whose results are close to or higher than the current regulatory limit. Values lower and which comply with the regulations are not considered, such as: rice oil from India, Italy, and Thailand [61], pumpkin seed oil from Serbia [62], edible oils from Iran [63], or hazelnut oil, sunflower oil, olive oil, and corn oil from Turkey [64].
Table
Table 2. Toxicity thresholds established in current national and international legislation for heavy metals, expressed in mg/kg.
Table 2. Toxicity thresholds established in current national and international legislation for heavy metals, expressed in mg/kg.
Heavy
Metals		CX-A 1WHO 2 EFSA 3Commission Regulation (EU) 2021/1317 4Commission Regulation (EU) 2021/1323 5GB/T23347-2021
(China) 6		RD
308/1983
(Spain) 7
CdNC 8NC/R 9,10NC/R 11NA 12NC 13NCNC
Pb0.1 14NC/R 15NC/R 160.10NANC0.1
CuNC 17NCNC/R 18NANA≤0.10.4
FeNC 19NCNCNANA≤3.010
1 CX-A, is Codex Alimentarius. 2 WHO, is World Health Organization. 3 EFSA, is European Food Safety Authority. 4 Commission Regulation (EU) 2021/1317 of 9 August 2021 amending Regulation (EC) No 1881/2006 as regards maximum levels of lead in certain foodstuffs [65]. 5 Commission Regulation (EU) 2021/1323 of 10 August 2021 amending Regulation (EC) No 1881/2006 as regards maximum levels of cadmium in certain foodstuffs [66]. 6 GB/T23347-2021 (China). Recommended national standard for olive and olive pomace oils exported to the Chinese market [67]. 7 Royal Decree (RD) 308/1983 (Spain), the Technical-Sanitary Regulation of edible vegetable oils is approved [68]. 8 NC, not considered in the regulations. 9 NC/R, not considered in the regulations but shows recommendations. 10 Provisional tolerable monthly intake (TMTI) of 25 µg/kg body weight [69]. 11 Tolerable weekly intake of cadmium of 2.5 µg/kg body weight [70]. 12 NA, regulation not applicable for this heavy metal. 13 Rapeseed 0.15 mg/kg and sunflower seeds 0.50 mg/kg, however the maximum levels do not apply to tree nuts or oilseeds for crushing and oil refining [66]. 14 [71]. 15 No defined maximum thresholds [72]. 16 Benchmark Dose Lower Level (BMDL01) of lead at 0.5 µg/kg of body weight per day [73]. 17 A product quality criterion is established with a maximum dose of 0.1 mg/kg for refined oils and 0.4 mg/kg for virgin oils [74]. 18 An adequate intake (AI) is established ranging from 0.4 mg/day for infants to 1.6 mg/day for the adult population of the male gender [75]. 19 A product quality criterion is established with a maximum dose of 1.5 for refined oils and 5.0 mg/kg for virgin oils [74].

2.3.1. Cadmium (Cd)

Cd is one of the most toxic heavy metals along with Pb and Hg. Although it is associated with different minerals distributed in nature, the human being has contributed to its dispersal through mining and metallurgical activities [76]. In the edible vegetable oil production and processing sector, its presence may have originated from the organic solvents used in the oil extraction process, which must meet a series of requirements to ensure safety. With this, it is intended to avoid transferring traces to the oil, controlling the presence of Cd in the final product and avoid exposure of this heavy metal to the consumer [77]. In addition to its high toxicity, its problem lies in its long half-life and its ability to accumulate in living beings. Some of the possible health effects from prolonged exposure may be kidney, lung, bone, stomach, and prostate toxicity problems, also developing metabolic syndrome and cancer [78]. More illustratively, Figure 4 represents the mobility of Cd in the soil and plants, and its incorporation through plant consumption [79].
Figure 2 show that Cd is the most analysed heavy metal in the edible vegetable oils collected in this literature review. However, and despite its importance due to the high toxicity it presents, there is no specific legislation for oils regarding the maximum limit allowed for consumption through the ingestion of edible vegetable oils.
In this sense, Commission Regulation (EU) 2020/1245 [66] established limit values of Cd in different foods such: as cereals, 0.10 mg/kg; bran, germ, wheat, and rice, 0.20; vegetables and fruits, 0.05; or leafy vegetables 0.10 mg/kg, among other foods. Furthermore, in 2010 the joint FAO/WHO expert committee on Food Additives [69] proposed a maximum tolerable monthly intake of 25 μg/kg of Cd with respect to weight. A year earlier, the EFSA established a tolerable weekly intake of 2.5 μg/kg, a figure significantly lower than that proposed by JECFA [70].
Therefore, in the absence of specific regulations for the presence of Cd in edible vegetable oils, the selected comparative criterion was the most restrictive normative value for the rest of the heavy metals analysed in this review, 0.10 mg/kg (Table 2). In this regard, it is possible to put into perspective those results that deserve mention due to their high concentration in these oils.
Table 3 shows both the results that are close to 0.10 mg/kg and the results that significantly exceed it. Due to its proximity, we find, among others, olive oils from Iran and Cyprus with values between 0.094 and 0.097, and 0.09 mg/kg, respectively [53,60], and rapeseed and sesame oils from Iran with values between 0.098 and 0.099, and between 0.090 and 0.097 mg/kg [53]. Likewise, among the results that significantly exceed this limit, China rapeseed oil stands out with a value of 0.88 mg/kg [80]; olive oil from Iran and Italy with a value of 4.181 mg/kg [81]; and sunflower oil from India with a value of 0.54 mg/kg [82]. The lack of evidence of toxicity thresholds by international organizations for Cd in oils—being the most analysed heavy metal—shows that oils with high values, such as those mentioned above, are being consumed without any type of restrictions with the aforementioned health risks associated with the prolonged intake of this heavy metal.

