Polycyclic Aromatic Hydrocarbon Occurrence and Formation in Processed Meat, Edible Oils, and Cereal-Derived Products: A Review

Abstract

The chemical group comprising polycyclic aromatic hydrocarbons (PAHs) has received prolonged evaluation and scrutiny in the past several decades. PAHs are ubiquitous carcinogenic pollutants and pose a significant threat to human health through their environmental prevalence and distribution. Regardless of their origin, natural or anthropogenic, PAHs generally stem from the incomplete combustion of organic materials. Dietary intake, one of the main routes of human exposure to PAHs, is modulated by pre-existing food contamination (air, water, soil) and their formation and accumulation during food processing. To this end, processing techniques and cooking options entailing thermal treatment carry additional weight in determining the PAH levels in the final product. With the background provided, this study aims to provide an improved understanding of PAH occurrence in meat, edible oils, and cereal products. The factors influencing PAH formation, including operational conditions and parameters, product composition, and storage settings, are described. The discussion also addresses reduction directions with respect to influencing factors informing the choice of the employed technique, fuel type, time–temperature settings, and ingredient variations. Considering the disparities caused by wide variations in PAH contamination, challenges associated with PAH control requirements are also outlined in the context of relevant preventive approaches during food processing.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are a large group of highly lipophilic organic molecules with two or more fused aromatic rings [1,2]. They contain only carbon and hydrogen atoms, unlike their parent category of polycyclic aromatic compounds (PACs). PACs include PAHs and their functional derivatives (alkyl, amino, chloro, cyano, hydroxyl, or thiol moieties) and may contain nitrogen, oxygen, or sulfur atoms in the aromatic structure (heteroatom PAHs) [3]. Given the limited state of knowledge regarding toxicological data on PACs, additional evaluation is required in order to provide an improved understanding of their toxicity [4].

PAHs are environmental and food-processing contaminants with suspected or confirmed carcinogenic properties [5,6]. Their conformational patterns, along with the respective ring number, determine, in a highly dependent manner, their physical and chemical properties [2].

Classification-wise, PAHs are generally divided based on their number of aromatic rings. As such, compounds containing two to four rings are classified as light PAHs (LPAHs) and are associated with increased (extremely high) volatility but low toxicity. Compounds with more than four rings are heavy PAHs (HPAHs), which are less volatile but more stable and exhibit higher toxicity [1,2,7].

In addition to unsubstituted PAH molecules, increasing attention has been given to PAH derivatives, which are perceived to exert greater toxicity than their precursor PAHs. These include substitutions on the aromatic ring and other groups of molecules, such as larger PAHs, alkylated PAHs, and/or compounds containing heteroatoms [8,9,10,11,12].

When considering their toxicity, a great deal of information is accessible. As a summary of PAHs’ “history” in terms of analysis, research was initiated in the context of industrial sources of PAHs through environmental studies [2,4,7,13]. Consequently, their migration to crops and food was found to be of great concern. Subsequently, regulatory bodies put in place guidelines with regard to the extent of their toxicity. In 1979, the United States Environmental Protection Agency (US-EPA) established a list of 16 PAHs that are considered priority pollutants. Later on (in 2008), given a series of analytical improvements, the 16 PAHs constituting the US-EPA list were deemed not representative enough for the entire PAH profile [2,8,13]. Accordingly, the European Food Safety Authority (EFSA) updated the list to align with the latest advances in the state of knowledge of toxicology [14]. The resulting EU 15+1 PAHs were established following a comprehensive exposure reassessment study (over 10,000 food samples from 18 European countries) and delineate the replacement of 8 LPAHs with another 8 HPAHs that manifest increased toxicity (Figure 1) [2,13,15].

PAHs are ubiquitous in the environment and are mainly generated from natural sources, including diagenetic processes (changes in sediments converting into rock) and anthropogenic sources (combustion of organic matter such as coal, wood, and vegetation) [2,3]. Consequently, PAH transport over long distances is accounted for by their airborne environmental contamination, enabled through their adsorption onto atmospheric particles, as well as their direct deposition onto soil and plants [2,16,17].

Moreover, PAHs undergo transformation processes in the environment over long periods, involving degradation reactions such as oxidation, nitration, and halogenation [10,12,17,18]. As PAHs are exposed to light in the environment, they may undergo photochemical processes. Oxidation reactions generate oxygenated PAHs and quinones [19], as well as photodegradation products upon extended photooxidation, while nitro-PAHs result from reactions with NO₂ or NO₃ radicals [17]. Additionally, combustion processes result in the tandem emission of NPAHs and OPAHs during soot formation [18,20]. Further heterogeneous oxidation and incomplete combustion reactions promote their high abundance in polluted air and particulate matter [17,18]. Additionally, the interaction between environmental PAHs and halogen-containing compounds during food processing and photochemical processes might result in the production of halogenated PAHs (XPAHs) [10,12,21].

Given their associated negative effects, the need for a systematic and comprehensive analysis of PAHs is increasing [4,22]. Accordingly, considerable efforts have been dedicated to screening the environmental transport and fate of PAHs in order to supply further insight with regard to their exposure and effects on human health [4,17]. For example, Andersson and Achten (2015) thoroughly explained how the priority PAHs (Figure 1) are standardized and widely accepted by scientists and routinely integrated into various environmental investigations. However, the team pointed out the difficulty of using a small number of representatives for a plethora of compounds (Figure 2) [2,8].

In terms of PAH compounds overseen by EU and US rules versus those not following such guidelines, future studies are encouraged to re-evaluate their genotoxicity and carcinogenicity and should include additional compounds pertaining to their occurrence in food.

Meat, edible oils, and cereal products are among the main food categories that are accompanied by relatively high daily intakes [23]. Given that they are usually consumed in large amounts, these foodstuffs represent concerning dietary exposure levels [24].

Current evidence addresses the amounts of PAHs stemming from food intake, which depends on both the initial food contamination (manufacturing/packaging) and the method of cooking [2,15,25]. However, under the combined action of mixed manufacturing and packaging processes, there are still uncertainties with regard to the source, fate, and health effects of PAHs. To this extent, a broad context is set for the continuous need for additional relevant research on the toxicity, occurrence, and analysis of PAHs and PAH derivatives (nitrated, oxygenated, halogenated, etc.) in meat, edible oils, and cereal products.

In this setting, this study aims to showcase the impact of thermal processing techniques on the formation of PAHs in meat, edible oils, and cereal-derived products. Moreover, this review attempts to bridge the characteristics and variation trends of PAH occurrence with the endeavor to minimize future exposure. Emphasis is placed on features related to PAH formation and accumulation patterns in these food groups. The overall goal is to improve the understanding of the conditions under which food contamination entails mitigation. Subsequently, focusing on the wide discrepancy in the available results in the scientific literature and the scarce screening of PAH derivatives, an overview of the associated challenges underlines the need for sustained PAH monitoring and inspection.

2. PAH Sample Pretreatment in Food and Quantitative Analysis

Sample preparation requires comprehensive extraction followed by purification before detection [26], entailing consistent improvement, as well as alternative approaches [2]. To provide repeatable data and satisfy legal criteria, proper sample preparation is necessary. Taking into account the complexity of food matrices, as well as the trace quantities of PAH molecules in comparison to other constituents, sample pretreatment translates into laborious and time-consuming tasks [15,27,28].

The most common PAH extraction methods include Soxhlet extraction, ultrasonication, and stirring/agitation [2]. The outcomes of these techniques usually involve large amounts of solvent and significant measurement errors [15,29].

