Aquaculture Nutrition

Transcription

1 Aquaculture Nutrition doi:./j x..., Applied and Industrial Biology, Department of Biology, University of Bergen, Bergen, Norway; Aquaculture Faculty, Nha Trang University, Nha Trang, Khanh Hoa, Vietnam Cobia, Rachycentron canadum (5 g) cultured in pond cages for a 3-month experiment were fed two moist diets based on raw fish with or without added fish silage. No significant differences in nutritional composition were observed between the fillet groups, which were of high quality with a balance of essential and non-essential amino acids (EAA/NEAA = ) and medium levels of omega-3 fatty acid composition ( g kg total fatty acids). The total quality index method and quantitative descriptive analysis from both groups were correlated throughout storage (r =.83.86). After 5 days iced storage, the scores of most attributes were low compared to maximum accepted values. The thiobarbituric acid reactive substances and microbial counts were also below the accepted limits after the storage trial. It might be concluded that the nutritional composition and the fillet quality were similar for the groups fed raw fish with or without added fish silage, and the estimated shelf life for cobia was >5 days. KEY WORDS: cobia, fish silage, quality index method, quantitative descriptive analysis, sensory evaluation, shelf life Received 3 January ; accepted May Correspondence: Diep T. N. Mach, Applied and Industrial Biology, Department of Biology, University of Bergen, PO Box 78, 5 Bergen, Norway. diep.mach@bio.uib.no Cobia, Rachycentron canadum Linnaeus (766), with excellent characteristics, for example good fillet quality, high commercial prices and fast growth, are considered to be a noteworthy candidate species for commercial aquaculture. As a potential species in aquaculture, cobia research has been devoted to nutritional demands, diseases and aquaculture conditions, while work on fillet quality and storage of cobia fillets has not been published. Sensory evaluation is considered as a rapid, cost-efficient and accurate method for the assessment of quality, shelf life and storage conditions of food (Nielsen 997; Martinsdottir ). The first sensory method was developed by Torry Research Station (Shewan et al. 953). The new method, the Quality Index Method (QIM), is based upon a scheme that was developed by the Tasmanian Food Research (Bremner 985). Now, QIM has been developed for many species in Europe and Nordic countries (Larsen et al. 99; Huss 995) including: Red fish Sebastes marinus (Martinsdottir & Arnason 99), Atlantic mackerel, horse mackerel and European sardines (Andrade et al. 997), brill, dab, haddock, pollock, sole, turbot and shrimp (Luten ), and gilthead sea bream (Huidobro et al. ), Atlantic salmon (Sveinsdottir et al., 3), herring (Jonsdottir 99; Nielsen & Hyldig 4), common octopus (Barbosa & Vaz-Pires 4), flounder (Massa et al. 5), Atlantic halibut Hippoglossus hippoglossus (Guillerm-Regost et al. 6), cuttlefish and shortfin squid (Vaz- Pires & Seixas 6), hybrid striped bass (Nielsen & Green 7), and Atlantic cod (Jonsdottir 99; Bonilla 4; Bonilla et al. 7). The most commonly used attributes for fish are the appearance of eyes, skin and gills, together with odour and texture. The development of QIM for a particular seafood or fish species involves the selection of appropriate and best fitting attributes to observe a linear increase in the QI during iced storage time. The maximum storage time and thus the limit of rejection of fish can be determined by the sensory evaluation of cooked samples using Quantitative Descriptive Analysis (QDA) (Stone & Sidel 993; Huss 995; Sveinsdottir et al.... ª Blackwell Publishing Ltd

2 , 3; Bonilla et al. 7; Nielsen & Hyldig 4). The results from QDA should be used as a reference when developing QIM for fresh fish. The aim of this study was to determine whether inclusion of fish silage in a raw fish diet has an influence on fillet quality and storage of cobia, through comparing the nutritional composition of fillets and shelf life of whole gutted fish by sensory evaluation throughout iced storage. In comparison with sensory evaluation, microbial growth and lipid oxidation were also investigated in fillets during iced storage. Vietnam. Lizardfish was minced and mixed with 5 g kg of formic acid (85%) and. g kg of potassium sorbate to prevent the growth of bacteria and fungi, respectively (Mach & Nortvedt 9). Because lizardfish is lean fish (crude lipid < g kg ), antioxidants were not added in the silage (Mach & Nortvedt 9). The silage was stored in -L plastic buckets indoors at ambient temperature (8 ± 3 C) and stirred daily. After weeks, the silage was solar-dried to achieve a moisture content of approximately 45 g kg. The feed was prepared with silage stored for month. Fresh moist pellets were made every 3 or 4 days and stored in a refrigerator at 5 C. All fish were healthy and survived until the termination of the experiment. Twenty-five cobia (5 g) were randomly placed in each of six cages ( diets 9 3 replicates) in a pond at the Institute of Aquaculture Research Nha Trang University at Cam- Ranh district, Khanh Hoa province, Vietnam, for 3 months. Temperature ( C), salinity ( g L ), ph ( ) and dissolved oxygen (4. 