Background

In human nutrition, fish and fish products play important role as good source of biologically valuable proteins, fats and fat-soluble vitamins (Belitz et al., 2009; Thaker et al., 2017). There has been dramatic increase in the fishing industry catch in tonnage during this century. In 1900, the catch was approximately 4 million tonnes, meanwhile it had risen to 129 million by 2001 (IOM, 2007). Fish consumption is recommended as an important part of the human diet, due to the high nutritional value of ω-polyunsaturated fatty acids present in fish (IOM, 2007).

In Nigeria, water bodies contain different types of fish species that serve as food and an economic resource for the country (Osibona et al., 2006). Some of the common species include croakers, catfishes, tilapias, threadfins, soles, and the clupeids accounting for about 90% of Nigeria’s fishery (FDF, 2004). About 69.6% of the total fish supply available to Nigeria is from fresh water fish (FOS, 1990).

In Africa, the fisheries sector provides means of livelihood to about 5 percent of the population, about 35 million people, mostly artisanal fisheries (FAO, 2001). In Nigeria fishing is done on a continuous basis in Nigeria, but a remarkable and significant harvest occurs around the seasonal catch from July to September each year. Hence, in order to ensure that fish is available throughout the year, especially during the lean season, it is imperative to process the fish in large quantities and preserve in good condition until its use is required (FAO, 2001).

One of the constraints to such an attainment aside seasonality of fish food of fish that makes it extremely perishable is the very high moisture content. Deterioration set in shortly after fish dies if no preservative or processing measures is applied to the fish, a number of microbial and physiological deterioration set in and thereby degrade the fish (Davies and Davies, 2009). According to the report of FAO (2001) in the tropics where the ambient temperature is high, fish spoils within 12-20 hours of being caught, depending on species and size therefore, a reasonable percentage of the landed catch is processed to extend the shelf life of most of their catch by artisanal methods (FAO, 2001).

Traditional methods of fish preservation such as drying, salting and smoking have been practiced perhaps longer than any other food preservation technique. Modern developments have centered on understanding and controlling these processes to achieve the standard of product demanded by today’s market (Horner, 1992). The various processing methods have different imparts on the nutritional compositions of fish. This is because during heating, freezing and exposure to high concentration of salt certain chemical and physical changes.  Therefore the quality of fish processed by the various methods cannot be the same and hence its subsequent effect on the fish’s shelf life also varies. Heavy metals according to Malik (2004) are regarded as the most important form of pollution of the aquatic environment because of their toxicity and accumulation by marine organisms. Several studies have been carried out on heavy metal contents in fish products (Bae and Lim, 2012), studies on the effect of storage on heavy metal contamination is scarce. The objective of this study was therefore to assess the effects of processing on the proximate composition of the fishes, and provide information on the effect of storage on heavy metal loads and the rate of deterioration.

1 Materials and Methods

1.1 Sample collection and preparation

Three species of fish (Sarotherodon galilaeus, Tilapia zillii and Clarias gariepinus) were separately harvested from Opa and Asejire reservoirs both situated in Osun state, Nigeria. The fishes were washed, descaled, trimmed, eviscerated, and rinsed with distilled water. Samples were handled and treated separately to avoid cross contamination. Each sample was divided into two parts and the first portion was salted (10:1 w/w; fish: salt) by rubbing the dry salt on the fishes. The second portion was left unsalted. Both the salted and unsalted samples were further subdivided into two portions. The first portion was dried (85°C, 14-18 h) while the second portion was smoked (80°C, 14-18 h) to obtain salted dried, salted smoked, unsalted dried and unsalted smoked fish samples respectively. An Afos type mini oven was used for both processes with the inclusion of sawdust for smoke production during the smoking process.

1.2 Storage stability study

Salted smoked, salted dried, unsalted smoked and unsalted dried mango tilapia (Sarotherodon galilaeus), red belly tilapia (Tilapia zillii) and catfish (Clarias gariepinus) from both locations were package in transparent polythene bags and stored on the shelf for a period of six weeks. Heavy metal concentrations during the first and sixth week were recorded.

1.3 Proximate composition determination

The proximate parameters (moisture content, protein, fat, ash) were determined using the methods of AOAC (2000). Carbohydrate content was calculated by difference while energy content was calculated using at water factors (17 KJ/g/ 4.0 kcal/g for protein and carbohydrate, 37 KJ/g/ 9.0 kcal/g for fat).

1.4 Heavy metal determination

The previously cleaned and fresh fish samples were, macerated and then homogenized thoroughly with a blender. Smoked and dried samples were pulverised/rushed separately with a pestle and mortar. Homogenized wet samples/crushed smoked and dried samples (0.5 g) were weighed separately into digestion tubes and 5 mL HNO₃ was added, and thereafter the mixture was allowed to digest at 90°C for 3 hours. Blanks (without the fish samples) were also prepared exactly in similar manner and subjected to similar conditions. Digested samples were allowed to cooled and thereafter diluted to 40 ml volume with distilled water and stored in plastic containers for analysis with the Flame Atomic absorption spectrophotometer (AAS Model 990 PG) (Hajeb and Jinab, 2012). Heavy metals determined were lead, cadmium, nickel and arsenic.

