Fish is very prone to decomposition, development of rancidity and microbial spoilage, immediately after harvest, therefore, there is a great need to process and preserve harvested fish items in order to extend its shelf life for human consumption [1]. Some of the processing and preservation techniques commonly used are salting, drying, smoking, canning, freezing, pickling and irradiation among others. In Nigeria, the greater portion of fish caught are preserved by smoking and sun drying [2]. In order to arrest fish spoilage, different processing methods are applied in the tropics. Some techniques are based on temperature control by using ice, refrigeration or freezing; while others based on the control of water activity through drying, salting, smoking and freeze-drying [3]. Most often a combination of different processing techniques is used to preserve fish, as different processing methods have different effects on nutritional composition of fish.  The simplest and most widely practiced method is smoking, usually with traditional methods of fish smoking. This is widely practiced in the riverine areas, fishing towns and villages and are scattered far away from consuming centers [4].

In most of the communities in tropical regions of the world, majority of the fish caught are smoked by traditional method using drum type smoking kiln in open space under unhygienic conditions [5]. Smoke drying has the advantages of increasing the shelf life of fish since water that create favourable condition for mould or bacteria growth has been drastically reduced. Also smoking enhances the flavour, colour and odour/aroma of the fish. Intramuscular phospholipids have been shown to be the most rapidly oxidized lipid component in smoked fish or meat [6]. Awareness of these constraints and limitations prompted many research organizations to develop improved method of smoking.

One of the improved types of kiln was developed at the Nigeria Institute of Oceanography and Marine Research (NIOMR), Lagos [7]. In this kiln, the central dome acts as the heat exchanger and also maintains a uniform temperature inside smoking kiln. The smoke produced enters the heat exchanger and then enters the smoking kiln through the tubes because of this arrangement, there is uniform distribution of smoke inside the smoking kiln and temperature is controlled by adjusting the addition of fire wood and wet saw dust inside the fire place. The smoke circulates uniformly inside the smoking chamber and then escapes at the top. A smoking kiln may be linked to a large square rectangular chimney, opened at the bottom as in the case of the fire place. The mechanical kiln is more easily controlled and gives a more uniform and more hygienic product with less labour [8].

Any fish can be smoked, however, fatter fish will absorb more smoke flavour, so fish like, mackerel, salmon and trout are perfect for smoking. According to Tobor [9], smoked products are known to possess an increased resistance to oxidative changes in fatty foods such as fish. The phenolic substances found in smoke are believed to be responsible for such effects on foods. Ito [10] recorded that smoking and drying treatment applied to fish account for 45% of the total preservation methods available to rural fishermen. According to Ihekoronye and Ngoddy the preservation and processing of fresh water and marine fishes largely depend on the technology developed in any particular area. Due to erratic power supply and the unavailability of storage facilities for most rural households, dried fish which store for several weeks before consumption represents the most inexpensive source of animal protein for rural dwellers. As a result of high temperature involved in fish drying and the long storage weeks, most of the proteins are denatured and this leads to reduction in functionality of essential amino acids. The totality of these, often results in fish products with low nutritive value, thereby exposing the populace who rely on dried fish products for their source of animal protein nutritionally insecure. However, consumers rarely have information about the nutritional quality of smoked fish and its quality changes during storage, thus the need for this study. This study aimed to determine the nutritional quality of smoked marine fishes as well as to examine the effect of ambient storage on its proximate qualities.

Experimental Location

The experiment was conducted at the Fish Processing Unit, in African Regional Aquaculture Centre, Aluu in Port Harcourt.

Source of Fish Materials

The fish materials that wereused in this research work consist of fresh samples of four fish species of marine origin: They are:Red snapper (Lutjanus goreensis); Croaker (Pseudotolithus elongatus); Mullet (Mugil cephalus) and Bonga  (Ethmalosa fimbriata).These were purchased from Creek Road Market in Port Harcourt. They were identified with the keys of [11] on marine and brackish water fishes. Red snapper (Lutjanus goreensis);Croaker (Pseudotolithus elongatus); Mullet (Mugil cephalus) and Bonga  (Ethmalosa fimbriata). The fish were purchased in fresh form from Creek Road main market in Port Harcourt, Rivers State. 

