Applications of Polarography for Assessment of Fish Freshness J.H.T. LUONG, K.B. MALE, and M.D. HUYNH ABSTRACT Fish freshness was assessed with an enzymatic method using a polar- ographic probe at 0.7 volt (platinum vs silver) to monitor degradation of inosine monophosphate(IMP), inosine (H x R), and hypoxanthine (Hx). The K-value, a ratio of H x R + Hx/IMP + H x R + Hx, was de- termined in Sockeye salmon, Pacific cod and Pacific herring in ice and at 12°C. Results were compared with those from HPLC and sen- sory scores. K-values by polarography correlated well with HPLC. Loss of freshnessaccompaniedrise in K-value in all studies. Sensory scorescorrelated relatively well with K-values in Sockeye salmon and Pacific herring but lesswith Pacific cod. K-value did not reflect eating quality of Pacific cod. INTRODUCTION CONSIDERABLE A’ITENTION has been focused toward a rapid and reliable method of fish freshness assessment.To date, a variety of methods have been used based on measure- ment of postmortem deteriorative changes associated with sen- sory quality, chemical changes and microbial growth. The traditional sensory method is subjective and costly becauseit requires specially trained personnel. Bacteriological assess- ment, and chemical methods measuring total volatile amine, trimethylamine or pH changes, do not measureearly postmor- tem deterioration. They measure bacterial spoilage instead of loss of freshness causedby autolytic deterioration shortly fol- lowing death. Nucleotide degradation has correlated well with loss of freshness in a wide range of species(Jonesand Murray, 1962, 1964; Spinelli et al., 1964). Concentration of thesecom- pounds within fish flesh is dependenton postmortem age, con- dition of the fish prior to death, sexual maturity, storage temperatureand other factors relating to handling and storage. Inosine 5’-monophosphate (IMP) is known to contribute to the pleasant flavor of fresh fish and its degradation to hypo- xanthine is a factor in progressive loss of desirable flavor. Procedures for following nucleotide degradation afford a basis for valid and useful indices of quality (Jones and Murray, 1964). Of tests for estimating nucleotides, the so-called K- value has received much attention. The K-value is defined as the ratio of inosine (H x R) plus hypoxanthine (Hx) to the total adenosine triphosphate (ATP) and related compounds (ADP, AMP, IMP, H x R and Hx) in fish muscle extract (Saito et al., 1959). Many studies confirmed the relationship between fish freshness and K-value (Ehira et al., 1970; Ehira and Uchi- yama, 1974). Assays for K-value, however, are difficult and time-consuming. Of the procedures available, high perform- ance liquid chromatography (HPLC) affords accurate and re- liable results but is not suitable for routine examination in a quality control laboratory. Consequently, considerable effort has been directed toward development of a rapid automated system for monitoring fish freshness(Uchiyama and Kakuda, 1984; Watanabeand Karube, 1986; and Karube et al., 1984). Luong et al. (1989) developed an enzymatic technique for de- termination of HX R, I-Ix and IMP of fish homogenates. A polarographic electrode (0.7 volt with respect to silver cathode Authors Luong and Male are with Biotechnology Research ln- stitute, National Research Council of Canada, Montreal, Quebec, Canada H4P 2R2. Author Huynh is with B.C. Research Corp., Vancouver, B.C. Canada V6S 2L2. and platinum anode) was attachedto a temperaturecontrolled reaction chamber where metabolites in fish muscles were en- zymatically degraded by xanthine oxidase, nucleoside phos- phorylase, or nucleotidaseto uric acid and hydrogen peroxide which were detectedby the polarographic electrode. Based on that approach a biosensor system was developed for reliable, rapid determination of K-value (Mulchandani et al., 1990). The objective of our study was to apply the analytical tech- nique proposed by Luong et al. (1989) for K-value determi- nation of some selectedfish species: Sockeye salmon, Pacific cod, and Pacific herring. The K-value obtained by polarogra- phy was compared with that obtained by HPLC. Concurrent sensory evaluation of the raw and cooked fish served for com- parison of fish quality. MATERIALS & METHODS Fish Three species, Sockeye salmon (Oncorhynchus nerka), Pacific cod (Gadus macrocephalus), and Pacific herring (Clupea harengus pal- lasii) caught during 1988 off the coast of Vancouver, B.C., were tested. Sockeye salmon was held in ice less than 24 hr before arrival at the laboratory. Eight fish were eviscerated, cleaned and kept with head and gill on for the storagestudy. Pacific cod and Pacific herring were less than 12 hr postmortem when tested at the laboratory. On arrival at the laboratory, cod were gutted, cleaned and filleted while herring were stored whole. Fresh fish of each snecies were divided equally into two groups and stored separately ii ice (about 0°C) or refrigerated at 12°C for UD to 2 wk. Sensorv and K-value analvsesfor freshness were measured&at 0, 2, 6, 9, 13-days for salmon; 6, 2, 5, 7, 10 days for cod; and at 0, 2, 3, 6 and 8 days for herring. To study quality changesof whole Sockeye salmon, four fish from each temperature were individually examined. At each sampling time, a cut of about 1.8 x 1.8 cm was made on the dorsal side and tissues without dark meat or skin were removed for K-value determination. The remaining fish was subsequently held at 12°C for 15 min for sensory evaluation. They were then packed and stored in ice or at 12°C until the next test. At each sampling time, care was taken not to contaminate the cut surface with slime, blood or other contami- nants. In the storagestudy of cod fillets, a set of four fillets from different fish were examined at each sampling time. After a small portion of the dorsal tissue had been taken for the K-value analyses, the re- maining fillet was used for sensory evaluation. Freshness was eval- uated on both raw and cooked fish. For herring, four whole fish from eachstoragetemperature were individually examined at each sampling time. Unlike Sockeye salmon, whole herring were discardedafter each sampling and a new fish from the same sourcewas used in subsequent tests. Sensory evaluation A trained panel of ten members was used. Fish were judged for overall freshnesson general appearance (eye, skin, belly cavity and gill), flesh texture and gill odor for whole salmon and herring. Cod fillets were assessed on appearance and odor for raw fish, and texture, odor and flavor for cooked fish. A scale of 1 to 10 was used with 10 indicating highest quality of very fresh fish, 5 indicating marginal quality and 1, lowest quality of spoiled fish. The sensory attributes were scored separately and an averagedcomposite score was used to evaluate quality. For evaluation of cooked cod fillets, about one cubic inch of fillet was wrapped in aluminum foil and steamed 10 minutes. Portions were served hot to panelists. Volume 56, No. 2, 1991-JOURNAL OF FOOD SCIENCE-335