CATALASE ACTIVITY AS AN INDEX OF MICROBIAL LOAD AND END‐POINT COOKING TEMPERATURE OF FISH

Abstract

AbstractPsychrotrophs that are responsible for spoilage of many aerobically cold‐stored foods are strongly catalase positive. This study was conducted to ascertain the feasibility of using the Pasteur pipette catalase test developed by Fung (1985) to estimate the microbial population in raw fish during aerobic cold storage and to determine the end‐point cooking temperature of fish. Fish samples (cod fillets, catfish, halibut fillets, rainbow trout, and salmon fillets in three replications each) were stored at 7C and monitored at 0, 6, 12, 24, 48, and 72 h and up to 7 days. Fish surface, fish juice, and fish meat samples were obtained by conventional microbiological methods and were serially diluted in 0.1% of peptone water to a final 1:100 dilution. To perform the catalase test 0.1 mL of the 1:10 or 1:100 dilution of the sample was placed into the wider portion of the heat‐sealed Pasteur pipette followed by 0.1 mL of 3% H2O2. The liquid mixture then was shaken into the heat‐sealed capillary portion. The pipette was then inverted and after 5 min, the lengths of the gas column and the liquid column were measured in mm; the gas column was expressed as a percentage of the total, which was considered as the percentage catalase activity (% CA). Viable cell counts (VCCs) of these samples also were made. As VCCs increased on fish surface, in fish juice, and in fish meat samples, concomitant increases occurred in % CA. The lowest level detect by this method was about 103‐4 CFU/cm2, mL, or g. Catalase activities of 0.0‐0.5% on fish surface, <4.3% in fish juice, and <2% in fish meat, corresponded to low total counts of 104‐5 CFU/cm2, 105 CFU/mL, and 103‐4 CFU/g, respectively. Catalase activities of 0.6‐1.5% on fish surface, 4.3‐8.2% in fish juice, and 2.3‐5% in fish meat, corresponded to high total counts of 106 CFU/cm2, 106 CFU/mL, and 105‐6 CFU/g, respectively. Catalase activities of 1.7‐2.5% on fish surface, 8.3‐12.4% in fish juice, and 5–6% in fish meat, indicated that fish was unacceptable for consumption. Fish samples were cooked at temperatures of 40C, 50C, 55C, 60C, 65C, 70C, and 75C to determine the end‐point temperature. Each sample was heated at each temperature for: 36 s, 1 min, 2 min, 3 min, and 5 min in a temperature‐controlled water bath. Samples that had reached an internal temperature of 65C showed no catalase activity in the test tube method or very little activity in the capillary tube method and contained no live bacteria, indicating that fish was safe for consumption.