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LWT 38 (2005) 201–211

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Quality characteristics of farmed Atlantic salmon (Salmo salar)fed diets high in soybean or fish oil as affected by

cold-smoking temperature

Anna Maria Bencze Røraa,�, Sveinung Birkelandb, Lisbeth Hultmannc, Turid Rustadc,Torstein Skarab, Bjørn Bjerkengd

aAkvaforsk, Institute of Aquaculture Research AS, P.O. Box 5010, NO-1432As-NLH, NorwaybNorconserv AS, Seafood Processing Research, P.O. Box 327, NO-4002 Stavanger, Norway

cDepartment of Biotechnology, Norwegian University of Science and Technology, NO-7491 Trondheim, NorwaydAkvaforsk, Institute of Aquaculture Research AS, NO-6600 Sunndalsøra, Norway

Received 19 March 2004; received in revised form 3 June 2004; accepted 22 June 2004

Abstract

Effects of cold-smoking temperature (range 21.5–29.9 1C) and dietary oil sources (pure Peruvian fish oil, FO, or pure soybean oil,

SO, supplements) on physical and chemical quality characteristics were determined in Atlantic salmon (Salmo salar). Weak but

statistically significant relationships were found between texture, composition, liquid holding capacity, colour parameters and yield,

and cold-smoking temperature. Coefficients of determination (R2) ranged from 0.01 to 0.48. Colorimetric parameters were largely

unaffected by temperature. Total phenol content exhibited the strongest positive linear correlation with temperature (Po0:01).Liquid holding capacities were reduced by increasing temperatures, but significantly only for salmon fed the FO diet (P ¼ 0:055 forsalmon fed the SO diets). Liquid loss during storage was not affected by prior processing temperature, but were significantly higher

in salmon fed the SO diet. Increased temperature caused decreased processing yield and water loss and increased fat loss during

vacuum storage, and firmer fillet texture. A significant (Po0:05) reduction in processing yield (1–2 g/100 g) was found at the highesttemperatures. The fish fed the SO diets had a lower carotenoid content and redness (a�-value) than salmon fed the FO diet

(Po0:05). Liquid holding capacity was lower in salmon fed the SO than the FO diets (Po0:05) possibly as a result of the differentfatty acid composition of the fillets. The effects of cold-smoking temperature and dietary oil source on quality were in general low to

moderate, and indicate that salmon represents a fairly robust raw material for cold-smoke processing.

r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.

Keywords: Cold-smoking; Process temperature; Liquid holding capacity; Texture; Total phenols

1. Introduction

Smoking and drying of fish have long worldwidetraditions as means to preserve the quality and increasethe shelf life of the products. The rapid increase in theAtlantic salmon processing industry during the last fewdecades has been boosted by the increase in salmonfarming, currently at an annual production rate of

0 r 2004 Swiss Society of Food Science and Technology. P

t.2004.06.008

ing author. Tel.: +47-6494-9526; fax: +47-6494-0502.

ess: mia.rora@akvaforsk.nlh.no (A.M.B. Røra).

approximately 465.000 metric tons for Norway alone in2002 (Statistics Norway, 2003). The salmon smokingtechnology has developed into a sophisticated industryand refrigeration and packaging technologies havereduced the necessity of heavy curing for preservationpurposes. Nowadays, smoke-cured Atlantic salmonproducts are popular food items due to the uniqueflavour, and a plethora of products differing in sensoryproperties such as colour, intensity and characteristic ofsmoke note, amine note and salt are found on theEuropean market (Cardinal et al., 2004).

ublished by Elsevier Ltd. All rights reserved.

ARTICLE IN PRESSA.M.B. Røra et al. / LWT 38 (2005) 201–211202

Cold-smoking is a combination treatment of salting,partial dehydration and smoke impregnation performedat temperatures between 20 and 30 1C. The smoke-processing industry frequently rely on in-house recipesand processes based on traditions, and often lacksfundamental knowledge about the properties of salmonas a raw-material for industrial processing. There is littlespecific documentation of process conditions and effectsof raw-material properties of Atlantic salmon on cold-smoked product quality parameters. However, proces-sing conditions may occasionally have profound effectson the final product quality. The effect of the processwith respect to quality and shelf life of the productdepends in general on raw material, type of smoking,relative humidity, air velocity, temperature, density inthe smoking chamber, composition of the smoke, andthe duration of the process (Doe, Sikorski, Haard,Olley, & Pan, 1998). Temperature is one of thefundamental process parameters that have been shownto affect quality characteristics of smoked Atlanticsalmon significantly. Thus, by increasing the cold-smoking temperature the colour parameters are alteredand carotenoid stability reduced (Cardinal et al., 2001;Birkeland, Haarstad, & Bjerkeng, 2004a), the texturemay be hardened in the event of case hardening(Cardinal et al., 2001), whereas liquid holding capacitywas unaffected (Montero, Gomez-Guillen, & Borderias,2003) in cold-smoked Atlantic salmon. Birkeland et al.(2004a) reported that drying and smoking at 20 1Ccaused significantly higher processing yield than dryingand smoking at 30 1C which is contrary to the findingsof Cardinal et al. (2001). Shear force was unaffected byincreasing the smoking temperature from 20 to 30 1C,but alterations in microstructure were more pronouncedfor the samples smoked at 30 1C compared to 20 1C(Sigurgisladottir, Sigurdardottir, Torrissen, Vallet, &Hafsteinsson, 2000). Cold-smoking of Atlantic salmonat increasing temperature in the range between 21.5 and29.9 1C causes alterations in the proteins that lead to amarked reduction in extractability of salt solublemyofibrillar proteins, whereas cathepsin B-like orgeneral proteolyic activity were unaffected (Hultmann,Røra, Steinsland, Skara, & Rustad, 2004).Few reports on effects of dietary oil on quality

