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JFS S: Sensory and Nutritive Qualities of Food

Effect of Soybean Meal-based Diets on theProduct Quality of Rainbow Trout FilletsNATASHA D’SOUZA, DENISE I. SKONBERG, DAVID A. J. STONE, AND PAUL B. BROWN

ABSTRACT: This study investigated the long-term effects of soybean meal (SBM)-based diets on rainbow trout quality.Two levels of SBM inclusion diets, 20% and 40%, were employed and compared with a fish meal control diet. Rainbowtrout were fed one of the three diets for 6 mo, then harvested and filleted. Proximate composition, color, lipidoxidation and sensory quality of the fillets were evaluated. Thiobarbituric acid reactive substances (TBARS) andheadspace propanal in ground fillets stored at 4 ◦C were measured over a 9-d period. Tristimulus color was measuredon day 2 and day 13 of refrigerated storage. Sensory evaluation included discriminatory (difference) and affective(acceptance) testing. TBARS and propanal in the trout fed the highest (40%) SBM level were significantly lower thanin the control and trout fed the 20% SBM diet. Significant differences in color were also recorded during refrigeratedstorage. Sensory difference testing revealed a significant difference between the trout fed the 40% SBM and thecontrol. No significant differences were observed in the acceptability ratings of trout fillets from the three differentdietary treatments.

Keywords: rainbow trout, quality, soybean meal, sensory

Introduction

Rainbow trout is a popular freshwater fish, primarily producedin Idaho (70% to 75% of US trout production). Trout feeds have

typically been based on fish meal and fish oil, from fish that are notconsumed as food. However, the fluctuation in costs of fish meal dueto limited availability of traditional fish stocks has led to a definiteneed for an alternate protein source to fish meal in order to sus-tain trout aquaculture (Naylor and others 2000; Hardy 1999). Theadvantages and disadvantages of several alternate protein sourcesincluding soybean meal (SBM), wheat gluten, lupin, and cottonseedmeal have been discussed by Hardy (1996), Tacon and others (1983)and Fowler (1980). SBM offers economic, availability, and nutritionaladvantages over several of the other alternate proteins sources (Limand others 2004; Hardy 1996). Increased use of SBM in aquaculturediets may also help to decrease dioxins, dieldrins, and polychlori-nated biphenyls (PCBs) found in muscle of farm-raised salmonidsgrown on traditional fish meal diets (Hites and others 2004). Theantinutritional components of SBM such as trypsin inhibitors andphytic acid which could limit its use in aquaculture diets may beovercome by pretreatment of the SBM before use in diets or reducedby processing treatments such as extrusion and pelleting (Vielmaand others 2000; Hardy 1996; Kaushik 1990).

The use of SBM as a substitute for fish meal has been investigatedfor several fish species including salmonids (Refstie and others 2000;Kikuchi 1999; Fowler 1980; Cho and others 1974). Commercial di-ets available for trout usually contain SBM in only limited amounts(maximum inclusion 20%). However, SBM can replace up to 47%fish meal in olive flounder and up to 33% in Pacific salmon diets, asdescribed by Kikuchi (1999) and Carter and Hauler (2000). The ef-fects of feeding diets containing soy on the growth, body proximate

MS 20050614 Submitted 10/13/05, Accepted 3/7/06. Authors D’Souzaand Skonberg are with the Dept. of Food Science & Human Nutri-tion, Univ. of Maine, Orono, ME 04469. Author Stone is with HagermanFish Culture Experiment Station, Univ. of Idaho, Hagerman, Idaho. Au-thor Brown is with Dept. of Forestry and Natural Resources, PurdueUniv., West Lafayette, Ind. Direct inquiries to author Skonberg (E-mail:denise.skonberg@umit.maine.edu).

composition, and quality attributes of rainbow trout have been in-vestigated to some extent (Vielma and others 2000; Kaushik andothers 1995; Smith and others 1988; Watanabe and others 1988).Kaushik and others (1995) reported that total substitution of fishmeal by a soy protein concentrate did not result in any negative ef-fects on growth and flesh quality. However, most of the studies thathave examined the effects of soy inclusion on product quality wereeither short-term (6 wk or less) or employed low levels of soy (30%or less).

