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Identification and Changes in Fatty AcidProfile of Rainbow Trout (Oncorhynchusmykiss) Fillet During Frozen Storage(-18°C)Maryam Sabetiana, Somayeh Torabi Delshadb, Sohrab Moinic, HoumanRajabi Islamib, Razmik Beglaryana & Abbasali Motalebida Department of Food Processing Technology, Armenian StateAgrarian University, Yerevan, Armeniab Department of Fisheries, Science and Research Branch, IslamicAzad University, Tehran, Iranc Department of Food Science and Technology, Faculty ofBiosystems, University of Tehran, Karaj, Irand Iranian Fisheries Research Organization, Tehran, IranAccepted author version posted online: 28 Feb 2013.Publishedonline: 03 Jun 2014.

To cite this article: Maryam Sabetian, Somayeh Torabi Delshad, Sohrab Moini, Houman Rajabi Islami,Razmik Beglaryan & Abbasali Motalebi (2014) Identification and Changes in Fatty Acid Profile ofRainbow Trout (Oncorhynchus mykiss) Fillet During Frozen Storage (-18°C), Journal of Aquatic FoodProduct Technology, 23:4, 321-332, DOI: 10.1080/10498850.2012.717592

To link to this article: http://dx.doi.org/10.1080/10498850.2012.717592

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Journal of Aquatic Food Product Technology, 23:321–332, 2014Copyright © Taylor & Francis Group, LLCISSN: 1049-8850 print/1547-0636 onlineDOI: 10.1080/10498850.2012.717592

Identification and Changes in Fatty Acid Profile of RainbowTrout (Oncorhynchus mykiss) Fillet During Frozen

Storage (−18◦C)

Maryam Sabetian,1 Somayeh Torabi Delshad,2 Sohrab Moini,3 Houman Rajabi Islami,2

Razmik Beglaryan,1 and Abbasali Motalebi4

1Department of Food Processing Technology, Armenian State Agrarian University, Yerevan,Armenia

2Department of Fisheries, Science and Research Branch, Islamic Azad University, Tehran, Iran3Department of Food Science and Technology, Faculty of Biosystems, University of Tehran,

Karaj, Iran4Iranian Fisheries Research Organization, Tehran, Iran

This study was carried out to determine proximate analysis, fatty acid (FA) composition, and theirchanges in rainbow trout (Oncorhynchus mykiss) fillet during 10-month storage at −18◦C. The resultsrevealed that monounsaturated fatty acids (MUFAs) (33.8 %) were the more predominant of the totalfatty acids, followed by saturated fatty acids (SFAs; 26.3%) and polyunsaturated fatty acids (PUFAs;24.62%). Palmitic acid (21.09%) and oleic acid (18.54%) were determined to be the most abundantfatty acids of SFAs and MUFAs at the end of the experiment, respectively. Among n-6 PUFAs, linoleicacid (3.04%) and arachidonic acid (0.25%) had the maximum values at the end of the storage period,respectively. Eicosapentaenoic acid (EPA; 2.36%), linoleic acid (2.53%), and docosahexaenoic acid(DHA; 6.43 %) also showed the final dominance among n-3 PUFAs, respectively. The present studydemonstrates that preservation of rainbow trout fillet under frozen storage could significantly affectits constituents by changes in SFA, MUFA, and PUFA and ratios of EPA + DHA/C16, n3/n6, andPUFA/SFA. Decrease in unsaturated fatty acids—especially PUFAs, EPA + DHA/C16, n3/n6, andPUFA/SFA ratios—showed that nutritional value of rainbow trout fillet had decreased, although it wasalways in the acceptable range for human consumption during the 10-month storage at −18◦C.