characteristics of Atlantic salmon are found. However,increasing the levels of n-3 polyunsaturated fatty acidsare reported to improve sensory evaluated colour tonein Atlantic salmon (Waagbø, Sandnes, Torrissen,Sandvin, & Lie, 1993). Atlantic salmon fed diets withPeruvian fish oil high in PUFA (polyunsaturated fattyacids) deposited about 13% more carotenoids than fishfed diets supplemented with herring oil (Bjerkeng,Hatlen, & Wathne, 1999). Atlantic salmon fed dietssupplemented with soybean oil had significantly lowercarotenoids in the flesh, and the fatty acid profilesresembled those of the respective feeds (Regost,

Jakobsen, & Røra, 2004). Also, in Atlantic salmon feddiets only supplemented with either fish or soybean oil,yield was improved by the latter (Røra, Regost, &Lampe, 2003). Fillet gaping is splitting of musclesegments due to disintegration of mycommata (cf. Love,1988), and may cause production loss due to down-grading of smoked salmon (Einen, Mørkøre, Røra, &Thomassen, 1999; Robb, Kestin, & Warriss, 2000; Lund& Nielsen, 2001; Torrissen, Sigurgisladottir, & Slinde,2001). The gaping score is increased by increasinginjection pressure during injection salting of salmonfillets for cold-smoking (Birkeland, Skara, Bjerkeng, &Røra, 2003).It is important to gain knowledge about the relation-

ships between the magnitude of the changes in qualitycharacteristics as related to specific process conditions,raw material properties, and their interactions. The aimof the present study was therefore to investigate theeffect of six different smoking temperatures in the rangebetween 21.5 and 29.9 1C, on processing yield, colour,texture, muscle gaping, liquid-holding capacity, phenoland salt content in fillets of cold-smoked Atlanticsalmon fed diets either supplemented only with highPUFA fish oil or soybean oil.

2. Materials and methods

2.1. Biological material

Atlantic salmon (Salmo salar) fed diets supplementedwith Peruvian fish oil (FO, whole body weight2798747 g) or soybean oil (SO, whole body weight2713758 g) as the sole oil supplements were sampledfrom a feeding trial conducted at Akvaforsk’s searesearch facilities (Averøy, Norway). The salmon wereobtained from Aqua Gen AS (Kyrksæterøra, Norway)stocked in seawater as 0+smolts, and are off-spring of afamily based selective breeding program (cf. Gjedrem,2000). Triplicate groups of salmon (initial weight 1.0 kg)were fed the fish meal-based diets (390 g/kg crudeprotein and 360 g/kg crude lipid) provided by Ewos(Ewos Innovation, Dirdal, Norway) for 135 days beforeslaughter. Feed formulation, proximate compositionand fatty acid profiles of the diets are reportedpreviously (Regost et al., 2004).Before slaugthering, the salmon were anaesthetised

using MS-222 (metacaine, approximately 0.1 g l�1). Thefish were killed by cutting the gill arch and bled in waterwith an ambient temperature of about 7 1C. Afterslaughtering, gutting and washing, 11 fish from eachnet-pen were packed and continuously kept in boxeswith ice before samples were prepared or the fish wereprocessed further. Three fish per net pen were sent toAkvaforsk (As, Norway) for proximate analyseswhereas the rest of the fish were cold-smoked at

ARTICLE IN PRESSA.M.B. Røra et al. / LWT 38 (2005) 201–211 203

Norconserv (Stavanger, Norway). Four days afterslaughter the raw fish were pooled for each net-penand analysed for dry matter, ash, protein and fatcontent, fatty acid (FA) profile, and concentrations ofastaxanthin and canthaxanthin. Samples were keptfrozen at �20 1C before being analysed within 1 monthof storage.

2.2. Smoking procedure

The salmon used for cold-smoking were mechanicallyfilleted five days after slaughter using a CarnitechFilletmaschine (Carnitech A/S, Støvring, DK), drysalted with refined salt (Akzo Nobel, Fint RaffinertSalt, minimum 99.8% NaCl, Dansk Salt A/S, Mariager,Denmark) for 18 h at 4 1C. A Bastramat C1500 smokingcabinet operated with a MC700 Microprocessor, and aBastra FR 100 smoke generator (Bayha StrackbeinGmbH, Arnsberg, Germany) with Reho Raucher GoldHBK 750/2000 chips (J. Rettenmaier & Sohne GmbH,Rosenberg, Germany) were used for controlled cold-smoking and drying cycles. The fillets were dried for150min before smoking and drying four consecutivetimes in alternating intervals of 50 and 10min,respectively. Airspeed was set to 1m s�1 and the cabinetwas loaded with 7.170.2 kg salmonm�3. Batches ofsalmon were cold-smoked at six different smokingtemperatures (21.5, 23.1, 24.3, 26.6, 28.2 and 29.9 1C,respectively), with concurrent relative humidities (RH)ranging from 74% to 68%. After cold-smoking thefillets were kept at 14 1C for 20–30min before weighing.Fillets for storage (4 1C, 14 days) were vacuum packagedat 99% vacuum (Webomatic C60 D/W/U, WebomaticMachinenfabrik GmbH, Bochum, Germany).