In a recent report of a long-term feeding study for 24 wk with plantprotein mixture, de Francesco and others (2004) found significanteffects on fillet color, lipid content, and organoleptic characteristicsof large commercial size rainbow trout. The plant mixture includedprotein sources such as corn and wheat gluten, rapeseed meal, andpeas. This study did not examine SBM inclusion. Thus, a shift inthe composition of trout feeds towards incorporating plant proteinsin place of fish meal may affect fillet quality. Few studies have fo-cused on the influence of alternate dietary protein source on lipidoxidation in trout fillets. However, a report by Lopez-Bote and others(2001) showed that dietary soy protein affected lipid peroxidation inrainbow trout and sea bass. Another study (D’Souza and others 2005)showed a significant effect of dietary genistein (a soy component)on reducing lipid oxidation in trout fillets.

The objective of this study was to evaluate the effects of feedingdiets containing 0%, 20%, and 40% SBM for 24 wk on several qual-ity attributes of the rainbow trout fillets. Since the consumer is theultimate user of the product, sensory testing was also performedto determine if the high inclusion levels of SBM in the diet wouldcause any differences in sensory quality of the edible product and, ifso, whether those differences would affect consumer acceptability.

Materials and Methods

Feeding studyThree experimental diets were formulated to contain 44% crude

protein and 20% crude lipid, and replaced fish meal with defatted

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Table 1 --- Diet formulation and chemical composition

0% SBM 20% SBM 40% SBM

Ingredients (%)Herring meal 50.00 30.00 10.00Soybean meal 0.00 20.00 40.00Wheat gluten 3.00 3.00 3.00Wheat flour 22.80 15.36 7.92Corn gluten meal 8.00 12.73 17.49Fish oil 13.62 15.47 17.31Vitamin C 0.30 0.30 0.30Choline 0.50 0.50 0.50TM salt 0.10 0.10 0.10Vitamin premix 1.50 1.50 1.50Lysine 0.18 0.78 1.37Methionine 0.00 0.26 0.51

Proximate composition (% dwb)Protein1 48.2 47.9 46.4Lipid2 18.1 19.2 19.3Ash3 9.0 7.1 5.5Moisture4 5.2 5.5 5.3Carbohydrate5 19.5 20.3 23.5

1,3,4Measured by AOAC (1995) methods. Protein (N × 6.25) by nitrogenanalyzer, ash by heating in muffle furnace at 550 ◦C and moisture by drying at105 ◦C.2Lipid measured by acid hydrolysis method.5Carbohydrate by difference.

SBM at 0% (control), 20%, and 40% of the diet on a dry-weight basis(Table 1). Rainbow trout (Oncorhynchus mykiss) with an initial meanweight of 19 g were grown at the Hagerman Fish Culture ExperimentStation (Hagerman, Idaho, U.S.A.) for 3 mo and then transferredto the Clear Springs Food Inc. research facility (Buhl, Idaho). Fishwere reared on each of the three diets in duplicate (150 L) tanks,each supplied with 4 to 6 L/min of untreated, constant temperature(14.5 ◦C), spring water. Trout were fed three times per day to apparentsatiation for the first half and twice per day during the second halfof the feeding trial, 6 days a week for a total of 6 mo. At the end of thefeeding trial, the trout were harvested, weighed, gutted, filleted, andshipped overnight on ice to Univ. of Maine. Upon arrival (day 0),fillets were rinsed in cold running water and sorted according totank and diet. Fillets intended for sensory evaluation were placedin Ziploc freezer bags, boxed, and frozen at −20 ◦C for 35 d. Groundsubsamples from each tank were prepared on day 0 by skinning andblending four fillets in a food processor for 45 sec. These sampleswere analyzed immediately for proximate composition. The groundsamples were stored in a refrigerator at 4 ◦C for a period of 13 d tomonitor lipid oxidation and effect of storage on color.

