Inactivation of Listeria monocytogenes on Hot-smoked Salmon by the Interaction of Heat and Smoke or Liquid Smoke †.

L. monocytogenes was inoculated onto the surface of brined salmon steaks and heat processed in a commercial smokehouse to simulate a hot process for preparing smoked fish. The minimum temperature required for inactivation of L. monocytogenes was 153°F (67.2°C) when generated smoke was applied throughout the entire process. When generated smoke was added only during the last half of the process, L. monocytogenes was recovered from steaks heated to temperatures as high as 176°F (80.0°C). When smoke was not applied during the process, L. monocytogenes survived on steaks heated to internal temperatures between 131°F and 181°F (55.0 to 82.8°C) but was not isolated from steaks heated above 181°F (82.8°C). When liquid smoke CharSol C-l0 was applied as a full-strength (100%) dip before processing, L. monocytogenes was inactivated in samples processed at temperatures as low as 138°F (58.9°C). When liquid smoke l0-Poly or CharSol C-l0 was applied at a concentration of 50%, the lethal temperature was increased to the range of 145 to 150°F (62.8 to 65.6°C). With further dilution of C-l0 to 25% and 10% the inactivation temperatures increased to 156°F (68.9°C) and 163°F (72.8°C). A full-strength dip of CharOil, the oil-soluble fraction of CharSol C-l0, was less effective, and L. monocytogenes survived in salmon steaks processed to an internal temperature of 166°F (74.4°C), the highest temperature tested with this liquid smoke. This study provides evidence that heat alone is not reliable for inactivation of L. monocytogenes during the hot-smoking process. The proper stage and duration of smoke application or proper composition and concentration of liquid smoke in combination with heat are critical for inactivation of the organism.

Incidence and Sources of Listeria monocytogenes in Cold-Smoked Fishery Products and Processing Plants.

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.

Inhibition of Listeria monocytogenes in Cold-process (Smoked) Salmon by Sodium Lactate.

Comminuted raw salmon containing various concentrations and combinations of sodium lactate, sodium chloride, and sodium nitrite was inoculated with 10 Listeria monocytogenes cells per g (150 cells/15-g sample), vacuum-packaged in oxygen-impermeable film and stored at 5 or 10°C. Samples were examined for growth of L. monocytogenes and total aerobic microorganisms at specific intervals for up to 50 d. Sodium lactate exhibited a concentration-dependent antilisterial effect that was enhanced by nitrite and/or increased concentrations of NaCl. At 5°C, total inhibition of L monocytogenes was achieved for up to 50 d by 2% sodium lactate in combination with 3% water-phase NaCl. At 10°C, total inhibition was achieved for up to 35 d by 3% sodium lactate in combination with 3% water-phase NaCl, or by 2% sodium lactate in combination with 125 ppm sodium nitrite and 3% water-phase NaCl. Sodium lactate and the other additives also inhibited growth of the aerobic microflora but to a lesser degree than L. monocytogenes .

Inhibition of Listeria monocytogenes in Cold-process (Smoked) Salmon by Sodium Nitrite and Packaging Method.

The behavior of Listeria monocytogenes in relation to sodium nitrite (NaNO2) in combination with sodium chloride (NaCl) was evaluated in cold-process (smoked) salmon during storage at 5 or 10°C in either oxygen-permeable film or vacuum-sealed impermeable film. Salmon slices containing either 3 or 5% water-phase NaCl, with or without 190-200 ppm of NaNO2, were inoculated with 10 or 327 CFU/g (150 or 4.9 × 103 CFU/15-g sample) of strain Scott A. The inhibitory contribution of NaNO2 was relative to inoculum size, storage time and temperature, packaging method, and concentration of NaCl. There was less growth of L. monocytogenes in vacuum-packaged samples as compared to those packaged in oxygen-permeable film. The most inhibition was achieved in vacuum-packaged products stored at 5°C, where NaNO2 in combination with 5% water-phase NaCl prevented any increase in a 10 CFU/g-inoculum during 34 d storage. At 10°C, inhibition was initially enhanced by NaNO2, but by 32 d L. monocytogenes populations had increased from a 10 CFU/g-inoculum to the range of 106 CFU/g in vacuum-packaged products and 108 CFU/g in permeable-film packaged products, regardless of NaNO2 or NaCl concentration. Growth of naturally occurring aerobic microorganisms was also inhibited by NaNO2 but to a lesser degree than L. monocytogenes .