2.3. Determination of proximate composition, salt

content, phenol and carotenoid concentrations, fatty acid

profiles, and estimated fat deposits

The raw samples were analysed gravimetrically forcontents of dry matter (also cold-smoked samples)following drying at 105 1C for 24 h, ash after flamecombustion followed by 3–4 h at 550 1C, and total lipidsafter ethyl-acetate extraction (Norwegian Standard,1994). The content of crude proteins was calculatedas N � 6:25 using the semi-micro-Kjeldahl method(KjeltecSystem, Tecator, Hoganas, Sweden).The salt content of the cold-smoked fillets was

determined conductivimetrically after accurately weigh-ing mince (20.0 g) in a chopping beaker, and addingdeionised water with a graded cylinder (200ml). Thesolution was homogenised using an electric blender(1min) and added through a strainer into a plasticbeaker (250ml). The NaCl content was measuredtwice in the filtered solution using a Dicromat 11-6 Salt

Analyser (PCL Control Instrumentation Ltd.,Leicester, UK).The content of total phenols in the smoked fillets were

determined after reacting an ethanol extract with 4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one and po-tassium hexacyanoferrate(III). Following extractionwith chloroform the total amount of phenols wasquantified spectrophotometrically according to theFrench standard for smoked salmon (NF V 45-065,1995).Astaxanthin and canthaxanthin concentrations were

determined in the raw fillets by HPLC using HewlettPackard Series 1050 Instrument as described byBjerkeng et al. (1997). All samples were analysedisocratically on a Spherisorb S5CN-4800 nitrile column(Hichrom Ltd., Theale, Berkshire, UK; length 250mm;internal diameter 4.6mm; particle size 5 mm) usingacetone (200ml/l) in n-hexane as mobile phase.Qualitative analysis of fatty acid compositions of

feeds and salmon fillets were undertaken by gaschromatography (GC) of fatty acid methyl esters(FAME) following transesterification of lipid extracts.The FAME were prepared overnight at room tempera-ture by transesterification in benzene (2ml) afteraddition of methanolic HCl (0.5mol/l, 2ml; Hoshi,Williams, & Kishimoto, 1973), and 2,2-dimethoxypro-pane (0.2ml; Mason & Waller, 1964). The samples wereneutralized with aqueous NaHCO3 (4ml, 60 g/l), andextracted with n-hexane (4ml), evaporated, and redis-solved in n-hexane. Samples of FAME were analyzedusing a gas chromatograph (PE Autosystem GC,Perkin–Elmer, Boston MA, USA) equipped with anautoinjector, programmable split/splitless injector, and aCP Wax 52 CB column (polyethyleneglycol-bondedfused silica; Varian Chrompack International, Middel-burg, The Netherlands) with length=25m, + ¼

0:25mm, film thickness=0.2 mm). The pressure of thecarrier gas (He) was 1.2 kg cm�2, and the injector anddetector temperatures were 280 1C. The oven tem-perature program was 50 1C for 1.2min followed byan increase of 10 1Cmin�1 until 180 1C and 0.7 1Cmin�1

until 240 1C was reached. Individual FAME wereidentified by comparison of the retention times withthose of authentic standards. The chromatograms wereprocessed using a Chem Station integrator system andquantification was based on peak areas, and fatty acidcomposition was expressed as percentage of totalFAME.The amount of estimated fat deposits describes the

proportion of a transverse section covered by discretefat deposits. The carcass was divided into fat, leanmuscle and bone (Rye, Bæverfjord, & Refstie, 1995),and analysed by computerized X-ray tomographyaccording to Rye (1991). The CT-data were analyzedby the AutoCAT-program (Jopson, Kolstad, Sehested,& Vangen, 1995), which is a fully automatic image

ARTICLE IN PRESSA.M.B. Røra et al. / LWT 38 (2005) 201–211204

analysis program. The fat deposits of raw fish wereestimated as the area of computer estimated fat depositsdivided by the total scan area.

2.4. Colour evaluation

Instrumental colour analyses (L�a�b�, CIE, 1994) ofcold-smoked fillets were performed with a MinoltaChroma Meter CR-200 (Minolta, Osaka, Japan) usinglight source D. Four measurements were made directlyon each fillet. Mean values were used for statisticalanalyses of the data. L� describes the lightness of thesample, a� redness (a�40) and b� yellowness (b�40).Chroma (C�-value; the relative purity or saturation of acolour) was calculated as C�

¼ ða�2 þ b�2Þ1=2, and the

hue angle (hab) was calculated as hab ¼ tan�1ðb�=a�Þ,where hab ¼ 01 for red hue and hab ¼ 901 for yellow hue(CIE, 1994).