Given the advancement toward more sensitive and accurate analytical techniques, the development of sample preparation processes has gained increasing attention. Automated equipment, shorter analytical times, greater quality, environmentally friendly processes, and smaller sample sizes are all benefits of technique optimization [25,26,28]. Modern extraction techniques for PAHs in food have achieved popularity through their increased efficiency and include pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), supercritical fluid extraction (SFE), high-temperature distillation (HTD), and fluidized-bed extraction (FBE) [2,13,25,30]. The purification step is commonly achieved by column chromatography, gel permeation chromatography (GPC), or solid-phase extraction (SPE), along with dispersive liquid–liquid microextraction (DLLME), solid-phase microextraction (SPME), magnetic solid-phase extraction (MSPE), and QuEChERS [2,13,25,28]. The advantages of these techniques include lower cost, less solvent, time savings, and increased yield through selective interaction with the molecules, thus ensuring great extraction performance [30,31].

The QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) method was first developed to screen pesticides in diverse food and agricultural products [32]. The original QuEChERS approach is aimed at simplifying the extraction and clean-up steps, as compared to time-consuming and laborious conventional procedures [33]. In recent years, the QuEChERS strategy has been persistently adjusted and systematically employed in routine analytical determinations [13,34]. Its foundation is based on dispersive solid-phase extraction (dSPE) to remove potentially interfering compounds (fats, pigments, sugars, etc.). Apart from multi-residue pesticide analysis, the method has been successfully implemented in food and environmental matrices to separate various target analytes, including mycotoxins, antibiotics, persistent organic pollutants (POPs), hormones, and PAHs, consistently attaining high selectivity, sensitivity, and specificity [2,13,28,34].

In addition to the original unbuffered method involving an optimal 1:4 ratio of the two salts (MgSO₄ and NaCl) for partitioning [32,35], two modified QuEChERS methods were established: (a) the EN official method—BS EN 15662:2008 standard (withdrawn) revised in 2018 [36], employing the addition of a citrate buffer (1 g of trisodium citrate dihydrate and 0.5 g of disodium hydrogen citrate sesquihydrate); (b) the AOAC official method [37], applying the addition of an acetate buffer (1.5 g of sodium acetate) and 6 g of MgSO₄ instead of 4 g.

In the purification process, MgSO₄ is used as a drying agent, whereas a primary secondary amine (PSA) is typically applied as a weak anion exchanger targeting the removal of fatty acids, sugars, organic acids, lipids, sugars, and some pigments [33,34]. Besides PSA and MgSO₄, the major sorbents employed are octadecyl silica (C18), which is able to hold vitamins and minerals and is highly effective in removing fats, and graphitized carbon black (GCB), which is effective in eliminating co-extracted pigments (e.g., carotenoids and chlorophyll) [34]. Other approaches are being steadily explored for improved clean-up efficiency, such as alumina, Florisil^®, chitosan, diatomaceous earth, zirconia-based sorbents (Z-Sep and Z-Sep⁺), Enhanced Matrix Removal-Lipid (EMR-Lipid), LipiFiltr^®, ChloroFiltr^®, CarbonX, and Cleanert^® NANO [26,34,38].

Common analytical approaches for the identification and quantification of PAHs in a wide range of food matrices make use of high-performance liquid chromatography (HPLC) with an ultraviolet (UV) or photo-diode array (PDA) detector and gas chromatography (GC) coupled with a flame ionization detector (FID) [2,15,28]. However, these techniques are exceedingly costly, time-consuming, and labor-intensive and no longer match today’s needs regarding selectivity and sensitivity requirements [15,28]. In contrast, LC and GC techniques coupled with mass spectrometry (MS) surpass the performance of DAD and FLD methods, being widely used in PAH determination in food matrices [2,13,39].

Official methods employing LC determination include ISO 15302:2007 for benzo[a]pyrene (BaP) (withdrawn), revised in 2017 [40], in crude or refined edible oils and fats using reverse-phase HPLC-FLD; ISO 15753:2006 (withdrawn), revised in 2016 [41], for 15 PAHs in animal and vegetable fats and oils, involving a clean-up on C18 and Florisil cartridges followed by HPLC-FLD; ISO 22959:2009 [42] for 17 PAHs in animal and vegetable fats and oils using LC-LC coupling and on-line donor–acceptor complex chromatography (DACC) and FLD; and CEN/TS 16621:2014 [43] for BaP, benzo(a)anthracene (BaA), chrysene (Chr), and benzo(b)fluoranthene (BbF) in foodstuffs using HPLC-FLD, based on SEC clean-up.

Methods for PAH determination employing GC techniques include CSN EN 16619:2015 [44] for BaP, BaA, Chr, and BbF in foodstuffs (extruded wheat flour, smoked fish, dry infant formula, sausage meat, freeze-dried mussels, edible oils, wheat flour) using pressurized liquid extraction (PLE); size exclusion chromatography (SEC) and SPE clean-up, followed by GC-MS; and PD ISO/TR 24054:2019 [45] for 27 PAHs (including 16 EPA) in animal and vegetable fats and oils using LL extraction and silica gel column clean-up, followed by GC-MS.

In order to address the increasing need for more precise PAH confirmation, modern LC-MS/MS and GC-MS/MS techniques are being developed in addition to conventional analytical methods [2,26,28,46].

3. PAH Formation—Mechanistic Features

Given their chemical diversity, PAH formation and development occur through a complex set of reactions, mainly through condensation and cyclization from smaller organic molecules; under an array of specific conditions (carbon source, environment), complex approaches entailing PAH structural expansion have been investigated [3,6,47]. As such, the endeavor to provide an appropriate growth model has yielded several representations. The most important types of PAH formation mechanisms identified over the last few decades are based on acetylene addition reactions, vinylacetylene addition reactions, and radical reactions (Figure 3) [11,47,48,49].

Hydrogen abstraction and acetylene or carbon addition (HACA). This mechanism was originally presented by Frenklach et al. (1984) [50] and later introduced as HACA [51] in the framework of PAH growth in ethane-, ethylene-, and acetylene-fueled flames. The mechanism is depicted as a two-step process involving hydrogen abstraction (activation of the aromatic molecule), followed by the addition of acetylene (C₂H₂) in the radical position. The advantages of HACA stem from its continuous PAH formation/growth. This is, in turn, attributed to the relatively low reversibility of the reaction and the increased affinity of hydrogen atoms due to low energy barriers throughout the process [52]. Nonetheless, given its reaction rate, the HACA growth mechanism is argued to underperform in comparison to the relatively quick PAH formation process [47,53].

Other pathways similar to HACA have been proposed in order to understand PAH chemistry. One of the alternatives is the Bittner–Howard process, involving the sequential addition of two C₂H₂ molecules to afford a C₄H₄ chain, which subsequently leads to additional ring formation [54]. However, these alternative pathways have been deemed unrealistic or limited under low-pressure or high-temperature flame conditions in terms of the residence time of unstable radicals [47,55].

The Diels–Alder mechanism, traditionally involving the reaction of acetylene with an olefin to afford a cyclic compound [56], was proposed by Siegmann and Sattler (2000) [57]. It involves the formation of a Diels–Alder adduct by the cycloaddition of acetylene, followed by the loss of hydrogen and the resulting bay region ring closure [57]. However viable, it should be considered that the DA mechanism is accompanied by high energy barriers throughout the process, along with low reaction rates [47,58].

