Effect of Gamma Radiation on the Quality and Shelf Life of Refrigerated Rainbow Trout (Oncorhynchus mykiss) Fillets

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1 1419 Journal of Food Protection, Vol. 72, No. 7, 2009, Pages Copyright, International Association for Food Protection Effect of Gamma Radiation on the Quality and Shelf Life of Refrigerated Rainbow Trout (Oncorhynchus mykiss) Fillets SOHRAB MOINI, 1 REZA TAHERGORABI, 2 * SEYED VALI HOSSEINI, 3 MOHAMMAD RABBANI, 4 ZOYA TAHERGORABI, 5 XESÚS FEÁS, 6 AND FEREIDOON AFLAKI 4 1 Department of Food Science and Technology, University of Tehran, P.O. Box , Karaj, Iran; 2 Animal and Nutritional Sciences, West Virginia University, P.O. Box 6108, Morgantown, West Virginia , USA; 3 Department of Environmental and Fishery Sciences, University of Tehran, , Karaj, Iran; 4 Department of Fishery, Azad University, Tehran North Campus, P.O. Box , Tehran, Iran; 5 Iran University of Medical Sciences and Health Services, P.O. Box , Tehran, Iran; and 6 Departamento de Química Analítica, Nutrición y Bromatoloxía, Universidade de Santiago de Compostela, E Lugo, Galiza, Spain MS : Received 18 October 2008/Accepted 1 February 2009 ABSTRACT The effect of gamma radiation (0, 1, 3, and 5 kgy) on the shelf life of farmed rainbow trout (Oncorhynchus mykiss) fillets that were treated with sodium acetate and vacuum packaged and subsequently stored under refrigeration was studied by measuring microbiological, chemical, and organoleptic changes. Radiation affected populations of bacteria, namely, H 2 S- producing bacteria and Enterobacteriaceae (P 0.05). Initial total viable counts of the control samples were ca log CFU/g, whereas the respective counts in samples irradiated at 1, 3, and 5 kgy were 3.08, 1.46, and 1 log CFU/g at day 1 of refrigerated storage. The maximum count of Enterobacteriaceae reached 2.29 and 1.45 log CFU/g at the end of storage for 1 and 3 kgy, respectively, but at a 5-kGy dose no growth of Enterobacteriaceae was observed. Of the biochemical indicators, thiobarbituric acid values for irradiated trout were higher than for nonirradiated fish (P 0.05). Sensory evaluation (taste) showed a reasonable and good correlation with bacterial populations with storage time. The results revealed that radiation at a high dose (5 kgy) might induce lipid and protein oxidation, although the growth of microorganisms was inhibited. Therefore, radiation at a low dose (3 kgy) could be used to control the microbial and safety biochemical indices of O. mykiss for up to 4 weeks at refrigerator temperature without adverse effects on quality and acceptability. The shelf life and safety of refrigerated fish and fish products are dictated by the presence of food spoilage and pathogenic microorganisms (38). Spoilage of refrigerated fish results from microbial growth and/or activity, which manifests itself as changes in the sensory characteristics (production of off-odor and off-taste, slime formation, production of gas, etc.) (19). Fish and shellfish are also known to be carriers of several pathogenic microorganisms that are implicated in foodborne diseases (13). The presence of spoilage and pathogenic microorganisms in seafood is a major concern for the fish processing industry, the administration, and consumers. Despite improved manufacturing facilities and implementation of effective process control procedures such as hazard analysis and critical control points in the food industries, the number of foodborne illnesses has increased (54). This risk factor has prompted food scientists worldwide to reassess their techniques of food safety assurance in order to preserve or extend the shelf life of various aquatic food products (7). Many methods, including low-temperature storage (21), reduction of water activity (e.g., smoking (5) and salting (28)), chemical treatments (e.g., antioxidant treatment (53)), and novel techniques such as high-pressure treatment (8) and use of lactic acid bacteria (LAB) metabolites (e.g., * Author for correspondence. Tel: ; Fax: ; Rtahergorabi@gmail.com. bacteriocins (6)) have been developed for extending the shelf life and hygienic quality of fish commodities. Recently, as a result of advances in irradiation technology, use of ionizing radiation has been practiced in fish industries (9, 41, 52). Gamma radiation at low doses is a cold process that has been accepted by several countries for