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Original article
Effect of gamma irradiation and frozen storage on chemical and
sensory characteristics of rainbow trout (Oncorhynchus mykiss)
fillet
Marjan Oraei,
1
* Abbasali Motallebi,
2
Ebrahim Hoseini
1
& Saeed Javan
3
1 Department of Food Science and Technology, Science and Research Branch, Islamic Azad University, PO Box 14155-4933, Tehran, Iran
2 Iranian Fisheries Research Organization, PO Box 14155-6116, Tehran, Iran
3 Iranian Fish Processing Research Center, PO Box 43145-1655, Anzali Port, Iran
(Received 9 April 2011; Accepted in revised form 8 December 2011)
Summary The present study was conducted to evaluate the combined effect of low-dose gamma irradiation (1, 3 and
5 kGy) and frozen storage (5 months at )20 C) on chemical and sensory characteristics of rainbow trout
(Oncorhynchus mykiss) fillet. Our statistical analysis showed that irradiation process and frozen storage time
had a significant effect (P< 0.05) on total volatile nitrogen (TVN), peroxide value (PV), thiobarbituric acid
(TBA) and pH. The level of all of these factors increased with increasing frozen storage time. At the end of
the fifth month of frozen storage, the lowest and the highest level of TVN, PV and TBA were corresponding
to the irradiated samples at 3 and 5 kGy, respectively. In terms of the overall acceptability of their texture,
odour, colour and taste, irradiated samples at 3 kGy had the best quality and remained acceptable after
5 months frozen storage. The optimum dose of gamma radiation of rainbow trout fillets according to
chemical and sensory analysis was obtained at 3 kGy.
Keywords Chemical analysis, frozen storage, gamma irradiation, rainbow trout (Oncorhynchus mykiss), sensory evaluation.
Introduction
The rainbow trout belongs to the Salmonidae, and is
one of the main fish species farmed in Iran. The market
demand for the rainbow trout nationally and interna-
tionally has increased significantly over the past decade.
This could be due to its desirable characteristics (aroma,
taste) along with nutritional value, mainly associated to
its high content of protein and essential fatty acids, n-3
fatty acids (Iranian Fisheries Organization, 2009; FAO,
2010a,b; Haghparast et al., 2010).
The major problem regarding the distribution and
storage of the seafood or fishery products is their
susceptibility to spoilage, mainly due to the contamina-
tion of spoilage and pathogenic microorganisms (O
¨zden
& Erkan, 2010). Fish spoilage occurs by growth and
activity of special microorganisms and associated dete-
rioration changes which causes off-odour and off-taste
by metabolite production changing sensory characteris-
tics and customer acceptability (Chen et al., 2007; Moini
et al., 2009; Rostamzad et al., 2010). Food irradiation is
a new and effective preservation method and a physical
treatment, in which, the products are exposed to a
radiant energy such as gamma rays, X-rays and electron
beams for a specific amount of time approved by the
Food and Drug Administration (FDA) (Yılmaz &
Gec¸ gel, 2007; Arvanitoyannis et al., 2009b). The radia-
tion process, when performed at usual absorbed doses
(<10 kGy), involves less chemical modifications than
other treatments such as heating (Arvanitoyannis et al.,
2009a). In Iran, according to the survey of International
Atomic Energy Agency (IAEA) and legal clearances of
the International Consultative Group on Food Irradi-
ation (ICGFI), food irradiation was started from 1991
(Mizani et al., 2009). Food irradiation extends its shelf
life and is recognized as a wholesomeness process. It
protects foods by reducing parasites, food-borne patho-
gens and spoilage microorganisms, and destroying pests
and insects (Brito et al., 2002; Chen et al., 2007; O
¨zden
& Erkan, 2010). Gamma rays are ionizing rays. When
ionizing rays have an impact on the material, they can
pick electrons from the atoms. Free electrons can take
part in chemical reactions or destroy DNA molecules
from the living organisms. This process is the basis
for killing the living organisms by irradiation (Arvani-
*Correspondent: Fax: +98 21 66420732;
e-mails: marjan.oraei@yahoo.com, marjan.oraei@srbiau.ac.ir
International Journal of Food Science and Technology 2012, 47, 977–984 977
doi:10.1111/j.1365-2621.2011.02930.x
2012 The Authors. International Journal of Food Science and Technology 2012 Institute of Food Science and Technology
toyannis, 2010). Freezing is a common preservation
method for long term storage by controlling or decreas-
ing the growth of microorganisms and biochemical
changes in fish occur during the storage period. How-
ever, frozen storage does not completely inhibit chem-
ical reactions (e.g. lipid oxidation) that lead to quality
deterioration of the fish (Motalebi et al., 2010). When
irradiation is used in combination with the other
preservation methods such as freezing, the overall
efficiency is reinforced through a synergistic action,
and the irradiation doses can be reduced without
affecting the product quality (Lacroix & Ouattara,
2000).
