Fish is one of the most important sources of animal protein available in the tropics and has been widely accepted as a good source of protein and other elements for the maintenance of healthy body (Karrar, 2007).
Fish and fishery products are highly nutritious, in addition to the high percentages of animal protein, they provide several other nutrients such as vitamins A and B especially in the liver, and E and K vitamins, and they are good sources of some minerals like calcium, phosphorus and iron (Lunven, 1982).
The global contribution of fish as a source of protein is high, ranging from 10 % to 15% of the human food across the world (Wilson, et al, 2007). Preservation of fish in developing countries is generally done by traditional methods i.e., salting, drying and smoking.
In the Sudan, nearly 70% of the total fish landings are consumed fresh, the rest is cured either by salting, fermentation or sun drying very little of the local fish supply is smoked, except in the southern Sudan where smoked and very dry fermented fish products are very popular among the local community (FAO, 1992a).
Salting is a traditional method of fish processing in many countries of the world. It can be used in combination with drying or smoking. Salting the fish removes water and lowers the water activity (water available for the support of microbial growth which causes the spoilage). Concentration of (6 - 10%) salt in the tissues will prevent the action of most spoilage bacteria (Clucas and ward 1996). Salting is performed either by dry, brine, or injection salting or a combination of these methods. Dry salting has been the most commonly used methods by the industry. Dry salting is the traditional salt-curing technique used during processing of salted fish in many countries (Gallart et al., 2007).
Clarias lazera which is selected for this study is the most important dietary fish in Sudan; it is amongst the top twenty species of the inland water resources of the Sudan they are found in abundance all the year round. They are consumed as fresh or treated products (Karrar, 1997).
The main objective of this research work is to study the effect of salting on the nutritive value of salted Clarias lazera product and to contribute in the development of fish processing to reduce post harvest losses.
Nutritive value of fish:
Fish is one of the most potential sources of animal protein. It contains 15% of total animal protein supplies. The chemical composition of sea food comes quite close to the land animals, the principle constituents are: water 66 _ 84 %, proteins 15 _ 24 %, lipids 0.1 _22 % (Awad El Karim,, 1998). Nutritional studies have proved that fish protein rank in the same class as chicken protein and are superior to beef protein, milk and egg albumin. Fish composition shows a wide range of variation according to age, size stage of sexual maturity, diet and other factors (Damberg, 1963). Recent epidemiological, clinical and nutritional studies on animals and humans have shown that marine fish oils, rich in polyunsaturated fatty acids of the omega-3 series, are useful in reducing the risk of coronary heart disease and atherosclerosis, as well as preventing certain forms of cancer. With the growing recognition of consumers and industry of the beneficial uses of dietary fish oil, the seafood market is expanding considerably (Saito and Udagawa, 1992).
Fish post harvest losses:
Quality of fish raw material plays an important role for the quality of the end-product. Once the fish raw material freshness and nutrition value is lost, it cannot be recovered in the processing stages. Products that are processed from low quality raw material is not always a safety risk, but the quality (nutrition value) and shelf life is significantly decreased (Connell, 1995). The quality of the freshly caught fish and its usefulness for further utilization in processing is affected by the fish capture method. Unsuitable fishing method does not only cause mechanical damage to the fish, but also creates stress and the conditions which accelerate fish deterioration after death. Fish is highly susceptible to deterioration without any preservative or processing measures (Clucas and Sctcliffe, 1987).
Despite the fact that the nutritional value of fish is well known, it nevertheless plays only a limited role in the diet of many countries. Therefore, it seems appropriate to find new processing methods for this compared valuable raw material so as to increase consumer interest. Compared to mammalian meat, fish meat has more water and less connective tissue, which contains very little elastin (Kolakowska, 2001).
Preservation for a long term is generally done by freezing or canning but in developing country like Nepal is not possible due to expensive operations. The principle aim of preservation is to delay, reduce or inhibit the spoilage. In case of fatty fish, the preservation may also aim at reducing or inhibiting oxidation and other undesirable changes in the fish oils, which are highly unsaturated and capable of going rancid at various stages of processing Afolabi, et al (1984).
Salting is one of the earliest techniques for preserving fish. Salting preserves by lowering the moisture content of the fish to the point where bacterial and enzymatic activities are retarded (Wheaton and Lawson, 1985). Salting is a popular procedure for preserving fish. Salting methods are simple and involve salt crystals or brine. There are three types of salting of fish: dry salting, wet salting and a combination of the two methods. Length of salting period as well as salt concentration depends on the expected final product (Bellagha et al., 2007).
Sodium chloride diffuses to muscles from the outside due to difference in osmotic pressure between the brine and fish muscle. This process does not continue indefinitely: sodium and chlorine ions form a water binding complex with protein which itself exerts an osmotic pressure and eventually equilibrium is reached (Horner, 1997).
In salted fish, where the salt concentration reaches about 20%, high ionic strength causes contraction of the myofibrils and dehydration of proteins. Also, pH of the medium and the type of salts used for salting can influence the degree of protein denaturation (Wheaton and Lawson, 1985).
Clarias lazera is a fresh water fish found mainly in shallow water which attracts attention as a potential fish for aquaculture owing to its breathing apparatus, enabling this fish to withstand low oxygen levels and wide range of temperature, it can withstand very harsh condition. It is an omnivorous feeder; young ones feed on aquatic insects, while adults feed on any potential food like zooplankton and molluscs, but mainly on fish like Oreochromis niloticus (Amirthalingam & Khalifa, 1965).
Clarias lazera is a mud catfish belongs to the family Clariidae. This specie has maintain its aquaculture quality because of its high growth rate, large size, good flesh quality, tolerance to poor water quality even at larval stages, acceptance of cheap feeds and ability to withstand high stocking densities, disease resistance and good taste (Hongendoorn, 1981).