extension of shelf life of marine and freshwater fishery products (25, 55). According to the Joint Expert Committee of Food and Agriculture Organization/World Health Organization/International Atomic Energy Agency (FAO/WHO/IAEA), irradiation of any food commodity up to an overall average dose of 10 kgy presents no toxicological hazard. There has been worldwide interest in using this technology for preservation of various foods, including fishery products (25, 56). In addition to the extension of shelf life, this treatment also improves the hygienic quality and safety of the products (32). According to Mendes et al. (41) there is no controversy among the experts that food irradiation is an economical and effective food preservation method that has been validated in at least 39 countries and approved for 49 different products. Although irradiation is an effective means for extending the shelf life of fishery products (45), indirect effects, including the acceleration of lipid oxidation, hydrolysis, and vitamin destruction, still limit its application in some food products (14, 46). In addition to irradiation, vacuum packaging of fresh fish prior to irradiation was considered essential in order to

2 1420 MOINI ET AL. J. Food Prot., Vol. 72, No. 7 prevent or delay microbial activity and also chemical changes during postradiation refrigerated storage (9, 29). Furthermore, several studies have shown that vacuum packaging in combination with chemical preservatives such as sodium acetate (37) will enhance the shelf life of fish during storage. So, the main aim of the present study was to evaluate the effect of low and medium doses of gamma radiation (1, 3, and 5 kgy), combined with sodium acetate and vacuum packaging, on the preservation and shelf life of aquacultured freshwater rainbow trout (Oncorhynchus mykiss) fillets, using microbiological, chemical, and organoleptic analyses during refrigerator storage. MATERIALS AND METHODS Preparation of the samples, irradiation, and storage conditions. A total of 84 freshwater rainbow trout (11 to 12 months old with average weight and length of g and mm, respectively) were obtained from a local aquaculture farm located at Noshahr, in the north of Iran. The fish had not been starved and were actively feeding on commercial fish feed (from the Chineh Company, Tehran, Iran). The fish were killed by immersion in ice-cold water (hypothermia) and then transported to the laboratory within 45 min in foamed polystyrene self-draining boxes with a suitable quantity of flaked ice (the ice/fish ratio was 3:1, wt/wt). After passing into rigor mortis, the fish were washed with potable water, skinned, and then filleted manually by use of a sterile scalpel within 2 h. The preparation process was carried out in a cold room at a temperature of 7 to 10 C. The average weight of the two fish fillets was 69.4% of the initial average weight. The fillets were divided into four lots (21 fish in each lot) and then immersed in separated prechilled (4 C) aqueous solution (2.5%, wt/vol) of sodium acetate for 10 min with a fish-to-dipping solution ratio of 1:25 (50). After dipping, fillets were allowed to drain for 4 to 5 min on a sterile stainless wire mesh screen at 10 C. Then, each fillet was placed separately in polyamide bags (S-gruppen, Vinterbro, Norway), 75 m in thickness with an oxygen transmission rate of 30 cm 3 /m 2 /24 h atm (the oxygen transmission rate was given by the supplier) (temperature, 23 C; relative humidity, 0%), labeled, and vacuum sealed using a BOSS N48 vacuum sealer (Boss GmbH, Bad Hamburg, Germany). Packed samples (both experimental and control) were delivered to the radiation plant in insulated polystyrene boxes with ice within 12 h of harvesting. Samples (except controls) were irradiated at the Atomic Energy Organization of Iran (Tehran) using a 60 Co radiation source. The strength of the source was 1, Ci with a dose rate capacity of Gy/s, and the Gamma Cell 220 (Point source AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, Ontario, Canada) was calibrated by standard Fricke dosimeter. The doses applied in this study were 1, 3, and 5 kgy, and the actual doses were within 2% of the target dose. To minimize variations in the radiation-dose absorption, the boxes were turned 180 halfway through the irradiation process. During irradiation, the fish were in flaked ice. After irradiation, the fish were transported