Hydrogen abstraction and vinylacetylene addition (HAVA). Similar to acetylene, vinylacetylene is widely available in combustion flames and involved in PAH formation through the hydrogen abstraction vinyl acetylene addition mechanism, hence the HAVA terminology. It can be achieved with vinylacetylene (VA), without the need for additional carbon species, through the addition of reactive VA (double and triple bonds) to a PAH radical, followed by cyclization [59,60]. Explorations of the HAVA mechanism deemed it considerably viable under different conditions (low energy barriers of reaction, wide temperature and pressure ranges) [59].

Methyl addition cyclization (MAC). The methyl radical is the most abundant alkyl radical in combustion flames among potential radical species. Due to the prevalence of the methyl radical, MAC plays an important role in PAH formation [61,62]. It is initiated with the formation of ethyl or propyl chains on the PAH structure (addition of 2–3 methyl radicals), followed by hydrogen elimination and subsequent ring closure [63]. As observed in a study simulating the formation of pyrene from phenanthrene, MAC appears not to be competitive with HACA, given that C₂H₂ surpasses the methyl radical in occupying the armchair position radical sites [64].

Ethynyl radical addition (HAERA). Although less likely to occur, another alternative to HACA is the mechanism involving the ethynyl radical, which could represent a potential reaction route at low temperatures [65]. The formation of phenylacetylene from benzene and the ethynyl radical was predicted by considering the ethynyl addition mechanism (EAM) [66]. More recently, the formation of benzo(a)pyrene from chrysene was predicted by using the hydrogen abstraction ethynyl radical addition (HAERA) [67].

Vinyl radical addition (HAVA*). Similar to the HAVA reaction, hydrogen abstraction vinyl radical addition (HAVA*) can also lead to the formation of PAHs. It was assessed through pyrolysis studies, and its suitability for the formation of cyclopentafused PAHs was pointed out, as well as its significance in the formation of pyrogenic PAHs at moderate temperatures [68,69].

Phenyl addition cyclization (PAC). During toluene pyrolysis, the proposed route involves the addition of the phenyl radical, hydrogen abstraction, dehydrocyclization, and subsequent conversion into thermally stable condensed PAHs [70]. It is considered not competitive with HACA due to the low prevalence of benzene in flames, as opposed to acetylene [71]. Its most important features are attributed to the formation of asymmetric PAHs, with high efficiency if the phenyl radical is present, and the potential for continuous growth by enabling multiple fusion sites during each reaction step [52,72,73].

Resonantly stabilized radicals (RSR). The combustion environment is a suitable setting for recombination reactions (radical–radical, radical–neutral) to occur involving resonantly stabilized radicals (RSR), being relatively stable and in high concentrations in flames. Computational studies involving RSR pathways pointed out three important mechanisms: the propargyl radical [74,75,76], cyclopentadienyl radical [77,78,79,80], and indenyl radical [81,82,83].

4. PAH Occurrence and Formation in Processed Meat, Edible Oils, and Cereal-Derived Products

Human exposure occurs in an inconsistent manner. Due to their prevalence in the environment, the wide distribution of airborne PAHs renders inhalation one of the major routes of exposure showing increased bioavailability [17]. However, for the non-smoking and non-occupationally exposed population, food consumption accounts for the highest percentage of human exposure to these contaminants [7,84,85]. With respect to the impact of pollution sources on human exposure, food intake of PAHs and derivatives plays an important role in the context of regional/localized contamination through their introduction to the food chain [6,85]. For instance, rural areas exhibit environmental contamination that may result from long-distance atmospheric movement, as evidenced by the corresponding unprocessed food contamination. In addition, the physical and chemical characteristics of PAHs, including solubility, volatility, reactivity, and degradability, highly influence their prevalence in food [2,23]. In this context, the use of contaminated water, crops, and deposition on agricultural food items, among others, contributes greatly to food contamination throughout the processing stages [7,15,84].

In 2002, the Scientific Committee on Food (SCF) established that BaP could be considered a marker for PAH occurrence and carcinogenic effects in food [86]. Harmonized maximum levels (MLs) for BaP were established in 2006 through Commission Regulation (EC) No 1881/2006 [87]. It was later amended by Commission Regulation (EU) No 835/2011 [88] to include Chr, BaA, and BbF in addition to BaP as a better indicator of total PAHs [89]. In accordance with the EU guidelines [88], the MLs of BaP and PAH4 set for the food categories addressed herein are as follows: 5.0 µg/kg (BaP) and 30.0 µg/kg (PAH4) for smoked meat and meat-derived products; 2.0 µg/kg (BaP) and 10.0 µg/kg (PAH4) for vegetable oils and fats; and 1.0 µg/kg (BaP/PAH4) for processed cereal-based foods and baby foods.

In spite of the inconsistencies surrounding the maximum levels of PAHs in food, the risk assessment of PAH carcinogenicity has been widely evaluated on the basis of BaP levels. Risk assessment, involving exposure assessment and risk characterization, represents an important aspect in the endeavor to thoroughly monitor PAH concentrations in food. Exposure assessment is generally performed using the toxic equivalency quotient of BaP (TEQBaP) [24]. BaP is used as an indicator to estimate the concentration and toxicity equivalency factor of each congener in the PAH mixture. Subsequently, cancer risk estimates are achieved via the incremental lifetime cancer risk (ILCR) and the margin of exposure (MOE) [24,90,91]. The ILCR approach is based on the estimation of the chronic daily intake (CDI) and the cancer risk of BaP. The MOE is estimated based on the benchmark dose lower confidence limit (BMDL) and the CDI [24,90].

In spite of the modest amounts that they contain, cereals and cereal products were determined to be significant sources of PAH ingestion due to their substantial consumption. Vegetable fats and oils, which have a greater proportion of PAHs than other dietary groups, are another significant factor. Similarly, meat and meat products tend to lead to increased PAH intakes when they make up a significant portion of the diet [2,23,89].

Table 1. Outline of PAH prevalence in processed meat, edible oils, and cereal-derived products.

