International Journal of Aquaculture, 2018, Vol.8, No.15, 112-120
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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 HNO
3
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).
