Development and Evaluation of Low Polycyclic Aromatic Hydrocarbons (PAHs) Improved Fish Smoking Kiln

Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic pollutants which get into foods during smoking. This research was used to develop an improved smoking kiln and to determine its effects of the methods of smoking in reducing the level of PAHs of a locally consumed fish in Nigeria. The fish specie clarias gariepenus (catfish) was smoked with different methods. They are the firewood in open-fire fish smoking drum (FOSD), charcoal using improved fish smoking kiln (CISK) and gas using improved fish smoking kiln (GISK). The PAHs was determined using the Gas Chromatography (GC) coupled with flame ionization detector (FID). Comparison of the concentration of PAHs in smoked fish samples were investigated with the aim of determining the process that has reduced the concentration of the PAHs in the fish samples. The total values of PAHs have shown that the methods FOSD, CISK and GISK have PAHs level of 80.01µg/g, 20.95µg/g and 14.69µg/g respectively. Benzo(a)pyrene, which is highly carcinogenic and posses health risk was found only in FOSD to be 9.20µg/g, while it was not detected in CISK and GISK respectively. As for other PAHs, they were all detected in FOSD, while fluoranthene was exceptionally high in all the methods used as 22.08µg/g, 10.23µg/g and 5.04µg/g respectively. However, naphthalene, phenanthrene, anthracene, pyrene, chrysene, benzo(b)fluoranthrene, benzo(k)fluoranthrene, benzo(a)pyrene and benzo(g,h)perylene were not detected (ND) in the developed improved fish smoking kiln. The developed improved fish smoking kiln has been able to reduce the level of PAHs considerably and is therefore recommended.


INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) constitute a large class of organic compounds that are composed of two or more fused aromatic rings. They are a class of ubiquitous environmental pollutants which can be generated during the preparation of food (Agerstad and Skog, 2005). They are primarily formed by incomplete combustion or pyrolysis of organic matter and during various industrial processes (Silva et al, 2011). PAHs generally occur in complex mixtures which may consist of hundreds of compounds. Humans are exposed to PAHs by various pathways. While for non-smokers the major route of exposure is consumption of food, for smokers the contribution from smoking may be significant (Agerstad and Skog, 2005). Food can be contaminated from environmental sources, industrial food processing and from certain home cooking practices (EFSA, 2008). On the other hand, PAHs are also found in foods as a result of certain industrial food processing methods such as smoke curing, broiling, roasting and grilling over open fires or charcoal which permits the direct contact between food and combustion products (Silva et al., 2011). In general, PAHs are not present individually but in mixtures. Sixteen PAHs that have been extensively monitored are the compounds included in the United States Environmental Protection Agency (USEPA) list of priority organic pollutants (USEPA, 1994). Of these 16 PAHs, Benzo(a)pyrene (BaP) is probably the most studied and has been described by the International Agency for Research on Cancer (IARC) as probable human carcinogen in 1987 (IARC, 1987). Thus, the determination of BaP has been widely used in environmental analysis as marker for the entire PAH content (Otahen, 2021).
Across Africa various traditional methods are employed to process, preserve and store fish. These include smoking, drying, salting, frying and fermenting and various combinations of these treatments (SFMP, 2016).
Smoking is a traditional processing technique used to preserve fish. It also gives colour and a range of tastes appreciated by consumers. It consists of exposing fish to the effects of heat and smoke, both produced by the combustion of biomass (wood, sawdust, coconut husks, etc) (Kamaldeen et al, 2016). The advantages of smoking fish are manifold: it prolongs shelf life, enhances flavour, reduces waste in times of bumper catches and permits storage for the lean season. It increases protein availability to people throughout the year and makes fish easier to pack, transport and market. It is a major source of income for coastal-dwelling communities and traders. In Nigeria, smoking is the most widely-used method for preserving fish and is the most common activity for women in fishing communities. Out of the total of 194,000 metric tons of dry fish produced in Nigeria, about 61% of it was smoked (Silva et al, 2011). It has been estimated that between 70 and 80 percent of the domestic marine and freshwater fish catch is consumed in smoked form (SFMP, 2016). Catfish (Clarias gariepinus) has been reported to be a very important freshwater fish in Nigeria. It has enjoyed wide acceptability in most parts of the country because of its unique taste, flavour and good texture. It is widely distributed, extensively cultivated in ponds. (Kamaldeen et al, Journal of Natural Sciences Research www.iiste.org ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online) Vol.12, No.12, 2021 40 2016). The actual levels of PAHs in smoked foods depend on several variables in the smoking process, including type of smoke generator, combustion temperature, and degree of smoking (Silva et al, 2011). The composition of the smoke and the conditions of processing affect the sensory quality, shelf life, and wholesomeness of the product. Potential health hazards associated with smoked foods may be caused by carcinogenic components of wood smoke; mainly PAHs, derivatives of PAHs, such as nitro-PAH or oxygenated PAH and to a lesser extent heterocyclic amines (Silva et al, 2011). The smoke for smoking of food develops due to the partial burning of wood, predominantly hardwood, softwood and bagasse (Rey et al., 2009 andSilva et al, 2011).
In Nigeria, kiln smoking ranges from traditional open fire to the mud brick, cylindrical drum, and brick. Recent studies have shown that fish smokers produce products with levels of Polycyclic Aromatic Hydrocarbons (PAH) that are well above current recommended levels for human health (Kamaldeen et al, 2016). Most of the smoking kilns available were unable to tackle the critical challenges faced by the end-users. In addressing the problems faced in traditional smoking process, mechanical smoking kiln was developed by NSPRI (2012). Some of limitations of the smoking kiln developed by NSPRI (2012) and the traditional stoves are: • poor control of the fire power resulting in high temperatures well above optimum • the smoking of fish using traditional kilns is generally carried out with wood at temperatures of 300°C to 700°C • in many cases cooking and smoking phases are concurrent • in those models, nothing preventing oil from dropping on the fire • effect of not collecting the falling oil drops and providing a roasting surface heated by the fire and the PAH rises into the fish above (Kamaldeen et al, 2016). • Effects of not using clean energy other than the firewood. This has provoked us to develop an Improved smoking kiln which cannot only significantly reduced fuel wood consumption and reduce human exposure to heat and smoke but also to reduce the PAHs content of the smoked fish. Based on this, a new and improved smoking kiln is fabricated with improved fish drying technologies and with adequately efficient fuel device that produces dried fish with low PAH levels. The constructed smoking kiln was also evaluated.

MATERIALS AND METHODS Collection of Materials
The clarias gariepenus (catfish) was purchased from Uchi market, Auchi, Edo state of Nigeria. The firewood used in the generation of heat and smoke were sourced from a local seller while the traditional Open-fire drum for the smoking was at the Department of Food Technology, Auchi Polytechnic Auchi. The new improved smoking kiln used for this research was constructed by the researcher, from the department of Food Technology, Auchi Polytechnic Auchi. The materials used for the construction of the new improved smoking kiln, mainly galvanized and stainless steel were purchased from a dealer in Auchi and the gas cylinder was also purchased in local store.

Components of the developed fish smoking kiln
The developed smoking kiln comprises of various components with their specific function include the following: • Smoke cabinet is the chamber where fish are kept for smoking and drying • Fish trays is made of wire mesh where the fish are placed for smoking and drying in the chamber • Oil collectors is for collecting oil dripping from the fish to prevent it into flame • Charcoal pot is design to contain charcoal for heating • Gas burner is used to generate heat into the kiln chamber • Chimney is used to conduct the smoke/heat coming from the chamber into the surrounding • Thermometer is used to know the temperature in the heated chamber • Rolling tyres is used to move the kiln to any location • Fibre glass which is embedded is used to prevent heat loss in the heated chamber • Silver paint spray was used the beautify the exterior part of the kiln Figure 1 shows some of the component parts of the developed improved fish smoking kiln