2.3.2. Lead (Pb)

Pb can be found in edible vegetable oils as a consequence of environmental contamination, refining processes, transfer during transport, or the packaging process [80]. Furthermore, Pb is a heavy metal that accumulates in the body altering cell metabolism and being a precursor of different harmful effects on human health [20], such as the decrease in the number of erythrocytes necessary for the synthesis of red blood cells and haemoglobin, due to the inhibition of enzymes because of exposure to this heavy metal [84].
Table 2 shows that there are several regulations that establish the maximum limit for the presence of lead in edible vegetable oils at 0.10 mg/kg. These are: WHO (Codex Alimentarius) at the international level, Commission Regulation (EU) 2021/1317 at the European level, and Royal Decree 308/1983 at the Spanish level. Table 4 shows both the results that are close to 0.10 mg/kg and the results that significantly exceed it. The results for sesame oil from Iran are close to the limit with values between 0.092 and 0.099 mg/kg [53]. Most of the results obtained equal or exceed the regulatory limit, such as olive oils from Iran, Italy and Pakistan, with values between 8.546 and 18,783, and 1.321 and 7.249 mg/kg, respectively [51,81], sunflower oil from the United Kingdom, with 0.274 mg/kg [42], and rapeseed oils from China and Pakistan, with values of 1.96 and between 1.301 and 6.765 mg/kg, respectively [42,80].

2.3.3. Copper (Cu)

Similarly to Cd, Cu has no specific international legislation for edible vegetable oils in its category as a heavy metal. The Codex Alimentarius refers to Cu—together with Fe—as a precursor for the catalytic oxidation of edible oils and fats, even at trace levels, and only refers to threshold concentrations in oils as quality criteria. Copper or its alloys—such as brass or bronze—must not be used in storage or transport tanks and circuits that come into contact with oils, such as hoses, pipes, valves, etc. These recommendations are associated with maximum levels that should not be exceeded in the different types of oils [74]:
Refined fats and oils: 0.1 mg/kg;
Virgin fats and oils: 0.4 mg/kg;
Cold-pressed fats and oils: 0.4 mg/kg.
Therefore, there is a view of copper from the point of view of the quality of the oils rather than from the point of view of chemical contamination in regards to the health of consumers. Among other harmful effects on the body, high exposure to Cu can cause neurodegenerative disorders, such as Wilson’s disease [87]. In this regard, in 2021, Chinese regulations for the export of olive oils and olive pomace to that country established a maximum tolerance threshold of 0.1 mg/kg [67]; likewise, in 1983, Spanish regulations considered Cu as a heavy metal and established the maximum limit allowed in edible vegetable oils at 0.4 mg/kg [68].
Given the current absence of regulation of toxicity thresholds in the case of Cu, the values issued by Chinese regulations for olive oils and olive pomace (0.1 mg/kg) and the values issued by the Spanish regulations for the rest of the edible vegetable oils (0.4 mg/kg) are considered in order to carry out the analysis of the results of this heavy metal in the present study.
Table 5 shows both the results that are close to 0.10 mg/kg and the results that highly exceed it, in the case of olive oils. Due to its proximity, olive oil from Iran is found with values between 0.091 and 0.098 mg/kg [53]; while among the results that exceed this limit are the oils from Ukraine and Cyprus with values of 0.355 and between 1.02 and 3.81 mg/kg, respectively [54,60]. In the case of the rest of the edible vegetable oils that are close to and exceeding the limit of 0.4 mg/kg established by Spanish regulations, there is coconut oil from India with values between 0.300 and 0.363 mg/kg [55] and rapeseed oil from Brazil with a value exceeded of 0.81 mg/kg, and sunflower oil, also from Brazil, with a value of 0.81 mg/kg [88], among others.