to the laboratory in ice via insulated polystyrene boxes within 3.5 h and maintained in a refrigerator (Yakhsaran, Tehran, Iran) at C for 42 days for microbiological, chemical, and organoleptic analysis. The fillets were sampled on storage days 1, 7, 14, 21, 28, 35, and 42. On each sampling occasion, three randomly chosen packages from every group were evaluated microbiologically, chemically, and sensorially. Microbiological analysis. At each storage interval, 10 g of sample was aseptically removed and homogenized for 1 min with 90 ml of prechilled (4 0.5 C) sterile peptone physiological saline solution (0.1% neutral peptone 0.85% NaCl; Merck, Darmstadt, Germany) in sterile deionized water (ph ) using a presterilized Stomacher Lab-Blender (Seward type 400, London, UK) (42). Further decimal serial dilutions were prepared from this homogenate in the same chilled sterile diluent. Microbiological data were expressed as log CFU per gram of fillet. All of the microbial analyses were performed in triplicate on three subsamples of each of the replicates. All media were purchased from Oxoid Inc. (London, UK). In this study, total viable count (TVC) and Pseudomonas count were determined using plate count agar and Pseudomonas Agar Base according to the methods of the American Public Health Association (4) and Mead and Adams (40), respectively. H 2 S-producing organisms were enumerated on iron agar Lyngby by a pour-plating method in anaerobic jars with disposable Anaerocult C bags (Merck) (20). Counts of Enterobacteriaceae and LAB were determined using violet red bile glucose agar and deman Rogosa Sharpe agar according to the method of the International Commission on Microbiological Specifications for Foods (26) and the method of González-Rodriguez et al. (16), respectively. Chemical analysis. After the sampling for microbial analysis, for all chemical methods, each side (half) of each fillet in each package was homogenized using a kitchen blender through a plate (4 mm) for 1 min (SAYA, Model Promeat W-1800, Tehran, Iran) and analyzed to determine total volatile base nitrogen (TVB-N), thiobarbituric acid (TBA), and ph, and the other half was used for sensory assessment. For the chemical analyses, all reagents were of analytical grade (Merck). In this study, the TVB-N content of experimental fish was determined according to the method of Goulas and Kontominas (17) and expressed in milligrams per 100 g of flesh. The TBA was determined according to the Kirk and Sawyer (30) method. The color development was measured at 532 nm by using a UVvisible spectrophotometer (Jenway 6305, Flested, Dunmow, UK). TBA value was expressed in milligrams of malondialdehyde (MDA) per kilogram of fish flesh. The ph of homogeneous mixtures of fillets and distilled water (1:10, wt/vol) was determined using a digital ph-meter (51). Briefly, a 10-g sample was homogenized in 100 ml of distilled water for 30 s, and the mixture was filtered through Whatman no. 42 filter paper (Maidenstone, England) and maintained at room temperature for 15 min. The ph value of the filtrate was measured using a digital ph-meter (Suntex Sp-701, Suntex Instrument Co., Taiwan) with an immersed electrode according to the manufacturer s instruction manual at ambient temperature. Sensory assessment. The sensorial attributes of cooked fillets were evaluated by a team of five semitrained panelists from the Department of Fishery, University of Tehran, Iran. Fish samples (100 g) were cooked individually in a microwave oven (MC- 2007TCR, LG, Korea) at 800 W. The samples were cooked on 50% power for 13 min (15) and served to the panelists after being cooled to ambient temperature (22 to 23 C). Each panelist evaluated approximately 20 g of fish. Panelists were asked to score the odor, taste, and texture of fish by using the Torry scale (44). Orange juice and water were provided to wash the oral cavity between treatments. The Torry scale, which is used to evaluate the freshness of cooked fillets, is a descriptive 10-point scale developed at the Torry Research Station. This scale has been developed for lean, medium fat, and fat fish species. Scores are given