Product	Processing	PAHs (µg/kg)		PAH Derivatives (µg/kg)			Reference
		BaP	Total Sum	OPAHs	NPAHs	XPAHs
Meat and meat-derived products
Chicken	Grilling	1.38–3.27	10.10–12.83 (8)	-	-	-	[92]
	Grilling	<LOD-22.09	118.10–473.78 (16)	-	-	-	[93]
	Boiling; Grilling	-	-	-	<LOD-195.4 (3)	-	[94]
	Grilling	<LOD-86.4	190.1–1781.4 (16)	-	-	-	[95]
	Smoking	<LOD-2.30	0.21–8.30 (4)	-	-	-	[39]
	Frying	-	-	-	<LOD-2.13 (5)	-	[96]
	Grilling	-	-	-	<LOD-14.43 (5)	-	[96]
Meatballs	Frying	0.10–1.64	0.10–3.66 (2)				[97]
Beef	Grilling	<LOD-43.63	328.78–817.18 (16)	-	-	-	[93]
	Boiling; Grilling	-	-	-	2.1–2.6 (3)	-	[94]
	Barbequing	<LOD-0.29	<LOD-2.63 (8)	-	-	-	[98]
	Grilling	2.20–5.07	9.20–23.81 (4)	-	-	-	[99]
	Frying	-	-	-	<LOD-2.19 (5)	-	[96]
	Grilling	-	-	-	<LOD-7.92 (5)	-	[96]
Satay	Grilling	0.95–32.60	69.14–350.38 (15)	-	-	-	[100]
Meatballs	Frying	<LOD-1.96	0.08–3.87 (2)	-	-	-	[97]
Patties	Barbequing	-	12.8–33.4 (6)	26.0–62.4 (7)	-	-	[101]
Patties	Barbequing	0.20–0.34	8.79–16.63 (11)	-	-	-	[102]
Pork	Grilling	<LOD-163.31	304.42–2290.31 (16)	-	-	-	[93]
	Boiling; Grilling	-	-	-	<LOD-4.2 (3)	-	[94]
	Grilling	<LOD-8.04	-	-	-	-	[103]
	Grilling	0.68–5.99	4.38–33.17 (4)	-	-	-	[99]
	Grilling	-	-	-	5.62 (16)	-	[104]
	Frying	-	-	-	<LOD-2.15 (5)	-	[96]
	Grilling	-	-	-	<LOD-11.46 (5)	-	[96]
	Grilling	-	3.0–570 (12)	-	-	0.02–0.08 (20)	[21]
	Frying	0.38–6.83	0.38–7.90 (4)	-	-	-	[105]
Mutton	Frying	-	-	-	<LOD-2.26 (5)	-	[96]
	Grilling	-	-	-	<LOD-7.63 (5)	-	[96]
Sausages	Commercial	<LOQ-1.05	<0.40–3.31 (4)	-	-	-	[106]
	Commercial	-	-	-	46 (16)	-	[104]
	Frying	-	-	-	<LOD-<LOQ (5)	-	[96]
	Grilling	-	-	-	<LOD-6.48 (5)	-	[96]
	Smoking	1.6–32.7	10.2–271.0 (4)	-	-	-	[107]
	Smoking	<LOD-6.20	1.21–35.90 (4)	-	-	-	[39]
	Smoking	<LOD-<LOQ	114–679 (16)	-	-	-	[108]
	Smoking	0.43–0.47	24.42–34.07 (16)	-	-	-	[109]
Edible oils
Coconut	Cold-pressed	<LOQ-1.01	<LOQ-4.95 (4)	-	-	-	[110]
Safflower	Cold-pressed	<LOD-0.90	1.43–3.16 (4)	-	-	-	[110]
Linseed	Cold-pressed	<LOD-<LOQ	<LOQ-2.44 (4)	-	-	-	[110]
Canola	Commercial	0.98–4.23	39.43–47.41 (15)	-	-	-	[91]
Palm	Storage	-	0.16–8.98 (8)	0.67–18 (5)	-	-	[111]
Corn	Commercial	0.32–4.39	1.83–47.0 (16)	-	-	-	[112]
	Commercial	0.23–0.69	12.41–20.64 (15)	-	-	-	[91]
Sunflower	Commercial	<LOD-5.29	30.6–75.9 (16)	-	-	-	[112]
	Commercial	0.71–1.56	21.68–26.76 (15)	-	-	-	[91]
Soybean	Refining	0.59–1.82	16.52–58.04 (16)	2.04–11.26 (5)	-	-	[113]
	Refining	0.32–0.76	19.71–48.72 (16)	2.04–20.04 (5)	-	-	[114]
	Commercial	<LOD-3.74	1.83–51.1 (16)	-	-	-	[112]
	Cooking waste	0.17–3.33	18.34–239.01 (16)	1.34–39.60 (4)	-	-	[115]
	Cooking waste	-	-	-	-	0.27–0.49 (2)	[116]
Rapeseed	Refining	0.03–0.94	20.02–52.03 (16)	0.93–10.88 (5)	-	-	[113]
	Refining	0.03–0.31	17.25–38.02 (16)	2.03–6.56 (5)	-	-	[114]
	Cooking waste	-	-	-	-	<LOD-0.27 (2)	[116]
EVOO	Commercial	<LOD	9.9–48.3 (16)	-	-	-	[117]
	Commercial	0.08–5.79	33.4–82.4 (16)	-	-	-	[112]
	Commercial	<LOQ	11.4–45.8 (16)	-	-	-	[118]
	Commercial	0.02–0.07	0.07–4.32 (16)	-	-	-	[119]
VOO	Commercial	0.26–6.71	39.4–96.7 (16)	-	-	-	[112]
	Commercial	0.04–0.06	4.42–6.36 (16)	-	-	-	[119]
OO	Commercial	0.27–0.89	17.90–55.55 (16)	-	-	-	[120]
	Commercial	0.01–0.44	11.55–16.65 (15)	-	-	-	[91]
OPO	Commercial	0.04–0.15	1.21–2.85 (16)	-	-	-	[119]
Cereal and cereal-derived products
Wheat	Commercial	0.08–0.20	9.74–23.87 (28)	-	-	-	[121]
Rice	Commercial	<LOD-0.97	-	-	<LOD-4.19 (2)	-	[122]
	Cooked	<LOD	248.3 (16)	-	-	-	[123]
Corn	Frying	-	-	-	<LOD-<LOQ (5)	-	[96]
	Grilling	-	-	-	<LOD-3.26 (5)	-	[96]
Bread	Commercial	<LOQ–0.20	0.11–0.22 (4)	-	-	-	[124]
	Commercial	≤LOD	1.29–4.80 (16)	-	-	-	[125]
	Baking	-	9.46–228.98 (13)	-	-	-	[126]
	Commercial	0.19–17.41	59.64–211.19 (16)	-	-	-	[127]
	Commercial	<LOD-0.95	1.60–16.91 (16)	-	-	-	[128]
	Commercial	0.11–0.25	0.16–0.46 (4)	-	-	-	[129]
	Frying	-	-	-	<LOD-<LOQ (5)	-	[96]
	Grilling	-	-	-	<LOD-1.96 (5)	-	[96]
	Commercial	<LOD	98.2–176.3 (16)	-	-	-	[123]
Youtiao	Frying	0.40–1.38	13.81–18.00 (16)	0.54–9.42 (5)	-	-	[5]
Youtiao	Frying	-	9.67–12.48 (16)	3.46–6.10 (5)	-	-	[130]
Youtiao	Frying	<LOQ-1.18	<LOQ-195 (15)	-	-	-	[131]
Breakfast cereals	Commercial	<LOQ–0.30	0.23–0.87 (4)	-	-	-	[124]
	Commercial	0.09–0.30	0.07–0.87 (4)	-	-	-	[129]
Cookies	Frying	<LOD-1.33	<LOD-26.92 (18)	-	-	-	[132]
Pasta	Commercial	≤LOD	0.16–1.98 (16)	-	-	-	[125]
Spaghetti	Commercial	0.4–2.0	9.0–200 (16)	-	-	-	[133]
Macaroni	Commercial	0.2–0.7	30–60 (16)	-	-	-	[133]
	Commercial	<LOD	176.2 (16)	-	-	-	[123]
Noodles	Frying	<LOD-11.9	<LOD-182.8 (18)	-	-	-	[132]
	Commercial	350–830	560–7889 (16)	-	-	-	[134]
	Commercial	0.3–3.0	300–800 (16)	-	-	-	[133]

Values are presented as a mean or range, as appropriate. The number in parentheses indicates the number of compounds included in the reported sum. LOD: limit of detection; LOQ: limit of quantification; EVOO: extra virgin olive oil; VOO: virgin olive oil; OO: olive oil; OPO: olive pomace oil.

provides an outline of the representative findings informing on PAH prevalence in meat, vegetable oils, and cereal products.