2.3.4. Iron (Fe)

Fe currently lacks specific international legislation for edible vegetable oils. It is also considered an accelerator of oxidation processes and, although it refers to consumer protection, the maximum permissible levels for the Codex Alimentarius are marked as quality criteria for oils [74]:
Refined fats and oils: 1.5 mg/kg;
Virgin fats and oils: 5.0 mg/kg;
Cold-pressed fats and oils: 5.0 mg/kg
Table 6 shows the value issued by Chinese regulations in the case of olive oils established for Fe at 3.0 mg/kg (Table 2), and olive oil from Saudi Arabia is above this toxicity threshold with a value of 7.861 mg/kg [91]. In the case of Spanish legislation, which establishes 10.0 mg/kg as the maximum permissible limit for Fe in edible vegetable oils (Table 2), Chinese walnut oil is above this limit with a value of 11.2 mg/kg [92].
It is necessary to indicate that overexposure to Fe, even in trace amounts and for prolonged periods of time, makes this heavy metal cause adverse risks to human health, such as risks of Parkinson’s, Huntington’s, Alzheimer’s, cardiovascular diseases, Diabetes Mellitus, hyperkeratosis, pigmentation changes, kidney, liver and respiratory tract disorders, and neurological disorders [93].

2.4. Presence of Antimony (Sb) Studies

One of the main problems that is currently arising in relation to heavy metals in food is their migration when they are present in plastic materials such as antimony (Sb).
Only two studies report the presence of this heavy metal in oils (Table 7). These studies do not focus on the migratory aspect, but instead focus on the analysis of the determination of different metallic traces present in oils. They only refer to the plastic container and mention that the samples for analysis were packaged in PET.
It is interesting to highlight the results obtained in this research to compare them with the maximum limit of specific migration of Sb established by the European Union at 0.04mg/kg for plastic materials intended to come into contact with food [94]. Walnut oil, sweet almond oil, soybean oil, bitter almond oil, and coconut oil are above this toxicity threshold with values between 1.16 and 1.66 mg/kg [95]. This explains the need to continue deepening the study of trace metals such as Sb to elucidate, among other issues, whether they are in the food from the plastic packaging.
Table
Table 7. Summary of data obtained from the presence of antimony (Sb) in edible vegetable oils.
Table 7. Summary of data obtained from the presence of antimony (Sb) in edible vegetable oils.
OilsAnalytical
Techniques		Limit of
Detection (LD) 1		Limit of Quantification (LQ) 1Results (Range or Average Value) 1Region References
Walnut oil ICP-OES6.01 × 10−30.0251.26–1.66Turkey[95]
Sweet
almond oil		ICP-OES6.01 × 10−30.0251.06–1.60Turkey[95]
Soybean oilICP-OES6.01 × 10−30.0251.24–1.47Turkey[95]
Bitter
almond oil		ICP-OES6.01 × 10−30.0251.02–1.47Turkey[95]
Coconut oilICP-OES6.01 × 10−30.0251.16–1.42Turkey[95]
Extra virgin
olive oil		ICP-MS2.50 × 10−30.5007.00 × 10−5–4.50 × 10−4Italy[96]
1 The values present the decimal places provided by the authors or, failing that, approximated to three decimal places.