3 J. Food Prot., Vol. 72, No. 7 EFFECT OF GAMMA RADIATION ON QUALITY OF RAINBOW TROUT 1421 FIGURE 1. Effect of gamma radiation on the total viable counts (A), Pseudomonas spp. (B), H 2 S-producing bacteria (C), Enterobacteriaceae (D), and lactic acid bacteria (E) of aquacultured freshwater rainbow trout (Oncorhynchus mykiss) treated with sodium acetate, vacuum packaged, and stored under refrigeration at 4 C., Nonirradiated;, 1-kGy radiation dose;, 3-kGy radiation dose;, 5-kGy radiation dose. Values of 1 log CFU/g of fillet are omitted, and significant differences are not shown. from 10 (very fresh) to 3 (spoiled), with a rejection level at 5.5. It is considered unnecessary to have descriptions below 3, as the fish is then no longer fit for human consumption (39). Statistical analysis. On each sampling occasion, three random, independent samples from each group were subjected to the microbiological and chemical analyses and sensory evaluation. All determinations were performed in triplicate. Descriptive statistics (means and standard deviations) of analysis results were calculated for each treatment. All data were subjected to one-way analysis of variance to test the effects of irradiation. The data were tested for homogeneity of variances with levels of significance set at P values of 0.05, and probability values of less than 0.05 were considered statistically significant (12). Excel and SPSS version 13.5 (SPSS Inc., Chicago, IL) were used for data manipulations and statistical analysis. RESULTS AND DISCUSSION Microbiological analyses. Spoilage of fish is caused by the growth and activity of specific spoilage organisms, which produce metabolites causing off-odors and consequently cause consumer food rejection (18). However, the specific spoilage organisms are not the same in every case and the microbial flora isolated from seafoods differs considerably from one study to another, depending on the species of fish, their environment, the mode of capture, and the type of fish product (whole, whole gutted, fillets, or slices), as well as the climatic and storage conditions (18). Nonetheless, Pseudomonas, H 2 S-producing bacteria, and LAB are generally predominant in spoiled fish flora, while different gram-negative bacteria, including Enterobacteriaceae, are frequently present (23). In the present work, changes in the microorganisms of the flesh of O. mykiss were observed for all measured groups during refrigerator storage, but all of these parameters were significantly lower than those of the control (Fig. 1A through 1F) (arithmetic means standard deviation, P 0.05). In the present study, irradiation (at 1, 3, and 5 kgy)

4 1422 MOINI ET AL. J. Food Prot., Vol. 72, No. 7 showed a significant impact on TVC, as its value was lower (P 0.05) than those of the control samples (Fig. 1A). Although it is widely accepted that the initial microbial load of freshwater fish varies depending on water conditions and temperature, most available literature on different freshwater fish species (tilapia, striped bass, rainbow trout, silver perch, and sea bream) reports bacterial counts of 10 2 to 10 7 CFU/g (12, 24). Initial total viable counts of the control samples were ca log CFU/g, whereas the respective counts in samples irradiated at 1, 3, and 5 kgy were 3.08, 1.46, and 1 log CFU/g at day 1 of refrigerated storage. The initial counts indicated an acceptable fish quality, considering the proposed upper limit for aerobic plate counts of CFU/g for fresh fish (26). Chouliara et al. (9) reported initial counts for control and irradiated (1 and 3 kgy) sea bream fillets salted with NaCl, vacuum packaged, and stored in a refrigerator of ca. 4.2, 2.4, and 1.0 log CFU/g, respectively. Also, Savvaidis et al. (52) reported a slightly higher initial count of 4.6 log CFU/g (day 1) for whole vacuum-packaged O. mykiss stored under refrigeration, whereas Jeevanandam et al. (27) and Lakshmanan et al. (32) reported counts of 4.2 and 4.0 log CFU/g for salted threadfin bream and whole anchovy stored in ice, respectively. Total mesophilic counts for the control and irradiated rainbow trout reached an average value of 7 log CFU/g with storage, which is close to the upper limit of acceptability for freshwater and marine fish as defined by the International Commission on Microbiological Specifications for Foods (26) after ca. 14 days (0 kgy) and 35 days (1 kgy), while for 3 and 5 kgy the fish never reached this population level after 42 days. Savvaidis et al. (52) reported counts of 7 log CFU/g for vacuum-packaged trout after 9, 14, and 24 days for nonirradiated and irradiated samples at 0.5 and 2 kgy, respectively. Pseudomonas spp. and H 