Fish can be described is one of the most inexpensive sources of animal protein, micro-nutrients, and other indispensable nutrients needed by human for the maintenance of their good body health and proper metabolism [1]. It has been reported that frequent consumption of fish and its products enhances the defence mechanism for protection against invasion of pathogens, because fish food has antimicrobial peptide that is capable of stimulating internal cells for disease resistance [2]. In addition, the impact of fish in enhancing food security in  both rural and urban dwellers  among developing and under developed nations across the world has been reported [3]. 

In Nigeria, fish supplies 38.2% of animal protein [4], while fishing communities with greater access to fish for direct consumption have higher animal protein intake with fish contributing as much as 75 % [5] Fish has a high nutritional value for humans. It is majorly composed of water, with protein value ranging from 60 to 80%. It is rich in mineral salts such as potassium phosphorous, iron and vitamins A and C, but, above all, the quantity and the composition of fats present in fish differ from other food [6]. Fish demand is increasing as a result of the increasing world population, higher standards of living, good perception and high acceptance of fish and fishery products among its consumers [7].In different parts of the country, fish is generally consumed in both fresh and smoked form. It is a highly cherished delicacy that cuts across socio-economic, age, religious and educational barriers [8]. 

However, fish is exceptionally a perishable item, because it provides a favourable environment for the enhancement of microorganism’s development immediately after its death [9]. Some factors accountable for this comprises; the prevailing high temperatures in Nigeria and the facilities for processing; storing and distributing the fish caught are frequently inadequate or non–existence in most cases especially in the rural areas. There is therefore enormous wastage through spoilage of both fresh and dried fish [10]. Generally, spoilage is a metabolic progression that makes food to be unacceptable and unwanted for human consumption due to changes in sensory and nutritional characteristics [11]. The processing and preservation of fresh fish were of paramount significance since fish is highly predisposed to deterioration instantaneously after harvest and also to prevent economic losses [12]. 

An estimate of 40% post harvest losses of total fish landings has been reported in Nigeria [13]. Fish spoilage in Nigeria is influenced to a large extent by considerable distances of landing ports to points of utilization and poor as well as inadequate infrastructure for postharvest processing and landing. Thus, it is imperative to process and preserve some of the fish caught in the period of abundance, so as to ensure an all year round supply. This will invariably reduce post-harvest losses, increase the shelf-life of fish, and guarantee a sustainable supply of fish during off season with a concomitant increase in the profit of the fishermen [14]. If fish is not sold fresh, preservation methods should be applied to extend the shelf-life.  Proper preservation starts the moment fish is harvested until it reaches the consumer’s table [15].

Inadequate processing techniques and machineries can lead to an increase in the microbial load of fish and fish products.  A lot of processing methods are in use in Nigeria. These include chilling, freezing, salting, canning, drying and smoking. Among these methods of long term preservation of fish, smoking is perhaps the simplest method as it does not require sophisticated equipment or highly skilled workers [16]. It is a method of preservation affected by a combination of drying and breakdown of naturally produced chemical resulting from thermal decomposition of wood [17]. Proper preservation starts the moment fish is harvested until it ultimately reaches the consumer’s table [18]. This ensures that fish and fish products get to the consumers in acceptable quality. Deterioration of marine fish products mainly occurs as a consequence of microbiological activity and chemical changes during processing and storage [19]. It is against this background that this study is designed to evaluate the proximate composition of these fresh and smoked marine fish species.

Experimental Location

The experiment was conducted at the Fish Processing Unit, in African Regional Aquaculture Centre, Aluu in Port Harcourt. Fresh samples of Lutjanus goreensis, Pseudotolithus elongatus, Mugil cephalus and Ethmalosa fimbriata were purchased from Creek Road Market in Port Harcourt. They were identified with the keys of Ames et al. [20] on marine and brackish water fishes. 

Experimental Procedure

In the first phase, 200.00 g (dry weight) each of the fish species in fresh form were properly packed in ice-chest and taken to the laboratory to be assessed for proximate composition. This was done in three replicates. In the second phase, 3kg each was descaled, viscera removed and the carcasses were weighed 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 [21]. The fish was again rinsed in fresh water. The fish samples were spread in trays and taken for smoking. The fish was smoked using the improved NIOMR Kiln for the first three hours at 155±5⁰C and then maintained at 55±5⁰C for 72 hours until constant weight was reached [22].  The kiln had charcoal as source of energy and was attached with thermostat and temperature reader.

