INTRODUCTION
Malaysia is one of the highest fish producing country in Asia. This is because
it has a vast water area. Fish is one of the main sources of protein to Malaysian
(MFD, 2008; Kumolu-Johnson and Ndimele,
2011). Fish contain high quality of protein with a complete range of the
essential amino acids and fat with polyunsaturated fatty acids that provide
a lot of health benefits including keeping our heart and brain healthy (Agusa
et al., 2007). In 2002, the per capita consumption of fish in Malaysia
was 53 kg, while in 2010 was 56 kg. In 2010, the total consumption of fish in
Malaysia was more than 1.58 billion kg.
Among various types of fishes, one of the popular fish in Malaysia is Indian
mackerel (Rastrelliger kanagurta). This is because it has a good taste
and it is available in all areas in Malaysia. Malaysia’s
Department of Statistics has reported the total consumption of Indian mackerel
has been increasing simultaneously since 2005 until today. Apart from Indian
mackerels, now-a-days, many Malaysians consume a shellfish, cockles (Anadara
granosa) as a source protein. Now-a-days, a few Malaysian also consume freshwater
eel (Monopterus albus) because of taste and soft texture. Generally,
Malaysians do not consider nutritive value during selection of fish for consumption.
The selection of fish is normally based on the availability, cost and taste.
In this study, an attempt was made to provide information on important nutrients
composition in Indian mackerel, freshwater eel and cockles. This information
would be very useful for consumer to conceptually increase their knowledge regarding
the nutrients contents in Indian mackerel, freshwater eel and cockles.
Frying and boiling (curry) are two popular methods generally used to cook fish
in Malaysia. However, frying is more popular than boiling to cook fish in Malaysia
because it is one of the fastest and simplest methods of fish cooking. Since
frying involves with a very high temperature (usually 170-180°C) it degrades
nutrients through hydrolysis and oxidation of the fatty acids (Rossel,
2001). The breakdown products can give rise to good flavor and taste. However,
cooking methods are very important parameter for nutritive value of fish. The
food nutritive value can be affected by deep-frying (Gokoglu
et al., 2004). Similarly, nutritive value of fish can be changed
by deep-frying. There are some published studies (Ersoy
and Ozeren, 2009; Erkan et al., 2010) which
discuss the effect of different cooking methods on nutritive value of different
fishes. However, information about the effects of deep-frying on the changes
of nutritive value of Indian mackerel, freshwater eel and cockles is still lacking.
Such information can improve our understanding on the preparation of healthy
diet with cockles, Indian mackerels and eels. This study helps to understand
the effect of deep-frying on proximate composition of cockles, Indian mackerel
and eel. The main objectives of this study were to determine proximate composition
and selected the mineral content of fresh cockle, Indian mackerel and eel and
to compare the proximate composition and selected mineral content between fresh
and fried cockle, Indian mackerel and eel.
MATERIALS AND METHODS
Sample collection and preparation: A total of 6 Indian mackerels (weight
50-100 g), 6 eels (100-160 g) and of 6 cockles (10-20 g) were purchased from
local fish market in Kuantan city and transported immediately to the laboratory
with an ice-box. On arrival in the laboratory, all samples were smoothly washed
by tap water to remove blood and slime. The Indian mackerels and eels were eviscerated,
de-headed, filleted. The cockle flesh was removed from shell. The bones of the
Indian mackerels and eels were separated from the flesh. All the samples were
then cut into pieces and washed with tap water several times to remove blood.
All pieces of samples of each species were mixed and divided into two groups
based on random selection. Each group were again divided into 3 sub-groups which
were considered as replications in this study. The first group was kept uncooked.
The second group was fried in a beaker of 400 mL palm cooking oil capacity at
a temperature approximately of 180°C for a 15 min period. All fresh and
fried samples were hand de-boned (if necessary) and ground in a mortar to ensure
homogeneity and representative samples taken for analysis. Samples were put
in a sterile container and kept under frozen conditions (-20°C) until analysis.