8. mg L ) of water in the pond were measured twice per day (6: and 4:) with YSI 556 Multi-parameter. Two moist diets based on raw lizardfish (Saurida undosquamis) with or without added lizardfish silage were fed in the present experiment (Table ). Lizardfish was purchased from a local market in Cam Ranh district, Khanh Hoa province, Table Formulation and composition of the experimental diets Diet A Diet B Ingredient (g kg ) Raw fish (moisture 78 g kg ) 8 6 Fish silage (moisture 44 g kg ) Fish meal 8 8 Wheat 53 4 Fish and plant oil ( : ) 5 5 Premix-Vitamin and mineral Sodium alginate 7 Proximate composition (g kg ) Dry matter Crude protein (dry wt) NPN (g kg total N) Crude lipid (dry wt) Ash (dry wt) ph Premix-vitamin and mineral (9-Vemedin) consisted of (mg kg wet diets): retinol 4 IU; cholecalciferol 8 IU; tocopherol ; ascorbic acid 6; menadione.4; niacin ; thiamin.6; riboflavin 3; pyridoxol ; folic acid.3; inositol 5; choline chloride 48; calcium pan 4; iron ; zinc ; manganese ; magnesium 75; copper ; cobalt.; iodine.4; methionin 3; lysine 5; Supplied by Veterinary Stock Company (Vietnam). NPN, Non-Protein Nitrogen. After 3 months, cobia (5 fish of each dietary group) were randomly sampled after 4-h starvation. Individual fish was killed by a strong blow to the head and the gills were cut. After measuring length and weight, the fish were immersed for a few seconds in water containing mg L chlorine and then gutted. Fish were rinsed with water containing 5 mg L chlorine before filleting or packing. Viscera and livers were weighed for the calculation of a viscera somatic index (VSI) and hepatic somatic index (HSI). The raw biological data (weight, VSI and HSI) showed no significant differences within replicated groups, and sampling was therefore designed based on pooled dietary groups. Seven cobia of each dietary group were sampled for chemical and microbiological analyses. One side of fillets was immediately collected after filleting and stored at 8 C for crude protein, total lipid, amino acid and fatty acid analyses, while the matching fillets from the other side of the fish were stored in ice ( C) and sampled after 5, and 5 storage days for lipid oxidation and microbiological determination. For shelf life study, 35 cobia of each dietary group were used. The gutted fish were packed in plastic bags and eventually stored in ice boxes at C for sensory evaluation of the shelf life after 3, 5, 7, 9,, 3 and 5 storage days. The remaining eight fish of each dietary group were used for training purposes. Quality assessment schemes for cobia in the study were developed based on references (EEC 976; Howgate et al. 99; Jonsdottir 99; Larsen et al. 99; Huss 995). QIM was applied to estimate the freshness and quality of the gutted cobia during storage. Moreover, QDA was also used... Aquaculture Nutrition ª Blackwell Publishing Ltd

3 as a reference for the developed QIM scheme to determine the maximum storage time of the fish. Flesh from the gutted fish was cooked and consumed by a panel to determine whether it was acceptable after storage. Ten trained assessors in a sensory testing panel from the Quality of Seafood Department, Seafood Processing Technology Faculty, Nha Trang University participated in the development and evaluation of the QIM and QDA schemes (Tables & 3). Thirty-five cobia from each group (five fish per storage time) were assessed after 3, 5, 7, 9,, 3 and 5 storage days. Each cobia was coded with a random Table The quality index method scheme developed for cobia Parameter Description Score Skin Colour Natural colour (black silver) Some reduction in lustre and colour Distinct reduction Mucus Transparent and not clotted Milky and clotted Yellow and clotted Odour Fresh seaweedy Slightly seaweedy, neutral Rancid Rotten 3 Eyes Pupils Bright and clear Cloudy Matt Shape Convex Flat Sunken Belly Blood in abdomen Red Light red Brownish colour Odour Fresh sea odour Neutral Slight sourness Strong sour to spoilt odour 3 Gills Colour Red Light red Grey, green Mucus Transparent Yellow, clotted Brown Odour Seaweedy Neutral Sour Rotten 3 Texture Elasticity Finger mark returns quickly (< s) Finger mark returns slowly (>3 s) Finger leaves mark Quality index total 5... Table 3 The quantitative descriptive analysis (QDA) (scheme developed for cooked cobia Parameter Description Score Odour Sweet fresh fish Metallic Oily Off odour 3 Colour White Whitish Yellow Texture Stiffness Little softness Softness Flavour Sweet of fresh fish Metallic Sourness Strong sourness 3 QDA total number unrelated to storage time. Fish were randomly and individually assessed according to the QIM principles. After QIM assessment, the fish were used for QDA evaluation. After filleting, three pieces (3 9 3 cm, with skin) were prepared from both fillets of each fish. The six coded pieces were individually placed in aluminium boxes and cooked in a steam oven at C for 7 min; afterwards, the pieces were randomly served to each panellist for sensory evaluation based on the QDA scheme. The samples were analysed at different laboratories in Vietnam. Total nitrogen (N) was determined by the combustion method (CHNS-O Analyzer Model FLASH EA series made by Thermo Finnigan, Italy) at Nha Trang Oceanography Institute, and crude protein was estimated as N Non-protein nitrogen (NPN) in diets was extracted in % tricloroacetic acid (Backhoff 976), and nitrogen was determined by the same method and laboratory as crude protein. Amino acid and fatty acid compositions were determined at the Advanced Laboratory, Can Tho University. Total amino acids were determined with the EZ:faast LC/MS kit (Phenomenex, Torrance, CA, USA) by LC/MS (Finnigan LCQ Advantage Max, Walham, MA, USA); however, tryptophan was not determined in this study because of the high costs of these specific analyses. The lipids from the fillets and diets were extracted using chloroform/methanol ( :, v/v) and analysed for fatty acid composition as described by Lie et al. (986) with GLC (Carlo Erba Vega GLC, CAE, Redwood City, CA, USA). Moisture, ash, ph and crude lipid were Aquaculture Nutrition ª Blackwell Publishing Ltd

4 determined at the laboratories of the Institute of Biotechnology and Environment, Nha Trang University. Moisture was determined by oven-drying at 5 C for 48 h. Ash content was determined by combustion at 55 C in a muffle furnace for 4 h. Crude lipid was determined gravimetrically after extraction with ethyl acetate (Losnegard et al. 979). Lipid oxidation was estimated by the thiobarbituric acid reactive substances (TBARS) method (Pikul et al. 989). The ph was determined according to Fagbenro & Jauncey with a digital ph meter (Omega, Stamford, CT, USA). Microbiological counts were determined immediately after sampling by the Aerobic Plate Count at 3 C: Surface plate method (Health Protection Agency 4). The Statistica TM (version 7.) software program (StatSoft, Inc. Tulsa, OK, USA) was used for one-way analysis of variance (ANOVA). Significant differences (P <.5) between means were tested by Duncan s multiple range test, according to Duncan (955). Multivariate correlations between objects and variables were revealed by principal component analyses (PCA) using the Sirius TM (version 7.) software program (Pattern Recognition Systems AS, Bergen, Norway), according to Kvalheim & Karstang (987). The reason for applying PCA is its ability to reveal complex correlation patterns between variables (loadings) and samples (scores), for example found in the present study between the loadings of the QIM variables and the storage days. The two diets had similar content of crude protein and crude lipids (Table ). Levels of dry matter, NPN and ash were higher in diet B than in diet A, but ph was lower in diet B than in diet A (Table ). The fatty acid and amino acid compositions of the two diets were mostly the same (Tables 4 & 5). Fatty acids in the diets consisted mainly of monounsaturated fatty acids [MUFA, >436 g kg total fatty acids (TFA)], while polyunsaturated fatty acids (PUFA) accounted for only 4 g kg TFA, which n-6 PUFA dominated (67 7 g kg total PUFA, Table 5). No significant differences (P >.5) in nutritional quality of the fillets were observed between the two cobia groups Table 4 Amino acid composition in the experimental diets (n = 3) and in the cobia fillets (n = 7) (g kg protein) Amino acids Diets Fillets A B A B Arginine ± ±.5 Serine ± ±.5 Hydroxyproline ±. 8.6 ±. Glycine ± ±.5 Threonine ± ±.6 Alanine ± ±.9 Proline ± ±.8 Methionine ± ±.7 Aspartic acid ± ±.3 Valine ±.7 5. ±. Histidine ±.9 3. ±. Lysine ±.6. ±.6 Glutamic acid ±. 6.8 ±. Leucine ±. 84. ±.3 Phenylalanine ± ±. Isoleucine ± ±.5 Cystine ±. 