Keywords: rainbow trout, fatty acid composition, proximate analysis, frozen storage, total volatile basicnitrogen (TVB-N), peroxide value (PV)

INTRODUCTION

Rainbow trout (Oncorhynchus mykiss) is a native species of North America and Russia and hasbeen widely farmed as a recreational and food fish around the world (Salem et al., 2010). Rapidgrowth rate, high nutritional value, and tolerance to the wide range of environment and handling

Correspondence should be addressed to Maryam Sabetian, Department of Food Processing Technology, Armenian StateAgrarian University, P.O. Box 37410, Yerevan 567411, Armenia. E-mail: Maryam_sabetian@yahoo.com

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322 SABETIAN ET AL.

caused rainbow trout to be cultured for human consumption in many temperate regions like theUnited States, Europe, Chile, Japan, Australia, and Iran (Celik et al., 2008). Based on the Food andAgriculture Organization of the United Nations (FAO, 2011) report, production of farmed rainbowtrout in 2003 was 195,032 tons, allocating 13.3% of total trout culture. Accordingly, much researchis aimed at culturing and marketing of rainbow trout by developing new technologies and improvingproduction efficiency of the farming industry (Fornshell, 2002; Henryon et al., 2002; Bobe et al.,2006; Cakli et al., 2006; Trattner et al., 2008).

Fatty acid composition of aquatic organisms and their impacts on human health have been stud-ied for the last few years (Robb et al., 2002; Wood et al., 2004; Görgün and Akpinar, 2007; Covaciand Dirtu, 2010). In particular, fish flesh is well-known as a rich source of long-chain n-3 polyun-saturated fatty acids (PUFAs), like eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoicacid (DHA, 22:6 n-3), which play a vital role in human nutrition, disease prevention, and healthpromotion (Puri, 2005; Erdem et al., 2009; Farooqui, 2009; Mazer et al., 2010). However, theseunsaturated fatty acids could be easily oxidized to hydroperoxide as a primary oxidation productduring the early phase of storage, even at low temperature (Aubourg and Gallardo, 2005; Sohn andOhshima, 2010; Maqsood and Benjakul, 2011).

Freezing is one of the common methods largely used to lessen many enzymatic and nonenzy-matic processes leading to putrefaction of aquatic products (Erickson, 1997; Erkan and Bilen, 2010).However, prooxidant agents and highly unsaturated lipid composition of fish flesh could encounterundesirable reactions which strongly limit their storage under frozen conditions (Miyazawa et al.,1991; Yesudhason et al., 2010). Different attempts were used by seafood industries to optimizefreezing conditions in order to prevent lipid oxidation, decrease drip loss, and improve other func-tional properties of fish meat (Srikar and Hiremath, 1972; Ozden, 2005; Asgharzadeh et al., 2010;Rodriguez et al., 2011).

As fish products have a prominent role in human nutrition, the aim of the present work was toconsider lipid, protein, ash, and moisture composition of rainbow trout fillet under frozen storage.Fatty acid profile and its changes as a food quality index were also determined during the frozenperiod to optimize freezing conditions.

MATERIALS AND METHODS

Sample Preparation

Specimens (30 individuals) of rainbow trout (O. mykiss), with average weight of 511.2 ± 7.0 g andlength of 35.2 ± 0.2 cm, were collected from a local trout farm in Jajrood, Mazandaran Province,Iran, and transferred alive by a truck equipped with an oxygen diffuser within 5 h to the FisheriesLaboratory of Islamic Azad University, Science and Research Branch, Tehran, Iran. The fish sam-ples were individually killed by a blow to the head after anesthetizing with 100 mg L−1 benzocaine.The specimens were then gutted, beheaded, filleted, and washed with tap water.

All 60 fish samples were randomly divided into six groups, each containing 10 fillets. One of thelots was examined on the dissection day, corresponding to initial time. The other lots were packed inhigh barrier polyethylene bags, frozen at −30◦C, and stored at −18◦C in a domestic refrigerator for amaximum period of 10 months. Sampling was done after 1, 2, 4, 8, and 10 months of freezing onset.At each time, one lot of the fillet samples was randomly thawed in a refrigerator (4◦C), homogenizedwith a mixture, and subjected to analytical determinations with four replications (n = 4).

Proximate Analysis

Moisture content was determined according to AOAC procedure (1998a) by drying an aliquot of gridfillet in a mechanical convection oven at 105◦C for 16 h and calculating the ground sample’s weight

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CHANGES IN FA OF RAINBOW TROUT DURING FROZEN STORAGE 323

loss. Results were reported as g/100 g edible part of fish flesh. Ash content was evaluated based onAOAC (1998b) method 938.08 by heating an aliquot of the sample in a muffle furnace at 550◦C for3 h and weighing the remaining material. Results were expressed as g/100 g edible part of fish flesh.Total lipid content was extracted according to the Bligh and Dyer (1959) method using a chloroform-methanol (1:1 by vol.) mixture. The resultant was calculated as g/100 g edible part of fish flesh.Crude protein content was measured by converting the nitrogen content, based on Kjeldahl’s method(AOAC, 1998c). Results were described as g protein/100 g edible part of the flesh.

