In most sub-saharan Africa countries, traditional fishing is practiced in almost all rivers, lakes, ponds, and seas. It represents an important part of total fish captures and is an important sector in the national strategies of fight against poverty, food security and food safety (Ahmed et al., 2011). Fish is an important dietary component of people all around the world and represents a relatively cheap and accessible source of high  quality protein for poorer households (Ikutegbe and Sikoki, 2014). In West Africa, fish has been reported to provide 40–70% of the protein intake of the population (Béné and Heck, 2005; Ikutegbe and Sikoki, 2014) and it is a critical source of  dietary protein that is not readily available in the carbohydrate-based staple foods of the populations. In Nigeria, fish has an edge over meat because it is cheaper and relatively more abundant (Eyo, 2001) and constitutes about 40% of the animal protein intake (Eyo, 2001; Abolagba and Melle, 2008). Fish can be consumed in several forms; fresh, dried, frozen, fermented and brined depending on the consumer preference. In a study by Mafimisebi (2012), it was discovered that majority of the Nigerian people had a preference for fresh fish; however limitations such as the low keeping quality of fish after harvested and the tendency of fish to spoil quickly make this very difficult. This results in a higher reported consumption of smoke-dried fish, which has relatively longer shelf life (Mafimisebi, 2012).

Fish is highly perishable and a considerable effort has been directed to extend the shelf-life of fish using preservation and processing techniques, such as refrigeration, freezing, canning, smoking, salting, and drying (Nwachukwu and Madubuko, 2013). Besides, some of these techniques can also be used to enhance the value of fish, such as smoked fish.

The smoking of fish smoldering dates back to civilization (Eyo, 2001). Smoking method mostly imparts a desirable flavour and inhibits the growth of micro-organisms (Swastawati, 2008). It has been used for centuries in food preservation, and is still widely used for this purpose among several communities in the third world where up to 70% of the catch is smoked for preservation (Omojowo and Raji, 2010). Consumers are rediscovering the good taste of smoked seafood, including smoked catfish. To satisfy the consumer demand, it is necessary to produce good quality and safe smoked seafood products (Omojowo and Raji, 2010).

There is insufficient data on the quality characteristics of traditional smoked wild fish from Nigeria despite the fact that it constitutes a substantial portion of fish available for Nigerians. This study is carried out to investigate the influence of smoking method on quality of traditional smoked bonga shad from Lagos State.

Fish used

Fresh bonga shad fish (100 samples) and smoked bonga shad fish (100 samples) were collected from 20 different processing centres from fishing communities of Badagry and Epe Local Government Areas of Lagos State, Nigeria. The fresh bonga shad fish samples were freshly harvested. The fresh samples were taken to the Institute of Food Security, Environmental Resources and Agricultural Research (IFSERAR) laboratory, Federal University of Agriculture, Abeokuta for smoking.

Area of study

Using a current geopolitical map of Nigeria, Lagos State (Figure 1) lies to the south-western part of Nigeria and has boundaries with Ogun State both in the north and east. It is bordered on the west by the Republic of Benin and in the south, stretches for 180 km along the coast of the Atlantic Ocean. It  therefore, has 22.5% of Nigeria's coastline and occupies an area of 3,577 sq km land mass with about 786.94 sq km (22%) of it being lagoons and creeks. The state is endowed with marine, brackish and fresh  water ecological zones with varying fish species that provide productive fishing opportunity for fishermen. Two local government areas (Badagry and Epe Local Government) were covered because they are highly densed fish processing centers. They were selected for the study and hazard analyses of the products.

Sampling procedure

Fresh samples (100) and smoked bonga shad samples (100) were collected from each of the 10 processing centres from each of the two local government areas (Badagry and Epe Local Government) by purposive sampling in sterile containers (Ziploc). 

All freshly harvested bonga shad samples were kept on ice during transportation to the laboratory and smoked on the same day. Smoked bonga shad fish samples were analyzed immediately.

Fish smoking process

Smoked fish was prepared following the method as described by Crapo (2011) with modifications. Bonga shad fish samples were carefully cleaned to remove slime, blood and harmful bacteria. The fish were eviscerated, leaving the skin on the fish. The fish were cut into uniform pieces (fillet) so that no parts will get overheated. 