3. Challenges and Future Prospects

As a consequence of this review, different details were extracted to clarify the trend lines of the current studies focused on the presence of heavy metals in edible vegetable oils. Likewise, it was possible to demonstrate which countries are most interested, which metals are the most studied, and which analytical techniques are chosen for the determination and quantification of these metals, even though there is no clear predominance in the latter.
However, this review has also made it possible to show existing deficiencies in the investigation of the presence of these heavy metals in edible vegetable oils. These are considered as new technical challenges and new opportunities to be addressed in future researches:
Choice of a suitable chromatographic standard.
The current bibliography shows the use of different chromatographic techniques of atomic absorption or mass spectrometry, and their different couplings used for the determination of heavy metals. Each one shows advantages and disadvantages regarding its use, but there is no specific criterion regarding sensitivity and detection of these metals. This lack of criteria may be due to the diversity of previous extraction processes as well as analyte extracting agents that, in combination with the different chromatographic techniques, broaden the range of possibilities for metal determination without a specific standard.
Current legislation is scarce and insufficient in relation to the presence of heavy metals in edible vegetable oils.
Taking Cd as an example, it was observed in this review as the heavy metal most analysed. However, there is currently no specific limit for this heavy metal established in international legislation for edible vegetable oils. In this regard, the most current regulation that deals with Cd is Commission Regulation (EU) 2021/1323 in which the limits established for oil seeds are annulled when they undergo an oil extraction and refining process. As it was possible to analyse in this study, different vegetable oils, whose process prior to commercialization was refining, have exceeded the established limits of toxicity marked for other heavy metals such as Pb. This implies that, despite the high toxicity for humans derived from prolonged intake, vegetable oils with a high Cd content may be being marketed and consumed. It is necessary to mention Pb at this point, whose selected results are matching or greatly exceeding the limit established by current legislation.
Therefore, the study of the presence of these metals in vegetable oils reveals the toxicity values found in the current bibliography, with the possible health conditions that these heavy metals can cause in humans, in order to be able to establish adequate toxicity thresholds.
Absence of migration studies of heavy metals to edible vegetable oils.
The heavy metal Sb, whose toxicity is high and its origin in food could be the result of packaging migration, lacks specific legislation regarding edible vegetable oils. The studies analysed so far are scarce and the existing ones do not take it into account as a migratory contaminant in edible vegetable oils. Taking into account, on the one hand, the ease of diffusion it has towards fatty foods and, on the other, that most of these oils are sold in PET containers, the study of the presence of these oils represents a challenge for future research into toxic traces such as of this heavy metal in vegetable oils for human consumption.

4. Conclusions

The present investigation examined the aspects related to the presence of heavy metals in edible vegetable oils. A global assessment was carried out to check those countries with the most consumers who, in turn, carry out a greater number of studies, these being Brazil, Turkey, China, Iran, and India.
The vegetable oils with the highest number of studies were found to be sunflower, olive, rapeseed, and corn oils and the heavy metals that have predominated are Cd, Pb, Cu, and Fe. In the case of Cd, being one of the most toxic heavy metals, it is the most analysed heavy metal and, despite this, there is currently no international legislation regulating its toxicity threshold in edible vegetable oils.
The predominant chromatographic techniques are ICP-OES/ICP-AES, FAAS, ICP-MS, and GF-AAS. However, the authors present different criteria for choosing the most appropriate technique or set of techniques for the determination of heavy metals.
Finally, the selected bibliography was reviewed to verify which studies contained the heavy metal Sb as a migration product from the packaging of oils. As a result, only two studies considered Sb as a heavy metal but not as a migratory product. In this sense, the current society could be exposed to these toxic contaminants without any restriction, taking into account, on the one hand, the ease of diffusion of these into fatty foods and, on the other hand, most of the oils studied are commercialized in PET containers.
This study has shed light on the new challenges facing research on the presence of contaminants in food and, more specifically, heavy metals in edible vegetable oils. Future prospects should aim at identifying the most effective and sensitive techniques detecting heavy metals in food, in the centralization of studies on migration contamination problems, and in the achievement of regulations limiting the heavy metals as shown in this study.

Author Contributions

Conceptualization, M.D.L.R. and P.G.-T.; methodology, P.G.-T. and M.D.L.R.; software, validation, and formal analysis, P.G.-T., J.G.P. and M.D.L.R.; investigation and data curation, M.D.L.R., P.G.-T. and J.G.P.; resources, P.G.-T. and M.D.L.R.; writing—original draft preparation, P.G.-T.; writing—review and editing, P.G.-T., J.G.P. and A.J.M.; supervision: M.D.L.R. All authors have read and agreed to the published version of the manuscript.

Funding

Research group “Bioprocesses” (TEP-138) of the Andalusia Regional Government (Spain).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors acknowledge the Andalusia Regional Government (Spain) for its financial support to our research group “Bioprocesses” (TEP-138).