2.5. Gaping evaluation and texture

Fillet gaping of cold-smoked fillets was assessedvisually according to Andersen, Strømsnes, Steinsholt,and Thomassen (1994) on a scale from 0 to 5, where 0 isno gaping and 5 is extreme gaping. The texture of thecold-smoked fillets was evaluated instrumentally using aTexture Analyser TA-XT2 (Stable Micro Systems,Surrey, England) equipped with a load cell of 5 kg anda cylindrical plunger (12.5mm diameter) during theTexture Profile Analysis (TPA). The measurements weremade in front of the dorsal fin, about 1.5 cm above thelateral line. The plunger was pressed into the fillets at aconstant speed of 2mm s�1 until 60% of the sampleheight was reached. The resistance force (samplefirmness) was recorded both at 60% of the breakpointof the curve and at the maximum force obtained duringcompression. The gradient (sample elasticity) up to 60%of the breakpoint was calculated. Three measurementswere performed on each fillet, and the mean values wereused for statistical analyses of the data.

2.6. Liquid holding capacity

Liquid holding capacity (LHC) was analysed accord-ing to a modification of the method described byGomez-Guillen, Montero, Hurtado, and Borderias(2000). A slice of muscle obtained from the cold-smokedfillet was cut about 3–4 cm in front of the dorsal fin,from the lateral line towards the belly. The skin and thebelly flap fat were removed. The sample (15 g) wasweighed and placed in a tube with a weighted filter paper(V1) (Schleicher & Schuell GmbH, Dassel, Germany).The tubes were centrifuged at 500� g for 10min at10 1C, and the wet paper was weighted (V 2) beforedrying at 50 1C to constant weight (V3). The percentageliquid loss (TLL) was calculated on a wet weight basis as

100� ðV22V 1Þ � S�1, where S=weight of musclesample, water loss (WL) as 100� ðV22V 3Þ � S�1 andfat loss (FL) as 100� ðV 3 � V1Þ � S�1, respectively.

2.7. Calculations and statistical analysis

Respective fillet weights were recorded for the freshfillets, and after cold-smoking and storage of thevacuum packaged cold-smoked fillets. Smoking yield(%) was calculated as 100ðWSM=W IÞ, where WSM wasweight of fillet after cold-smoke processing (g) and W I

of initial fillet (g). Coefficient of variation (CV) wascalculated as CV=100(standard deviation of mean)/mean.To reveal the effects of temperature on the measured

responses, regression analyses using net-pen as unit ofobservation were performed. Both linear and curvilinearregressions were tried for best fit of mean smokingtemperature effects. Regression analyses of the datausing fillet as unit of observation and whole body weightas predictor were performed to reveal correlationsbetween the measured responses and whole body weight.Both linear and curvilinear regression models werefound to give best fit of whole body weight effects. Whensignificant correlations were observed, a one-wayANOVA was performed using groups fed different oiltypes as fixed factor and weight (or in the case of L�-value the squared value of weight) as covariate. Leastsquares means for each net-pen thus corrected for wholebody weight were used in further regression analyses ofresponses to temperature. Data were tested for homo-scedasticity of the variance by examination of theresiduals. Arcsin transformation of percentage data(Sokal & Rohlf, 1995) did not improve the coefficientsof determination and were not used during analyses.Analysis of variance (GLM) was performed to investi-gate the main effects of dietary oil supplements andinteractions to smoking temperatures, using net pen asthe unit of observation. Statistical analyses wereperformed using the MINITABt Statistical Software(Release 13, Minitab Inc., USA) with a confidence levelof 95%, and results are presented as mean7s.e.m.

3. Results

3.1. Raw material

The mean contents of protein, fat, dry matter and ashin the raw material were similar (P40:05) for salmonfed diets added Peruvian fish oil (FO) or soybean oil(SO), whereas the former fish had a significantly higher(Po0:05) concentration of total carotenoids, Table 1.Dietary oil source did not significantly influence the sizeof the estimated fat deposits (P40:05). There were nosignificant differences in body weights for groups of

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Table 1

Protein, fat, dry matter, and ash contents (g/100 g), carotenoid

concentration (mg/kg) and estimated fat deposits (% of total scan

area) of raw fish fed diets supplemented with 100% Peruvian fish oil

(FO) and 100% soybean oil (SO). Mean7s.e.m., n ¼ 3 netpens per

dietary treatment, with samples of three pooled fish per netpena

Diet FO SO

Protein 19.370.3 19.470.2

Fat 15.670.5 15.170.5

Dry matter 35.270.8 34.770.3

Ash 1.670.1 1.970.1

Astaxanthin 4.670.1b 4.070.1c

Canthaxanthin 2.270.1b 1.970.1c

Estimated fat deposits 7.470.2 7.670.2

aMeans with different lower case superscripts in a row are

significantly different (Po0:05) by one-way ANOVA and Fischer’s

pairwise comparison tests.

Table 2

Profiles of major fatty acid (% of total FAME) of the diets and

corresponding raw fillets of Atlantic salmon fed diets supplemented

with either Peruvian fish oil (FO) or soybean oil (SO).