The bonga shad fish were smoked to 80°C internal temperature for 18 h. The kiln temperature was adjusted as needed throughout this smok­ing period to maintain the 80°C internal tem­perature. Hands, utensils and work surfaces were cleaned when transferring fish from smoker to oven to cool down to avoid cross-contamination. Smoking was done for 24 h until the fish is fully dried. 

Proximate analysis 

The following proximate analyses were carried out on fresh bonga shad and the smoked bonga shad samples collected from the processing centres. The moisture content of the fresh bonga shad and smoked bonga shad were determined by the oven-drying method. Protein contents of the bonga shad were extracted and fractionated by the method of AOAC (2000) method. The crude fat, crude fibre and ash content of the fresh bonga shad  and smoked bonga shad were determined by AOAC (2000) method.

Physico-chemical analysis

Kent pH meter (Kent Ind. Measurement Ltd., survey) model 7020 equipped with a glass electrode was used to measure the pH of the flesh, employing 10 g of fish homogenized in 10 ml of distilled water. Triplicate determinations were made in all cases. The pH meter was calibrated using pH 4.0 and pH 7.0 buffers.  The total volatile base- nitrogen, trimethylamine value (TMA), thio-barbituric acid value, peroxide value and free fatty acid value of the fresh fish and smoked fish were determined by AOAC (2000) method. All chemicals used in this study were of the analytical grade unless stated otherwise.

Data analysis

All data analyses were done in triplicates. The data obtained were subjected to descriptive statistics using IBM SPSS version 21.0 software. One  way analysis of variance (ANOVA) was done using Duncan’s Multiple Range Test (p<0.05) to study the difference between means.

Moisture content of fresh bonga shad samples ranged from 72.96 - 76.89% (Table 1) and that of smoked bonga shad samples obtained using local drum kiln and conventional smoke kiln ranged from 11.22 - 14.64% and 8.64 - 10.32% (Table 2). In contrary to protein, fat, and ash, the moisture content of fresh bonga shad samples decreased sharply after the smoking process. This decrease was due to loss of water during smoking. Moisture content of fish is of great importance as it promotes microbial growth. In the present study, the protein content of the fresh bonga shad samples ranged from 15.18 - 16.95% and that of the smoked bonga shad samples ranged from 51.86 - 60.24% and 58.86 - 64.84%. There was an inverse relationship between the moisture and protein content in the smoked bonga shad. Fat content of fresh bonga shad samples ranged from 6.46% - 8.84% and that of smoked bonga shad samples ranged from 16.13 - 20.84% and 12.87 - 17.34%. Crude fibre content of fresh bonga shad samples ranged from0.21 - 0.37% and crude fibre content of smoked bonga shad samples ranged from 2.14 - 4.32% and 2.26 - 4.56%. Ash content of fresh bonga shad samples ranged from 0.12 - 0.16% and ash content of smoked bonga shad samples ranged from 1.18 - 1.46% and 1.24 - 1.79%. The increase in mineral content, ash and crude fibre can be attributed to an increase in the dry matter content per unit weight following sample dehydration and during the smoking process. In this study, carbohydrate content is given by difference that is the percentage of water, protein, fat and ash subtracted from 100. The carbohydrate content of fresh unsmoked bonga shad samples ranged from 0.92 - 1.24% and carbohydrate content of smoked bonga shad and control samples ranged from 6.43 - 10.00%   and 8.01 - 10.24%. Carbohydrate content of smoked bonga shad samples is low because it is a proteinous food.