8.3 ±. Tyrosine ± ±.5 Aspartic acid included asparagine; Glutamic acid included glutamine; Tryptophan was not analysed in the AA analyses. fed diets with or without added fish silage (Tables 4 6). Nutritional composition of fish fillets varies greatly from species to species and individual to individual, which depends on feed intake, sex, size, reproductive status, geographic location, seasonal changes and tissue. According to Stansby (96) and Love (97), lipid content in fish fillets ranges from to 5 g kg, while protein levels range from 6 to g kg total fillets. In the present study, lipid content of cobia fillet was g kg and protein was 97 3 g kg. Cobia fillets showed a balance of essential amino acid (EAA), and non-essential amino acid (NEAA) compositions with ratios of EAA/NEAA were approximately equal. EAA comprised high levels of lysine (. 6.5 g kg protein) and leucine ( ). Amino acid profile of the cobia fillets was quite similar to that of rainbow trout (EAA/NEAA ratios about.) (Unusan 7), and no significant difference in the fatty acid composition was found between the two fillet groups. The three groups of fatty acids saturated fatty acid (SFA), MUFA and PUFA in cobia fillet were divided into quite similar proportions from 3 to 33 g kg TFA (Table 5). PUFA accounted for approximately 33 g kg TFA. Contrary to the diets, PUFA in the cobia fillet comprised mainly n-3 PUFA (69 g kg total PUFA), which consisted mainly of docosahexaenoic acid (DHA, :6n-3; g kg total PUFA) and eicosapentaenoic acid (EPA, :5n-3; g kg total PUFA). MUFA shared... Aquaculture Nutrition ª Blackwell Publishing Ltd

5 Table 5 Fatty acid composition in the experimental diets (n = 3) and the cobia fillets (n = 7) (g kg total fatty acids) Fatty acids Diets Fillets A B A B 4: ±. 4.8 ±.9 5: ±. 7.9 ±. 6: ±. 3.5 ±.6 6:n ±. 6. ±. 6:n ±.6 6. ±. 7: ±.3 9. ±.3 6:n-4. ±..7 ±. 8: ± ±.8 8:n ± ± 5.4 8:n-7 3. ±. 9.7 ±.9 8:n ± ±. : 3 3 8:3n-3 6. ±. 6.9 ±. :n-. ±.6.7 ±.7 :n ±. 7. ±.5 :n-7.8 ±.4.9 ±.5 8:4n ±. 4.8 ±. :n-6.6 ±..4 ±. : :3n-6.7 ±..8 ±. :4n ±. 7.9 ±.4 :n- :n-9 :4n-3.9 ±..9 ±. :5n ± ±.3 4:. ±.5.9 ±.4 4:n ±. 3.5 ±. :4n-6 7. ±. 6.9 ±. :5n ± ±. :5n ± ±.8 :6n ± ±. SFA ± ± 3.6 MUFA ± ± 8. PUFA ± ± 4.4 sum n ± ±.8 sum n ±.8 9. ± 3.7 n3/n ±.6.8 ±.3 MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; SFA, saturated fatty acid. Table 6 Length, weight and biological indices of cobia (n = 3); and dry matter, lipid, protein and ash content of fillet cobia fed with the experimental diets (n = 7: seven cobia per pooled dietary group); Mean ± SEM (g kg, wet wt) Cobia A Cobia B Final weight (kg).49 ±.3.56 ±.4 Total length (cm) 6.8 ± ±.66 Viscera somatic index 69.9 ± ±.9 Hepatic somatic index. ±.5. ±. Fillet yield ± ±.7 Dry matter in fillets 46.3 ± ±.6 Fat in fillets 36.3 ± ±. Protein in fillets 3.3 ± ±.8 Ash in fillets 3.6 ± ±.4 by Liu et al. As a marine fish, cobia can convert n-6 PUFA from diets into n-3 PUFA and their PUFA consist mainly n-3 PUFA. Similar results were reported in Atlantic salmon where they were fed diets with and without added fish silage (Lie et al. 988; Heras et al. 994). Minor differences in fatty acid composition were observed between the two salmon fillet groups, and PUFA levels accounted for 45 9 g kg total TFA, with n-3 PUFA dominant (68 8 g kg total PUFA). By comparison with Atlantic cod fillets, PUFA composition in cobia fillets was lower (3 versus 5 6 g kg TFA) (Ackman & Burgher 964; Jangaard et al. 967; Addison et al. 968; Lie et al. 986), but similar to sea bass or sea bream fillets (Testi et al. 6; Yanar et al. 7; Yildiz et al. 8). In cod fillets, DHA accounted for 56 6 g kg total PUFA and the ratios of n-3/n-6 ranged between , whereas the ratio for the cobia fillets in the present study was.3. The total fat content (3 36 g kg ) in the present cobia fillets was, however, higher than observed in Atlantic cod. approximately 3 g kg TFA and was composed dominantly of C8:n-9 (64 65 g kg total MUFA). Similarly, SFA accounted for 38 g kg TFA and comprised mainly C6: (6 6 g kg total SFA). The present result was consistent with results in commercial cobia (5 6 kg) by Liu et al. (9). PUFA, MUFA and SFA consisted mainly of DHA ( g kg total PUFA), C8:n-9 (75 7 g kg total MUFA) and C6: ( g kg total SFA), respectively (Liu et al. 9). PUFA in the present study was higher (33 versus 77 g kg TFA), but n-3 PUFA composition was lower (69 versus 838 g kg total PUFA) than that in the study Sensory evaluation for gutted cobia No significant differences were observed in scores for the attributes and total QI (sum of all attributes) between two cobia groups throughout trial (Fig. ). The correlations (r =.83.84) between the total QI scores and the storage time in the present study was higher than that (r =.74) in the study by Martinsdottir et al. (), but lower (r =.85.99) compared to the studies by Sveinsdottir et al. (3), Nielsen & Hyldig (4), Nielsen & Green (7) and Bonilla et al. (7). The values showed that the attributes gradually and naturally decayed throughout the storage time. Scores for the attributes increased more sharply in gills... Aquaculture Nutrition ª Blackwell Publishing Ltd