Color analysesInstrumental color was recorded using a HunterLab ColorScan

XE colorimeter (Hunter Associates Laboratory, Reston, Va., U.S.A.)on day 2 and day 13 of storage at 4 ◦C. L∗ (white intensity), a∗ (redintensity), and b∗ (yellow intensity) values (CIE 1976) were recordedfor ground sample from each tank. The values recorded were anaverage of three readings, each taken at a 45 ◦ turn. The colorimeterwas calibrated using white and black tiles prior to taking readingson each day.

Proximate analysesProtein, fat, moisture, and ash were determined using standard

AOAC (1995) methods. Moisture determination was done by dryingsamples overnight in an oven at 105 ◦C. Dried samples were groundand used for determination of crude protein by the Rapid N nitrogenanalyzer (Elementar, Analysensysteme Gmbh, Germany). Crude fatwas determined by the acid hydrolysis method and ash by heatingat 550 ◦C in a muffle oven for 6 h. All analyses were performed, induplicate, on the ground subsamples from each tank.

Lipid oxidation analysesLevels of thiobarbituric acid reactive substances (TBARS) formed

in the ground sample from each tank from each of the three dietgroups (0%, 20%, and 40% SBM) were monitored over a 9-d storageperiod at 4 ◦C. All analyses were performed in duplicate (two anal-yses of each subsample per tank). The extent of lipid oxidation wasdetermined by measuring levels of the malonaldehyde-TBA com-plex (Raharjo and others 1993).

Propanal, an aldehyde formed during oxidation of ω-3 fatty acids,was monitored in the trout fillets over a 9-d storage period at 4 ◦C. Theground sample (1 g) was sealed in a 22 mL glass vial and broughtto 100 ◦C in a headspace oven Teklink 7000 (Tekmar Dohrmann,Mason, Ohio, U.S.A.), and volatile components collected in theheadspace were then separated and analyzed by a gas chromatog-raphy system Agilent 6890 Series (Agilent Technologies, Palo Alto,Calif., U.S.A.). The following chromatographic conditions were used:Back Inlet–Splitless mode, injection temperature 225 ◦C, pressure11.66 psi, purge flow 259.6 mL/min for 1.5 min, helium gas. Cap-illary column–Crossbond Carbowax-PEG (Restek Stablwax), length30 m, 320.0 μm dia with 2.6 mL/min initial flow and average velocity,41 cm/sec, Flame-ionization Detector - temperature 180 ◦C, hydro-gen flow @ 40.0 mL/min, air flow @ 450 mL/min, combined flow @45.0 mL/min. A standard curve prepared from a series of propanalconcentrations (0, 3.125, 6.25, 12.5, 25, 50, and 100 μM) analyzedconcurrently was used to calculate the amounts of volatiles givenoff from trout samples.

Sensory evaluationDifference testing. Trout fillets were thawed in the refrigerator

at 4 ◦C for 2 d prior to sensory testing. Thirty panelists who wereseafood consumers were recruited through flyers posted through-out the Univ. of Maine campus as well as by the intranet email sys-tem. Each of the 30 untrained panelists first performed triangle tests(ISO 1983) on four sets of patties (1′′ dia and 1

4′′ thick) made from

skinned, ground fillets from each diet group. Each set consisted ofthree patties coded and served in ceramic bowls where two wereduplicate samples (patties from the same diet group) and one wasfrom the other diet group. Presentations were randomized such thatall samples were evaluated in a particular order an equal numberof times. Panelists evaluated the patties from the control group (0%SBM) against patties from each of the SBM-containing groups (20%and 40% SBM) in both raw and cooked form. They were asked toidentify the odd sample in each set based on visual and aromaticdifferences when evaluating raw samples and on overall differences(including taste) when evaluating cooked patties. All patties weremade on the day of the test. Patties served raw were kept refriger-ated at 4 ◦C. Cooked patties were baked in a gas oven at 204 ◦C for7 min and held warm in a holding oven at 66 ◦C until served. All tri-angle tests were followed by paired comparisons in which panelistscompared the two groups and indicated which sample had a whiterflesh color, which had a more fishy aroma (or taste), and which onethey preferred overall.

Hedonic testing. Since triangle tests revealed significant differ-ences, acceptability testing was further performed to assess if thedifferences influenced acceptability. Fifty untrained panelists par-ticipated in the study, of which 29 were male. Some of the panelistswho participated in hedonic testing had previously participated indifference testing, but had no knowledge that samples were fromthe same study.