Proximate composition analysis: Moisture content of the samples were
determined according to AOAC (1993) with slight modifications
by Tee et al. (1996). One gram of the sample was
weight out in a sterile, flat, aluminum dish and dry up to constant weight at
100°C in an oven. The percentage of moisture content was calculated according
to equation below:
Ash content was also determined according to AOAC (1993)
with slight modifications by Tee et al. (1996).
One gram of the sample was weighed and put into a dry, tarred porcelain dish
and then placed in a muffle furnace at 500°C for 22 h. Then, the samples
were cooled in desiccators and weighed. The percentage of ash content was calculated
using the following equation:
Crude protein contents were determined according to Kjeldahl method described
by AOAC (1993). For this, 1 g of sample was weighed into
digestion tubes. Two Kjeltabs Cu 3.5 (catalyst salts) were added into each tube.
About 15 mL of concentrated acid sulphuric (H2SO4) was
carefully added into the tube and then shaken gently. Digestion procedure was
performed using pre-heated (420°C) digestion block of Kjeltec 2200 (Foss
Analytical, Hoganas, Sweden) for 60 min until clear blue/green solution was
obtained. Digested samples were cooled for 10-20 min. Distillation procedure
was then performed using distillation unit of Kjeltec 2200. Distillate was titrated
with 0.1 N hydrochloric acid (HCl) until blue end point achieved. Volume of
acid required in the titration was recorded. Blank was prepared with the exclusion
of sample. The percentage of crude protein content was calculated according
to the following equation:
Protein (%) = Nitrogen (%)x 6.25 |
where, 6.25 is the conversion factor for nitrogen to protein:
where, T is the titration volume for sample (mL), B is the titration volume
for blank (mL), N is the normality of acid to 4 decimal places.
Crude lipid assay was carried out by a Soxtec extraction procedure with a Soxtec
2050 automated extraction system (Foss). Approximately 1 g of sample was weighed
into tarred cellulose thimbles. A defatted cotton plug was placed on top of
each sample to keep the material immersed during the boiling step and to prevent
any sample loss from the top of the thimble. Samples were extracted with petroleum
ether (boiling range 40-60°C) solvent. The extraction thimble was set into
the weighed aluminum cup (Foss) and approximately 70 mL of petroleum ether (40-60)
was added to each cup. Crude fat was extracted by immersing the sample in the
boiling solvent under reflux for 30 min. The sample thimble was raised and rinsed
with condensed solvent for an additional 45 min. The reflux rate was adjusted
to approximately 3-5 drops during the extraction and rinse steps. Then, the
extraction Petroleum ether (40-60) was removed by a final evaporation step (10
min). The sample cups were lifted about 1 cm during the evaporation cycle to
avoid excessive sample heating and then pre drying 1-2 min. After completion
of the extraction process, sample cups were dried at 105°C for at least
30 min and transferred to a desiccator and cooled to ambient temperature. Weight
of the crude fat extracted was determined dry-matter basis (DMB) using following
equation:
Carbohydrate was calculated using indirect method. The following standard equation
was used for carbohydrate content estimation:
Carbohydrate content (%) = 100%-
(% moisture+% ash+% protein+% lipid) |
Cd, Mn, Cu and Zn content analysis: All samples were digested before
analysing Cd, Mn, Cu and Zn content using Atomic Absorption Spectrometry (AAS).
For digestion, a representative sample of up to 0.3 g was extracted and dissolved
in 6 mL concentrated nitric acid and 1 mL of hydrogen peroxide for 45 min using
microwave heating unit named multiwave 3000. The sample and acids were placed
in a quartz microwave vessel or vessel liner. The vessel was sealed and heated
in the microwave unit. After cooling, the vessel contents were filtered, centrifuged
and allowed to settle and then diluted to 15 mL in falcon tubes. The tubes were
sealed and kept under room temperature prior using Atomic Absorption Spectrometry
(AAS).
All the digested samples were then analysed three times for several minerals
like Cd, Mn, Cu and Zn using SIMAA 6100 Perkin Elmer Atomic Absorption Spectrometry
(AAS). This machine detected the presence of the selected minerals using graphite
furnaces atomic absorption spectrometry (GFAAS).