According to previous study, Oraei et al. (2011), the
effect of gamma irradiation (1, 3 and 5 kGy) and
increasing frozen storage time on reduction of microor-
ganism’s population had been investigated, indepen-
dently. The objective of this study was to determine the
combined effect of irradiation process using low-dose of
gamma irradiation (1, 3 and 5 kGy) and frozen storage
time on the chemical and sensorial characteristics of
rainbow trout fillets.
Materials and methods
Sample preparation
A total of 25 kg Oncorhynchus mykiss (with weight
range of 300–500 g per fish) was obtained from a local
fish farm in north Iran. The fish from the same batch
were killed by immersion in ice-cold water (hypother-
mia) and then were transferred to a laboratory in
foamed polystyrene self-draining boxes with a suitable
quantity of flaked ice (the ice ⁄fish ratio was 2:1, w ⁄w).
They were washed with tap water, skinned, beheaded
and gutted immediately after passing into rigor mortis,
and then filleted by a sterile scalpel and washed again.
Each fish was divided into four fillets, each of them
being approximately 60–70 g. Each fillet was separately
placed in a plastic film bag. The fillets were divided into
four lots (50 fillets in each lot): one lot with no radiation
exposure as control (0 kGy) and three lots were irradi-
ated with the intensity levels of 1, 3 and 5 kGy,
respectively. Packed samples were delivered to the
radiation plant in insulated polystyrene boxes with
ice ⁄fillets weight ratio of 2:1 to keep at 0–4 C. The ice
was placed in plastic film bags.
Irradiation
Gamma irradiation was carried out in Nuclear Research
Center for Agriculture and Medicine, Karaj, Iran. Fish
samples were gamma irradiated using a
60
Co source
irradiator (Gamma cell Px-30, Russia, dose rate
0.23 Gy s
)1
). The applied dose levels were 0 (control),
1, 3, and 5 kGy (Moini et al., 2009). Fish samples were
maintained at 2 ± 2 C during irradiation using sealed
ice covering the samples. The dose rate was established
using alanine transfer dosimeter (Berthold, Germany)
(De Angelis et al., 2005).
Storage conditions
After irradiation, irradiated and non-irradiated fillets
were transported back to the laboratory in insulated
polystyrene boxes with ice ⁄fillets weight ratio to 2:1 to
keep at 0–4 C. In the laboratory, fillets were exposed to
rapid freezing in a spiral freezer (Koppens SVR
C400 ⁄17-50, UK). Fillet depth temperature reached
)20 C within 25 min. Then frozen fillets were kept in a
cold storage at )20 C up to 5 months.
Chemical analysis
Chemical attributes (e.g. TVN) were analysed and
monitored, periodically with the interval of 1 month
up to 5 months in two replications per analysis. Details
of each analysis and measurements are provided in the
following sections.
Total volatile nitrogen (TVN)
Measurements of TVN were performed according to the
Kjeldahl method (AOAC, 2005). Ten grams of fillet was
blended and added to 300 mL tap water and 2 g
magnesium oxide in a distilling flask of macro-Kjeldahl
for distillation. The distillate was collected in 2% boric
acid, methyl red and bromocresol green and was titrated
with 0.1N H
2
SO
4
. The TVN as mg N ⁄100 g of fish flesh
was obtained multiplying the titre by 14.