Lipid Extraction and Fatty Acid (FA) Analysis

Lipid content of rainbow trout flesh was extracted and identified following AOAC (1998d) method948.15. In brief, lipids of the sample were converted to fatty acid methyl esters (FAME) usingacetyl chlorine and assessed with a gas chromatograph (Hewlett-Packard 5890 series II, Palo Alto,CA, USA), equipped with a SGE BPX70 capillary column (length 30 m, internal diameter 0.25 mm,thickness of film 0.25 µm) and a Flame-Ionization Detector (FID). The oven temperature was raisedfrom 50 to 180◦C at the rate of 10◦C/min, then increased to 240◦C at the rate of 1◦C/min. Columninjector and detector temperature were held at 250 and 260◦C, respectively. Nitrogen was usedas a carrier gas at a column inlet pressure of 10 psi with flow rate of 0.6 mL/min. Identificationof fatty acids was performed by comparison of retention time with those in authentic standards(Supelco 37 Components FAME Mixture, Kat Nr 4-7885, Bellefonte, PA, USA). The FA amountswere computed as a weight percentage basis by measuring area under the peak corresponding tothat FA.

Determination of Lipid Damage and Protein Degradation

Peroxide values (PVs) were measured based on the procedure of Chapman and Mackay (1949) byperoxide reduction in the lipid extract with ferric thiocyanate. Results were stated as meq activeoxygen/kg lipids. Thiobarbituric acid reactive substances (TBARS) were also assessed accordingto the method proposed by Vyncke (1970) for quantification of malondialdehyde (MDA) as theend-products of lipid oxidation. TBARS values were expressed as mg MDA/kg fillet. Furthermore,total volatile basic nitrogen (TVB-N) was evaluated by steam distillation, following the titrationprocedure of Antonacopoulos and Vyncke (1989).

Statistical Analysis

All analytical determinations were carried out with four replications to determine shelf life ofthe fillets (up to 10 months). Significant differences between storage times of the samples weredetermined using one-way analysis of variance (ANOVA) followed by Tukey’s honestly significantdifference (HSD) test. Results were expressed as mean ± standard error (SEM), and a p-value lessthan 0.05 was considered as significance level.

RESULTS AND DISCUSSIONS

Proximate Composition

Moisture, ash, lipid, and protein contents of the rainbow trout fillet during 10-month storagetime are given in Figure 1. The results revealed that moisture content of the fillet graduallyreduced from 71.39 ± 1.91 g/100 g of edible part to a minimum of 69.84 ± 0.78 g/100 g after10-month storage at −18◦C (Figure 1a), although no significant differences were found during the

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324 SABETIAN ET AL.

Moi

stur

e C

onte

nt (

g/10

0g)

(a)

Ash

Con

tent

(g/

100g

)

1.35 1.39 1.41 1.47 1.511.54

1.56

0

1

2

3(b)

Pro

tein

Con

tent

(g/

100g

)

(c)

Lip

id C

onte

nt (

g/10

0g)

4.48 4.81 5.215.52 5.82

6.12

6.84

0

5

10

Initial 1 2 4 6 8 10

Initial 1 2 4 6 8 10

Month

(d)

FIGURE 1 The level of moisture (g/100 g), ash (g/100 g), crude protein (g/100 g), and lipid (g/100 g) in the filletof rainbow trout during frozen storage for 10 months at −18◦C. Results are presented as mean ± SEM (n = 4).

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CHANGES IN FA OF RAINBOW TROUT DURING FROZEN STORAGE 325

experiment (p < 0.05). Sahari et al. (2009) recorded similar results for moisture content of mackerel(Scomberomorus commerson) and shark (Carcharhinus dussumieri) after 6-month frozen storage.Not significant reduction of moisture content during storage period may be explained by temperatureuniformity, resulting in a slight difference between the vapor pressure of frozen product and air.