Experimental diets Fillets

FO SO FO SO

R SFAa 28.7 17.5 25.45 17.3

18:1n-9 9.9 16.8 11.8 17.8

20:1n-9 2.8 2.4 3.6 2.8

22:1n-11 0.3 2.4 0.4 2.0

R MUFA 30.6 24.8 32.0 28.0

18:2n-6 4.3 43.6 2.4 32.0

18:3n-6 0.2 ndb 0.1 0.3

20:2n-6 0.2 nd nd nd

20:3n-6 0.1 nd nd nd

20:4n-6 0.8 0.1 0.6 0.2

R n-6 PUFA 5.8 43.7 4.6 34.8

18:3n-3 1.2 5.5 0.9 3.9

18:4n-3 3.1 0.9 2.4 0.9

20:3n-3 0.3 nd 0.5 0.5

20:4n-3 0.7 0.2 1.5 0.7

20:5n-3 13.2 2.2 11.6 2.9

22:5n-3 1.5 0.2 4.0 1.4

22:6n-3 11.8 3.2 11.2 4.5

R n-3 PUFA 31.8 12.2 32.0 14.8

n-3/n-6 5.5 0.3 7.0 0.4

aSFA are saturated, MUFA monounsaturated, and PUFA poly-

unsaturated fatty acids, respectively.bnd: Not detected.

A.M.B. Røra et al. / LWT 38 (2005) 201–211 205

sampled salmon smoked at different temperatures orsalmon fed different diets (P40:05). Large differences infatty acid profile were obtained for Atlantic salmon fedthe diets added Peruvian fish oil (FO) or soybean oil(SO), Table 2. Thus, fillets of Atlantic salmon fed SOhad 7.5-fold higher total amount of n-6 PUFA(polyunsaturated fatty acids), and approximately halfthe amount of n-3 PUFA compared to salmon fed FO.

Salmon fed FO had a higher amount of SFA (saturatedfatty acids, about 7 g/100 g).

3.2. Smoking and drying

The mean logged chamber temperatures of thesmoking cabinet were 21.5, 23.1, 24.3, 26.6, 28.2 and29.9 1C, respectively, for the six different temperatureprocessing regimes. The coefficient of variation (CV)ranged from 5% to 8% for temperature duringprocessing. Relative humidity (RH) in the smokingchamber decreased from 73% at 21.5 1C to 68% at29.9 1C. The linear correlation coefficient between RHand processing temperature was relatively high(R2 ¼ 0:79), and the CV for RH ranged from 6% to10% during processing.

3.3. Effect of processing temperature on quality

parameters

The best fit for the regressions between temperatureand most of the physical and chemical parameters thatwere tested in the present study were obtained for linearmodels. Although a polynomial expression gave slightlybetter fit for total phenol content, a linear model waschosen for simplicity of interpretation. The regressioncoefficients, intercepts at 20 1C, coefficients of determi-nation (R2) and levels of significance for cold-smokedfillets of Atlantic salmon fed diets added either FO orSO are presented in Table 3. No significant effects oftemperature were observed for gaping score, lightness(L�), redness (a�), hue (Hab), chroma (C�), liquid lossduring storage and dry matter content (P40:05).Parameters that were significantly affected by tempera-ture will be presented in the following.Significant relationships were found between proces-

sing yield, b�-value, total liquid loss, force, and totalphenol content (Po0:05) when net-pen was used as unitof observation for all data, but the coefficients ofdetermination were low, Fig. 1. The linear relationshipsbetween quality parameters and temperature wereimproved when data on salmon fed either SO or FOwere treated separately. Significant positive regressionswere observed between temperature and the textureparameters force and gradient for cold-smoked fillets ofAtlantic salmon fed diets supplemented either SO orFO. The mean force and gradient at 20 1C were26.170.8N and 8.870.4N s�1, and 25.970.4N and8.570.4N s�1 for fillet samples of Atlantic salmon feddiets supplemented with FO or SO, respectively. Theobserved force and gradient were 4–14%-units and8–14%-units higher, respectively, at 29.9 than at 21.5 1C.Among the colorimetric parameters only yellowness (b�)exhibited significant correlations with temperature(Po0:05). Although statistically significant, the increase

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Table 3

Regression coefficients (b), and the predicted intercept at T ¼ 20 1C (a20), coefficients of determination (R2) and levels of significance for the

regression (P) for texture, colour, proximate composition, liquid holding capacity parameters and yield using temperature (21.5, 23.1, 24.3, 26.6, 28.2

and 29.9 1C) as regressor and effect of feed type and interaction effects of feed and temperature on cold-smoked fillets of Atlantic salmon fed diets

supplemented with either Peruvian fish oil (FO) or soybean oil (SO)

a20 b R2 P ANOVAa

FO SO FO SO FO SO FO SO Feed Feed*temp

Texture

Force (N) 25.0 25.3 0.42 0.42 0.21 0.28 0.056 0.024b ns ns

Gradient (N/s) 7.9 8.4 0.21 0.15 0.35 0.23 0.010b 0.043 ns ns

Gaping score 1.2 0.8 �0.07 0.05 0.08 0.07 0.253 0.298 ns ns

Colour

L* 43.0 41.1 �0.02 0.07 0.01 0.05 0.764c 0.361 ns ns

a* 8.3 7.5 0.03 0.10 0.03 0.16 0.502 0.103c * ns

b* 16.6 17.6 0.61 0.16 0.13 0.30 0.148 0.018 ns ns

Hab 65.6 67.1 0.09 �0.09 0.02 0.03 0.610 0.471 ns ns

C* 10.2 9.5 0.07 0.09 0.14 0.18 0.132 0.077b ** ns

Proximate composition

Salt content (g/100 g) 3.2 3.8 0.07 �0.009 0.23 0.02 0.046b 0.623b ns ns

Dry matter (g/100 g) 49.7 49.3 0.33 0.10 0.06 0.06 0.347 0.767 ns ns

Total phenol content (mg/100 g) 0.9 0.9 0.04 0.03 0.48 0.38 0.001b

0.006 ns ns

Liquid holding capacity

Total liquid loss (g/100 g) 2.3 2.7 0.09 0.10 0.26 0.21 0.033 0.055 * ns

Lipid fraction in TLL (g/100 g) 71.0 69.9 0.62 0.81 0.23 0.22 0.044 0.051 ns ns

Liquid loss during storage (g/100 g) 3.5 4.6 0.07 0.03 0.07 0.02 0.287 0.629 ** ns