Experimental Procedure

Each of the four species was weighed with weighing scale. In each of the species,4kg was weighed, decaled, and the viscera removed and washed in clean water.Brining was done by dipping the fish into 75% saturated brine which was made by dissolving 27g of salt (NaCl) in 100ml of water for 30 seconds [12]. The fish was again rinsed in fresh water. The fish samples were spread in trays and taken for smoking. 

In NIOMR Kiln, the fish was smoked for three hours at 155±5⁰C and then maintained at 55±5⁰C until constant weight was reached.  While in Half Drum, the fish was smoked for three hours at 95±5⁰C and then maintained at 45±5⁰C until constant weight was reached.

The smoked fish of different species were preserved in three different ways for three weeks:

• Freezer (-5⁰C)

• Refrigerator (4⁰C)

• Room temperature (27- 30⁰C).

The stored fish were then be analyzed for proximate values.

Determination of Proximate Composition of Fish

Proximate chemical composition analysis including determination of moisture content, crude protein, crude fat, crude fiber, carbohydrates and total ash of the smoked fish was performed according to AOAC official methods [13]. Petri dish was cleaned and weighed. Then 1.0 g of each of the grounded fish samples was measured in each petridish and then weighed. They were each transferred into the oven at 105°C for 3 hours. After the first 3 hours, the petridish was removed from the oven, allowed to cool and weighed. The petridish was returned into the oven and was brought out after an hour and weighed again; this process was repeated until a constant weight was achieved and moisture content was determined. The crude protein was determined using Kjeldahl method. The fish sample (either smoked or frozen) was ground into a fine or smooth texture. A known weight (5.0 g) of the fish sample is then weighed into a long necked Kjedahl flask along with 5 g of copper sulphate anhydrous and 5 g of sodium sulphate anhydrous. Then, 25 ml of concentrated sulphuric acid (H2SO4) was added. The flask was gently placed and the content was heated, the heating continued until a clear solution was obtained. The digestion was performed between 3 to 5 hours. The clear hot solution obtained was allowed to cool and solution was filtered using filter paper. Then, 5 ml of the filtered digested sample was poured into the protein determination equipment and 10 ml of 40% NaOH was added followed by a distillation process. The steam being passed in the reactor condenses and drops into a conical flask containing boric acid (5 ml) until the mixture changes color. After changing color, 50 ml of the liquid was collected and titrated with 0.01 M of HCl until the color (green) changed to deep blue. For the estimation of fat content, the dried samples left after moisture determination were finely ground and the fat was extracted for 4 hours with a non-polar solvent (i.e. ethyl ether) using soxhlet extraction method. After extraction, the solvent was evaporated and the extracted fat was weighed. Ash was determined by burning the dried sample in a furnace at 550°C for 4 h. The difference in weights before and after burning gave the total ash content. Crude Fibre was determined using trichloroacetic acid method as recommended by AOAC [13]. The total carbohydrate content was determined by subtracting the sum of the percentage moisture, ash, crude lipid, and crude protein from 100%.

Statistical Analysis 

The data obtained from the study was collated and analyzed using Statistical Package for Social Sciences (SPSS 22.0). A two way analysis of variance (ANOVA) was employed to reveal significant differences in measured variables. When a difference was detected (p<0.05), Tukey’s multiple comparison test was applied to identify differences between the means [14].