On the other hand, ash content in muscle tissue of rainbow trout steadily increased, but notsignificantly (p < 0.05), from 1.35 ± 0.12 g/100 g in fresh fillet to 1.56 ± 0.1 g/100 g edible partat the end of the experiment (Figure 1b). Increase of ash content could be elucidated by decrease ofmoisture content of the fillet, as reported previously (Ozden, 2005; Keyvan et al., 2008).

Initial content of protein was 19.61 ± 1.24 g/100 g in fresh rainbow trout; it decreased signifi-cantly (p < 0.05) to 18.37 ± 1.07 g/100 g edible part at the end of storage (Figure 1c). Beklevik et al.(2005) also reported a decline in protein content of sea bass (Dicentratchus labrax) during frozenstorage from initial content of 19.75 ± 0.17 to 19.31 ± 0.12% after 9-month storage. Decrease inprotein content could be related to the protein breakdown by enzymatic activities and production ofvolatile basic substances (Erkan and Bilen, 2010; Kaneniwa et al., 2000).

In addition, lipid content varied from 4.46 ± 0.22 to 6.84 ± 0.14 g/100 g of edible part in raw andstored fillets over the 10-month storage period (Figure 1d). The present findings are in accordancewith those reported previously illustrating a converse correlation between the moisture and lipidcontents of fish meat (Ben-Gigirey et al., 1999; Beklevik et al., 2005). Accordingly, the rise in lipidcontent during the experimental time could be elucidated by decline in the moisture content causingan increase in lipid amount per 100 g edible part of the rainbow trout flesh.

The present results for moisture, ash, protein, and lipid contents of rainbow trout fillet weresimilar to earlier findings (Alasalvar et al., 2002; Orban et al., 2002), and slight differences could berelated to seasonal factors, geographical location, diet, size, and physiological conditions (Tzikaset al., 2007).

Fatty Acid Composition

Fatty acid composition of fresh rainbow trout flesh and its changes during 10-month frozen stor-age are given in Table 1. The findings indicate that values of saturated and unsaturated fatty acidschanged over the experimental period. The lowest changes in fatty acids were observed in ligno-ceric acid (C24:0), which had no significant differences throughout the experiment. Palmitic acid(C16:0), palmitoleic acid (C16:1), and oleic acid (C18:1 n-9c) showed the most changes during thefrozen storage, although they allocated the highest value among the other fatty acids of rainbowtrout fillet.

The results revealed that MUFAs (33.03 ± 1.00%) were more predominant in the total fattyacids, followed by SFAs (26.34 ± 1.30%) and PUFAs (23.80 ± 0.45%) at the beginning of theexperiment. Palmitic acid (C16:0), stearic acid (C18:0), and myristic acid (C14:0) were the mostabundant SFAs by the end of experiment. Among n-6 PUFAs, linoleic acid (C18:2) and arachidonicacid (C 20:4 n-6) also showed the final dominance, respectively. EPA (C20:5 n-3), linoleic acid(C18:3 n-3), and DHA (C22:6 n-3) were also dominant among n-3 PUFAs, respectively.

In addition, the∑

SFA increased from 26.34 ± 1.30% in the fresh sample to 36.57 ± 0.44% oftotal fatty acid composition at the end of experimental period. Conversely,

∑MUFA reduced during

the freezing period from 33.03 ± 1.00% in the fresh sample to 28.95 ± 0.34% after 10 months,although percentage of total fatty acids in the fillet did not change throughout the experiment. El-Sawy et al. (1988) reported similar findings for frozen tilapia (Tilapia nilotica) during cold storageat −20◦C. This change could be due to the breakdown of PUFA’s double bonds and their conversionto SFAs.

The present research has shown that all fatty acids of rainbow trout fillet were significantly alteredduring the storage period at −18◦C, except lignoceric acid (C24:00) which did not significantlychange (p < 0.05). The PUFA/SFA ratio steadily decreased from 0.90 ± 0.05 to 0.47 ± 0.03%

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TAB

LE1

Cha

nges

inpe

rcen

tage

offa

ttyac

ids

(Mea

n±

SE

M)

inra

inbo

wtr

outfi

llets

tore

dat

−18◦

Cfo

r10

mon

ths

(n=

4)

Fatt

yac

ids

Init

ial

1stm

onth

2nd

mon

th4t

hm

onth

6th

mon

th8t

hm

onth

10th

mon

th

C14

:03.