Food processing entails internal physical and chemical modifications of the particular raw material, the extent of which (size, nature) dictates the selection of the technique [135,136]. With no precise formation mechanism established, three potential routes have been pinpointed in connection to PAH formation in thermally treated food products: the pyrolysis of organic materials (protein, fat); fat dripping onto the heat source or cellular sap leaking; and incomplete organic matter combustion, which, in this case, refers to the fuel type [15,85,137]. In addition, during pyrolysis, high-molecular-weight (HMW) compounds may be formed through pyrosynthesis from low-molecular-weight (LMW) precursors and PAH-forming radicals [85,138].

The subsequent sections summarize the features falling under each category of influencing factors, either physical or chemical, within the context of PAH contamination and occurrence in processed food.

4.1. Features Underlying PAH Formation in Meat and Meat-Derived Products

In the case of meat and meat-derived food products, domestic preparation options involve thermal processing, such as smoking, drying, barbecuing, roasting, baking, frying, or grilling [1,23,27,107,139]. Figure 4 provides an overview of the elements modulating PAH levels in meat and meat-derived products undergoing thermal processing. Further, a detailed interpretation is supplied for the corresponding factors involved in PAH formation and occurrence.

Smoke curing, comprising direct and indirect smoking, produces the highest PAH content compared with other processing techniques [27,140]. Direct smoking consists of the thermal degradation of wood and is further divided into cold smoking (15–30 °C) and hot smoking (80 °C) [27,141]. By contrast, indirect smoking is based on smoke generation (friction, touch, steam) techniques [27,142] and aims to reduce PAH contamination in food [143]. For instance, industrial smoking resulted in lower BaP, BaA, BaF, and Chr levels compared to traditional smoking in meat sausages and pork ham [39]. A similar outcome was reported for “Petrovská klobása” dry fermented sausages, attaining considerably lower PAH contents by industrial smoking compared to traditional smoking [144]. The positive effect of the indirect process was also revealed in Frankfurter sausages cured by friction smoke and steam smoke, showing diminished contamination in comparison to traditional smoking [142].

In addition to smoke curing, techniques entailing thermal processing include direct and indirect (electric grilling, pan grilling) contact between the product and the heating source [15,103]. A charcoal grill yields a greater BaP content in pork bacon, as opposed to a modified charcoal grill, which enables direct contact with the product but avoids fat dripping onto the heating source [103]. On the other hand, infrared and electric grills render BaP undetectable in grilled bacon [103]. Similarly, PAH formation was higher in beef grilled on a stone barbeque than on a wire barbeque [98]. In the same vein, the classic barbeque produced higher PAH content than the electrical oven in grilled duck meat and skin [145].

Generally referred to as “doneness”, cooking intensity translates into the extent of exposure of food materials to the heating source. Hence, time–temperature settings synergistically modulate PAH formation [7]. The progressive rise in PAH levels over the course of a prolonged processing time and an increased temperature has been ascribed to the formation and accumulation of additional PAHs in meat products [15,27,142,146,147,148,149]. The cooking degree of beef steak, including rare, medium, well done, and very well done, generates a directly proportional level of PAH accumulation [98]. Correspondingly, doneness had a significant effect on promoting PAH content in charcoal-grilled meat samples, with chicken displaying higher contamination than beef for both medium and well-done cooking levels [150]. In meat model systems (lyophilized ground beef), PAH levels increased gradually as a function of both temperature and time. However, temperature had a greater effect than time [7,148]. In addition, grilled meat subjected to a temperature gradient (heating to 320 °C followed by a steady decrease to 200 °C) resulted in elevated PAH accumulation during the initial heating stage [99]. In this regard, the preboiling or partial cooking of meat products might minimize the time of the subsequent cooking stage (involving high temperatures) [7]. Rose et al. (2015) evaluated how cooking time affected contamination with PAHs in a variety of fried, grilled, barbecued, toasted, and roasted animal and plant food products. They noted overall increased PAH levels, attributed to a 50% increase in cooking time [151]. The impact of time is also evident during smoke curing. Pork sausages show an increasing trend in PAH accumulation with the extension of the smoking time [107]. Similarly, prolonged smoking from 0 to 5 days revealed a considerable increase in BaP levels in smoked meat samples, followed by a decreasing trend up to day 7 [139].

Another important factor to consider is the type of fuel. Unlike spontaneous combustion, the pyrolytic process may involve secondary condensation and cyclodehydrogenation reactions to afford more condensed and relatively stable HMW PAHs [1,138]. In a typical combustion process for open wood burning, hemicellulose degrades first, then cellulose, and finally, lignins [138,152,153]. Prior to the generation of the final PAHs, precursor and intermediate compounds go through a number of primary and secondary processes that have an impact on pyrolysis-derived chemicals [138]. In the case of smoking, irrespective of moisture and chip size [153,154], the effect of temperature is translated into the choice of wood type. In turn, this may lead to significant variations in PAHs, depending on the associated smoke generation temperature [109,154]. For instance, the use of wood containing low cellulose and hemicellulose contents determines the formation of lower PAH levels than other wood types [109,154]. In the same vein, three charcoal classes promoted PAH accumulation during grilling, irrespective of the meat product (pork, beef, or chicken), depending on the charcoal type in the order white > black > extruded [93]. Under such conditions, however, the type of fuel should be assessed in conjunction with additional parameters, such as the distance of the product from the heat source, the fat content of the sample, the batch cooking order, and the presence of skin or casing, among others [108,136,147,151,155].

Moreover, foods prepared using open-flame sources are linked to elevated PAH levels as a result of smoke production from fuel combustion and fat droplets [7,156]. Smoke and PAHs migrate together and settle on the food surface before potentially being retained in the hydrophobic (fat-rich) areas [15,85,137,155]. Alternatively, gas used as an open-flame heating source could minimize the production of PAHs compared to charcoal and wood [7,157]. Unlike a charcoal- or wood-generated open flame, by using gas nozzles to create and spread the flame, flame-free areas are created between each nozzle (small gas flame). This may decrease the contact between the flame and fat droplets [157]. Limiting the direct contact between meat and smoke could aid in reducing the PAH content in grilled meat products [99]. Consequently, cooking techniques not involving smoke generation, such as infrared and electric heating, are advised to inhibit PAH formation [7].

The contents of PAHs produced during processing also vary depending on the nature of the food material. In fat-rich food products, the lipidic constituents act as PAH carriers, resulting in their accumulation in both raw and processed products. Moreover, the amounts of PAHs are influenced by the fatty acid profile of the fat [7,158]. Alternatively, the use of low-fat approaches can lead to reduced PAH levels. In addition, some practices involve replacing animal fat with vegetable oils in certain meat-derived products, such as sausages, patties, etc. In turn, the stability of the final product to high-temperature cooking is enhanced, whereas the sensory characteristics are fairly well maintained [7,159].

With a less significant impact compared with fat content, protein types and levels also influence PAH formation in food [158,160]. Although less studied, the protein content has been associated with increased protein carbonyl levels in deep-fried meat products, along with elevated lipid peroxidation values [97], suggesting its contribution. Moreover, additional investigation of amino acid profiles during thermal processing might impart further insight on the mechanistic implications of the resulting PAH accumulation [161].

Antioxidants and antioxidant-rich ingredients have been shown to reduce the generation of PAHs in processed food [7,162]. For example, the addition of either garlic (15 g/100 g of meat) or onion (30 g/100 g of meat) was shown to reduce PAH levels in pan-fried pork meat by up to 54% and 60% on average, respectively [163]. Similarly, Lu et al. (2018) detected the PAH inhibitory effects of onion, garlic, red chili, paprika, black pepper, and ginger addition (5 g/kg) to deep-fried beef meatballs (65%, 57%, 65%, 87%, 47%, and 98%) and chicken meatballs (86%, 86%, 79%, 74%, 97%, and 97%), respectively [97]. The reduction in PAH levels under the influence of antioxidant molecules has been attributed to the free radical species scavenging capacity, which may interfere with the fragmentation and cyclization reactions involved in PAH formation [97,163]. However, different scavenging potencies may be observed depending on the chemical structure of the antioxidant [97]. Correspondingly, antioxidant compounds may exert a scavenging effect on the free radicals formed during thermal decomposition and result in decreased lipid peroxidation (TBARS), accompanied by decreased PAH levels in the food product [164].