Materials and methods
Fresh fish specimens of Clarias lazera were obtained for this study from Omdurman fish market (Al Mowrada). Fish specimens were washed with tap water, gutted, eviscerated and washed again. Then the fish sample was divided into two parts. One was taken and analyzed as a control fresh sample. The second one was salted using 20% sodium chloride of the fish weight.
The moisture content was calculated by determining the difference in weight before and after drying one gram of the sample in a drying oven adjusted at 100 – 105 Cº, as described by AOAC (1990). Then the moisture content was calculated using the following formula:
Protein content was determined by the Micro – Kjeldahl method, and applying the factor 6.25 to the nitrogen content of the sample, as described by AOAC (1990). The protein percentage was given by the following formula:
V1 = Volume of HCl used in titration.
V2 = Volume of HCl used in blank titration.
N = Normality of HCL used in titration.
14/1000 = Conversion ratio from ammonium sulphate to nitrogen. Wt. = Weight of sample. 6.25 = Conversion factor from nitrogen to protein.
Fat content was determined by extracting 1 gm of sample with petroleum ether (boiling point 60 – 80 Cº) for six hours in Soxhelt apparatus. The extract was then dried in an oven at 100 – 105 Cº for removal of extra ether traces, following the method described by AOAC (1990). The fat content was given by the following formula:
Ash content was determined after incineration of 2 gm of sample in a Muffle furnace at 450 -550 Cº for 5 hours, as described by Pearson (1976) and AOAC (1990), the ash percentage was given by the following formula:
Minerals were determined by further analysis of ash following the method described by Koddebuch (1988).
Fresh and salted fish samples were analyzed for the determination of the total count of bacteria, and the detection and identification of total coliforms, Escherichia coli and Salmonella spp. Following the standard methods for analysis, described by (FAO, 1992b).
The data obtained throughout the course of this study was statistically analyzed using the computer package (SPSS). T-test with significant level(0.05) was used for the comparison of means.
A comparative study of fresh and salted clarias lazera (Garmout) meat composition was conducted to determine the effect of the salting on the nutritive value of fish.
Results concerning the proximate composition of fresh and salted clarias lazera are summarized in table (1). Table (2) shows the effect of salting on the minerals content of Clarias lazera .
From the results obtained, the moisture content was found to be 70.754 % and 23.138 % for control and salted fish samples respectively (figure 1). Ash content was 9.998 % and 17.853 % for control and salted fish samples respectively (figure 2). Protein content was found to be 72.345 % and 66.825 % for control and salted fish samples respectively (figure 3). Oil content was found to be 13.165 % and 15.987 % for control and salted fish samples respectively (figure 4).
From the statistical analysis of data using T-test it was noticed that there were significant differences between fresh and salted fish in concerning moisture, ash and protein contents (P0.05).
Table (1): Effect of salting on the proximate composition of Clarias lazera.
Table (2): Effect of salting on the minerals content of Clarias lazera.
Figure (1): Effect of salting on moisture content of Clarias lazera
Figure (2): Effect of salting on ash content of Clarias lazera
Figure (3): Effect of salting on protein content of Clarias lazera
Figure (4): Effect of salting on oil content of Clarias lazera
From the statistical analysis of data of minerals content it was noticed that there were significant differences between fresh and salted fish in concerning sodium and iron contents (P0.05).
The total count of bacteria of fresh and salted fish was determined, as well as the identification of some of the most dominant species of bacteria found in fish. Results are shown in details in table (3).
Table (3): Total viable bacterial count and presence of some dominant microbial species in salted Clarias lazera.
Fish is a perishable food commodity; it requires preservation for future uses. Several preservation methods are followed over the world for preserving fish. Aims of all these methods are same to extend the shelf-life of fish so that the fish can be used in future properly (Adesiyun, 1993).
Thus, certain processes such as salting and drying could be used to obtain a product which maintains almost all its nutritional characteristics with a longer shelf life. Salting process could be wet, dry or a combination of the two (Bellagha et al., 2007).
The effect of fish processing on the final product quality ultimately determines the usefulness and commercial viability of the processing method applied. Quality losses are often due to poor processing practices (e.g. laying fish directly on the ground before and after cleaning, using unclean storage and carrying containers and processing equipments and the usage of unclean water for washing of fish (Karrar, 2007).
The nutritive value and the chemical composition of fish is an important aspect in fish processing as it influences both the keeping quality and the technological characteristics of the fish. It is directly related to the moisture, protein, fat and ash contents of the muscles (Huss, 1988).
In all instances, the initial quality of raw fish material strongly influences subsequent performance in processing and storage. Fish freshness and related quality control problems were studied by Jackson (1971) and Huss (1988).
The proximate composition of the fresh and salted samples of Clarias lazera was determined. The moisture content was 70.754 % and 23.138 %; Ash content was 9.998 % and 17.853 %; Protein content was found to be 72.345 % and 66.825 % and Oil content was 13.165 % and 15.987 % for control and salted fish samples respectively. These results are coincides with the results reported for fish by Mahmoud (1977), Omer (1984), Awouda (1988) and Karrar (1997).
In concerning the statistical analysis a very clear significance difference was found in moisture, ash and protein contents (P0.05). From the statistical analysis of data of minerals content it was noticed that there were significant differences between fresh and salted fish in concerning sodium and iron contents (P0.05).
The results obtained from microbiological examination of fresh and salted fish samples, indicated that the fresh and the salted products of the Oreochromis niloticus are within the acceptable ranges of the specified microbiological limits recommended for fish and fishery products (Jay, 1992).
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