Reducing Total Aerobic Counts and Listeria monocytogenes on the Surface of King Salmon (Oncorhynchus tshawytscha)

Abstract

A trial industrial-scale fin-fish washing system was assessed for its effectiveness in removing bacteria associated with the skin of gilled and gutted king salmon (Oncorhynchus tshawytscha). Exposure of the salmon to 200 ppm free chlorine at a turnover rate for the total volume of the wash solution of 2.25 cycles h−1 for 120 min resulted in decreases in the aerobic plate count (APC) recovered from the salmon ranging from 96.6 to 99.2%. In order to optimize the washing regime a laboratory-scale fin-fish washing system was developed. Twenty-six washing treatments were used to generate a model to relate efficacy of bacterial removal with chlorine concentration, flow rate, and duration of washing. The model gave two local maxima of percentage APC reduction, one of 99.3% at a concentration of 126.3 ppm chlorine with a turnover of 0.75 cycle h−1 and a duration of 71.3 min and a second of 100.6% at a concentration of 126.3 ppm chlorine with a turnover of 3.75 cycles h−l and a duration of 120 min. In additional experiments, it was determined that washing could eliminate 99.79% of Listeria monocytogenes cells that had been artificially inoculated onto the surface of gilled and gutted salmon. It was concluded that while chlorinated wash regimes have the potential to reduce the carriage of bacteria, including L. monocytogenes, into fish-processing facilities, they will not ensure an L. monocytogenes-free product. Further, the use of such a system has to be assessed with regard to allowable chlorine levels (subject to regulation), the effect of washing on the quality of the finished product, and the cost of water purchase and disposal.