6 Gill colour Skin colour Texture Pupil colour Gill mucus Skin mucus Abdomen blood colour Eye shape 3 3 Gill odour Skin odour Belly odour IQ score y A =.78x +.4; r A =.83 y B =.7x +.79; r B =.84 Figure Mean ( ± SEM) scores of each quality attribute and sum of all attributes (QI) assessed with QIM scheme for gutted cobia (n = 5) fed the experimental diets versus storage time; fish fed diet A; * fish fed diet B. QIM, quality index method. (colour and mucus), and eyes (shape) than in skin (colour, mucus and odour), and texture during storage (Fig. ). PCA showed clearly the correlation between the parameters and storage time. All attributes received high scores related to storage time (gills, belly odour and eye shape), which were located on the right side of the first principal component axis that explained approximately 76.3% of the variation between the samples, while skin colour, texture, pupil colour and abdomen blood, which received low scores, were located on the opposite side (Fig. ). At the end of the storage time, the scores of the attributes were quite low with the exception of those for gill colour, gill odour and 7.6 * Day 3 Comp. (.%) Pupil color A Skin color Texture B B Skin color A Abdomen blood A Texture A Skin mucus A Day 5 Skin mucus B Skin odour B Eye shape A Abdomen blood B Pupil color B Skin odour A Gill mucus B Eye shape B Belly odour B Belly odour A Gill Gill mucus odour A Gill color B Day 7 Comp. (76.3%) Gill color A Day 5 Day 9 Gill odour Day A Day 3 Figure Principal component analyses (PCA) loading plot of QIM data from gutted cobia (n = 5) fed the experimental diets A and B against storage time. The two-component PCA model explained 87.4% of the total variation in the quality attributes. QIM, quality index method.... Aquaculture Nutrition ª Blackwell Publishing Ltd

7 3 Odour 3 Flavour Score Colour Texture Figure 3 Mean ( ± SEM) scores of quantitative descriptive analysis (QDA) of cooked cobia (n = 5) fed the experimental diets against storage time; fish fed diet A; * fish fed diet B. Maximum potential QDA score is QDA score y A =.6x +.63; r A =.86 y B =.5x +.8; r B = Days in ice... IQ score Days in ice Figure 4 Average QI given by each QIM panellist in the shelf life study of the gutted cobia fed diet A throughout iced storage., *, 3, 4, 5, 6, 7, 8, + 9,. QIM, quality index method. eye shape, which ranged 33 63% of the maximum scores ( or 3) given in the QIM scheme. Eventually, the total QI score values were quite low ( ), compared to the maximum total value given in the scheme (5), (Fig. ) after 5 days stored in ice; therefore, QDA played an important role in determining the shelf life of the fish in the study. Similarly, there were insignificant differences in scores for the attributes and total QDA between the two cobia groups throughout the trial (Fig. 3). The average scores for the individual attributes fluctuated, but the total of all the attributes increased with storage in ice. The correlation (r =.86) between the total QDA and the storage time indicates that the quality of the fillets gradually deteriorated with time. The highest scores for odour and flavour attributes were approximately 4% of maximum values given in the QDA scheme by the end of the storage, while the highest values in colour and texture were 6 7% of maximum (Fig. 3). At the end of the storage trial, the total QDA score values were , compared to scores of a total QDA. Total QDA scores were significantly different at the sampling times, but the distinctions were not clear enough to be used as references for assessing the fish quality during storage. The fluctuated and low scores of the attributes of sensory evaluation may be caused by confusion about attributes, individual differences in the use of the scale, or individual differences in precision (Næs et al. 994), which are reflected in Fig. 4. The QIM evaluation of cobia given by the individual panellist was variable. Panellists,, 4, 5 and 9 gave stable increases in scores during storage time, while the QI scores of the other panellists fluctuated. The variation was lowest at day 3 and 5, but highest at day 7 of storage in ice. This was probably due to clearer quality attributes at the beginning and at the end of the storage trial. Lipid oxidation of cobia fillets Lipid oxidation is considered to be one of the most important factors responsible Aquaculture Nutrition ª Blackwell Publishing Ltd