Panelists were asked to answer questions about trout and soyconsumption, liking, and purchasing behavior, before evaluatingsamples. They were then served baked trout fillet portions from thethree diet treatments and asked to rate each fillet for color, texture,

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aroma, flavor, appearance, and overall acceptability on a 1 (dislikeextremely) to 9 (like extremely) acceptability scale using the SIMS(Sensory Computer Systems, Morristown, N.J., U.S.A.) computer-ized data collection system. Trout fillets with skin on were baked ina gas oven at 204 ◦C for 10 min and held warm in a holding oven at66 ◦C until served. All sensory testing received prior approval fromthe Univ. of Maine Human Subjects Committee and was performedunder standardized (ASTM) conditions (Eggert and Zook 1986).

StatisticsData were subjected to analysis of variance (ANOVA) using

SYSTAT (SYSTAT Software Inc, Point Richmond, Calif.). Means werecompared by Tukey’s honest significant test (HSD). Standard de-viations and average values were calculated in EXCEL (Microsoft,Redwood, Wash., U.S.A.). Significance for triangle tests and pairedcomparisons results were analyzed by probability tables found inMeilgaard and others (1999). Statistical significance was acceptedat p < 0.05.

Results and Discussion

GrowthSignificant differences (p < 0.05) were found in the body weight

of trout (Table 2) in our study. The trout fed the 40% SBM had loweraverage weights (approximately 290 g per fish) than trout fed the 0%SBM or the 20% SBM diets (approximately 360 g per fish). Mambriniand others (1999) reported a lower growth rate of rainbow troutwith increasing soy protein concentrate content and reasoned itto be due to a deficiency of methionine. A study by Medale andothers (1998) found a lower feed intake in rainbow trout consum-ing soy protein concentrate (SPC)-based diets when compared tothose consuming fish meal-based diets. These diets contained highamounts of isoflavones which were suggested as a possible reasonfor the decreased feed intake. Fagbenro and Davies (2001) also re-ported a significant reduction in growth and feed utilization in cat-fish with increasing levels of soybean flour. Weight gain was signif-icantly depressed at the highest replacement levels of 75% withoutmethionine. However, after supplementation with methionine, cat-fish weight gain significantly improved. Since in this study neitherisoflavones nor methionine deficiency could have played a role, it isassumed that the lower growth could be due to poor digestibility ofsoy carbohydrates, or other antinutritional factors.

ColorThe fillets from trout fed the 40% SBM diet were significantly

whiter (L∗ value) in color (Table 3) than trout from the 0% SBM and20% SBM diets. The red color (a∗ value) significantly decreased overtime for fillets from all three diet groups. A decrease in dietary fishmeal content showed a corresponding decrease in a∗ values. The

Table 2 --- Mean weights of harvested fish fed 0%, 20%, or40% soybean meal (SBM) diets

Mean Number Average weightweight (g) of fish (g) of fish per

Diet Tank per fish harvested diet group1 ± SD

0% SBM Tank 1 374 9 374a ± 0.0Tank 2 374 8

20% SBM Tank 1 365 8 352a ± 17.6Tank 2 340 14

40% SBM Tank 1 290 13 289b ± 1.4Tank 2 288 14

1Different superscript letters (a, b) in the same column indicate significantdifferences among treatments.

Table 3 --- Instrumental color determinations of groundrainbow trout fillets from fish fed 0%, 20%, or 40% SBMdiets and held at refrigerated storage (4 ◦C)

Days ofParameter storage 0% SBM 20% SBM 40% SBM

L∗ Day 2 60.53a ± 2.0 58.53a ± 0.1 64.17b ± 1.7Day 13 59.89a ± 1.8 59.87a ± 1.8 64.20b ± 2.8

a∗ Day 2 5.62a,A ± 0.3 4.84a,A ± 0.0 4.13a,A ± 1.1Day 13 1.73a,B ± 0.8 2.12a,B ± 1.3 2.07a,B ± 0.8

b∗ Day 2 20.30a ± 0.1 19.21a ± 1.6 20.81a ± 0.3Day 13 19.71a ± 0.7 19.10a ± 0.7 20.96a ± 0.0