Yield

Processing yield (%) 85.7 84.9 �0.30 �0.16 0.40 0.30 0.011b

0.036b ns ns

ans=not significant; *indicates 0.01oP-valueo0.05; **indicates 0.001o P-valueo0.01.bLeast square means used in regression are corrected for body weight.cLeast square means used in regression are corrected for the squared body weight.

A.M.B. Røra et al. / LWT 38 (2005) 201–211206

in b�-values from 21.5 to 29.9 1C only represented 2%-units of the mean values of 10.070.2 at 21.5 1C.A significant positive correlation was found between

salt content and temperature for cold-smoked fillets ofAtlantic salmon fed diets supplemented with FO(Po0:05), but not for salmon fed diets with SO. Whenthe effect of temperature on salt content was determinedusing dry matter content as a covariate this observa-tion was strengthened (FO, R2 ¼ 0:45, P ¼ 0:002; SOR2 ¼ 0:01, P ¼ 0:66). The mean salt content at 20 1Cwas 3.270.0 g/100 g, and an increase by 16 g/100 g insalt content was observed between 21.5 1C and 29.9 1C inthe former fillets. The total phenol content of fillets ofAtlantic salmon fed diets added FO or SO werepositively correlated to cold-smoking temperature(P ¼ 0:001). The mean total phenol content in the FOand SO groups at 21.5 1C were 1.170.1 and 1.070.1mg100 g�1, respectively. The difference obtained at smok-ing temperatures of 29.9 and 21.5 1C were 27% and30%-units for the SO and FO-groups, respectively.Significant positive correlations with temperature

were observed in cold-smoked fillets in total liquid loss(TLL) and lipid fraction in TLL for Atlantic salmon feddiets added FO (Po0:05). The mean total liquid loss inthe FO and SO-groups at 21.5 1C was 2.570.2 g/100 gand 2.670.2 g/100 g, respectively, and an increase in

TLL after processing at 29.9 compared to 21.5 1C of40%-units for the FO-group and 46%-units for the SO-group were observed. The mean percentages of lipidfraction of TLL at 20 1C were 71.072.0 g/100 g and67.972.5 g/100 g for the FO and SO-group, respectively.An increase by 7% and 12%-units in the lipid fraction ofTLL was found by processing at 29.9 1C compared to21.5 1C for the FO and SO-group, respectively.Processing yield correlated negatively with cold-

smoking temperature in Atlantic salmon fed diets withboth FO and SO (Po0:05). The mean processing yieldsat 21.5 1C were 84.970.6% and 84.570.5% for the FOand SO-group, respectively. However, the observeddecreases in processing yields corresponded to differ-ences of 2%-units for the FO-group and 1%-unit for theSO-group at 21.5 and 29.9 1C, respectively.

3.4. Effect of dietary oil on smoked fillet quality

An ANOVA revealed that dietary oil source only hadsignificant effects on redness (a�-value), chroma (C�-value), total liquid loss and liquid loss during storage ofthe cold-smoked fillets, Table 3. Atlantic salmon feddiets with FO were slightly more red (10%-units,Po0:05) than salmon fed SO (a�-values 8.470.2 and8.170.2). Furthermore, the FO-group had significantly

ARTICLE IN PRESS

Fig. 1. (A–E) Regression lines for processing yield, b�-value, total

liquid loss, force, and total phenol content for cold-smoked fillets of

Atlantic salmon fed diets supplemented with Peruvian fish oil (FO) or

soybean oil (SO) using temperature (21.5, 23.1, 24.3, 26.6, 28.2 and

29.9 1C) as regressor. R2 is coefficient of determination.

A.M.B. Røra et al. / LWT 38 (2005) 201–211 207

higher C�-values than the SO-group (10.670.2 versus10.170.2, Po0:01). There was a slight, but notstatistically significant higher salt content (3 g/100 g,Po0:05) in cold-smoked fillets of salmon fed diets withSO than salmon fed diets with FO, and mean valueswere close to 3.7 g/100 g.A significant effect of dietary treatment was observed

on smoked fillet liquid holding capacity. The SO-grouphad significantly higher (Po0:01) liquid loss duringstorage than the FO-group (4.770.2 g/100 g versus3.870.2 g/100 g). No significant effects on texture, drymatter content, phenol concentration, total liquid loss,or processing yield were found, and no interactioneffects were found for dietary treatment and processingtemperature, Table 3.