The quality indices of the fresh and smoked bonga shad were studied (Tables 3 and 4). Fats undergo changes during storage which result in production of an unpleasant taste and odour which is commonly referred to as rancidity. The peroxide value (PV) results are similar in pattern to TBA. In this study PV of fresh bonga shad samples was 6.11 – 8.59 mg Eqperoxide/kg and 6.22 – 10.41 mg Eqperoxide/kg and 7.12 – 9.86 mgEq.peroxide/kg for smoked bonga shad and control samples. These values are below the recommended value of between 20 and 40 mgEq.peroxide/kg for rancid taste to begin.  Free fatty acids values (FFA) of fresh unsmoked bonga shad samples was 0.86 - 1.13% while that of smoked bonga shad and control samples ranged from 1.05 - 1.26% and 1.00% - 1.19%. These values are very low and below the threshold for rancidity detection in smoked fish. The thiobar-bituric acid value (TBA) is used to assess the degree of fish spoilage especially in fatty fish. The TBA test measures a secondary product of lipid oxidation, malonaldehyde. The TBA values of fresh bonga shad samples ranged from 0.84 – 1.11 mgMol/kg and thio-barbituric acid value (TBA) of smoked bonga shad samples and control samples ranged from 1.01 – 1.23 mgMol/kg and 1.01 – 1.15 mgMol/kg. The TBA (1.00 to 1.15 mg TBA/kg) did not exceed 1 to 2 mg TBA/kg which was well within acceptable limits.

The sensory threshold level for detecting rancidity in fresh meat was reported to be between 1 and 2 TBA (Calhoun et al., 1999). The increased TBA values in the smoked fish probably originated from the breakdown of oxidation products, mainly malonaldehyde, during smoking due to the high temperature (Goktepe and Moody, 1998).  Beltran and Moral (1991) reported that high TBA values are correlated with the degree of oxidation of fats in hot smoked sardines.

The legislative standard for TVB-N include: 20 mgN/100 g for fresh fish, 30 mgN/100 g stale fish and 40 mgN/100 g for fish that is unfit for human consumption but can be used for animal feed (da Silva, 2002).  In this study, the total volatile base- nitrogen (TVB-N) of fresh bonga shad ranged from 13.38 – 15.57 mgN/kg and TVB-N of smoked bonga shad and control samples ranged from 16.43 – 19.36 mgN/kg and 15.49 – 18.83mgN/kg. These values are within the range of legislative standard for TVB-N which is 20 mgN/100 g for fresh fish. This suggests that the level of protein decomposition or breakdown in all the samples is low.

Trimethylamine (TMA) is a reduction product of trimethylamine oxide during spoilage and ammonia is mainly formed as a product of protein break-down. Trimethylamine (TMA) is one of the volatile amines plus ammonia which can be used as an index of spoilage (da Silva, 2002). In this study, the trimethylamine value (TMA) for fresh unsmoked bonga shad samples ranged from 1.93 –2.56 mgN/kg and 2.17 – 2.81mgN/kg and 2.05 – 2.61 mgN/kg for smoked bonga shad and control samples. The trimethylamine value (TMA) of 1.93 –2.56mgN/kg for fresh bonga shad samples and  2.17 – 2.81 mgN/kg and 2.05 – 2.61 mgN/kg for smoked bonga shad  samples are within the range of < 3 mgN/100g for fresh fish, >8 mgN/100g for spoiled fish and > 5 mgN/100g for doubtful quality specified U.S.F.D.A (da Silva, et al., 2008). pH is the most critical factors affecting microbial growth and spoilage of foods. The pH value of fresh unsmoked bonga shad samples ranged from 6.81 – 7.24 and pH value of smoked bonga shad samples ranged from 6.20 – 6.71 and 6.56 – 6.93. The pH values of the fresh bonga shad samples was high compared to smoked bonga and control samples; this may be due to biochemical reactions and enzyme action as a result of delay in reaching the shore from the sea because most of the fishermen had no cooling system in their boats or canoes. However, the pH in fish tissues drops due to smoking.

This research work revealed that in contrary to protein, fat, and ash, the moisture content of fresh bonga shad samples decreased sharply after the smoking process.This decrease had been found to be due to loss of water during smoking. Protein content in smoked bonga shad fish and control samples has been found to increase due to an increase in the dry matter content per unit weight following sample dehydration during smoking compared to fresh samples this is important as so many people in West Africa eat smoked dried fish and are involved in producing smoked dried fish has a high protein content lends support to national strategies of fight against poverty, food shortage and malnutrition. There was an inverse relationship between the moisture and protein content in the smoked bonga shad fish. There were increases in mineral content, ash, crude fibre, TBA and pH values. Convective smoking kiln did not significantly affect pH and proximate composition of smoked fish. It, however, significantly (p<0.05) reduced the moisture content of smoked fish and the quality indices such as FFA, TBA and PV. 

The authors did not declare any conflict of interest.