Statistical analysis: All data were analyzed using SPSS version 16.0.
All data were checked for normality before analysis. Only the percent data had
to be arcsine-transformed before analysis. Nutrients contents of Indian mackerel,
eel and cockle were compared through one way ANOVA. If an ANOVA was significant,
differences between the means were analyzed by Tukey test for unplanned multiple
comparisons of means (p<0.05). The t-test was used to compare the nutrients
content between raw and fried samples.
RESULTS
Nutrients content of fresh Indian mackerel, eel and cockle: Indian Mackerel,
cockle and eel were significantly different (p<0.05) in term of moisture,
protein, lipid, carbohydrate, ash, Cd, Mn, Cu and Zn contents (Table
1). Moisture content was highest in cockle, followed by eel and Indian mackerel.
Protein content of Indian mackerel and eel were significantly higher (p<0.05)
than the protein content in cockle. There was no significant difference (p>0.05)
between the protein content of Indian mackerel and eel. Lipid content was highest
in Indian mackerel, followed by cockle and eel. Carbohydrate content was higher
in the Indian Mackerel than in the eel. Indian mackerel had higher ash content
than eel and cockle (p<0.05). There was no significant difference between
ash content of eel and cockle (p>0.05).
Cockle had higher Cd, Mn and Zn contents than Indian mackerel and eel (p<0.05).
However, Indian mackerel and eel are statistically similar on Cd, Mn and Zn
contents. Cu content was comparatively highest in Indian mackerel, followed
by cockle and eel.
Nutrients contents of fried cockle, Indian mackerel and eel: Average
nutrients content of fried Indian mackerel, cockle and eel were significantly
different (p<0.05) (Table 2). Highest moisture was observed
in fried Indian mackerel, followed by fried eel and fried cockle. However, opposite
results of moisture content were observed in case of lipid and Mn contents.
Protein content was higher in fried eel than fried mackerel, while lowest quantity
of protein was observed in fried cockle.
| Table 1: |
Macronutrient and micronutrient contents in raw Indian mackerel,
eel and cockle |
 |
| Values (Mean±95% CI) in the same row with no superscript
in common differ significantly at p<0.05, if the effects are significant,
ANOVA was followed by Tukey test, **p<0.01, ***p<0.001 |
| Table 2: |
Macronutrients and micronutrients of fried Indian mackerel,
Eel and Cockle |
 |
| Values (Mean±95% CI) in the same row with no superscript
in common differ significantly at p<0.05, if the effects are significant,
ANOVA was followed by Tukey test, **p<0.01, ***p<0.001 |
|
| Fig. 1(a-b): |
Effects of frying on (a) Macro-nutrients and (b) Micronutrients
contents in Indian mackerel based on t-test, *Significant difference between
treatment (raw and fried), ns: No significant difference |
Fried cockles had highest ash content, followed by Indian mackerel and eel.
Mn of fried cockle was higher than the Mn content of Indian mackerel and eel
(p<0.05). In case of Mn content, fried Indian mackerel and fried eel are
statistically same (p>0.05). Cu content of fried mackerel was higher than
the Cu content of fried eel and cockle (p<0.05). Eel had highest Zn content,
followed by Indian mackerel and cockle.
Effects of deep-frying on nutrients content of cockle, eel and Indian mackerel:
All macronutrients content were statistically different between raw and fried
Indian mackerel except carbohydrate content (Fig. 1a). Frying
reduced moisture content of Indian mackerel, whereas an opposite result was
observed in case of ash, protein and lipid contents of Indian mackerel. Eel
and cockle showed similar trend of Indian mackerel when compare with macronutrients
content between fresh and fried samples (Fig. 2a, 3a).
Frying reduced Cu and Zn content of Indian mackerel, whereas an opposite result
was observed in case of Cd content in Indian mackerel (Fig. 1b).