4. Discussion

4.1. Effect of temperature on quality

Documentation of production process unit operationsand their effects on quality may assist in development ofquality programmes and may improve reputation andconfidence in product quality. Production of cold-smoked Atlantic salmon needs to be developed into amore firmly controlled industrial process with a highlypredictable outcome. In this study, we documented theeffects of cold-smoking temperature on fillets of Atlanticsalmon with different fatty acid composition onimportant product quality characteristics (Sigurgisla-dottir, Torrissen, Lie, Thomassen, & Hafsteinsson,1997), in particular colouration, texture, liquid holdingcapacity, total phenol and salt contents and yield. Thecold-smoked Atlantic salmon obtained had good qualityusing the specific processing conditions in the presentexperiment. Notably, no tendency to case hardening wasobserved and indicated that adequate RH was used (cf.Cardinal et al., 2001). Also, no severe discolouration atthe surface indicated that the extraction of astaxanthin-protein complex was low (cf. Birkeland & Bjerkeng,2004). The relationships between temperature and theinvestigated quality parameters were generally ratherweak, and temperature only explained a low tomoderate amount of the observed variation (1–48%).Although statistically significant, the maximum effectswere moderate, except for up to 30%-units increase inphenol content. This implies that Atlantic salmonrepresents a quite robust raw material with respect tothe temperatures that are usually employed by theindustry. However, our results are not valid for otherprocessing parameter settings or other unit operationsthan those used specifically in this experiment. Thus,other results may be obtained when fillets are brine orinjection-salted, or when fillets are desalted in water, orwhen other processing temperatures, cold-smoking and

ARTICLE IN PRESSA.M.B. Røra et al. / LWT 38 (2005) 201–211208

drying cycles, or salmon fillets with less good quality areused.The mean relative humidity (RH) decreased in

response to increasing processing temperature in accor-dance with basic thermodynamics (Sonntag & VanWylen, 1982). According to a Mollier diagram for moistair (RH 70%) an increase in temperature from 24 to25 1C, causes a rise in RH by approximately 5–6%.Thus, small changes in the smoking temperature maycause substantial alterations in RH. The observeddifference between the highest and lowest mean RHduring different temperature treatments was 6%-units,which corresponds to an approximate change in thetemperature by 1 1C. The salmon fillets respondrelatively slowly to the fluctuations in the smokingtemperature (Hultmann et al., 2004). Thus, temperatureis regarded as the major contributor to the observeddifferences in quality characteristics of the smoked filletsat the RH obtained in this study.Increased temperature caused a firmer texture and

increased elasticity of the smoked fillets. Increasedfirmness of cold-smoked salmon fillets may be causedby decreased moisture content (Indrasena, Hansen, &Gill, 2000). A higher degree of firmness at highercold-smoking temperature is reported for farmedAtlantic salmon (Sigurgisladottir et al., 2000; Monteroet al., 2003). Smoking processing causes decreasedprotein solubility (Gomez-Guillen et al., 2000) and ahigher shear strength of the myotomes (Gomez-Guillenet al., 2000; Montero et al., 2003). Increasing tempera-tures during cold-smoking causes alterations in theproteins that lead to a marked reduction in extractabi-lity of salt soluble myofibrillar proteins, and this maycontribute to the increased firmness of the fillets(Hultmann et al., 2004). Although the differences intextural properties observed between fillets processed at21.5 and 29.9 1C were significant, the coefficients ofdetermination were low (0.21–0.35), indicating thattemperature explains less than 35% of the variation inthe textural properties in the smoked fillets. Thus, thetextural property of the smoked fillets in this experimentseems to have some resilience to the processingtemperature. Nevertheless, careful adjustment of theprocessing temperature may be an option to targetdifferent textural properties of cold-smoked Atlanticsalmon fillets. Muscle gaping, separation of muscleblocks due to breakage of the myosepta, is a seriousproblem in the salmon processing industry, and thecause of an estimated 40% of quality downgradingduring secondary processing of Scottish salmon (Michie,2001). The gaping score of cold-smoked salmon issignificantly affected by processing conditions in parti-cular when brine injection technology is used (Birkelandet al., 2003; Birkeland, Røra, Skara & Bjerkeng, 2004b).However, in the present study the gaping scores werelow, and unaffected by temperature.

Visual colour is among the most important qualitycriteria of Atlantic salmon (Sigurgisladottir et al., 1997),and among the main discriminating factors of cold-smoked Atlantic salmon products (Cardinal et al.,2004). Furthermore, the redness of salmon has beenimplied as signalling value as an indicator of productquality (Sylvia, Morrissey, Graham, & Garcia, 1996).We observed no effects of processing temperature oncolorimetric parameters (lightness, redness, chroma,hue), except for yellowness of fillets of salmon fed SO,and corresponding R2-values were low (0.01–0.30).Thus, the colour of fillets from Atlantic salmon feddiets with fish- or vegetable oil were relatively robust tothe processing temperatures applied in the present study.The total phenol content of the cold-smoked fillets

exhibited the highest correlation to temperature amongthe parameters investigated, although in the mediumrange (R2 from 0.38 to 0.48). This relationship isconsistent with previously reported data on smokedSpanish mackerel (Chan, Toledo, & Deng, 1975) andAtlantic salmon (Cardinal et al., 2001). The higherabsorption of phenolic compounds at higher tempera-ture may be explained by increased solubility at highertemperatures. There was a weak significant positiverelationship between salt content and temperature inAtlantic salmon fed diets with FO supplement, but notin salmon fed diets with SO supplement, also when datawere corrected for dry matter content. This differencecannot be explained satisfactorily at present, but it maybe speculated that membrane fluidity is involved.Dietary fatty acid composition greatly affects the fattyacid composition of Atlantic salmon muscle, includingthe composition of polar phospholipids (Jobling &Bendiksen, 2003). PUFA have a greater disorderingeffect on lipid bilayer membranes than saturated fattyacids, in particular the bottom part of the moleculeembedded inside the membrane (cf. Saiz, & Klein, 2001).The extent to which temperature has an effect onmembrane order and the diffusivity and in- andoutfluxes of salt may depend on the fatty acidcomposition of the membrane and thus explain thedifferent effects of dietary oil sources in the presentexperiment.Lipids constituted a major fraction of the total liquid