Proximate Analysis

Proximate chemical composition analysis which includes determination of moisture content, crude protein, crude fat and total ash of the smoked fish was performed according to AOAC official methods [23].

Statistical Analysis

The data was collated and were subjected to analysis of variance (ANOVA) in a completely randomized design. Turkey’s multiple comparison tests at 5% probability level were used for the separation of the means.

The results in Table 1 indicates that the values of moisture content of the fresh samples were within the same range, except in Lutjanus goreensis, that had a higher value of 75.27± 0.03%.The value of the moisture content of the smoked fish samples indicates that lower values of moisture content were recorded in all the species, with the highest value (5.16± 0.02%) from Pseudotolithus  elongatus, while the lowest (2.47± 0.02 %) was in Lutjanus goreensis. The result presented in Table 2 shows that the crude protein values in fresh samples Ethmalosa fimbriata and Lutjanus goreensis were within the same range. While that of Pseudotolithus elongatus and Mugil cephalus were within the same range. Mugil cephalus had the highest protein content of 22.18±0.04. For the smoked samples, the crude protein values in Ethmalosa fimbriata and Pseudotolithus elongatus were within the same range. While that of Lutjanus goreensis and Mugil cephalus were within the same range. The least percentage crude protein of 53.00±0.05 was recorded for Lutjanus goreensis, while the highest was recorded for Ethmalosa fimbriata (74.56±0.03). In Table 3, higher values of crude fat were recorded in fresh Ethmalosa fimbriata and Pseudotolithus elongatus when compared to the other species. For the smoked fish, highest value of crude fat was recorded in Lutjanus goreensis (35.13±0.05) when compared to other species.  In Table 4, the values of ash in fresh Ethmalosa fimbriata and Mugil cephalus were within the same range. While that of Pseudotolithus elongatus and Lutjanus goreensis were within   the   same   range. For the smoked fish, highest value of ash (17.58±0.02%) was observed in Mugil  cephalus and the lowest (7.18±0.02%) in Pseudotolithus  elongatus.

Table 1: Percentage of moisture content in fresh and smoked fish samples

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

Table 2: Percentage of crude protein content in fresh and smoked fish samples

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

Table 3: Percentage of  crude fat content in fresh and smoked fish samples

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

Table 4: Percentage of crude ash content in  fresh and smoked fish samples

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

Moisture is the principal constituent of fish, quantity-wise. Moisture content in many fish varies between 60-80%. Moisture content is also found to vary considerably within the same species of fish depending on the age, fat content, feeding condition, spawning etc. Fatty fish exhibit an inverse relationship between fat and moisture contents [24]. In the present investigation, moisture contents were very high in the fresh species. The percentage of moisture in fish muscles was within the acceptable level in all the samples without any significant difference due to the stable water levels in the environmental location from where the fish were collected. The moisture content did not differ among all the marine fishes and the values were close to each other, slight differences present may be attributed to the difference in fish species. Similar result was found in some marine fishes as reported by Ashraf et al. [25] in some marine fishes evaluated for their proximate composition in coastal areas of India. 

The moisture content of the smoke-dried fish samples decreased considerably after the smoking process. This decrease was caused by loss of water during smoking which led to the reduction of moisture. A similar result was reported for smoked Atlantic salmon, hot smoked catfish and the smoked Nile tilapia [26-28].  The moisture levels in all the smoke-dried fishes examined were below 20% which is good or acceptable for smoke-dried fishes as suggested by Clucas and Ward [29]. A similar result was also found by Akinneye et al. [9] in some smoke-dried marine fishes from coastal areas of Ondo State, Nigeria. Moisture content differences could be due to certain factors such as genetic makeup, feed intake, metabolic efficiency, size, sex, and season of the year. 