8 TBARS (nmol g fillet) Days in ice CFU g Days in ice Figure 5 Means ( ± SEM) of lipid oxidation (TBARS) and aerobic plate counts (APC, cfu g fillet) of cobia fillets (n = 7) from the dietary trial versus days in ice; fish fed diet A; * fish fed diet B. TBARS, thiobarbituric acid reactive substances. for quality deterioration of fish during storage. In the present study, no statistically significant differences in lipid oxidation were observed between the two fillet groups during iced storage (Fig. 5). TBARS values of the both groups rapidly increased from day 5 to day (7 7 nmol g fillet), but slightly decreased at day 5 (6 nmol g fillet). The fatty acid composition of the two groups was similar, which might explain the absence of significant differences in rancid development between them. The reduction in TBARS values at day 5 was probably due to deficiency of substrate, for example free fatty acids. It is well known that the initiation of lipid oxidation probably involves nonenzymatic and enzymatic reactions. The development of lipid oxidation depends on several factors, such as storage period, temperature, presence of inhibitors or catalysts, availability of oxygen, and degree of unsaturated fatty acids (Maclean & Castell 964; Castell et al. 966; Castell & Bishop 969; Aubourg & Medina 999; Erickson ). Unsaturated fatty acids are known to be more susceptible to oxidation than SFA because of lowered activation energy in the initiation of free radical formation for triplet oxygen auto-oxidation (Holman & Elmer 947; Lea 95). Seafood, particularly fatty fish with highly unsaturated fatty acid composition, is sensitive to oxidation during storage, especially in iced storage. According to Nunes et al. (99), the limit of acceptability of lipid oxidation for fish stored in ice is 7 nmol TBARS g flesh (equal to 5 8 mg of malondialdehyde kg flesh). The TBARS values in the present study were low compared to the limitation during storage. Microbial counts of cobia fillets No significant differences were found in total aerobic plate counts (APC) between the two fillet groups throughout the trial; even the mean APC of fish fed diet A was lower than that of fish fed diet B at day 5 (Fig. 5). The International Commission on Microbiological Specifications for Food (ICMSF) recommends that total APC should not exceed 7 cfu g wet weight during iced storage (ICMSF 978). In the present trial, the total aerobic bacterial counts slowly increased from day 5 to day ( cfu g ) in both groups. The values sharply increased at day 5 ( in fish fed diet A and for fish fed diet B), but still satisfied this recommendation. Based on the above results from QIM and QDA for the gutted cobia, and for lipid oxidation and microbial counts of the fillet, the quality of the cobia was probably acceptable after 5 days stored in ice. There were no significant differences in nutritional composition between the 9 3 pooled replicated cobia fillet groups after 3 months feeding trial given the diets with or without added fish silage, and no significant differences were observed in the shelf life study between the two cobia groups. With high nutritional composition, particularly balance of EAA and NEAA and high levels of n-3 PUFA, cobia fillets demonstrate good quality, compared to Atlantic cod or Atlantic salmon. The use of the QIM and QDA schemes developed for the gutted cobia in the present study showed clear correlations between the attributes and storage time in ice. However, the scores for most attributes were low compared to maximum values given in the schemes by the end of the trial, which was probably due to the short period of storage. Moreover, the TBARS values and microbial counts were below acceptable limits in the cobia fillets at the end of storage. Consequently, the shelf life of the cobia was estimated to be >5 days; thus, further studies are needed in the future to estimate more accurately the shelf life of this species.... Aquaculture Nutrition ª Blackwell Publishing Ltd

9 The authors are grateful for financial support from the project Improving training research capacity at University of Fisheries funded by NORAD (The Norwegian Agency for Development Cooperation) (NORAD SRV 7 project). We also thank the Institute of Aquaculture Research, the Institute of Biotechnology and Environment, and the Faculty of Seafood Processing Technology Nha Trang University, Nha Trang Oceanography Institute, and the Advanced Laboratory Can Tho University in Vietnam for assistance with facilities and to their colleagues for advice and collaboration in the execution of the present study. Ackman, R.G. & Burgher, R.D. (964) Cod flesh: component fatty acids as determined by gas-liquid chromatography. Fish. Res. Board Can.,, Addison, R.F., Ackman, R.G. & Hinglev, J. (968) Distribution of fatty