58±

0.04

a3.

89±

0.10

b4.

43±

0.10

c4.

61±

0.06

d4.

58±

0.11

d4.

87±

0.05

e4.

62±

0.04

dC

15:0

0.32

±0.

02b

0.35

±0.

04b

0.44

±0.

04c

0.59

±0.

05d

0.52

±0.

03e

0.61

±0.

03d

0.82

±0.

03f

C16

:016

.03

±1.

00a

18.0

2±

0.12

b18

.83

±0.

12c

20.6

3±

0.11

d20

.80

±0.

1d20

.98

±0.

18d

21.0

9±

0.10

dC

17:0

0.47

±0.

05a

0.56

±0.

06b

0.68

±0.

03c

0.79

±0.

09d

0.85

±0.

04d

0.88

±0.

03d

0.97

±0.

03e

C18

:04.

06±

0.06

b4.

52±

0.08

c4.

68±

0.03

d5.

28±

0.13

e5.

58±

0.08

f5.

79±

0.07

g6.

07±

0.07

hC

20:0

0.69

±0.

04a

0.70

±0.

05a

0.72

±0.

02a

0.76

±0.

06b

0.77

±0.

04b

0.78

±0.

04b

0.98

±0.

06c

C22

:00.

45±

0.03

b0.

56±

0.04

c0.

99±

0.09

d0.

94±

0.04

d0.

96±

0.03

d1.

06±

0.04

d1.

14±

0.04

eC

24:0

0.67

±0.

06a

0.69

±0.

09a

0.64

±0.

04a

0.67

±0.

05a

0.77

±0.

01a

0.64

±0.

04a

0.89

±0.

07a

∑SF

A26

.34

±1.

30a

29.3

5±

0.59

b31

.41

±0.

45c

34.2

7±

0.55

d34

.83

±0.

43d

35.6

1±

0.4e

36.5

7±

0.44

eC

14:1

0.16

±0.

02b

0.15

±0.

03b

0.18

±0.

01a

0.21

±0.

02c

0.17

±0.

02b

0.16

±0.

01b

0.14

±0.

01b

C15

:10.

11±

0.01

a0.

10±

0.01

a0.

08±

0.01

b0.

05±

0.01

c0.

04±

0.01

c0.

02±

0.01

d0.

02±

0.01

dC

16:1

8.66

±0.

12a

8.62

±0.

10a

8.18

±0.

08b

8.47

±0.

03c

8.39

±0.

04c

8.37

±0.

07c

8.32

±0.

10b

C17

:10.

84±

0.04

a0.

80±

0.05

a0.

75±

0.05

b0.

71±

0.03

b0.

70±

0.06

b0.

64±

0.04

c0.

59±

0.09

cC

18:1

n-9t

0.05

±0.

01a

0.07

±0.

01b

0.09

±0.

02c

0.07

±0.

01b

0.06

±0.

01a

0.06

±0.

01a

0.04

±0.

005a

C18

:1n-

9c21

.58

±0.

70a

20.6

2±

0.10

b19

.18

±0.

10c

18.9

3±

0.15

c18

.92

±0.

12c

18.6

9±

0.1c

18.5

4±

0.10

cC

20:1

n-9

1.33

±0.

03b

1.31

±0.

06b

1.30

±0.

04b

1.28

±0.

04b

1.27

±0.

03b

1.22

±0.

02c

1.19

±0.

02c

C22

:1n-

90.

17±

0.05

b0.

15±

0.02

b0.

15±

0.01

b0.

10±

0.02

c0.

08±

0.01

c0.

07±

0.02

c0.

05±

0.01

dC

24:1

n-9

0.13

±0.

02a

0.14

±0.

02a

0.34

±0.

02b

0.16

±0.

01c

0.15

±0.

01c

0.12

±0.

01a

0.07

±0.

01d

∑M

UFA

33.0

3±

1.00

a31

.96

±0.

40a

30.2

5±

0.34

b29

.98

±0.

27b

29.7

8±

0.31

b29

.35

±0.

28b

28.9

5±

0.34

cC

18:2

-n-6

t0.

04±

0.01

b0.

03±

0.01

b0.

03±

0.01

b0.

05±

0.01

b0.