Peroxide value (PV)
According to AOAC (Association of Official Analytical
Chemists). (2005), 5 g of extracted fish oil sample was
placed in a 250 mL Erlenmeyer flask containing 30 mL
acetic acid–chloroform (3:2) solution, 0.5 mL saturated
KI solution, 30 mL distilled water and was titrated with
0.01N sodium thiosulphate. The peroxide value was
reported as mequiv. of peroxide per kg of oil using the
formula (1); where Sis mL of Na
2
S
2
O
3
,N= 0.01, the
concentration of the Na
2
S
2
O
3
solution and W= weight
of oil sample in grams:
PV ¼SN1000=Wð1Þ
Thiobarbituric acid (TBA)
The thiobarbituric acid was determined as mg malonal-
dehyde (MA) per kg of fish flesh by the method of
Pokorny & Dieffenbacher (1989). A total of 200 mg of
fish sample was homogenized in a 25 mL balloon with
Chemical and sensory characteristics of rainbow trout M. Oraei et al.978
International Journal of Food Science and Technology 2012 2012 The Authors
International Journal of Food Science and Technology 2012 Institute of Food Science and Te chnology
1-butanol. Approximately 5 mL of the homogenate was
mixed well with 5 mL of TBA agent in a capped test tube.
The mixture was heated in a boiling water bath (95 C)
for 2 h and then cooled at room temperature. Blank
reagent was run, and absorbance at 538 nm (A
b
) was
recorded. TBA value (mg MA kg
)1
of fish flesh) was
obtained by the formula (2). ‘A
s
’ was the absorbance of
the sample against a blank sample of distilled water:
TBA ¼50ðAsAbÞ=200 ð2Þ
Measurement of pH
According to AOAC (Association of Official Analytical
Chemists). (2005) the samples were homogenized in
distilled water in ratio of 1:10 (w ⁄v). The pH level was
measured using a digital pH meter (AZ 86502, Taiwan).
Sensory evaluation
Sensory attributes of frozen irradiated and non-irradi-
ated rainbow trout fillets were evaluated as grilled fish
fillets at the first day of frozen storage and 1-month
sampling intervals for 5 months. The irradiated and
non-irradiated rainbow trout fillets were grilled in an
electric toaster oven (Sun Star SO-20, Iran) for about
1 h. The panel consists of 10 qualified fish tasters, staff
members of Fish Processing Technology Research
Center at Anzali port in Iran, who were familiar with
fish characteristics. An orientation session was con-
ducted before participating in the formal panel. The
sensory attributes evaluated were texture, odour, taste
and appearance (colour) according to the method of
Watts et al. (1989) using a scale of 1–5. Where 5
represented excellent, 4 = very good, 3 = good,
2 = consumable (acceptable) and 1 represented non-
consumable (unacceptable), with 3 being the lower limit
of acceptability.
Statistical analysis
All data from chemical and sensory analysis were
subjected to factorial analysis of variance (anova) and
Duncan’s multiple range test (P< 0.05) to evaluate the
effect of irradiation and different applied doses in this
study and frozen storage time on chemical and sensory
characteristics of rainbow trout fillets. Differences be-
tween means were considered significant when P< 0.05.
SPSS version 18.0 was used for the statistical analysis.
Results and discussion
Chemical analysis
The measured chemical attributes including TVN, PV,
TBA and pH of irradiated (1, 3 and 5 kGy) and
non-irradiated (control) rainbow trout fillets during
5 months frozen storage at )20 C are shown in
Fig. 1.
Total volatile nitrogen
TVN is considered as a quality index for fish and fishery
products. Increase in TVN value corresponds to the
activity of spoilage microorganisms and endogenous
enzymes. The function of such enzymes and microor-
ganisms results in the formation of some of the volatile
nitrogen substances such as ammonia, indol, scatol,
cadaverin, monoethylamine, dimethylamine and trim-
ethylamine imparting characteristics of off-flavours to
fish (O
¨zden et al., 2007; Sedeh et al., 2007; Moini et al.,
2009). There is a direct correlation between the
amount of TVN and fish spoilage. TVN values of
15–20 mg N ⁄100 g of fish muscle confirms a good
quality, whereas level of 35 mg N ⁄100 g is usually
regarded as indication that the fish is spoiled (Moini
et al., 2009; Gunsen et al., 2010).