1.5 Effect of storage on moisture content and total volatile base nitrogen (TVB-N)

Salted smoked, salted dried, unsalted smoked and unsalted dried mango tilapia (Sarotherodon galilaeus) and catfish (Clarias gariepinus) from both locations were package in transparent polythene bags and stored on the shelf for a period of six weeks during which various analyses were carried out weekly. The Moisture content and pH were determined by the standard methods of AOAC (2000), while the Total Volatile Basic Nitrigen (TVB-N) was determined using the methods of Atonacopulus (1968) and (Senturk and Alpas, 2013).

2 Statistical Analysis

All determinations were done in triplicate and subjected to statistical analysis of variance (ANOVA) using SPSS version 18 statistical package (SPSS, Inc., USA) to determine variation between means. Duncan Multiple Range Test (DMRT) was used to separate means. Significant variation was accepted at p<0.05.

3 Results and Discussions

Table 1, Table 2 and Table 3 present the proximate composition of mango tilapia (Sarotherodon galilaeus), red belly tilapia (Tilapia zillii) and catfish (Clarias gariepinus) respectively. In most fish species the proximate composition is majorly water, proteins, and lipids. These constituents account for 98% of the total mass, and the other minor constituents include, carbohydrate, vitamins, and minerals (FAO/WHO, 2011).

In general, the moisture contents of all the fish species used in this study ranged from 77.16 to 82.46%, 11.85 to 17.67% and 10.20 to 12.97% for fresh, smoked and dried fish samples respectively. The protein content ranged from 14.02 to 17.32%, 53.24 to 61.95% and 62.61 to 73.46% for fresh, smoked and dried samples respectively. The fat content ranged from 0.77 to 6.48%, 16.71 to 21.69% and 9.36 to 17.99% for fresh, smoked, and dried fish samples respectively. Ash content ranged from 0.57 to 1.67%, 4.63 to 7.18% and 4.50 to 5.79% for fresh, smoked and dried fish respectively.

All the fresh samples contained high moisture, low fat and ash. However, there were significant difference between the species, and in some cases, significant difference between same species from different locations were recorded. The geographical locations, season, diet, stage of maturity, and sizes of fish determine the chemical composition of fish most of the time (Guner et al., 1998; Tanakol et al., 1999).

Edible fish tissue is reported to contain about 60-84% water, 15-24% protein and 0.1-22% lipids. The proportion of the constituents are species-specific and the main variations in proximate composition between species occur in moisture and lipids content (FAO/WHO, 2011; Boran et al., 2011; Kocatepe et al., 2012).

Fresh fish tissue has high moisture content. The high moisture content recorded in the fresh samples is a disadvantage as it predisposes the fish to microbial spoilage, oxidative degradation of polyunsaturated fatty acids and consequently decreases in the quality of the fishes. Hence, fresh fish is highly perishable. The moisture content of all the processed samples ranged from 10.20 to 17.67%. Reducing the moisture content of fresh fish by drying to 25% water content will stop bacterial growth and reduce autolytic activity (Oparaku, 2010). The low moisture content recorded after processing indicates that the processed fish have the tendency to be stable compared to fresh fish. Quantitatively, protein ranked second among the components in muscle tissue of fish. The protein content vary less widely from one species to another (FAO/WHO, 2011). Protein content of fish is considered low if it is below 15% (Stancheva et al., 2013). There was a strong inverse relationship between the moisture and the crude protein contents (Stancheva et al., 2013). The protein content after drying and smoking was higher than 50% for all samples. This indicates that processed fish are rich source of protein.

On the basis of their fat contents, fish species has be classified into four majorcategories: high fat (more than 8%), medium fat (4-8%), low fat (2-4%), and lean (less than 2%) (Achman, 1989; Doğan and Ertan, 2017; Martins et al., 2017). Based on the result obtained in this study, red belly can be classified as lean fat fish, while catfish can be classified as low to medium fat fish. Fat content of mango tilapia was more variable. There exists an inverse relationship between the water and lipid content of fish and the summation of both frequently spans a range of 78 to 88% (Stancheva et al., 2013).

This study confirms that the ash and carbohydrate contents were low in all the fish species from both locations. Fishes have variable composition of proteins and fat, and the energy content is dependent on this distribution. The lipid level in fish contribute significantly to the its calorie content.

In general, drying and smoking had significantly influence of on the proximate compositions of all the fish types used for this study. The relative increase in protein, lipids and ash content due to loss/reduction of moisture were the most prominent changes after the drying and smoking process. This was expected because of increased dried weight as a result of loss of moisture.