A further influence is exerted by water content, with its rise leading to reduced PAH contents [7,15]. Variations in the water retention rate have a crucial role in minimizing the incomplete combustion of organic compounds during meat cooking [148,165]. In this regard, chicken thawed in the fridge prior to air frying or deep frying resulted in lower PAH levels compared to microwave or water immersion defrosting [165]. In addition, Min et al. (2018) described a decreasing rate of PAH levels when water was added in an equal proportion (50%) to minced meat, depending on the cooking temperature, which ranged from 80 °C to 200 °C [148]. Aside from water content, pH has been documented to affect PAH formation [7]. The Maillard reaction promotes aromatization and dehydrocyclization, caramelization, and the breakdown of fats and sugars [100]. With higher pH values promoting the Maillard reaction, a subsequent rise in PAH levels is expected [6,95,161]. The use of acid marinade on raw food material seems to be a potential way to prevent PAH development, despite the limited research investigating the impact of pH on the PAH formation rate in processed food [7,166].

Casings act as an effective barrier to mitigate PAH contamination. To this end, the characteristics of the casing surface, such as smoothness and material porosity, can contribute to inhibiting PAH penetration, implying reduced adsorption by the product [7,143,167]. For example, Youssef et al. (2016) noted a 78% reduction in PAH content when replacing the original sheep intestine with a cellulose casing for smoked beef sausages [168]. Similarly, collagen is also a suitable alternative to natural animal intestinal casings in suppressing PAH contamination in sausages [108,143,169]. The effect of PAH penetration inhibition was also evidenced for banana leaves (40%) and aluminum wrapping (46%) in grilled beef meat. On the other hand, grilled chicken meat showed a reduction of 80% irrespective of the wrapping [170], emphasizing the importance of casing removal before consumption [143].

Other features bearing additional weight on PAH content cover the storage settings and operational circumstances [7,15]. Depending on the type of food, the storage period has varying effects on the PAH concentration. For example, the maximum PAH concentration in smoked sausages is detected just after smoking [168,171]. Nevertheless, during storage, PAHs may migrate inside the product and become less readily decomposable, or may interact with other substances, such as antioxidants, and undergo photochemical oxidation [171]. Nutmeg oil and ginger oil nanoemulsions were investigated for their protective effects in grilled beef patties after a 90-day storage period (after cooking), resulting in a significant improvement in antioxidant activity, a decrease in the lipid peroxidation rate, and a corresponding PAH reduction [172]. By contrast, should the meat products be subjected to storage prior to cooking, the present antioxidants might undergo autooxidation and shift to pro-oxidant action. In turn, undesirable changes in the product (e.g., lipid peroxidation) may be promoted. In this setting, pork sausages grilled after 8 days of storage showed elevated PAH levels, increased lipid peroxidation (TBARS), and decreased antioxidant levels (DPPH, ABTS) when compared to sausages grilled after 4 days of storage [164]. Notably, the effect of storage may be regulated by the casing type in conjunction with the surface area. This is attributed to the smooth morphology and low porosity of edible coatings/synthetic casings, limiting PAH permeability into the product [15,136,139,143,169,173].

4.2. Features Underlying PAH Formation in Edible Oils

The relatively high PAH concentrations (μg/kg) detected in edible oils account for approximately 50% of dietary exposure. It is attributed to their extensive use in cooking, seasoning, and margarines, as well as their incorporation into cereal-based products (e.g., cakes and biscuits) [2,174,175]. In the food chain, the main causes of PAH occurrence in vegetable oils include cultivation with contaminated soil and water, the presence of mineral oil residues, and soil burning [2]. In addition, contamination during solvent extraction and the contamination of packaging materials also foster the formation of PAHs [175]. Moreover, the steps involved during processing, such as artificial drying and roasting of seeds and raw materials, seed dewaxing or grinding, and pomace compression, are factors contributing to PAH contamination in crude oil [2,23,176,177]. Figure 5 provides an overview of the important properties modulating PAH levels in edible oils undergoing thermal processing. Further, corresponding factors involved in PAH formation and occurrence are presented and discussed.

The analysis of raw peanuts and the corresponding crude oil revealed higher PAH content in pressed oil than in solvent-extracted oil (leached). Moreover, due to the temperature applied during roasting, hot-pressed oil results in more abundant PAHs than cold-pressed oil [178]. Similarly, roasting olive fruits results in a significantly higher PAH content in olive oil [13,179]. The occurrence of PAHs in extra virgin olive oil (EVOO) is generally fairly low and mainly accounts for the environmental load. With lower sensory quality than EVOO and no additional treatment besides mechanical processing, virgin olive oil (VOO) contains comparable PAH levels to EVOO. However, despite undergoing refining steps, olive oil (OO) (consisting of various blends of refined OO and VOO) and olive pomace oil (OPO) contain higher PAH levels than EVOO [180].

Among vegetable oils collected from Egypt, canola oil showed a higher residual PAH concentration than corn, olive, and sunflower oils extracted by cold-pressing. Moreover, heat treatment of canola oil (pan-fried at 180 °C for 15 min) resulted in a significant increase in PAH levels [91].

In this context, in contrast to crude oils, refining and purification result in considerably lower to non-detectable levels of PAHs in refined oils [114,120,175,181,182]. For example, PAH levels in sunflower oil throughout processing resulted in decreased contamination upon neutralization (32.02%), bleaching (54.14%), or deodorization (71.77%) compared to the crude oil. Moreover, soybean oil showed comparable refining yields for both bleaching and neutralization (32%), whereas deodorization led to more than an 80% reduction in both soybean and olive oils, respectively [182]. The differences in PAH removal capacity might be attributed to the efficiency variations among activated charcoal and activated earth [182]. Similarly, Brazilian soybean oil registered up to 88% reduction in PAH levels during the refining steps (neutralized, bleached, and deodorized oils) [181]. To this end, notable importance is given to reducing the contamination of oils with OPAHs [114]. Correspondingly, Hua et al. (2016) revealed a significant reduction in total PAH levels in refined soybean oil (59.54%) and rapeseed oil (54.63%). Concomitantly, declines in OPAHs obtained for soybean and rapeseed oils through refining reached 89.82% and 69.05%, respectively [114]. Similarly, olive oil investigated throughout the refining steps rendered neutralization 90% effective in decreasing the total PAH concentration. Moreover, altering the proportion of activated carbon during bleaching from 0.3% to 0.9% resulted in a reduction in PAH content from 86% to 91% [183].