Results obtained from TVN measurement were the
indicator of good storage conditions of the samples. The
amount of TVN increased in all irradiated and non-
irradiated samples with frozen storage time but increas-
ing rate for irradiated samples at 5 kGy was higher.
TVN increase had a much higher rate at the very early
stages of freezing during the storage time and the rate
decreased subsequently. This can be explained by
reduction of activity of enzymes and microorganisms
as time proceeds. At the end of fifth month of frozen
storage, maximum and minimum levels of TVN were
found to be associated with the samples irradiated at 5
and 3 kGy, respectively.
Moini et al. (2009) reported similar result, showing that
TVN values increased gradually with refrigeration period
in the control and irradiated (1, 3 and 5 kGy) rainbow
trout samples. Ahmed et al. (2009) reported that TVN
values of degutted fresh Chinese pomfret, Pampus chin-
ensis increased with the progress of storage period at
)20 C. O
¨zden et al. (2007) reported that TVN values
increased according to time of storage in ice at +4 C.
According to the results reported by Sedeh et al.
(2007) in the irradiated bovine meat samples (0.5, 1, 2
and 3 kGy), no significant difference was found for
TVN by increasing irradiation dose level. Increasing the
applied dose decreased the rate of TVN formation
during frozen storage by reducing the initial levels of the
common spoilage bacteria. Javanmard et al. (2006) also
reported that increasing the applied dose decreased the
rate of TVN formation in irradiated (0.75, 3 and 5 kGy)
chicken meat samples during frozen storage by reducing
the initial levels of the common spoilage bacteria. Badr
(2004) showed that gamma irradiation of rabbit meat at
1.5 and 3 kGy had no significant effect on their TVN
contents, while storage in refrigerator significantly
Chemical and sensory characteristics of rainbow trout M. Oraei et al. 979
2012 The Authors International Journal of Food Science and Technology 2012
International Journal of Food Science and Technology 2012 Institute of Food Science and Technology
increased TVN of irradiated and non-irradiated sam-
ples. Chouliara et al. (2004) reported that TVN values
increased slowly, attaining a value of 60.52 mg N ⁄100 g
for non-irradiated salted sea bream during refrigerated
storage. Whereas for irradiated fish, lower values of
48.13 and 37.21 mg N ⁄100 g muscle were recorded at 1
and 3 kGy at the 42nd day, respectively.
Peroxide value
The most important aspects of fish quality deterioration
after death are related to peroxide increase with highly
unsaturated fatty acids (HUFA) decrease as a result of
lipid hydrolysis and unpleasant smell formation (Yildiz
et al., 2006). The peroxide value is used as an indicator of
lipid oxidation. The lipid oxidation is attributed to the
formation of hydroperoxides as the combination of free
radicals with O
2
. A PV below 5 mequiv. kg
)1
shows that
the fat is fresh or the hydroperoxides (the primary
oxidation products) have degraded into volatile com-
pounds such as aldehydes and ketones (the secondary
oxidation products). A PV between 5 and 10mequiv. kg
)1
determines commencing rancidity (Javanmard et al.,
2006).
In this research peroxide values were 0 mequiv. kg
)1
in
non-irradiated and irradiated samples at 1 and 5 kGy
until the end of the first month of frozen storage, while PV
of irradiated samples at 3 kGy was still 0 mequiv. kg
)1
until the end of the third month of frozen storage. The PV
level of all irradiated and non-irradiated samples
increased gradually with frozen storage time. Also the
peroxide value never decreased during 5 months of frozen
storage. Because the oxidation was slow and the produced
peroxide compounds did not seem to degrade. In addi-
tion, the rainbow trout contains high amounts of carot-
enoids in its flesh relative to other fish. Due to the
antioxidant role of carotenoids, the rainbow trout is less
susceptible to the lipid oxidation than other fish (Decker
et al., 2000). Regardless of storage time, maximum levels
of PV values corresponded to irradiated samples at
5 kGy, while the minimum values were obtained for the
samples irradiated at 3 kGy. Thus increasing gamma
irradiation dose (5 kGy) relative to 1 and 3 kGy increased
lipid oxidation in rainbow trout fillets.