All values are mean ± SD. Different superscript letters in the same row (a, b) orin the same column (A, B) indicate statistically significant differences at p < 0.05within the same color parameter.

increased “lightness” in color of the trout fed the 40% SBM diet isin accordance with the results of de Francesco and others (2004)who found a significantly lighter color in the fillets from trout fedthe plant protein mixture. The lighter color of the fillets has beenattributed to lower pigmentation (astaxanthin) in the soy diets ascompared to diets containing fish meal where the fish meal servesas a source of astaxanthin (Hardy 1996; de Francesco and others2004). The decrease in a∗ values (redness) observed in our studyduring storage at 4 ◦C may have been due to oxidation of carotenoidpigments, however, astaxanthin concentration of the fillets was notmonitored. Skonberg and others (1998) reported an increase in yel-lowness in raw fillets when trout were fed diets containing corngluten. This is contradictory to results from our study, where no sig-nificant differences in b∗ value (a measure of yellow intensity) werefound between trout from the three diet groups, although the SBMcontaining diets had higher corn gluten content than the 0% SBMdiet.

Proximate compositionNo significant differences in moisture, protein, fat, or ash con-

tent were found among trout fillets from the three dietary treat-ments (Table 4). Average values for moisture, protein, fat, and ashwere 76.8%, 18.4%, 3.3%, and 1.3%, respectively, similar to values ob-tained by Adelizi and others (1998) for rainbow trout. In that study,feeding rainbow trout a 20% SBM diet resulted in fillet proximatecomposition values of 76.1%, 17.8%, 3.7%, and 1.4% for moisture,protein, fat, and ash, respectively.

Lipid oxidationDiet had a significant influence on the extent of lipid oxidation

during refrigerated storage as indicated by both the TBARS andthe headspace volatiles analyses. The amount of TBARS (Figure 1)formed in fillets from the 40% SBM dietary treatment were notice-ably lower on day 7 and significantly lower (p<0.05) on day 9 as com-pared to the levels in trout from the other two treatments. ANOVA didnot show any significant differences between TBARS formed in thetrout fed the three different diets on the first 2 d of analysis (day 2 andday 5). Headspace propanal (Figure 2) in the fillets of trout fed thethree diets followed a similar pattern as the TBARS concentrations

Table 4 --- Proximate composition of rainbow trout filletsfrom fish fed 0%, 20%, or 40% SBM diets

Diet % Protein % Fat % Ash % Moisture

0% SBM 18.7 ± 0.3 3.3 ± 0.2 1.3 ± 0.1 76.8 ± 0.120% SBM 18.0 ± 0.0 3.5 ± 0.3 1.4 ± 0.0 76.4 ± 0.140% SBM 18.5 ± 0.1 3.2 ± 0.2 1.3 ± 0.0 77.2 ± 0.4

All values are mean ± SD. (N = 4 for each analysis). Values are given on awet-weight basis.

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Figure 1 --- TBARS levels in ground rainbow trout fillets dur-ing refrigerated storage. Values are mean TBARS levels(N = 2) determined in ground fillets from each treatment.Different superscript letters (a, b) indicate significant dif-ferences (p < 0.05) among the three dietary treatmentsat each time-point (day) analyzed.

Figure 2 --- Headspace propanal levels in ground rainbowtrout fillets during refrigerated storage. Values are meanheadspace propanal levels (n = 2) determined in groundfillets from each treatment. Different superscript letters(a, b) indicate significant differences (p < 0.05) amongthe three dietary treatments at each time-point (day) an-alyzed. n.d = not determined.

measured. Headspace propanal was analyzed on day 2, 5, 7, and 9.However, due to instrumental error on day 2, the results from thattime-point are not presented. Similar to the results from the TBARSanalyses, propanal concentrations found in headspace of trout fil-lets from the 40% SBM diet treatment were noticeably lower thanthose found in headspace of trout fed either the 0% SBM or 20%SBM diets, although the differences were not significant due to thelarge between-tank variability within dietary treatments. TBARS andpropanal data from day 7 and 9 of refrigerated storage showed astrong positive correlation (R = 0.94 and 0.99, respectively).