Table 4, Table 5 and Table 6 present the changes in the heavy metals concentration after a storage period of six weeks. From the tables, it can be observed that all the heavy metals investigated in this study were present in all the stored fishes from both locations. The lead, cadmium, nickel and arsenic concentrations in most of the samples showed significant variations after storage. Significant reductions (p<0.05) were observed in most cases, while in some of the samples, there was no effect at all. The arsenic concentration of the samples was more variable, as increases and decreases were observed after storage.

The packaging materials used were transparent low density polythene bags. Observed reductions may be due to the ability of the packaging materials in protecting the processed fishes from environmental pollution as heavy metals are ubiquitous and are usually found in the environment, also the packaging material used probably did not release any of the metals into the fish during storage.

The European Community (No 1881/2006) sets maximum permitted level for lead at 0.4 mg/kg. Cadmium is non-essential and highly toxic. The European Community (No 1881/2006) has set the maximum level permitted for fish food as 0.05 mg/kg. In most food products, the nickel content is less than 0.5 mg/kg (IARC, 1990). The FAO/WHO (2004) recommended the maximum levels permitted for arsenic in sea food as 5 mg/kg. All the values reported in the study are lower than their maximum limits, hence the fishes were safe both before storage and after storage.

3.1 Changes in the moisture content during storage

Figure 1, Figure 2, Figure 3 and Figure 4 present the changes in the moisture contents during storage. The smoking, drying and salting processes resulted in significantly reductions in all the samples. Moisture content reduction during processing is advantageous; it reduces the fish susceptibility to microbial spoilage, oxidative decomposition of polyunsaturated fatty acid, and consequently improves the quality of the fish and also extend the shelf life (Frankel, 1991; Bassey et al., 2014). Moisture content increased during the storage of the samples. Throughout the storage period, all samples had moisture contents that could be regarded as shelf stable (<25%). However, increase in the moisture content is unfavourable, as it increases the susceptibility of the processed fish to microbial spoilage, thereby decreasing their shelf life. Increase in moisture content is probably due to absorption of moisture from the surrounding environment.

3.2 Changes in TVB-N during storage

Figure 5, Figure 6, Figure 7 and Figure 8 present the changes in TVB-N during storage. TVB-N is one of the most widely used parameter to evaluate fish quality (Pankyamma et al., 2017). TVB-N is present in very small quantity in fresh fish and produced mainly by bacterial. TVB-N captures all the volatile Nitrogen containing compounds during decomposition. After smoking, significant increases were observed in some of the samples. TVB-N values increased significantly (p<0.05) in all the samples after the third week of storage. Towards the end of storage, salted smoked samples generally had lower values than unsalted samples. This may be due to the preservative effect of salt. Also, catfish generally had higher TVB-N, this may be as a result of the higher moisture contents. There was steady increase in TVB-N in all the samples, this is a sign of deterioration. Drying of the fish samples resulted in the reduction of TVB-N in most of the samples.

FAO (2001) has recommended and classified samples with TVB-N value less than 25 mg-N/10 g has ‘perfect quality’, samples with 30 mg-N/100 g has ‘good quality’, samples with up to 35 mg-N/100 g has ‘marketable quality’, and samples with TVB-N of more than 35 mg-N/100g has ‘spoiled’ (Çelik et al., 2012). It is however difficult to fix the limit of TVB-N for cured and processed products due to variety and diversity of products and their processing procedure. The limiting level for rejection of TVB-N is 30-40 mg-N/100g for storage at ambient temperature (Connell, 1995). In freshly caught fish TVB-N content is generally less than 10 mg-N/100g and does not exceed 15 mg-N/100g except for pelagic fish (Pérez et al., 2008; Nollet and Toldra, 2009). Values obtained in this study falls within this range. Based on results obtained in this study, all the samples were of perfect quality during the first week. Thereafter, their quality deteriorated in varying proportions during the storage period. During the sixth week of storage, most unsalted smoked fishes had values exceeding 30 mg-N/100g, hence they were close to being rejected.

4 Conclusion

Smoking and drying of the fishes resulted in changes in the proximate composition of the fishes. Protein, fat, ash and carbohydrate were all observed to increase after processing in all the fish types. Moisture content reduced after processing, however, it increased during storage. There was increase in the total volatile base nitrogen during the storage period which was a sign of deterioration. Storage of the processed fishes at room temperature reduced the heavy metals concentration as concentrations were considerably reduced.

Authors’ contributions

The third author conceived and designed the study. The first author carried out the experiment under the supervision of second and third authors. The second and third authors analysed the laboratory data. The second author produced the first and final draft, presented the draft for publication. All authors read and approved the final manuscript.

Acknowledgements

The authors acknowledge the contributions of the Department of Food Science and Technology, Obafemi Awolowo University, Ile-Ife for the provision of the laboratory facilities for the study.

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