Design Consideration
In order to reduce PAHs levels in smoked fish to a level nearer to European Union (EU) standard, the following are considered in the improved kiln design: • The smoking of fish can be carried out with wood, charcoal or gas • There is good control fire power resulting at controlled temperatures.
• There advantage of using a very clean combustion with low PAHs • In case of fire wood, cooking and smoking phases should be done separately: the separation of the two phases presents the advantage of cooking first without or with very low levels of PAHs, then smoking Journal of Natural Sciences Research www.iiste.org ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online) Vol.12, No.12, 2021 41 during a reduced duration. • In this model, there is oil collectors preventing oil from dropping on the fire which could have generated PAHs rising into the fish kept above.

Smoking of fish
Clean water was used to wash the fish before gutting and they were washed again and drained. The fish were spread in trays and taken for smoking. The firewood was used in this case of traditional open-fire drum. The smoking was done for about 7-8hours. As for the new developed improved smoking kiln, the charcoal and gas were used as the source of heat. The kiln was opened for fish to be loaded in the trays and gently closed again for the drying process. The gas/charcoal was ignited to provide fire for heating at the burner section. The kiln temperature was regulated between 80 and 100 0 C and the fish were dried for about 10 to 12hours. After the drying processes, the smoked and dried fish were cooled and packaged and samples were taken to the laboratory for PAHs analysis.

Analysis of PAHs Extraction:
The samples were each pulverized using a manual grinding machine to ensure homogenization. Five grams (5g) of the pulverized sample was thoroughly mixed with 10g of anhydrous sodium sulphate in a mortar (Amos-Tautua et al, 2013) to absorb moisture. The homogenate was placed into an extraction thimble and covered with a Whatman filter paper (125 mm diameter).This was then inserted into a Soxhlet extraction chamber of the Soxhlet extraction unit. Extractions were then carried out with 50mL mixture of redistilled n-hexane and dichloromethane in the ratio 3:1 for effective recovery. Subsequently, the crude extract was filtered through a layer of anhydrous sodium sulphate. The obtained filtrate was evaporated to near dryness. Clean up: The clean-up of the extract was carried out using activated silica gel and anhydrous Na2SO. The silica gel column was prepared by loading an activated silica gel (12g) onto a chromatographic column (id=1cm). About 1g of anhydrous Na2SO4 was added to the top of the silica gel in the column. After conditioning the columns with 20mL hexane the sample was applied and eluted with 200ml of mixture of Dichloromethane : Hexane (3:1).The eluate was collected into an evaporating flask and rotary evaporated to near dryness. The dry eluate was then dissolved in 1mL n-hexane for Gas Chromatographic analysis (Amos-Tautua et al, 2013). Instrumental analysis: The polycyclic aromatic hydrocarbon analysis was carried out using gas chromatograph system. The system consisted of a Hewlett Packard Model 5890 gas chromatography (GC) equipped with a flame ionization detector (FID) and a data processor (Hewlett Packard, Wilmington, DE, USA). The column used was HP-1932530, a non-polar, fused-silica capillary column (30m length × 25µm inner diameter × 0.25µm film thickness). The oven temperature was set initially at 60°C (5min hold), increased to 250°C at 15°C/min. (14min hold), and a final temperature of 320°C at10ºC/min (4min hold). Nitrogen gas was used as the carrier gas at a flow rate of 1mL/min at a pressure of 30psi. The injector temperature was set at 250°C, injection volume was 1mL and the detector temperature was set at 320°C. Verification of peaks was carried out based on retention times compared to those of external PAHs standards (Amos-Tautua et al, 2013).

Front view Side view
Gas hose 12 trays loaded fish Oil collector Charcoal Plate Burner chamber Figure 1: Some component parts of the developed improved fish smoking kiln