2 S-producing bacteria have been reported to be the specific spoilage microorganisms in various fish species, including sea bass (43) and rainbow trout (10) harvested from different arctic, temperate, and tropical waters. In this study, of the bacterial groups examined, Pseudomonas spp. had the lowest count in both groups (irradiated and nonirradiated O. mykiss fillets) (Fig. 1B). It is clear that irradiation reduced (P 0.05) the populations of Pseudomonas, and especially at 3 and 5 kgy Pseudomonas organisms were totally eliminated. Lewis et al. (34) reported that Pseudomonas spp., like other gramnegative bacteria, are known to have a very low resistance to irradiation. Therefore, elimination of Pseudomonas by irradiation could be beneficial to the preservation of fish products in view of the major role that these species play in the spoilage of fish (9). Furthermore, one must not forget that Pseudomonas spp. are obligate aerobic bacteria and their low population in the control group can be related to the absence (or very low amount) of oxygen in the package. In addition, the H 2 S-producing bacteria in the control reached a maximum count of 4.89 log CFU/g on day 35 and H 2 S-producing bacteria were not observed at dose levels of 1, 3, and 5 kgy for 7, 21, and 42 days, respectively (Fig. 1C). This shows that radiation can reduce the population of sulfite-reducing bacteria. This is in agreement with the reduction of H 2 S-producing bacteria in freshwater and marine fish (tilapia and Spanish mackerel) by irradiation at a dose of 1.5 kgy (2) and also of sea bream by irradiation at 1 and 3 kgy (9). Enterobacteriaceae were also found to be part of the spoilage microflora of rainbow trout in salted samples in refrigerated storage. This finding is consistent with results reported for different fish species, including fresh Atlantic salmon (3), sea bass (43), sea bream (9), and rainbow trout (10), which showed the presence of Enterobacteriaceae at the end of the storage time of the product under refrigerated condition. The initial count of Enterobacteriaceae was 1.22 log CFU/g (day 7), and it reached a maximum count of 3.29 log CFU/g for the control sample at the end of the storage period. The maximum count reached 2.29 and 1.45 log CFU/g at the end of storage for 1 and 3 kgy, respectively, but for 5 kgy no growth of Enterobacteriaceae was observed (Fig. 1D). The presence of Enterobacteriaceae in the microflora of fish and its spoilage potential must be considered when fish are obtained from polluted water or there is a delay in chilling after the catch (35). Furthermore, the presence of Enterobacteriaceae may occur by crosscontamination during postprocessing and the filleting process. Although this group can grow at low temperatures, their abundance decreases during ice storage, as they are poor competitors. LAB have received particular attention from the food industry in recent years due to their potential application as natural preservatives. Short-chain fatty acids such as lactic acid, propionic acid, and butyric acid produced by LAB may help maintain an appropriate ph and protect against pathological changes in fish during storage. In this study the initial LAB count was 1.09 log CFU/g in the control samples. A final count of 5.07, however, was reached in the control on the 35th day, whereas samples treated with 1, 3, and 5 kgy reached 4.66, 4.04, and 3.55 log CFU/g, respectively, at the end of storage. The low LAB count in this study was expected since LAB tend to grow slowly at refrigeration temperatures (22). LAB were found to be the major spoilage microorganisms in fresh vacuum-packaged Atlantic salmon portions stored at 4 C (47). Chemical analyses. The approximate composition of the measured edible portions of rainbow trout studied was as follows: moisture, 72.6% 1.12%; fat, 5.73% 0.31%; protein, 18.9% 0.63%; and ash, 1.47% 0.03%. The changes in all the biochemical indices measured (TVB-N, TBA, and ph) observed for the control and the irradiated rainbow trout during the 42-day storage period under refrigeration are shown in Fig. 2A through 2C (P 0.05). The TVB-N may be considered to be a quality index for unprocessed fishery products. Its increase is related to the activity of spoilage bacteria and endogenous enzymes (49). The function of such enzymes results in the formation of compounds including ammonia, monoethylamine, and dimethylamine, as well as trimethylamine, imparting characteristics of off-flavors to fish. A level of 35 to 40 mg of TVB-N per 100 g of fish muscle is usually regarded as an indication that the product is spoiled (31). However, various