Sedeh et al. (2007) reported that PV was greater in
gamma irradiated (0.5, 1, 2 and 3 kGy) bovine meat
aw bw
cw
dw
ew
fw
ax
bx
cx
dx ex
fx
ay
by cy
dy
ey
fy
az
bz
cz
dz
ez
fz
9
11
13
15
17
19
21
23
25
First day Month 1 Month 2 Month 3 Month 4 Month 5
TVN (mg 100 g
–1
)
Storage time
az az
bw cw
dw
ew
ax ax
bx
cx
dx
ex
aw aw by cy
dy
ey
ay ay
bz
cz
dz
ez
0
0.5
1
1.5
2
2.5
3
3.5
PV (meq kg–1)
Storage time
0 kGy
1 kGy
3 kGy
5 kGy
First day Month 1 Month 2 Month 3 Month 4 Month 5
cw
dw
ew fw
ax bx cx
dx
ex fx
aw bw
ay by cy
dy
ey fy
aw bw cw
dw
ew
fw
0
1
2
3
4
5
6
First day Month 1 Month 2 Month 3 Month 4 Month 5
TBA (mg MA kg–1)
Storage time
0 kGy
1 kGy
3 kGy
5 kGy
First day Month 1 Month 2 Month 3 Month 4 Month 5
Storage time
aw bw
cw
cw dw
dw
aw
bw
cw cw
dw
dw
ax
bx
cx cx
dx
dx
ax
bx
cx cx
dx
dx
6
6.1
6.2
6.3
6.4
6.5
6.6
6.7
pH
0 kGy
1 kGy
3 kGy
5 kGy
0 kGy
1 kGy
3 kGy
5 kGy
Figure 1 Chemical attributes (TVN, PV, TBA and pH) of the
irradiated (1, 3 and 5 kGy) and non-irradiated (control) rainbow trout
fillets during 5 months frozen storage. In each chemical attribute, values
with different letters (
a-f
) within a similar radiation dose and (
w-z
) within a
similar storage time are significantly different (P< 0.05).
Chemical and sensory characteristics of rainbow trout M. Oraei et al.980
International Journal of Food Science and Technology 2012 2012 The Authors
International Journal of Food Science and Technology 2012 Institute of Food Science and Te chnology
samples, but there was no significant difference between
irradiated and control group. Javanmard et al. (2006)
also reported that there was no significant difference in
PV between irradiated (0.75, 3 and 5 kGy) and control
chicken meat samples and also in the irradiated groups,
no significant difference was found as irradiation dose
level increased. But Ouattara et al. (2002) showed
increase in lipid oxidation after gamma irradiation (1,
2 and 3 kGy) of ground beef samples.
Thiobarbituric acid
TBA measures secondary lipid oxidation products,
which are also responsible for the rancid odour and
taste developing during storage (Decker et al., 2000).
TBA is an index of lipid oxidation measuring malonal-
dehyde (MA) content which is formed through hydro-
peroxides. TBA values of 1–2 mg MA kg
)1
of fish flesh
are usually regarded as the limit beyond normal odour
or taste (Moini et al., 2009). TBA value should be less
than 3 mg MA kg
)1
and should not be more than
5mgMAkg
)1
in perfect quality and good quality fish.
Consumption limits are from 7 to 8 mg MA kg
)1
(Gunsen et al., 2010).
In this study TBA values were 0 mg MA kg
)1
and
near zero in irradiated (1, 3 and 5 kGy) and non-
irradiated samples in first day of frozen storage. These
results are justifiable because PV was zero and lipid
oxidation was at the minimum level the first day of
storage. TBA increased in all irradiated and control
samples with the progress of frozen storage time. The
maximum and minimum levels of TBA at the end of
fifth month of frozen storage were corresponding to
irradiated samples at 5 and 3 kGy, respectively. This
may be attributed to a higher concentration of free
radicals formed in the substrate upon higher dose
ionizing irradiation (5 kGy) relative to 0, 1 and 3 kGy
which led to carbonyl compounds formation. Irradiated
samples at 3 kGy also had the lowest level of TBA in all
the time of storage. According to Gunsen et al. (2010)
control and irradiated samples at 5 kGy did not have a
good quality at the end of fifth month of frozen storage
because of a TBA more than 5 mg MA kg
)1
.
