Cold-smoked salmon processing plants were surveyed to determine the occurrence and sources of L. monocytogenes contamination. Sanitation and cleanup procedures adequately eliminated L. monocytogenes from the processing line and equipment, but recontamination occurred soon after resumption of processing. The primary source of contamination proved to be the surface areas of frozen or fresh raw fish coming into the plant. Listeria species were not isolated from the flesh except when they were introduced during the filleting operation, by the injection of recirculated brine, or in localized areas where there were pre-existing bruises. Penetration of L. monocytogenes into intact flesh via the vascular system did not occur when fresh fish were immobilized with carbon dioxide and bled, nor when frozen, headed, and eviscerated fish were thawed for 20 h in water inoculated with 44 L. monocytogenes organisms per ml. Populations of L. monocytogenes inoculated onto the surface of brined salmon portions changed very little during a cold-smoke process at 72 to 87°F (22.2 to 30.6°C) for 20 h, with or without applied smoke; but when the processing temperature was lowered to 63 to 70°F (17.2 to 21.1°C), populations decreased 10- to 25-fold when smoke was applied. L. monocytogenes injected into the interior of these portions increased 2- to 6-fold at 63 to 70°F (17.2 to 21.1°C) and 100-fold at 72 to 87°F (22.2 to 30.6°C), regardless of the presence of smoke. L. monocytogenes was enumerated in 48 contaminated finished products collected from six different processing plants. L. monocytogenes populations ranged from 0.3 to 34.3 cells per g, with a mean of 6.2 per g and a median of 3.2 per g.