5 J. Food Prot., Vol. 72, No. 7 EFFECT OF GAMMA RADIATION ON QUALITY OF RAINBOW TROUT 1423 FIGURE 2. Effect of gamma radiation on total volatile base nitrogen (A), thiobarbituric acid (B), and ph (C) of aquacultured freshwater rainbow trout (Oncorhynchus mykiss) treated with sodium acetate, vacuum packaged, and stored under refrigeration (4 C)., Nonirradiated;, 1-kGy radiation dose;, 3-kGy radiation dose;, 5-kGy radiation dose. authors have reported different acceptability levels for different fish species, specific treatments, and processing conditions for TVB-N values of 35 to 40 mg/100 g (11), 25 to 30 mg/100 g (36), and 25 to 35 mg/100 g (1). In the present study, TVB-N values were found to increase gradually with storage period in the control and treated samples (Fig. 2A). Changes in TVB-N values showed no significant increase in the control and irradiated samples at 1 and 3 kgy until day 14 of storage. TVB-N values (P 0.05) of ca. 17 to 25 mg of N per 100 g of flesh were observed. After day 14, TVB-N values for control samples increased steadily, attaining a final value of 79.8 mg of N per 100 g of flesh at day 35, whereas the respective values for irradiated samples were 52.2 (1 kgy), 39.4 (3 kgy), and 28.2 (5 kgy) mg of N per 100 g of flesh. Since TVB-N is produced mainly by bacterial decomposition of fish flesh, the higher level of TVC of nonirradiated samples throughout the period of storage under refrigeration could account for the higher TVB-N values of rainbow trout. The TVB-N acceptance limit of 35 to 40 mg of N per 100 g of flesh (11) was reached between days 14 and 21 for nonirradiated samples and between days 21 and 28 for samples irradiated at 1 and 3 kgy. Samples irradiated at 5 kgy never reached this level. Suppression of TVB-N values following irradiation at 1.5, 2, and 3 kgy has also been reported for other fish species such as salted and vacuum-packaged sea bream, refrigerated carp, whole anchovies, and salted threadfin bream (9, 24, 27, 32). Therefore, low levels of TVB-N in samples were due to either a reduced bacterial population (irradiation) or a decreased capacity of bacteria for oxidative deamination of nonprotein nitrogen compounds (vacuum packaging) or both (9). TBA is a widely used indicator for the assessment of the degree of lipid oxidation. That is an index of lipid oxidation measuring MDA content. MDA is formed through hydroperoxides, which are the initial reaction products of polyunsaturated fatty acids with oxygen. According to Connell (11), TBA values of 1 to 2 mg of MDA per kg of fish flesh are usually regarded as the limit beyond which fish will normally develop an objectionable odor and/or taste. The results of the present study showed that TBA values for the control and irradiated rainbow trout samples (Fig. 2B) increased gradually from initial values of ca. 0.4 to 0.9 mg of MDA per kg of flesh to maximum values of ca (nonirradiation) on day 35, and 6.03, 7.26, and 8.21 mg of MDA per kg of flesh for samples irradiated at 1, 3, and 5 kgy, respectively, on day 28 of storage. After this, TBA values for both control and irradiated samples decreased gradually to final values of 4.1, 5.53, 5.95, and 6.47 mg of MDA per kg of flesh on day 42. It is interesting that the TBA values for samples irradiated at 1, 3, and 5 kgy were higher than those for the control samples throughout the entire storage period. This may be attributed to a higher concentration of free radicals formed in the substrate upon irradiation. It is well documented (33) that ionizing radiation is the cause of free radical formation in lipids, which constitutes the first step of the lipid oxidation chain reaction leading to carbonyl compound formation. Thus, the higher the dose, the higher the degree of oxidation (i.e., higher TBA values) is. The decrease in TBA values after 28 days of storage may represent the breakdown of MDA to tertiary degradation products. Similar behavior of TBA values for nonirradiated and irradiated sea bream (filleted) and whole anchovy was reported by Chouliara et al. (9) and Lakshmanan et al. (32). The present results are also comparable to those obtained for irradiated Indian fish species (14).