04±

0.01

b0.

03±

0.01

b0.

02±

0.01

cC

18:2

-n-6

c3.

34±

0.04

b3.

31±

0.02

b3.

27±

0.10

b3.

24±

0.06

b3.

20±

0.13

b3.

18±

0.04

b3.

04±

0.04

cC

18:3

-n-6

0.41

±0.

01a

0.37

±0.

03b

0.33

±0.

03b

0.30

±0.

04c

0.26

±0.

04c

0.21

±0.

02d

0.17

±0.

02d

C20

:20.

79±

0.06

a0.

75±

0.04

a0.

71±

0.03

b0.

64±

0.02

c0.

61±

0.06

c0.

59±

0.03

c0.

54±

0.04

dC

20:4

n-6

0.38

±0.

04a

0.34

±0.

02b

0.31

±0.

04b

0.31

±0.

05b

0.28

±0.

01b

0.26

±0.

03c

0.25

±0.

02c

C22

:22.

52±

0.04

b2.

29±

0.07

c2.

13±

0.10

d1.

99±

0.05

e1.

94±

0.01

e1.

88±

0.10

e1.

83±

0.03

fC

18:3

n-3

2.81

±0.

10a

2.78

±0.

08a

2.75

±0.

05a

2.73

±0.

06b

2.63

±0.

03b

2.58

±0.

04c

2.53

±0.

10c

C20

:3n-

30.

09±

0.02

b0.

08±

0.01

b0.

06±

0.01

c0.

07±

0.01

b0.

06±

0.01

c0.

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±0.

05d

0.42

±0.

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0.41

±0.

04d

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0.05

a0.

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0.05

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0.04

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d

Dif

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ntle

tters

ina

sam

ero

war

ere

pres

enta

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p<

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.

326

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CHANGES IN FA OF RAINBOW TROUT DURING FROZEN STORAGE 327

throughout the experiment, indicating the susceptibility of rainbow trout fillet to significant changeduring frozen storage (Sahari et al., 2009; Pirestani et al., 2010; Strik et al., 2010).

The ratio of n-3/n-6 is another useful indicator to compare relative nutritional values of fish oils(Gibson, 1983; Maazouzi et al., 2011). It is suggested that a ratio of 1:1 to 1:5 would constitutea healthy human diet (Osman et al., 2001; Zuraini et al., 2006). This study has also shown thatrainbow trout is an n-3 rich species in comparison to the other sources of PUFAs like n-6. Levelsof

∑n-3 and

∑n-6 were 16.34 ± 0.10 and 7.46 ± 0.19% in fresh fillet, which changed to 11.32 ±

0.1 and 5.85 ± 0.16% at the end of storage time, respectively. The ratio of∑

n-3/∑

n-6 for freshrainbow trout was 2.19 and declined to 1.93 at the end of the experiment. However, it was alwaysin suitable range of nutritional consumption as reported by Connell (1975).

It has been suggested that the EPA + DHA/C16:0 ratio is a good index to determine lipid oxida-tion (Jeong et al., 1990). Although this ratio in rainbow trout was 0.84 ± 0.03, it decreased to 0.41 ±0.04 toward the end of frozen storage. The same result was found for Japanese oyster (Crassosteragigas), shark (Carcharhinus dussumieri), and mackerel (Scomberomorus commerson) flesh (Jeonget al., 1990; Sahari et al., 2009). The negative correlation between EPA + DHA/C16:0 ratio andstorage time shows that oxidation mechanisms and enzymatic breakdown were an active processduring frozen storage.

Lipid Oxidation

Peroxide Value.

Hydroperoxide production, as an off-flavor factor affecting color and nutritional value, was raisedfrom an initial level of 0.1 ± 0.01 to 0.68 ± 0.05 meq oxygen/kg at the 6th month and then tendedto decline to 0.49 ± 0.06 meq oxygen/kg toward the end of the experiment (Figure 2). However,there were significant differences between initial and final hydroperoxide content (p < 0.05). Otherstudies have also reported a similar pattern of hydroperoxide content in fish flesh over frozen stor-age (Lehmann and Aubourg, 2008; Erkan and Bilen, 2010). According to the AOAC (2000), the

FIGURE 2 Comparison of PV (peroxide value) content in rainbow trout during frozen storage (−18◦C). Results arepresented as mean ± SEM (n = 4).