Generally, the proteins are important for normal function, growth, and maintenance of body tissue and hence protein content is considered to be an essential tool for the evaluation of biochemical and physiological standards of a given organism [30].  From the nutritional point of view, protein is the most important constituent of fish. Protein content in most fish averages 18-20%, though the general variation is in the range of 15-24%. Variations in protein content occur in relation to age, fat content, spawning, starvation etc [31]. In this study protein content was slightly higher in the muscle of fresh Mugil cephalus than in the other three species. Although, slight variations were observed for the protein levels and statistically no significant difference (p>0.05), indicating that protein levels were within the same range in the species. The results of the range of protein content were within the range of variations reported by Fawole et al. [32] in some marine species from Lagos Lagoon. The fish species examined are highly proteinous. The high tissue protein content of the fish species in this study may be related to the high protein contents of their common diets as they fed mostly on fish items, crustaceans, molluscs, algae and diatoms [33]. The relatively average percentage of protein obtained in this work, may be attributed to the fact that marine fishes are good sources of protein, but the differences observed in values obtained, could also be as a result of fish consumption or absorption capability and conversion potentials of essential nutrients from their diets or effect of their local environment into such biochemical attributes needed by organisms’ body [34]. 

Fish protein is of high quality and contains sufficient amounts of all the essential amino acids required by the body for growth, maintenance of lean muscle tissue and active metabolism [35]. Moreover, Dawson and Grimm [36] reported that smoke drying methods increased the protein in Plaice pleuronetes which is in line with the present research work. Moreover, Oyeleye [36] observed that the proximate composition of fish varies with species, body size, season, environmental factors and nutritional status. The increase of the protein content observed in this work in all the species may be due to product dehydration which concentrated the protein during the heat treatment of the fish, thus increasing the nutritive value of the fish, similar findings were also reported by Idah and Nwankwo [37] who compared crude protein levels of Oreochromis niloticus dried with local cut drum oven and NSRRI developed smoking. Debnath [38]  also reported that increase in protein may be attributed to the fact that fish are a good source of crude protein and the difference in crude protein could be due to the fish consumption, absorption capacity and conversion potentials of essential nutrients from their diet or local environment into such biochemical attributes needed by the organisms.

After smoke-drying, there was an increase in fat content. This could be the result of evaporation of moisture contents which is in agreement with the previous works of Diakoku et al. [39], in Poecilia reticulate. The fat content increase could be as a result of heat produced by smoking kiln which results in moisture content losses, increasing the concentration of nutrient in the reaming mass of fat as related to lipid oxidation, which produced volatile compounds known to be unsaturated and very prone to oxidation. These findings are in line with Dirar, [40]  who reported that during fish smoking, fish loses its moisture content, which results in an increase in the concentration of nutrient in the remaining mass of fats. 

Ash is a measure of the mineral content of a food item. It is the inorganic residue that remains after the organic matter has been burnt off [41]. Ash is constituted by the minerals present in the flesh. The ash content gives a measure of the total mineral content in the tissue [42]. The variability of ash in the body tissue composition of the fish has been attributed to several factors such as environment, age, size, diet and species [43]. In the present study, the percentage of ash content in the fishes analyzed were very low, this is comparable to the study of Fagbenro  et al.,[44] who reported the same trend in some commercial West African food fishes. However, the ash levels were found less in all the selected marine fishes in this study, when compared to the results of other workers including Hardy and Keay [45] in marine fish from Lake Geriyo. This is also in line with the observation of Mesomya et al. [46] in some marine fishes from India. The lesser amount of ash contents found in these fishes may be due to less amount of skeleton. Yana [47], reported that the inorganic content remains as ash after the organic matter is removed by incineration. Salan et al. [48] observed increase of ash content in smoked Atlantic salmon  and the authors further noted that the increase in the ash content in the smoked fish was due to the loss of humidity and as a result of the loss in moisture during hot smoking which was in agreement with the present study. 

In conclusion, the outcome of this study indicates that moisture contents of fish are of great importance, as most of the biochemical reactions and physiological changes in fish depend on moisture content. Protein content in smoked fish samples was observed to increase, as a result of an increase in the dry matter content per unit weight following sample dehydration during smoking. Therefore, the increase in protein content, ash content and ash content can be due to an increase in the dry matter content per unit weight following sample dehydration. The smoking of the fish samples is to ensure that the fish come out with low moisture content, higher nutrient values and longer shelf life. 

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