Nonetheless, oil refining has the drawback of removing the antioxidant molecules in oil, which might lead to a rise in PAHs during storage [113]. In this regard, temperature settings are to be considered jointly with the storage period. A lower storage temperature is often linked to a decreased risk of PAH formation [7,113]. For example, soybean and rapeseed oils were investigated over a 270-day storage period at 4 °C and 25 °C. Increases of 80.88% and 117.5% at 25 °C were registered, whereas only 14.77% and 27.07% increases were observed at 4 °C for crude and refined soybean oil, respectively. Similarly, the OPAH content increased by 72.69% and 82.35% at 25 °C and by 22.69% and 43.13% at 4 °C in crude and refined soybean oils, respectively [113]. Additionally, PAH levels in crude and refined rapeseed oils showed similar trends (89.51% and 98.14% increase at 25 °C; 61.13% and 55.09% increase at 4 °C), followed by OPAHs (65.85% and 429% at 25 °C; 19.66% and 212.9% at 4 °C) [113]. Guillén et al. (2008) also revealed a great increase of 14–23-fold in the PAH concentration of sunflower oil subjected to 112 months of storage at room temperature [184]. It is suggested that a number of processes, including polymerization, oxidation, degradation, and volatilization of unsaturated fatty acids, as well as the production of radicals throughout the storage period, might account for the elevated PAH concentrations [7].

Moreover, the fatty acid composition of the oil also affects PAH content [185], similar to the case of fatty acids from animal sources, where the number of carbon atoms and the respective degree of unsaturation influence PAH formation during processing [158]. The findings of Liu et al. (2019) indicated that the increase in the fatty acid carbon number and unsaturation leads to a proportional rise in PAH levels [158]. In this respect, Chiang et al. (2022) reported that palm oil generated more particle-bound PAHs in comparison with soybean and olive oils. In addition, saturated fatty acids (SFAs) in general, and palmitic acid in particular, were found to contribute to PAH formation as a result of significant positive correlations between palmitic acid and acenaphthene (r = 0.74, p < 0.05) and benzo(e)pyrene (r = 0.79, p < 0.05), as well as between SFAs and chrysene (r = 0.86, p < 0.01) [185].

The replacement of the fat source in the final product may, for example, enhance monounsaturated fatty acids (MUFAs) when using olive oil or promote polyunsaturated fatty acids (PUFAs) when using sunflower oil and grape seed oil [159]. By substituting pork back fat with different vegetable oils, under the effect of temperature, sunflower and grape seed oils had a significant influence on PAH concentrations in pork patties owing to their lower smoking points (227 °C and 216 °C) compared to that of olive oil (242 °C), mainly due to the presence of PUFAs [159].

Model lipids or food lipids subjected to thermal treatment foster the production of cyclohexane or hydroperoxide through lipid oxidation and breakdown, which, upon further oxidation or cyclization, lead to the formation of naphthalene or naphthalene-like compounds [186,187]. Specifically, the interaction of oleic and linoleic acids during heating might result in cyclic molecules, subsequently leading to the synthesis of PAHs or PAH derivatives through further polymerization [186].

The development of PAHs is also affected by the choice (type) of cooking oils employed when deep frying, pan frying, and pan grilling [7,188]. At this stage, the use of air frying as an alternative might reduce PAH formation in French fries by up to 90% [189] and chicken meat by 20% [165]. Aside from adopting a cooking technique lacking oil, refined oil represents a suitable option for reducing oil-derived primary contamination [114]. Oils with higher smoke points are more suited for processing at high temperatures [7]. For instance, the smoke points of brown rice oil, sesame oil, and perilla oil are 257 °C, 165 °C, and 161 °C, respectively. By altering the proportions of the oil mixtures to include more brown rice oil, a reduction in PAH levels was observed in seasoned-roasted laver (snack food) by up to 32% at 380 °C [190]. The deep frying of dough sticks in peanut, sunflower, rapeseed, rice bran, soybean, and palm oils revealed a considerable rise in polar compounds and PAH levels with the increase in frying time, further informing on the choice of oil type used for frying [191]. Olive oil was attributed to higher toxic emissions during deep frying compared to soybean and palm oils, which is in line with the recommendations indicated by other reports [2,6,7,192].

To this end, the generation of PAHs in oil during high-temperature cooking may be favored by the aromatization and dehydrocyclization of monounsaturated fatty acids [140]. Moreover, extending the practice of reusing the frying oil leads to the progressive accumulation of PAHs in oil [175,192,193]. Consequently, it is crucial to discard spent oil, replace it often, and utilize the right frying conditions (i.e., temperature and duration) and oil type to lower the PAH levels in fried food products [7,175].

Other physical factors, such as exposure to heat or UV rays, have also been investigated for their exerted effects on PAH occurrence in vegetable oils. Mocek and Ciemniak et al. (2016) explored the change in PAH content in rapeseed and sunflower oils under various UV radiation and temperature settings, resulting in a significant PAH decline in both cases that was strongly correlated with the duration of exposure [194]. However, new PAHs (photoinduced) with higher toxicities than the parent compounds may be produced [2].

4.3. Features Underlying PAH Formation in Cereal and Cereal-Derived Products

Existing research examining PAH occurrence in cereal products attaches high importance to PAH formation throughout the food chain. Similar to meat products and edible oils, PAH contamination in cereal and cereal-derived products mainly differs depending on the processing technique [175]. At this stage, the quality of cereal-derived products is greatly influenced by their pre-processing method [195]. Aside from the temperature reached during drying, processing, or cooking, PAH contamination of cereal products is also affected by the type of fuel and the method of heat delivery [125,156,196]. Figure 6 provides an overview of the representative features modulating PAH levels in cereal and cereal-derived products undergoing thermal processing. Further, additional factors pertaining to PAH formation and occurrence are described.

In corn, De Lima et al. (2017) reported PAH levels within the range of 5.9–127 μg/kg as a result of the drying process [197]. Other raw materials, including wheat and rye grains, bran, and flour, ranged from 1.07 to 3.65 μg/kg [198]. In addition, wheat, rye, and the resulting bread products were found to contain PAH levels between 0.22 and 1.62 μg/kg [199]. Similarly, Lee et al. (2018) analyzed the PAH content in cereal products as a sum of eight PAHs, including barley (0.11 μg/kg), brown rice (1.26 μg/kg), and instant noodles (2.38 μg/kg) collected from the Korean market [24]. Notably, in cereal samples collected from Pakistani households, the highest total PAH levels were detected in wheat (169 μg/kg), followed by maize (159 μg/kg) and rice (53 μg/kg) [200]. The levels of PAHs in wheat grain samples collected in Poland showed similar profiles for 2017 and 2018 harvests, ranging between 9 μg/kg and 11 μg/kg EU PAH (15+1) [121]. In addition, toasted guarana seeds subjected to husk removal resulted in 0.78 μg/kg PAHs, as opposed to unhusked seeds, where no PAHs were detected [201]. Other studies place PAH contamination in cereal-based products within different ranges based on the employed processing technique and type of fuel [175,176,202]. In order to maintain a certain moisture degree for preservation, grains are subjected to drying for moisture reduction involving high temperatures, which in turn promote PAH formation [175]. The accumulation of PAHs in rice under various drying conditions showed increasing PAH levels depending on the heating source in the order electric heating (7.7 μg/kg), liquefied petroleum gas (15.9 μg/kg), rice husk (45.7 μg/kg), and wood (131.6 μg/kg) [203]. Furthermore, the drying temperature showed no significant differences in PAH accumulation, whereas polishing resulted in reduced PAH content in rice when compared to parboiling [203].