acids in cod flesh lipids. Fish. Res. Board Can., 5, Andrade, A., Nunes, M.L. & Batista, I. (997) Freshness quality grading of small pelagic species by sensory analysis. In: Methods to Determine the Freshness of Fish in Research and Industry. Proceedings of the Final Meeting of the Concerted Action Evaluation of Fish Freshness (Olafsdóttir, G., Luten, J., Dalgaard, P., Careche, M., Verrez-Bagnis, E., Martinsdótirr, E. & Heia, K. eds), pp International Institute of Refrigeration, Paris. Aubourg, S.P. & Medina, I. (999) Influence of storage time and temperature on lipid deterioration during cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) frozen storage. J. Sci. Food Agric., 79, Backhoff, H.P. (976) Some chemical changes in fish silage. J. Food Technol.,, Barbosa, A. & Vaz-Pires, P. (4) Quality index (QIM): development of a sensorial scheme for common octopus (Octopus vulgaris). Food Control, 5, Bonilla, A.C. (4) Development of a Quality Index Method (QIM) scheme for fresh cod (Gadus morhua) fillets and consumer acceptance of different cod products. Final project 4. UNU Fisheries Training Programmer. Bonilla, A.C., Sveinsdottir, K. & Martinsdottir, E. (7) Development of Quality Index Method (QIM) scheme for fresh cod (Gadus morhua) fillets and application in shelf life study. Food Control, 8, Bremner, H.A. (985) A convenient, easy to use system for estimating the quality of chilled seafood. Fish Process. Bull., 7, Castell, C.H. & Bishop, D.M. (969) Effect of hematin compounds on the development of rancidity in muscle of cod, flounder, scallops and lobster. Fish. Res. Board Can., 6, Castell, C.H., Moore, B.A., Jangaard, P.M. & Neal, W.G. (966) Oxidation rancidity in frozen stored cod fillets. Fish. Res. Board Can., 3, Duncan, D.B. (955) Multiple range and multiple F-tests. Biometrics,, 4. EEC (976) Council Regulation No. 3/76 freshness ratings. Official Journal European Communities No. L. Erickson, M.C. () Lipid oxidation of muscle food. In: Food Lipids Chemistry, Nutrition, and Biotechnology (Akoh, C.C. & Min, D.B. eds.), pp Marcel Dekker, New York, Basel. Fagbenro, O. & Jauncey, K. (995) Water stability, nutrient leaching and nutritional properties of moist fermented fish silage diets. Aquac. Eng., 4, Guillerm-Regost, C., Haugen, T., Nortvedt, R., Carlehög, M., Lunestad, B.T., Kiessling, A. & Rørå, A.M.B. (6) Quality characterization of farmed Atlantic halibut during ice storage. J. Food Sci., 7, Health Protection Agency (4) Aerobic plate count at 3 C: Surface plate method National standard method F Issue. Heras, H., Mcleod, C.A. & Ackman, R.G. (994) Atlantic dogfish silage vs. herring silage for Atlantic salmon (Salmo salar): growth and sensory evaluation of fillets. Aquaculture, 5, Holman, R.T. & Elmer, O.C. (947) The rates of oxidation of unsaturated fatty acids and esters. J. Am. Oil Chem. Soc., 4, 7 9. Howgate, P., Johnston, A. & Whittle, A.D.J. (99) Multilingual Guide to EC Freshness Grades for Fisheries Products. Tommy Research Station, Aberdeen. Huidobro, A., Pastor, A. & Tejada, M. () Quality index method developed for raw gilthead sea bream (Sparus aurata). J. Food Sci., 65, 5. Huss, H.H. (995) Quality and Quality Changes in Fresh Fish. FAO Fisheries Technical Paper 348. Food and Agriculture Organization of The United Nations, Rome. ICMSF (978) Microorganisms in Foods.. Their Significance and Method of Enumeration, nd edn. University of Toronto Press, Toronto, ON. Jangaard, P.M., Ackman, R.G. & Sipos, J.C. (967) Seasonal changes in fatty acids composition of cod liver, flesh, roe, and milt lipids. Fish. Res. Board Can., 4, Jonsdottir, S. (99) Quality index method and TQM system. In: Quality Issues in the Fish Industry (Olafsson, R. & Ingthorsson, A.H. eds), pp The Research Liaison Office, University of Iceland, Reykjavik. Kvalheim, O.M. & Karstang, T.V. (987) A general purpose program for multivariate data analysis. Chemom. Intellig. Lab. Syst.,, Larsen, E.P., Heldbo, J., Jespersen, C.M. & Nielsen, J. (99) Development of a method for quality assessment of fish for human consumption based on sensory evaluation. In: Quality Assurance in the Fish Industry (Huss, H.H., Jakobsen, M. & Liston, J. eds), pp Elsevier Science Publishing, Amsterdam. Lea, C.H. (95) Methods for determining peroxide in lipids. J. Sci. Food Agric., 3, Lie, Ø., Lied, E. & Lambertsen, G. (986) Liver retention of fat and of fatty acids in cod (Gadus morhua) fed different oils. Aquaculture, 59, Lie, Ø., Waagboe, R. & Sandnes, K. (988) Growth and chemical composition of adult Atlantic salmon (Salmo solar) fed dry and silage-based diets. Aquaculture, 69, Liu, S.C., Li, D.T., Hong, P.Z., Zhang, C.H., Ji, H.W., Gao, J.L. & Zhang, L. (9) Cholesterol, lipid content, and fatty acid composition of different tissues of farmed Cobia (Rachycentron canadum) from China. J. Am. Oil Chem. Soc., 86, Losnegard, N., Bøe, B. & Larsen, T. (979) Undersøkelse av ekstraksjonsmidler for bestemmelse av fett. Fiskeridirektoratet, Bergen. Method no. /79 (in Norwegian). Love, R.M. (97) The Chemical Biology of Fishes. Academic Press, London.... Aquaculture Nutrition ª Blackwell Publishing Ltd