Moini et al. (2009) showed that peak values of TBA
for the control samples were obtained at thirty-fifth day
and for irradiated (1, 3 and 5 kGy) rainbow trout
samples obtained at twenty-eighth day of refrigeration
storage. Then TBA values decreased gradually until day
42 of storage. The decrease in TBA values may represent
the breakdown of MA to tertiary degradation products.
TBA values for irradiated samples were higher than
controls throughout the entire storage period. Chouliara
et al. (2004) also reported that TBA values for irradiated
(1 and 3 kGy), salted sea bream samples were higher
than respective non-irradiated (salted) fish throughout
1
2
3
4
5
First day Month 1 Month 2 Month 3 Month 4 Month 5
Score (texture)
Storage time
0 kGy
1 kGy
3 kGy
5 kGy
First day Month 1 Month 2 Month 3 Month 4 Month 5
Storage time
1
2
3
4
5
Score (odour)
First day Month 1 Month 2 Month 3 Month 4 Month 5
Storage time
1
2
3
4
5
Score (colour)
0 kGy
1 kGy
3 kGy
5 kGy
0 kGy
1 kGy
3 kGy
5 kGy
1
2
3
4
5
Score (taste)
First day Month 1 Month 2 Month 3 Month 4 Month 5
Storage time
0 kGy
1 kGy
3 kGy
5 kGy
Figure 2 The scores of sensory evaluation (texture, odour, colour,
taste) of the irradiated (1, 3 and 5 kGy) and non-irradiated (control)
rainbow trout fillets during 5 months frozen storage.
Chemical and sensory characteristics of rainbow trout M. Oraei et al. 981
2012 The Authors International Journal of Food Science and Technology 2012
International Journal of Food Science and Technology 2012 Institute of Food Science and Technology
the entire storage period, and increased slowly until day
28 of storage and after this period, decreased gradually
until day 42 of storage. O
¨zden et al. (2007) reported that
TBA value of non-irradiated and irradiated (2.5 and
5 kGy) sea bass samples increased to a maximum level
during storage up to the fifth, ninth and seventh day,
respectively and decreased subsequently. TBA values for
irradiated samples were lower than that for non-irradi-
ated samples. Badr (2004) reported that irradiation of
rabbit meat samples (1.5 and 3 kGy) and refrigeration
storage significantly increased the TBA for irradiated
and non-irradiated samples.
pH
The pH of the muscle tissue of live fish is close to the
value of 7.0; however, post-mortem pH can vary from
6.0 to 7.1 depending on species, season and other factors
(Castillo-Ya
´n
˜ez et al., 2007). In this study, the pH
values of irradiated (1, 3 and 5 kGy) and non-irradiated
rainbow trout fillets increased after 5 months relative to
the first day of frozen storage. Increase in pH indicates
the accumulation of alkaline and volatile base com-
pounds such as ammonia and trimethylamine as well as
the other biogenic amines, mainly derived from micro-
bial action (O
¨zden et al., 2007; Moini et al., 2009). In
each irradiation dose, the differences of pH value in first
day with other frozen storage times and in month 1 with
other storage times were significant (P< 0.05), but
there were no significant differences (P> 0.05) of pH
values in months 2 and 3 together and also in months 4
and 5 together. According to irradiation dose, there was
no significant difference (P> 0.05) between the pH
value of 0 and 1 kGy together and 3 and 5 kGy
together. But the differences of pH values between 0
or 1 kGy and 3 or 5 kGy were significant (P< 0.05).
The pH increases are in agreement with the findings of
Chouliara et al. (2004), Moini et al. (2009), O
¨zden et al.
(2007) and Reale et al. (2008) for gamma irradiation of
different fish species during storage.
Sensory evaluation
While a fishery product is consumed, its quality is
evaluated with the relationship between collection of
organoleptic characteristics such as colour, taste, odour
and texture. Thus when undesirable changes occur in
this product, many of these changes will be searchable
by human senses (Watts et al., 1989). Figure 2 illustrates
the result of texture, odour, colour and taste evaluation
of control and irradiated rainbow trout fillets during
5 months of frozen storage. As it was mentioned before,
a score of 3 was taken as the lower limit of acceptability.