The results from TBARS analyses in our study correspond wellto in vitro studies by Lopez-Bote and others (2001) who found

Table 5 --- Triangle tests (raw and cooked) between trout patties from control (0% SBM) diets and SBM diets

Number of correct Critical number of Difference DifferenceFish pairs responses (of 30) correct responses (p < 0.05) (p < 0.001)

0% SBM vs. 20% SBM (Raw) 14 15 No No0% SBM vs. 20% SBM (Cooked) 15 15 Yes No0% SBM vs. 40% SBM (Raw) 29 15 Yes Yes0% SBM vs. 40% SBM (Cooked) 25 15 Yes Yes

that muscle of trout fed diets containing fish meal as the mainsource of protein showed higher susceptibility to forced peroxida-tion than did muscle from trout fed plant protein-based diets. Incontrast, de Francesco and others (2004) did not observe any differ-ences in lipid oxidation in trout fed plant protein-based diets andtrout fed fish meal-based diets. Unlike this study, the latter com-pared TBARS levels in the muscle of trout fed the two diets at asingle time-point and not over a continuous period of refrigeratedstorage.

Lipid oxidation is a primary concern in fish owing to its high con-tent of unsaturated fats. Moreover, pro-oxidants such as heme ironand metals may accelerate the process. The reduced lipid oxida-tion seen in fillets from trout fed the 40% SBM could lead to thepossibility of an increased shelf-life for SBM-raised trout. Fornshell(2002) reported shelf-life to be ranked among the top three market-ing attributes for rainbow trout buyers. Although no significant dif-ferences were observed in fillet fat content among the three groups,there could very well have been differences in the fatty acid pro-file of the fillets which would influence their susceptibility to lipidoxidation.

Sensory evaluationDifference testing. There were no significant sensorial differ-

ences found between raw patties from ground fillet from the 0% SBMand the 20% SBM dietary treatments (Table 5). However, untrainedpanelists (N = 30) found a significant difference (p < 0.05) betweenthe cooked patties from the 0% and 20% SBM treatments. Signifi-cance for results from triangle tests are based on the critical numberof correct responses (Table 5) required for an appropriate samplesize (N = 30) in probability tables (Meilgaard and others 1999). Tri-angle test panelists found a highly significant difference (p < 0.001)between both raw trout patties from the 0% and 40% SBM diets andcooked patties from the 0% and 40% SBM diets. Results from onlythose panelists who correctly identified the odd sample (in triangletests) were taken into consideration when determining significancefor paired comparisons. Paired comparisons revealed a significantlywhiter color (Table 6) for the raw trout patties from the 40% SBMdiet.

Hedonic testing. The majority of consumers that participated inthe acceptability or hedonic test were in the age range of 18 to 33 y.Prior to the actual testing, consumers were asked a few questionsabout eating habits. Of the 50 panelists that participated in the study,35 said they consumed trout 1 to 10 times a year, 15 consumed soyproducts once a month, 24 said they did not normally buy farm-raised fish, and 19 panelists said they did not know if they coulddifferentiate between farm-raised and wild fish. Although the troutfrom the 20% SBM diet received slightly higher mean scores (Table7) than for the trout fed the 0% SBM and the 40% SBM diets onalmost all attributes, there were no significant differences (p > 0.05)in consumer acceptability (N = 50) between baked trout fillets fromtrout fed the 0%, 20%, and 40% SBM diets.