10 Luten, J.B. () Development and implementation of a computerised sensory system (QIM) for evaluation fish freshness. CRAFT FAIR CT Final report for the period from to RIVO. The Netherlands Institute for Fisheries Research, Wageningen, the Netherlands. Mach, T.N.D. & Nortvedt, R. (9) Chemical and nutritional quality of silage made from raw or cooked fish and crab. J. Sci. Food Agric., 89, Maclean, J. & Castell, C.H. (964) Rancidity in lean fish muscle. I. A proposed accelerated copper-catalyzed method for evaluating the tendency of fish muscle to become rancid. Fish. Res. Board Can.,, Martinsdottir, E. () Quality management of stored fish. In: Safety and Quality Issues in Fish Processing (Bremmer, H.A. ed.), pp Woolhead Publishing, Cambridge, UK. Martinsdottir, E. & Arnason, A. (99) Redfish. In: Nordic Industrial Fund, Quality Standards for Fish: Final Report Phase II, pp. 35. Nordic Industrial Fun, Oslo, Norway. Martinsdottir, E., Sveinsdottir, K., Luten, J., Schelvis-Smit, R. & Hyldig, G. () Sensory Evaluation of Fish Freshness. Reference manual for the fish sector. QIM Eurofish, Ijmuiden, the Netherlands. Massa, A.E., Palacios, D.L., Paredi, M.E. & Crupkin, M. (5) Postmortem changes in quality indices of ice-stored flounder (Paralichthys patagonicus). J. Food Biochem., 9, Næs, T., Hirst, D. & Baardseth, P. (994) Using cumulative ranks to detect individual differences in sensory QDA. J. Sens. Stud., 9, Nielsen, J. (997) Sensory analysis of fish. In: Methods to Determine the Freshness of Fish in Research and Industry. Proceedings of the Final Meeting of the Concerted Action Evaluation of Fish Freshness (Olafsdottir, G., Luten, J., Dalgaard, P., Careche, M., Vererez-Bagnis, V., Martinsdottir, E. & Heia, K. eds), pp International Institute of Refrigeration, Paris. Nielsen, D. & Green, D. (7) Developing a quality index grading tool for hybrid striped bass (Morone saxatilis 9 Morone chrysops) based on the quality index method. Int. J. Food Sci. Technol., 4, Nielsen, D. & Hyldig, G. (4) Influence of handling procedures and biological factors on the QIM evaluation of whole herring (Clupea harengus L.). Food Res. Int., 37, Nunes, M.M., Batista, I. & Morao, D.C.R. (99) Physical, chemical and sensory analysis of sardine (Sardine pilchardus) stored in ice. J. Sci. Food Agric., 59, Pikul, J., Leszczynski, D.E. & Kummerow, F.A. (989) Evaluation of three modified TBARS methods for measuring lipid oxidation in chicken meat. J. Agric. Food Chem., 37, Shewan, J.M., Mackintoch, R.G., Tucher, C.G. & Erhenberg, A.S. C. (953) The development of a numerical scoring system for the sensory assessment of the spoilage of wet fish stored in ice. J. Sci. Food Agric., 6, Stansby, M.E. (96) Proximate composition of fish. In: Fish in Nutrition (Heen, E. & Kreuzer, R. eds), pp Fishing News Books, London. Stone, H. & Sidel, J.L. (993) Sensory Evaluation Practices, nd edn. Academic Press, San Diego, CA. Sveinsdottir, K., Martinsdottir, E., Hyldig, G., Jørgensen, B. & Kristbergsson, K. () Application of Quality Index Method (QIM) scheme in shelf-life study of farmed Atlantic salmon (Salmo salar). J. Food Sci., 67, Sveinsdottir, K., Hyldig, G., Martinsdottir, E., Jørgensen, B. & Kristbergsson, K. (3) Quality Index Method (QIM) scheme developed for farmed Atlantic salmon (Salmo salar). Food Qual. Prefer., 4, Testi, S., Bonaldo, A., Gatta, P.P. & Badiani, A. (6) Nutritional traits of dorsal and ventral fillets from three farmed fish species. Food Chem., 98, 4. Unusan, U. (7) Change in proximate, amino acid and fatty acid contents in muscle tissue of rainbow trout (Oncorhynchus mykiss) after cooking. Int. J. Food Sci. Technol., 4, Vaz-Pires, P. & Seixas, P. (6) Development of new quality index method (QIM) schemes for cuttlefish (Sepia officinalis) and broadtail shortfin squid (Illex coindetii). Food Control, 7, Yanar, Y., Kucukgulmez, A., Ersoy, B. & Celik, M. (7) Cooking effects on fatty acid composition of cultured sea bass (Dicentrarchus labrax L.) fillets. J. Muscle Foods, 8, Yildiz, M., Sener, E. & Timr, M. (8) Effects of differences in diets and seasonal changes on the fatty acid composition in fillets from farmed and wild sea bream (Sparus aurata L.) and sea bass (Dicentrarchus labrax L.). Int. J. Food Technol., 43, Aquaculture Nutrition ª Blackwell Publishing Ltd