Statistical analysis of scores obtained from the
sensory evaluation of cooked rainbow trout fillets
showed that radiation process and frozen storage had
a significant effect (P< 0.05) on texture, odour, colour
and taste of rainbow trout fillets. Initial sensory attri-
butes scores of rainbow trout fillets gradually decreased
with the progress of frozen storage period in control and
irradiated samples at 1 and 5 kGy.
The lower limit of acceptability of texture was reached
at the third month of frozen storage for the control
samples and at the fourth month of frozen storage for
the irradiated samples at 1 and 5 kGy. But the quality
scores of texture never reached less than 3.5 ± 0.6 for
irradiated samples at 3 kGy.
The lower limit of acceptability of odour was reached
at the fourth month of frozen storage for the control
and irradiated samples at 1 kGy. But the quality scores
of odour reached 3.7 ± 0.6 and 3.0 ± 0.5, respectively,
for irradiated samples at 3 and 5 kGy until the end of
fifth month of frozen storage that were desirable. The
odour scores showed a negative correlation with the
levels of TVN (r=)0.56, correlation is significant at
the 0.01 level); the quality scores of odour decreased
with increasing TVN levels.
The lower limit of acceptability of colour was reached
at third, fourth and fifth months of frozen storage for
the control and irradiated samples at 1 and 5 kGy,
respectively. However, quality score of colour was
4.0 ± 0.6 for irradiated samples at 3 kGy at the end
of fifth month of frozen storage which was very
attractive.
The lower limit of acceptability of taste was reached at
third month of frozen storage for the control samples
and at fifth month of frozen storage for the irradiated
samples at 1 and 5 kGy. But the quality score of taste
for irradiated samples at 3 kGy remained acceptable
until the end of fifth month of frozen storage.
According to overall acceptability of all sensory
attributes in present study irradiated samples at 3 kGy
had best quality and remained acceptable after
5 months in frozen storing.
Ahmed et al. (2009) reported that the organoleptic
scores of non-irradiated fish samples decreased gradu-
ally with the progress of storage period whereas irradi-
ated samples showed the acceptable score up to 90 days.
Fallah et al. (2008) reported that gamma irradiation of
camel meat at doses 1.5 and 3 kGy had no significant
effect on the initial sensory attributes of the meat
samples. O
¨zden et al. (2007) reported that acceptability
scores for odour, taste and texture of cooked sea bass
decreased with the storage time. According to the
sensory scores of sea bass stored in ice (+4 C) the
shelf life of 13, 15 and 17 days was determined for non-
irradiated and irradiated samples at 2.5 and 5 kGy,
respectively. Javanmard et al. (2006) reported that there
was no significant difference in sensory quality during
freezing storage between irradiated (0.75, 3 and 5 kGy)
and non-irradiated chicken meat. Chouliara et al. (2004)
reported that on the basis of sensorial evaluation, a shelf
Chemical and sensory characteristics of rainbow trout M. Oraei et al.982
International Journal of Food Science and Technology 2012 2012 The Authors
International Journal of Food Science and Technology 2012 Institute of Food Science and Te chnology
life of 27–28 days was obtained for vacuum-packaged,
salted sea bream irradiated at 1 or 3 kGy, compared to a
shelf life of 14–15 days for the non-irradiated, salted
sample. Badr (2004) showed that gamma irradiation had
no significant effect on the sensory properties of raw
rabbit meat. Moreover, fried burgers prepared of
irradiated rabbit meat showed high sensory acceptability
similar to those prepared of non-irradiated rabbit meat.
Conclusion
All the results from chemical and sensory analysis in this
study indicates that low dose gamma irradiation
(3 kGy) can be applied to improve biochemical safety
indices and to extend the shelf life of rainbow trout for
up to 5 months at frozen state without adverse effects on
the quality and acceptability. Gamma irradiation at
3 kGy was the most effective in maintaining sensorial
and chemical characteristics of rainbow trout fillets. The
results (higher levels of PV and TBA) revealed that the
radiation at a high dose (5 kGy) might induce lipid
oxidation due to high levels of free radicals generated
compared to radiation at 3 kGy. In addition, the current
study showed the synergistic effect of the two preserva-
tion methods, food irradiation and freezing, on extend-
ing the shelf life of rainbow trout fillet.
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