Incidence, Growth, and Inactivation of Listeria monocytogenes in Cooked and Peeled Cold-Water Shrimp

Abstract

ABSTRACT Cooked and peeled cold-water shrimp (Pandalus jordani) naturally contaminated with Listeria monocytogenes were obtained from a processor for a series of studies to determine the level of contamination and growth characteristics of this bacterium in the naturally contaminated product. L. monocytogenes was isolated from every 25-g sample of individually quick frozen (IQF) shrimp that was tested. The level of contamination in each composite sample ranged from 5 to 16 colony forming units (CFU) per 25 g. When individual shrimp taken from the 25-g sample portions were tested separately, samples positive for L. monocytogenes ranged from 1 of 12 to 5 of 15 shrimp tested. The project also evaluated the effectiveness of three methods to inactivate the bacterium: ozone, chlorine dioxide, and steam as possible product reconditioning strategies. Ozone and chlorine dioxide were both found to be ineffective reconditioning treatments for shrimp naturally contaminated with L. monocytogenes. Experiments with steam conducted at the laboratory and later at the shrimp processing plant verified that shrimp contaminated with L. monocytogenes could be safely reconditioned by steam pasteurization. Steam was used successfully to pasteurize several thousand pounds of contaminated shrimp in the processing plant. When the naturally contaminated product was packaged in either oxygen-permeable or impermeable films and stored at 5°C and 10°C, the product was deemed spoiled by sensory evaluation after 9 days of storage, at which time the L. monocytogenes population were 3 × 104 CFU per g. By comparison, when an isolate (strain 4311) from naturally contaminated shrimp was inoculated onto the pasteurized shrimp at a concentration of 12 cells /25g, the L. monocytogenes population reached 3.0 × 108 per g after 9 days of storage. The pasteurization process used in this study would not be effective in inactivation of Clostridium botulinum. Ready-to-eat-shrimp must therefore be stored below 3°C or frozen.