The proximate compositions of marine fishes smoked with Half Drum Kiln and preserved under room temperature are presented in Table 1. The results indicated the highest value of moisture content (6.53±0.03%) was observed in P. elongatus, this was closely followed by that ofM.cephalus (6.52±0.02%), while the lowest (5.16±0.02 %) was recorded in E. fimbriata. The lowest crude protein value (58.67±0.03%) was recorded in E. fimbriata, while other species were within the same range with no significant difference (p>0.05). Higher values of crude fat were recorded in L.goreensis and E. fimbriata when compared to other species. The highest values of ash (16.47±0.01%) was observed inP. elongatus and the lowest (9.21±0.02%) inM.cephalus. However, zero values were recorded in crude fiber and carbohydrates in all species under consideration. The proximate compositions of marine fishes smoked with Half Drum Kiln and preserved with freezer are shown in Table 2. The results obtained revealed that the highest value of moisture content (8.53±0.01%) was observed in L.goreensis, this was closely followed by that of E. fimbriata (8.41±0.03%), while the lowest (5.10±0.02 %) was recorded in M.cephalus. The lowest crude protein value (46.00±0.05%) was recorded in E. fimbriata, while other species were within the same range with no significant different (p>0.05).  Higher values of crude fat were recorded in L.goreensis and P. elongatus when compared to other species. The highest values of ash (14.90±0.02%) were observed in E. fimbriata and the lowest (12.54±0.01%) inP. elongatus. However, zero values were recorded in crude fibre and carbohydrates in all species under consideration. In the fishes smoked with half drum kiln and preserved with a refrigerator (Table 3), the moisture content of E. fimbriata and M. cephalus were within the same range. While that of L.goreensis and P.elongatuswere within the same range. The highest value of crude protein (78.67±0.05%) was recorded in E. fimbriata, while other species were within the same range with no significant difference (p>0.05).  Higher values of crude fat content were recorded in L.goreensis and M.cephalus when compared to other species. The highest values of ash (15.52±0.02%) was observed in M.cephalus and the lowest (6.84±0.02%) inE. fimbriata . However, zero values were recorded in crude fibre and carbohydrates for all species. 

Furthermore, the proximate compositions of marine fishes smoked with NIOMR Kiln and preserved under room temperature are presented in Table 4.The results revealed that the highest value of moisture content (7.44±0.04%) was recorded in M.cephalus , while other species were within the same range with no significant difference (p>0.05). The values of crude protein and crude fat  varied significantly among the species  under consideration. The highest values of ash (18.59±0.01%) was observed in P. elongatus and the lowest (7.16±0.03%) inM.cephalus . However, zero values were recorded in crude fiber and carbohydrates in all species under consideration.

The proximate compositions of marine fishes smoked with NIOMR Kiln and preserved with freezer are shown in Table 5.The results obtained revealed that the highest value of moisture content (10.35±0.03%) was observed in E. fimbriata, while other species were within the same range with no significant difference (p>0.05). The lowest crude protein value (57.29±0.08%) was recorded in L.goreensiswhile higher values were recorded in other species and these  were within the same range with no significant difference (p>0.05).  The specie L.goreensis had the highest value (29.73±0.08%) of  crude fat, whereas lower  values were recorded in other species and these  were within the same range with no significant difference (p>0.05). The lowest values of ash (4.29±0.02%) was observed in E. fimbriata, the ash value in other species were within the same range. However, zero values were recorded in crude fibre and carbohydrates in all species under consideration. In the fishes smoked with NIOMR kiln and preserved with a refrigerator (Table 6), the highest value (8.75±0.06%) of moisture content was recorded in M.cephalus and the lowest (4.36±0.02%) was recorded in L.goreensis.The highest value of crude protein (79.88±0.02%) was recorded in E. fimbriata, and the lowest (56.54±0.04%) was inL.goreensis.  The values of crude fat varied significantly among the species, with the highest value (31.70±0.06%) in L. goreensis and the lowest (7.25±0.02%) obtained in E. fimbriata. The highest values of ash (15.94±0.04%) was    observed in P. elongatus  and the lowest (7.39±0.71%) inL.goreensis. However, zero values were recorded in crude fibre and carbohydrates in all species under consideration.

Table 1: Proximate Composition of Marine Fish Species Smoked with Half Drum and Preserved under Room Temperature (Mean±SD)

Mean with different superscript within the same row are significantly different (p< 0.05)1

Table 2: Proximate Composition of Marine Fish Species Smoked with Half Drum and Preserved with Freezer (Mean±SD)

Mean with different superscript within the same row are significantly different (p<0.05)

Table 3: Proximate Composition of Some Smoked (Half Drum) Marine Fish Species Preserved with Fridge (Mean±SD)

Mean with different superscript within the same row are significantly different (p<0.05)

Table 4: Proximate Composition of Marine Fish Species Smoked with NIOMR Kiln and Preserved under Room Temperature (Mean±SD)