In bread, PAH levels may range from 1.59 to 365 μg/kg under the effect of raw materials, the bread type, thermal treatment, and microbial culture during fermentation, as well as the heating source during thermal processing [156,198]. Rascón et al. (2018) found that home toasting elevated the content of PAHs in bread depending on the raw material, with the highest levels in white bread (17.8 μg/kg), followed closely by wholegrain bread (16.9 μg/kg), multiseed bread (16.5 μg/kg), black bread (13.0 μg/kg), and sliced bread (10.3 μg/kg) [125]. The effect of raw materials on PAH levels in bread revealed lower contamination in Baltonowski bread and wholemeal rye bread compared with plain rye bread. Moreover, irrespective of the baking temperature, the crumb had lower PAH levels than the loaf and crust [198]. In addition, increasing the baking temperature resulted in proportionally elevated PAH content irrespective of the bread type [198]. Similarly, the PAH content was lower in traditional Iranian Lavash (9.46–152.07 μg/kg) and Taftoon (18.19–169.26 μg/kg) bread baked for 2.5 min at 315 °C in comparison with industrial baguette bread baked for 14 min at 245 °C (20.78–228.98 μg/kg), which may be attributed to the time–temperature settings of the thermal process [126]. The effect of the baking method on the PAH content in bread displayed reduced contamination in tandoori bread baked on a plate (tawa plate) by up to 71.7% when compared to those baked in the oven [127]. In addition, toasting entails variations in PAH accumulation depending on the heating source. No PAHs were detected in electric-oven-baked bread compared to 350 μg/kg when using a toaster. Additionally, switching from coal to oak could favor a reduction in PAH levels in toasted bread [156].

In youtiao (wheat-based fried stick), PAH levels between 9.9 and 90 μg/kg were found [204]. Moreover, the temperature effect emphasized lower PAH levels in youtiao fried at 160 °C, as opposed to 170–200 °C, which generated an increase of up to 30% [5]. In addition, the effect of time was assessed through continuous frying. PAH concentrations in youtiao fried in soybean oil were measured to be 22.25 μg/kg PAHs at 2 h, followed by a subsequent decrease in the 4 h–8 h interval (2.01–3.63 μg/kg), which was attributed to degradation processes. However, replacing soybean oil with shortening oil resulted in significantly lower PAH levels, irrespective of time [5]. Although addressed in the previous sections covering PAH formation in meat products and vegetable oils, the effect of cooking oil on PAH formation also extends to cereal products. In conjunction with antioxidant extracts, marked changes were observed in youtiao fried in soybean and palm oil [130]. The presence of rosemary extract, tea polyphenol, and bamboo antioxidants in soybean oil showed 23.47%, 11.38%, and 28.85% inhibition of total PAHs in comparison to the 30.30% induced by tert-butyl hydroquinone (TBHQ) in fried youtiao. The same antioxidant compounds included in palm oil also exerted changes in PAH reduction (27.56%, 9.45%, and 39.26%) in fried youtiao, distinct from those enabled by TBHQ (38.94%) [130].

Other forms of cereal-based products, such as biscuits, including shortcake, digestives, cookies, shortbread, wafers, and crackers, showed variable PAH concentrations (35.7–645.3 μg/kg, 75.9–490.7 μg/kg, 91.5–537 μg/kg, 18.4–522.2 μg/kg, 123.5–393.8 μg/kg, and 167.2–880 μg/kg), stemming from differences in the employed baking methods, along with the choice of fuel type, oven conditions, and raw materials [205]. With fairly high popularity, pasta, including noodles, spaghetti, and macaroni, among others, contains variable PAH concentrations, ranging from 9 to 800 μg/kg in Nigerian brands and 2 to 7 μg/kg in imported brands [133]. Similarly, Charles et al. (2018) found total PAHs ranging from 0.564 to 7.889 mg/kg in noodles from a Nigerian market [134]. Moreover, fried noodles displayed approximately 3-fold higher PAHs than unfried noodles in a Vietnam market [132].

Aside from the operational conditions employed during processing, cereal and cereal-derived products (e.g., flour and bread) undergo PAH contamination through the deposition of atmospheric particles, as well as through polluted water and soil [125]. Cereal products generally carry fairly low amounts of PAHs and have not yet been thoroughly regulated in terms of establishing the maximum limits [125,206]. Given the available data, the EU regulation established the maximum levels for processed cereal-based foods and baby foods at 1.0 μg/kg for both BaP and the sum of four PAHs [88]. However, through dietary intake, they contribute greatly to human exposure [7,206].

4.4. Outlook on Associated Knowledge Gaps and Challenges

Within the legislative framework, certain challenges arise with respect to future directions toward assessing PAH transformation and monitoring within the food chain.

The wide discrepancy among PAH results found in products subjected to similar processing may be attributed to differences in the composition and the conditions under which the products are processed [207,208]. In the context of processing parameters affecting PAH formation and accumulation, prioritizing the awareness of reduction strategies is crucial. Various approaches have been identified for mitigating PAH contamination in processed meat, edible oils, and cereal products. In conjunction with the previously presented features underlying PAH formation, an overview of the process-based means to control their accumulation is given in Figure 7.

However, these approaches targeting specific food items do not account for the diversity of cooking practices, cultural features, and food types. Given the different distribution patterns in the environment and in food, limited data on dietary exposure in different demographic regions may provide subjective solutions and oversight.

Moreover, additional information from combustion/flame experiments is required to provide better insight on the particularities under which PAH structural expansion or degradation relates to specific formation mechanisms in food. This highlights the importance of broadening the range of regulations covering PAH derivatives. To this end, improvements in instrumental analysis capabilities during the past decade have generated the need for updates in PAH screening. Compounds containing heteroatoms, such as OPAHs, NPAHs, and XPAHs, have drawn attention owing to their considerable impact on the environment and were assessed to impose significant risks to human health [10,12]. In addition to their presence in atmospheric particulate matter, their occurrence was also revealed in meat products [21,94,96,101,209], vegetable oils [111,113,114,115,116,210], and cereal products [5,122,130].

Determination challenges involve the lack of relevant reference materials, product instability, reagent selection, by-products, and adverse reactions that can reduce analytical sensitivity. Increased interest is currently being given to tailoring the use of QuEChERS for clean-up efficiency [15,28,211]. The outcome is expected to avoid overlaps between food matrix composition and target PAHs. In addition, both chromatography and mass spectrometry techniques are subjected to continuous improvement. However, considering the complexity of PAH investigations, further tools are deemed necessary to provide precise identification and quantification.

Having examined the prospective effect of industrial and household cooking options on PAH formation and accumulation in food, improved monitoring is needed in order to provide further systematic regulations for additional classes of PAHs/PACs, including technical detection capabilities. These requirements relate to the endeavor to formulate relevant prevention recommendations during food processing for consumers and producers alike.

5. Conclusions

In this study, an overview has been presented on PAH occurrence and formation patterns in processed food that undergoes thermal treatment. Given the broad consumption of processed meat, edible oils, and cereal products, our study aimed to provide further insight into features related to PAH formation and accumulation in these food categories. The presentation covered the most relevant reports with respect to the conditions and factors affecting PAH contamination of the final product, such as processing methods and parameters, exposure and the type of heating source, the nature and composition of the raw food materials, pretreatment techniques, the type of oil used for cooking, and storage conditions. Along with these features, process-driven PAH reduction considerations were addressed. Nonetheless, the concentrations of PAHs in meat, edible oils, and cereal products show wide variations, rendering the maximum levels set for these food groups exceeded in certain instances. Moreover, the discrepancy in the outlined results is accompanied by the limited evaluation of PAH derivatives, or the lack thereof. In this context, proper PAH analytical methods are crucial for assessing their prevalence in food, ultimately leading to improved monitoring of these contaminants. In conjunction with the identified challenges, the current trends reflecting PAH occurrence in processed food emphasize the developing need to establish relevant preventive approaches and control during food processing.