6 1424 MOINI ET AL. J. Food Prot., Vol. 72, No. 7 FIGURE 3. Effect of gamma radiation on odor (A), taste (B) and texture (C) score of aquacultured freshwater rainbow trout (Oncorhynchus mykiss) treated with sodium acetate and vacuum-packed stored under refrigeration (4 C)., Nonirradiated;, 1-kGy radiation dose;, 3-kGy radiation dose;, 5-kGy radiation dose. The ph value of live fish muscle is close to 7.0; however, postmortem ph can vary from 6.0 to 7.0 depending on the season, the species, and other factors. In the present study, changes in ph during storage were not statistically significant (P 0.05) between treatments. Values of ph for the control and irradiated rainbow trout samples remained between 6.25 and 6.8. Similarly, Chytiri et al. (10) found ph values between 6.43 and 6.52 for rainbow trout (filleted) stored in ice and Chouliara et al. (9) reported ph values of between 6.6 and 6.8 for nonirradiated and irradiated sea bream (filleted) treated with sodium acetate, vacuum packaged, and stored refrigerated. An initial slight decrease in ph values may be attributed to the dissolution of CO 2 in the fish muscle, and a secondary increase in ph may be attributed to the production of volatile base compounds such as ammonia and trimethylamine as well as other biogenic amines by fish spoilage bacteria (49). Sensorial analysis. Freshness is the single most important attribute when assessing fish quality. Microbiological, biochemical, and sensory changes are associated with deterioration of fish quality during handling and storage. Although a variety of biochemical, physical, and microbiological methods have been used to assess fish freshness, sensory evaluation is still the most satisfactory method of achieving such a goal (48). Acceptability scores for odor, taste, and texture of cooked control and irradiated rainbow trout decreased with the time of refrigerated storage as shown in Figure 3A through 3C. A score of 5.5 was taken as the lower limit of acceptability, which is equivalent to a slight off-odor or offtaste development (39). Odor (Fig. 3A) and taste (Fig. 3B) showed a similar pattern of decreasing acceptability. The lower limit of acceptability of odor and taste was reached between days 7 and 14 for the control samples and between days 21 and 28 (for doses of 1 and 3 kgy) and between days 28 and 35 (for doses of 5 kgy) for the irradiated samples. Texture scores for both control and irradiated rainbow trout (Fig. 3C) decreased at a slower rate than odor and taste scores. Interestingly, the limit of acceptability for texture was reached only for the control samples at 42 days. In general, the higher texture scores of both control and irradiated fish samples throughout the entire period of refrigerated storage may be due to salting (treating of fillets with sodium acetate), which slows down textural changes by decreasing the activity of proteolytic enzymes (cathepsins) in fish muscle. Jeevanandam et al. (27) reported that salting of threadfin bream prior to irradiation improved the texture of irradiated fish samples. Taste data (Fig. 3B) of cooked rainbow trout correlated rather well with the microbiological data (Fig. 1A). Given that specific spoilage-causing microorganisms cannot be detected by organoleptic or chemical testing, it is useful to conduct microbiological, chemical, and organoleptic analyses when assessing the quality of fish.

7 J. Food Prot., Vol. 72, No. 7 EFFECT OF GAMMA RADIATION ON QUALITY OF RAINBOW TROUT 1425 In this research work, a shelf life of 4 weeks was obtained for aquacultured rainbow trout, dipped in sodium acetate, vacuum packaged, and irradiated at 1, 3, and 5 kgy under refrigeration, in comparison to a shelf life of only 2 weeks for the nonirradiated vacuum-packaged and sodium acetate dipped rainbow trout. ACKNOWLEDGMENTS The authors thank J. M. Regenstein from the Department of Food Science at Cornell University, Ithaca, NY, and Barbara A. Rasco from the Department of Food Science and Human Nutrition at Washington State University, Pullman, for their critical review of the manuscript. REFERENCES 1. Ababouch, L. H., L. Souibri, K. Rhaliby, O. Ouahdi, M. Battal, and F. F. Busta Quality changes in sardines (Sardina pilchardus) stored in ice and at ambient temperature. Food Microbiol. 13: Abu-Tarboush, H. M., H. A. Al-Khatami, M. Atia, A. A. Abou-Arab, A. S. Bajaber, and M. A. 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