loss (TLL) in the cold-smoked fillets, and showed aweak positive correlation to temperature for salmon fedFO (trend only for salmon fed the SO diet). A TLLpredominately consisting of lipids is consistent withprevious findings in cold-smoked salmon (Røra et al.,2003; Regost et al., 2004). The fatty acid profile of thelipid fraction reflects that of the flesh (Røra et al., 2003).It is common to distinguish between different forms ofwater present in muscle foods (Fennema, 1985). Thus,bound water denotes water that is firmly associated toproteins and other polymers such as DNA (ca. 0.5 g/gprotein), entrapped or immobilized water that is held by

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steric effects or is attracted to bound water, and freewater that is held by weak surface forces. Alterations inprotein configuration and unfolding involve weakintramolecular interactions (not covalent bonds), inter-actions with hydration bound water and salts. It usuallyrequire little energy and changes in free energy change,DG1, are small, although separate enthalpy and entropyterms may be substantial (Dunitz, 1995). Decreasedextractability of salt soluble myofibrillar proteins withincreasing temperature in cold-smoked salmon (Hult-mann et al., 2004) indicates a certain extent of proteindenaturation. Alterations in the structures entrappingwater and holding free water are those likely to beinvolved in the decreased liquid holding capacity.However, the detailed structural basis for this decreaseneeds further elucidation.Optimising the yield throughout the value-adding

process of smoking is a major area of focus to improveeconomic success. Profit margins in the fish processingindustry are often small and even small differences inprocessing yield may have considerable impact on theeconomy in the salmon processing industry. A decreasein processing yield (1–2 g/100 g) with increasing proces-sing temperature was observed in the present study. Thisis in accordance with previously reported data forsmoked Atlantic salmon (Birkeland et al., 2004a). Case-hardening may be detrimental to product quality since itprevents effective drying and can reduce product shelflife, and was not observed at the temperatures and RHsused in the present experiment. Cardinal et al. (2001)related the higher losses in Atlantic salmon fillets atsmoking temperatures of 30 compared to 20 1C to casehardening. A plausible explanation for the case hard-ening observed by Cardinal et al. (2001) is the appliedcombination of low RH (50%), air velocity (2m s�1) andhigh temperature (30 1C) during drying and smoking.Case hardening can be avoided if optimised processingconditions regarding air velocity, temperature, RH anddrying and smoking time is applied during cold-smokingof salmon fillets.

4.2. Effect of raw material type on quality

Aquaculture production has been the engine ofgrowth in food fish production globally, and Atlanticsalmon represented 48% of high-value finfish produc-tion and thus generated about 19% of all export revenuefrom fisheries in 1997 (Delgado, Wada, Rosegrant,Meijer, & Ahmed, 2003). Fish meal and fish oil are themajor ingredients of salmon feeds. The current con-sumption of fish oil for salmon farming representedabout 19% of the global fish oil production in 1999(Delgado et al., 2003). Since the global production offish oil is expected to be at the same level in 2020 as itwas in 1985 there is a demand for alternative oils fromplant sources to accommodate the soaring production of

high-value fishes. Soy products, including soy bean oil,are currently of wide use in intensive aquaculture andmay successfully be incorporated in feeds for Atlanticsalmon (Storebakken, Refstie, & Ruyter, 2000). How-ever, the characteristics of the farmed salmon as rawmaterial for smoking are changed when farmingstrategies and feeds are altered. It is therefore importantto ensure that the quality and properties of the feedingredients are in harmony with the requirements of theprocessing industry and the consumer to obtain fishproducts with adequate quality.Omega-3 fatty acids, notably eicosapentaenoic and

docosahexaenoic acids, are abundant in fish productsand prevent cardiovascular disease (Schacky, 2004).Alterations in fatty acid composition of the food maythus have an impact on consumer health and nutrition.It should be noted that alterations in fatty acidcomposition of Atlantic salmon depends on the feedand can be tailored (cf. Torstensen, Frøyland, Ørnsrud,& Lie, 2004), but further discussion is outside the scopeof the present work.Only minor effects of dietary oil source were found on

quality parameters in the present experiment. However,a significantly lower total carotenoid content in thesalmon fed the SO diet than salmon fed the FO diet wasreported previously (Regost et al., 2004). This was alsofound to influence redness. Total liquid loss and liquidloss during storage were lower in the cold-smokedsalmon fed the FO diets. A study on pork has shownthat water holding capacity of pork loin was improvedby increased intramuscular fat apparently caused bydietary conjugated linoleic acid (Joo, Lee, Ha, & Park,2002). No difference in total area of fat deposits wasobserved in the present experiment. However, arelatively high amount of MUFA, and less SFA andPUFA in the fillets of salmon fed the SO diets may altermembrane structures, and thus explain differences inliquid holding capacity.

Acknowledgements

This work was supported by the grants 135841/140and 140733/130 financed by The Research Council ofNorway.

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