What are Polynuclear Aromatic Hydrocarbons?
Polynuclear aromatic hydrocarbon or polycyclic aromatic hydrocarbons (PAHs) are a group of toxic organic compounds. These organic contaminants are often byproducts of the processing of crude oil or incomplete combustion consisting of many aromatic rings joined together. (1)
Occurrence of Polynuclear Aromatic Hydrocarbons in Foods
PAHs tend to be Lipophilic in nature and there occurrence in foods can be linked to contamination by soil, water and/or air or from cooking practices at home such as roasting, grilling smoking etc.(2) High temperatures and exposure to sunlight decreases the concentration in food substances. There are over 500 PAHs but only 15 have been identified that affect foods. Figure 1 shows a list of the fifteen PAHs along with their structures. (see appendix)

Sources of airborne PAHs are forest fires and volcanoes. The most important stationary anthropogenic sources include residential burning of wood, oil, gas and charcoal as well as industrial power generation, incineration, production of aluminium, iron and steel, petroleum catalytic cracking and production of asphalt, coal tar and coke.
The most important mobile sources are vehicular exhausts from gasoline and diesel-powered engines. In combustion processes the formation of PAHs is reduced when combustion is more thoroughly performed but this will increase the formation of nitrogen oxides.
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Formation of PAHs
The formation of PAHs in foods is varied based on the processing method chosen. The Formation of PAHs compound occur at temperatures of 100-150˚C. It has been postulated that other than PAHs forming as a result of byproduct of the processing of petroleum that they may be form from through radical mechanism.
A traditional method of dealing with these polynuclear aromatics is to hydrogenate them, cleave the rings and convert the aromatics into generally more valuable molecules for fuels. However, those processes which use molecular dihydrogen as a process reactant, such as hydrocracking, hydrogenation, and hydrogen addition processes, incur the well-known problems associated with the use of molecular hydrogen, namely cost and materials handling difficulties associated with compressing and containing large quantities of gaseous hydrogen. When hydroprocessing polycyclic aromatic components to lower ring number or lower formula weight products, hydrogenation usually begins with a terminal ring and degrades rings successively inward, followed in each case by cracking of the then-saturated rings. If a center ring becomes hydrogenated, it usually undergoes dehydrogenation rather than cracking under mild hydroprocessing conditions. Thus, low yields of single ring (gasoline boiling range) aromatics are realized, high hydrogen consumption is required, and high yields of gas are produced
Oxidation of PAH and soot particles is a process competing with the formation of these species. It decreases the mass of PAH and soot material through the formation of CO and CO2. Oxidation may occur simultaneously with formation as in premixed aromatics flames and well-mixed combustors. Figure 2 shows an example of an oxidation reaction to form PAHs. The main oxidation reactants are OH, O and O2, the largest contributor in general being OH under fuel-rich conditions and O2 under fuel-lean conditions.(4)
Analysis and Implications
The method that is highly utilized to isolate PAHs is saponification of lipids using methanolic KOH followed by two chromatography technique; liquid-solid and liquid- liquid.(3)
Both hydrogen addition and carbon rejection processes, which often require the use of large quantities of solid catalysts, are also susceptible to reduced throughput and high catalyst replacement costs resulting from catalyst poisoning when processing heavy oils. This poisoning usually results from the deposition of contaminant metals, high molecular weight refractory compounds (or coke derived there from), or sulfur or nitrogen containing heterocyclic compounds onto the catalyst surface. means that no prescribed fixed official methods have to be followed but laboratories can use each method of analysis, provided it can be demonstrated in a traceable manner that they strictly fulfil the analytical requirements laid down in the respective legislation. As a general requirement, methods for benzo[a]pyrene analysis used for food control purposes must comply with the provisions of points 1 and 2 of Annex III (characterisation of methods of analysis) to Regulation
(EC) No 882/2004 of the European Parliament and of the Council of on official controls performed to ensure the verification of compliance with feed and food law, animal health and animal welfare rules 20.
The extraction method used for extracting PAHs from food samples greatly depends on the nature of the food matrix. Saponification followed by liquid-liquid extraction and extraction with organic solvent are the most often used methods for solid food samples (e.g. meat, fish and their products), while liquid-liquid extraction is often used for liquid samples (e.g. vegetable oils). For solid food matrices automated extraction techniques, such as pressurised liquid extraction (PLE) and supercritical fluid extraction (SFE), are also applied, but less frequently. Column chromatography, solid phase extraction (SPE) and gel permeation chromatography (GPC) are the main sample purification techniques used for isolating PAHs from interfering matrix substances.
Recently especially the automatic GPC technique has been applied for cleaning up PAH sample extracts (Fontcuberta et al., 2006; Llobet et al., 2006; Fromberg et al., 2007; Reinik et al., 2007).
Nowadays the two main analytical techniques for determining PAHs in foods are high performance liquid chromatography (HPLC) coupled to a fluorescence detector (FLD) and gas chromatography-mass spectrometry (GC-MS). Both of these methods are sufficiently sensitive for determining PAH concentrations usually found in foods. Earlier HPLC with an ultraviolet
(UV) or a photo-diode array (PDA) detector and GC with a flame ionisation detector (FID) were also methods often applied but today they are not state of the art anymore due to their poorer selectivity and sensitivity. Programmed temperature vaporization (PTV) injection used in the
GC-methods allows injection of high sample volumes and hence low detection limits are achieved for PAHs. On-line methods simplify the analytical procedure of the PAH analysis because several separate manual sample preparation steps can be omitted. In the on-line HPLC method the sample extract is injected into a donor-acceptor complex chromatographic column which is coupled on-line to an analytical column in the HPLC-FLD system A

References 1. Toxic Substance hydrology program http://toxics.usgs.gov/definitions/pah.html accessed March 15, 2013 2. Beata Janoszka Food Chemistry: HPLC-fluorescence analysis of polycyclic aromatic hydrocarbons (PAHs) in pork meat and its gravy fried without additives and in the presence of onion and garlic. Volume 126, Issue 3, 1 June 2011, Pages 1344–1353 ( Department of Chemistry, Medical University of Silesia, Faculty of Medicine and Division of Dentistry, Jordana 19, 41-808 Zabrze, Poland)

3. J. Agric. Food Chem. 1997, 45, 1394−1403 Formation of Polycyclic Aromatic Hydrocarbons during Processing of Duck Meat
B. H. Chen* and Y. S. Lin Department of Nutrition and Food Science, Fu Jen University, Taipei, Taiwan 242, Republic of China