Mean with different superscript within the same row are significantly different (p<0.05)

Table 5: Proximate Composition of Marin Fish Species Smoked with NIOMR Kiln and Preserved with Freezer (Mean±SD)

Mean with different superscript within the same row are significantly different (p<0.05)

Table 6: Proximate Composition of Marine Fish Species Smoked with NIOMR Kiln and Preserved with Fridge (Mean±SD)

Mean with different superscript within the same row are significantly different (p<0.05)

The moisture content can be described as an indicator to the frequency at which deterioration occurred in fish samples resulting in the early decomposition. During storage at refrigeration, freezing and room temperature, percentage of moisture content was found to vary from one specie to another in the two smoking kilns. There was a slight increase in the moisture content of the smoked fish stored at room temperature and those preserved in refrigerator and freezer.  The increase can be attributed to absorption of moisture from the surrounding since there was no re-drying during storage period [15].  Conversely, Clucas and Ward, reported that moisture content of 12% is the level beyond which fish products begin to develop moulds after few days. In this study the final moisture of the preserved smoke-dried fish samples was less than 12% in all the species.

During storage, the value of crude protein in smoked fish samples preserved under room temperature and refrigerator were within the same range. However, lower values of crude protein were observed in the fish stored in freezer. An indication that frozen storage reduced the percentage protein content of the samples. This result agrees with the findings of Wu and Mao [16] in smoked grass carp stored in the freezer.  Also, Xing-hua et al. [17] reported the same trend in smoked Sinilabeo rendalii during frozen storage. They attributed this protein loss to the leaching effect of amino acids and water soluble protein leaching out with melting ice. Moreover, Zaglol and Eltadawy [18] noted that protein decreased with increasing duration of frozen storage. Disadvantages such as product dehydration, rancidity, drip loss and product bleaching have an overall effect on the quality of frozen food. According to Window et al.  [19] proteins exposed to oxidizing environments are very susceptible to chemical modification, such as amino acid destruction, peptide scission and formation of protein-lipid complexes that results in decrease in protein content.  On the other hand, Eyo [12] attributed this loss to gradual degradation of the initial crude protein to more volatile products such as Total Volatile Bases (TVB), Hydrogen sulphide and Ammonia. 

The crude fat values obtained in the smoked marine fishes stored under different conditions such as refrigeration and freezing showed a slight decrease in lipid content during storage when compared with those that were stored at room temperature. This result was supported by Fagbenro et al. [20] in smoked fresh water fish stored in the freezer, and that of Gomez-guillen et al. [21] who found a significant loss in total lipid content during frozen storage of smoked Atlantic salmon. These workers attributed this loss to oxidation of lipid. As Hasan [22] noted that reduction in lipid content could be attributed to oxidation of poly-unsaturated fatty acids (PUFA) contained in the fish tissue to products such as peroxides, aldehydes, ketones and free fatty acids. Ikeme [23] also reported that reduction in lipid is associated with higher PV and FFA in five different species of smoked freshwater fish stored in ambient temperatures for eight weeks.

In this study, slight decrease was observed in ash contents of the smoked marine fishes preserved in freezer and refrigerator, when compared to the one stored at room temperature. The decrease in ash content during storage as observed in this work is in agreement with the observation of Robinson et al., [24] in channel catfish, who reported a decrease in total ash content during frozen storage. However, Rossano et al. [25] observed that the ash content remained almost the same throughout the 20 days of frozen storage of mora fish (Mora moro). The decrease in ash content was attributed to the drip loss [26]. There are slight changes in the mineral profiles of fish stored in freezer and refrigerator, when compared to the one stored at room temperature. This result is in accordance with Sadiku and Oladimeji [27] who observed some changes in mineral contents of smokedLates niloticus stored at different temperatures. The changes with respect to frozen and refrigeration period in all the minerals evaluated could be attributed to drip loss and dehydration that is associated with cold and frozen storage [28]. 

This study has also shown that using any of the storage conditions such as room temperature, refrigeration and freezing, there were significant differences in the quality of smoked marine fishes. There were changes in the proximate, mineral and microbial quality during the storage period. However smoked marine fishes stored under room temperature had a less reduction in quality.

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