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 03020 g001 550
Figure 1. Percentage of studies per country related to the presence of heavy metals in edible vegetable oils and global domestic consumption in December 2022. 1 Data expressed in million metric tons.
Figure 1. Percentage of studies per country related to the presence of heavy metals in edible vegetable oils and global domestic consumption in December 2022. 1 Data expressed in million metric tons.
Applsci 13 03020 g001
Applsci 13 03020 g002 550
Figure 2. Study frequency of each heavy metal in each type of oil.
Figure 2. Study frequency of each heavy metal in each type of oil.
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Applsci 13 03020 g003 550
Figure 3. Frequency of use of the predominant techniques.
Figure 3. Frequency of use of the predominant techniques.
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Applsci 13 03020 g004 550
Figure 4. Flow of cadmium in soils and plants and ingestion by humans through plants.
Figure 4. Flow of cadmium in soils and plants and ingestion by humans through plants.
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Table
Table 3. Cadmium (Cd) content in edible vegetable oils, expressed in mg/kg.
Table 3. Cadmium (Cd) content in edible vegetable oils, expressed in mg/kg.
Types
of Oils		Analytical
Techniques		Limit of
Detection (LD) 1		Limit of
Quantification (LQ) 1		Results
(Range or
Average Value) 1		RegionReference
Rice oilICP-MS0.50-<LD–0.891USA (Peoria, Illinois)[31]
Rapeseed oilICP-OES--0.88China (Shizhuyuan, Chenzhou)[80]
Sunflower oilGC-MS-AAS--<LD–0.54India (Tamil Nadu)[82]
Corn oilICP-OES1.00 × 10−42.00 × 10−40.02–0.14Morocco, Algeria,
Jordan		[83]
Olive oilICP-OES1.00 × 10−42.00 × 10−4<LQ–0.10Morocco, Algeria,
Jordan		[83]
Olive oilICP-MS--0.02–0.09Cyprus[60]
Olive oil
flavoured 2		ICP-OES--<LD–1.004Iran, Italy[81]
Olive oilICP-OES--0.396–4.181Iran, Italy[81]
Olive oilFAAS7.40 × 10−52.48 × 10−30.026–0.097Turkey (Istanbul)[58]
Rapeseed oilICP-OES1.00 × 10−33.00 × 10−30.098–0.099Iran[53]
Corn oilICP-OES1.00 × 10−33.00 × 10−30.098–0.100Iran[53]
Olive oilICP-OES1.00 × 10−33.00 × 10−30.094–0.097Iran[53]
Sesame oilICP-OES1.00 × 10−33.00 × 10−30.090–0.097Iran[53]
Sunflower oilICP-OES1.00 × 10−33.00 × 10−30.091–0.100Iran[53]
Olive oilICP-OES3.80 × 10−41.28 × 10−32.24 × 10−3–0.1052Turkey (Istanbul)[50]
1 The values present the decimal places provided by the authors or, failing that, approximated to three decimal places. 2 Different seasonings added to flavour the oil, such as pepper, truffle, and vegetable.
Table
Table 4. Lead (Pb) content in edible vegetable oils, expressed in mg/kg.
Table 4. Lead (Pb) content in edible vegetable oils, expressed in mg/kg.
Types
of Oils		Analytical TechniquesLimit of
Detection (LD) 1		Limit of
Quantification (LQ) 1		Results
(Range or
Average Value) 1		RegionReference
Rice oilICP-MS0.10-<LD–0.127USA (Peoria, Illinois)[31]
Soybean oilAAS0.12-0.31–2.35Bangladesh[85]
Rapeseed oilICP-OES--1.96China (Shizhuyuan,
Chenzhou)		[80]
Rapeseed oilGF-AAS--0.012–0.100Poland (Lubelskie,
Mazowieckie, S’laskie, Opolskie, Wielkopolskie)		[86]
Olive oilICP-MS--0.15–1.48Cyprus[60]
Olive oil
flavoured 2		ICP-OES--0.984–12.33Iran, Italy[81]
Olive oilICP-OES--8.546–18.783Iran, Italy[81]
Rapeseed oilICP-OES1.00 × 10−33.00 × 10−30.099–0.100Iran[53]
Corn oilICP-OES1.00 × 10−33.00 × 10−30.099–0.100Iran[53]
Olive oilICP-OES1.00 × 10−33.00 × 10−30.099–0.100Iran[53]
Sesame oilICP-OES1.00 × 10−33.00 × 10−30.092–0.099Iran[53]