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acceptable level of peroxide content is less than 5 meq oxygen/kg. Thus, rainbow trout fillet showsa maximum shelf time of 10 months during frozen storage under −18◦C.

Thiobarbituric Acid Reactive Substances.

Thiobarbituric acid reactive substances have been widely applied as a secondary product of lipidoxidation (Clarkson, 1995; Pezeshk et al., 2011). Because of higher level of PUFAs such as DHA,fish flesh sensitivity to TBARS is higher than other animal meat (Tokur et al., 2004; Matsushitaet al., 2010). The results illustrate an increase in TBARS from 0.17 ± 0.07 mg MDA/kg fillet atthe beginning of the storage time to 0.78 ± 0.43 mg MDA/kg fillet by the end of the experimentalperiod, indicating an aggregation of free radicals throughout 10-month freezing at −18◦C (Figure 3).However, it was always lower than the maximum value (above 1–2 mg MDA per 1 kg fish flesh)determined as rancid flesh (Connell, 1975; Ke et al., 1984; Kilinc et al., 2006).

Total Volatile Basic Nitrogen.

Total volatile basic nitrogen is another quality factor to identify decomposition levels in foodand their qualitative changes during frozen storage (Aubourg et al., 1998; Cakli et al., 2006). Thesecompounds are created by the breakdown of protein constituents and, therefore, illustrate the decom-position level of proteins (Mendes et al., 2005; Zhang et al., 2011). The current results demonstratethat TVB-N of rainbow trout flesh was always within an acceptable level (less than 20 mg/kg)throughout the experiment (Pearson, 1973; Connell, 1995). The TVB-N value for fresh meat ofrainbow trout was 8.11 ± 0.5 mg/kg and showed a rising trend to the maximum value of 15.09 ±1.0 mg/kg after 10-month frozen storage at −18◦C (Figure 4).

The study conducted by Castell et al. (1974) revealed that enzyme activity under constant coolingconditions exhibits a descending trend over time. Therefore, the gradual increase of TVB-N duringcold storage in the present study may be related to low activity of the volatile nitrogen decomposingenzymes or the low contents of other nonprotein nitrogenous compounds such as dimethylamine(DMA), trimethylamine (TMA), or trimethylamine oxide (TMAO).

FIGURE 3 Comparison of thiobarbituric acid reactive substances (TBARS) value as mg malonaldehyde/kg fillet inrainbow trout (Oncorhynchus mykiss) during frozen storage (−18◦C). Results are presented as mean ± SEM (n = 4).

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8.11

9.8311.06

11.7513.16

14.35

15.09

5

10

15

20

25

Initial 1 2 4 6 8 10Month

TV

N (

mg/

kg)

FIGURE 4 Comparison of total volatile basic nitrogen (TVB-N) content as mg/kg of muscle content in rainbowtrout (Oncorhynchus mykiss) during frozen storage (−18◦C). Results are presented as mean ± SEM (n = 4).

CONCLUSION

The present study is in agreement with those previously reported for other species demonstratingthat preservation of rainbow trout fillets under frozen storage could significantly affect its con-stituents, specifically an increase of SFA and decrease of MUFA and PUFA percentages as wellas ratios of EPA + DHA/C16, n3/n6, and PUFA/SFA (Simeonidou et al., 1997; Vareltzis et al.,1997; Aubourg and Gallardo, 2005; Mizuguchi et al., 2011). Decrease in unsaturated fatty acids,especially PUFAs in addition to EPA + DHA/C16, n3/n6, and PUFA/SFA ratios showed that thenutritional value of rainbow trout fillet has decreased during 10-month storage at −18◦C, although itwas always in an acceptable range for human consumption (Uzun et al., 2008; Yoshida et al., 2008;Asim, 2009; Haggarty, 2010). Regardless of reduction during the experimental period, other qualityindices including TBA, PV, and TVB-N were also under acceptable levels.

ACKNOWLEDGMENTS

This work was done in the fisheries laboratory complex of Zakarya-e-Razi Laboratory, Science andResearch Branch, Islamic Azad University, Tehran, Iran. The authors are grateful to Dr. ShahlaJamili for her technical advice throughout the experiment.

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