Selective and Differential Medium for Isolation of Listeria monocytogenes from Foods.

Hemolytic ceftazidime lithium chloride agar (HCLA) has been developed as a selective and differential plating medium for the isolation of Listeria monocytogenes from fishery products. Selectivity is based upon lithium chloride, colistin methane sulfonate, and ceftazidime. When horse blood was incorporated in the agar overlay, L. monocytogenes colonies were easily distinguished by their characteristic blue-gray color surrounded by narrow zones of ß-hemolysis. Listeria innocua , a species commonly present in foods, does not produce hemolysis on the medium. Therefore, one or more colonies of L. monocytogenes were easily distinguished from large populations of L. innocua . When used in combination with Food and Drug Administration and U.S. Department of Agriculture enrichment methodology, HCLA was effective in inhibiting competitive organisms, differentiating colonies of L. monocytogenes by their ß-hemolysis, and shortening the incubation time at 35°C for presumptive identification to 17-24 h.

Parameters for Control of Listeria monocytogenes in Smoked Fishery Products: Sodium Chloride and Packaging Method.

The behavior of Listeria monocytogenes (10 Scott A cells per g) in cold-process (smoked) salmon containing 3, 5, or 6% water-phase NaCl was evaluated during 30 to 40 d storage at 5 or 10°C in either oxygen-permeable film or vacuum-sealed impermeable film. At 10°C, L. monocytogenes grew to 106 to 108 CFU/g by the second week, with no differences attributed to NaCl concentration except for an initial lag in the 6% NaCl samples. Vacuum packaging suppressed growth of L. monocytogenes by 10- to 100-fold in samples with 3 or 5% NaCl. Inhibition related to NaCl concentration was most apparent at 5°C and L. monocytogenes populations were held below 102 CFU/g by 6% NaCl. Growth of a 327 CFU/g inoculum was about 10-fold greater than a 10 CFU/g inoculum at 10°C and 100-fold greater at 5°C. Growth of two strains isolated from naturally contaminated, commercially prepared, cold-smoked fish did not differ from Scott A. The use of sugar in the product did not influence growth of L. monocytogenes . Maximum populations of aerobic microorganisms reached at 5 and 10°C were similar, although the rate of growth was somewhat delayed at 5°C, and some inhibition was shown by 5 and 6% NaCl and by vacuum packaging.

Comparison of Selective Direct Plating Media for Enumeration and Recovery of L. monocytogenes from Cold-process (smoked) Fish.

Three selective media were evaluated for direct plating recovery and enumeration of Listeria monocytogenes in the presence of high levels of a variety of microorganisms occurring on cold-process (smoked) salmon products. Sliced salmon was brined to contain either no added salt, 3, or 5% water-phase NaCl, or 3 or 5% NaCl plus 140 ppm NaNO2. The slices were packaged in oxygen-permeable film or sealed under vacuum in oxygen-impermeable film, and stored at 10°C or 5°C until total microbial loads reached 106 to 109 CFU/g. Oxford formulation of Listeria selective agar and Lee's modification of Listeria selective agar achieved quantitative recovery of 102 cells per ml of L. monocytogenes strain Scott A in the presence of diluted slurries of these fish containing 104 to 108 CFU/ml of background organisms. A modification of lithium chloride-phenylethanol-moxalactam agar containing an iron-esculin indicator system sometimes failed because of interfering growth by the background microflora.