Sunflower oilICP-OES1.00 × 10−33.00 × 10−30.098–0.100Iran[53]
Coconut oilICP-OES--0.158United Kingdom
(London)		[42]
Olive oilICP-OES--0.143United Kingdom
(London)		[42]
Rapeseed oilICP-OES--0.181United Kingdom
(London)		[42]
Sunflower oilICP-OES--0.274United Kingdom
(London)		[42]
Sesame oilFAAS3.04 × 10−41.02 × 10−31.370–6.641Pakistan (Mardan)[51]
Olive oilFAAS3.06 × 10−41.03 × 10−31.321–7.249Pakistan (Mardan)[51]
Rapeseed oilFAAS3.06 × 10−41.03 × 10−31.301–6.765Pakistan (Mardan)[51]
1 The values present the decimal places provided by the authors or, failing that, approximated to three decimal places. 2 Different seasonings added to flavour the oil, such as pepper, truffle, and vegetable.
Table
Table 5. Copper (Cu) content in edible vegetable oils, expressed in mg/kg.
Table 5. Copper (Cu) content in edible vegetable oils, expressed in mg/kg.
Types
of Oils		Analytical
Techniques		Limit of
Detection (LD) 1		Limit of
Quantification
(LQ) 1		Results
(Range or
Average Value) 1		RegionReference
Olive oilGF-AAS5.00 × 10−42.00 × 10−30.355Ukraine[54]
Soybean oilHR-CS ET-AAS0.0410.140.83Brazil (Salvador)[88]
Sunflower oilHR-CS ET-AAS0.0410.140.81Brazil (Salvador)[88]
Rapeseed oilHR-CS ET-AAS0.0410.140.81Brazil (Salvador)[88]
Linseed oilFAAS--0.10Poland[89]
Soybean oilAAS0.11-9.75–30.50Bangladesh[85]
Mustard oilGF-AAS0.04-0.571–0.582India
(Hyderabad)		[55]
Sunflower oilGF-AAS0.04-0.436–0.455India
(Hyderabad)		[55]
Sesame oilGF-AAS0.04-0.150–0.175India
(Hyderabad)		[55]
Coconut oilGF-AAS0.04-0.300–0.363India
(Hyderabad)		[55]
OlivaICP-MS--1.02–3.81Chipre[60]
Olive oilICP-OES1.00 × 10−33.00 × 10−30.091–0.098Iran[53]
Castor oilAAS0.003-0.158-0.417Russia[39]
Sunflower oilFB-EIEBH0.00630.02090.025–0.127Greece
(Thessaloniki)		[90]
Olive oilFB-EIEBH0.00630.02110.033–0.139Greece
(Thessaloniki)		[90]
1 The values present the decimal places provided by the authors or, failing that, approximated to three decimal places.
Table
Table 6. Results obtained from the presence of iron (Fe) in edible vegetable oils, expressed in mg/kg.
Table 6. Results obtained from the presence of iron (Fe) in edible vegetable oils, expressed in mg/kg.
Types
of Oils		Analytical
Techniques		Limit of
Detection (LD)1		Limit of Quantification (LQ) 1Results (Range or Average Value) 1RegionReference
Pequi oil 2ICP-OES0.020.071.05–9.64China (Hangzhou, Zhejiang)[92]
Walnut oil 2ICP-OES0.020.07<LD–11.2China (Hangzhou, Zhejiang)[92]
Olive oilICP-OES0.002-0.025–7.861Arabia Saudi (Riad)[91]
1 The values present the decimal places provided by the authors or, failing that, approximated to three decimal places. 2 Different varieties of walnut studied individually, such as pequi and walnut.
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González-Torres, P.; Puentes, J.G.; Moya, A.J.; La Rubia, M.D. Comparative Study of the Presence of Heavy Metals in Edible Vegetable Oils. Appl. Sci. 2023, 13, 3020. https://doi.org/10.3390/app13053020

AMA Style

González-Torres P, Puentes JG, Moya AJ, La Rubia MD. Comparative Study of the Presence of Heavy Metals in Edible Vegetable Oils. Applied Sciences. 2023; 13(5):3020. https://doi.org/10.3390/app13053020

Chicago/Turabian Style

González-Torres, Pablo, Juan G. Puentes, Alberto J. Moya, and M. Dolores La Rubia. 2023. "Comparative Study of the Presence of Heavy Metals in Edible Vegetable Oils" Applied Sciences 13, no. 5: 3020. https://doi.org/10.3390/app13053020

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