The significant differences between the control (0% SBM) troutand the trout fed the SBM-containing diets are likely due to filletcolor, as suggested by the panelists who reported that their choice

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Table 6 --- Paired comparisons between raw trout pattiesand between baked trout patties from control diet andSBM diets

Panelists response SignificanceCondition (n = 14) (p < 0.05)

Attribute 0% SBM 20% SBMMore white 5 9 NoMore fishy aroma 8 6 NoOverall preference 8 6 No

Panelists response SignificanceRaw (n = 29) (p < 0.05)

Attribute 0% SBM 40% SBMMore white 0 29 YesMore fishy aroma 18 11 NoOverall preference 13 16 No

Panelists response SignificanceCondition (n = 15) (p < 0.05)

Attribute 0% SBM 20% SBMMore white 11 4 NoMore fishy flavor 11 4 NoOverall preference 10 5 No

Panelists response SignificanceCooked (n = 25) (p < 0.05)

Attribute 0% SBM 40% SBMMore white 9 16 NoMore fishy flavor 14 11 NoOverall preference 15 10 No

of the odd sample was based primarily on color differences. Theseresults are consistent with data from instrumental measurements.When panelists were forced to indicate their overall preference be-tween the control group and the SBM-containing groups, no definitetrend could be seen for the raw patties. Panelists had a slight prefer-ence (not significant) for the cooked patties from the 0% SBM dietgroup. Secondly, results from the hedonic testing showed no signif-icant differences between mean overall acceptability scores for allthree diet groups. These results demonstrate that even the visuallyobservable differences in color did not negatively affect consumeracceptability. Kaushik and others (1995) and de Francesco and oth-ers (2004) both found organoleptic differences between trout fedeither 100% fish meal diets or a 100% plant protein-based diet. How-ever, these studies employed descriptive evaluation and not accept-ability testing. Descriptive panelists are usually trained to perceiveselect attributes and do not represent the average untrained con-sumer. The results from hedonic testing are consistent with the re-sults from paired comparisons in which no significant overall pref-erence for either diet group was seen. Most of the scores obtained forall attributes evaluated by hedonic testing were around 6.0, which

Table 7 --- Mean acceptability scores for baked rainbowtrout fillets from fish fed 0%, 20%, or 40% SBM diets

Diet

0% SBM 20% SBM 40% SBM

AttributeColor 6.1 ± 1.4 6.3 ± 1.3 5.9 ± 1.6Appearance 5.8 ± 1.4 6.4 ± 1.4 6.0 ± 1.6Aroma 5.7 ± 1.7 6.0 ± 1.6 5.6 ± 1.4Texture 6.2 ± 1.7 6.4 ± 1.7 6.3 ± 1.4Flavor 6.3 ± 1.8 6.3 ± 1.7 6.0 ± 1.7Overall 6.3 ± 1.6 6.4 ± 1.6 6.1 ± 1.6

Values are mean scores (±SD) for N = 50 panelists. Scores are based on a9-point hedonic scale where 1 = dislike extremely to 9 = like extremely.

on the scale would represent “like slightly.” Sample size and bonynature of the fillets could be reasons for the low scores. Sensoryevaluation results across studies are difficult to compare due to dif-ferences in the age of fish, time between slaughter and analyses, levelof inclusion, and method of cooking. However, color differences be-tween trout fed fish meal diets and trout fed plant protein-baseddiets remain the common finding for all studies. It would also beinteresting to see if the significant differences in lipid oxidation pat-terns in the three groups of trout would bring about any changesin taste if sensory testing were performed on refrigerated fillets asopposed to frozen fillets.

Conclusions

SBM inclusion in the diets of rainbow trout at levels up to 20%did not affect proximate composition, lipid oxidation, color, or

consumer acceptability of fillets when compared with trout fed thefish meal-based control diet. A 40% SBM inclusion level resulted insignificant color differences but these did not affect consumer ac-ceptability of the fillets. A significantly slower lipid oxidative mecha-nism was also seen during refrigerated storage in muscle of rainbowtrout fed the higher SBM levels. With regard to product quality, thisstudy supports the use of SBM as a suitable alternate protein sourcefor growing rainbow trout. The incorporation of SBM in diets oftrout may help provide a less expensive and constant supply of thisfamiliar species.

AcknowledgmentsThe authors thank Dr. Scott LaPatra, Clear Springs Food Inc. (Buhl,Idaho, U.S.A.) for coordinating the second phase of the feeding studyand Drs. Calder and Perkins, Univ. of Maine, for technical assis-tance. Funding for this project was received from the United SoybeanBoard. This manuscript is nr 2858 of the Univ. of Maine Agriculturaland Forest Experiment Station.

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