Eel and cockle showed similar trend of Indian mackerel when comparing Cd, Cu
and Zn contents between fresh and fried samples (Fig. 2b,
3b). Frying reduced Mn content in cockles whereas there is
no significant effect of frying on Mn content of Indian mackerel and eel.
DISCUSSION
In this study, some selected nutrients contents of fresh and deep-fried Indian
mackerel, eel and cockle were studied. Among many nutrients, protein content
is one of most important criteria to evaluate food quality. It is an important
constituent of foods for a number of different reasons such as growth, replacement
of metabolic losses and damaged tissue as well as general well-being. Proteins
are the major source of energy, as well as containing essential amino acids
which are needed to human health. In the present study, when compared among
raw fish (Indian mackerel and eel) and shellfish (cockle); fish has higher protein
content than shellfish.
|
| Fig. 2(a-b): |
Effects of frying on (a) Macro-nutrients and (b) Micronutrients
contents in eel based on t-test, *Significant difference between treatment
(raw and fried), ns: No significant difference |
|
| Fig. 3(a-b): |
Effects of frying on (a) Macro-nutrients and (b) Micronutrients
contents in cockle based on t-test, *Significant difference between treatment
(raw and fried) ns: No significant difference |
However, overall heavy metals content were higher in shellfish than finfish.
Almost similar results were observed by Nurnadia et al.
(2011). In case of moisture and lipid content, no definite trend was observed
when compare uncooked finfish and shellfish. However, an inverse relationship
between the lipid and moisture content was observed in uncooked fish and shellfish.
The fat content of Indian Mackerel was slightly higher than the value obtained
by Nurnadia et al. (2011). However, the differences
in proximate composition in fresh fish could be due to many factors, such as
age, size, habitat, species, etc.
In this study, a comparison between raw and fried Indian mackerel, eel and
cockle was made and found significant difference between them. Fried samples
had higher fat, protein and ash content compare to raw samples. There is no
previous study which compared the effects of frying on nutrient content of eel
and cockles. However, Arias et al. (2003) and
Erkan et al. (2010) observed similar effects
of frying on nutrients content in fish samples. The increase in fat content
was most obvious in fried fillets mainly due to the absorption of oil and leaching
out of the water by the fish during deep-frying. According to Saguy
and Dana (2003), cooking oil penetrates into the fillets during frying.
This ultimately increases fat content in fried fish. The results of this study
is similar to the results observed by Rosa et al.
(2007), Gokoglu et al. (2004) and Gall
et al. (1983) who reported significantly higher lipid content in
fried fish than raw fish. The plausible reason of higher protein and ash content
in fried samples might be the decreased in moisture content which subsequently
increased all other nutrients. Steiner-Asiedua et al.
(1991), Unlusayin et al. (2001) and Erkan
et al. (2010) reported similar findings.
In micronutrient, there were significant different between raw and fried Indian
mackerel, eel and cockle. Similar finding was observed by Bandarra
et al. (2009). In this study, micronutrients were mostly heavy metal.
The result showed that shellfish had overall higher heavy metal than fish. However,
macronutrients in cockle are slightly lower compared to eel and Indian mackerel.
The similar result was also observed in fried sample. These indicate that freshwater
fish and marine fish are better than shellfish in the nutritional point of view.
CONCLUSION
Protein content was higher in fish than cockle (shellfish) while overall heavy
metal contents were higher in cockle than Indian mackerel and eel. Therefore,
fish is better than shellfish in the nutritional point of view. However, more
research is needed to analyze a complete nutritional profile of these fish and
shellfish. Fried fish and shellfish had very high fat content due to absorption
of fat during frying, although they had higher protein, fat and ash. Therefore,
frying cannot be recommended to prepare a healthy diet. More research is needed
including all cooking methods of fish to know the nutritional changes by each
cooking method. This would be very helpful to select a healthy diet. Age, size
and location are very important factor for nutritional value of fish. Therefore,
these factors should be considered in the future study. Fish contains many important
fatty acids and amino acids which might be lost during frying. This was not
considered in this study. Therefore, future study should also include the analysis
of the composition of amino acids and fatty acids.
