Contaminated surfaces of food processing equipment are believed to be a significant source of Listeria monocytogenes to foods. However, very little is known about the survival of Listeria in processing environments. In a mixed bacterial biofilm of L. monocytogenes and Flavobacterium spp., the number of L. monocytogenes cells attaching to stainless steel increased significantly compared to when L. monocytogenes was in a pure culture. The L. monocytogenes cells in the mixed biofilms were also recoverable for significantly longer exposure periods. On colonized coupons held at 15 degrees C and 75% humidity, decimal reduction times were 1.2 and 18.7 days for L. monocytogenes in pure and mixed biofilms, respectively. With increasing exposure time, the proportion of cells that were sublethally injured (defined as an inability to grow on selective agar) increased from 8.1% of the recoverable cell population at day 0 to 91.4% after 40 days' exposure. At 4 and -20 degrees C, decimal reduction times for L. monocytogenes in pure culture were 2.8 and 1.4 days, respectively, and in mixed culture, 10.5 and 14.4 days, respectively. The enhanced colonization and survival of L. monocytogenes on "unclean" surfaces increase the persistence of this pathogen in food processing environments, while the increase in the percentage of sublethally injured cells in the population with time may decrease the ability of enrichment regimes to detect it.

The use of multiple freeze (-20 degrees C)-thaw cycles in combination with isoeugenol and polysorbate 80 was investigated as a method for the reduction of numbers of Listeria monocytogenes cells in a bacteriological medium. Three freeze (1 h, -20 degrees C)-thaw cycles in the presence of isoeugenol at concentrations of 0, 100, and 300 ppm resulted in average L. monocytogenes reductions of 0.69, 2.65, and 3.3 log10 MPN (most probable number) per ml, respectively. Increasing the number of freeze-thaw cycles further decreased cell numbers, with reductions of nearly 5 log10 MPN/ml being obtained with six freeze-thaw cycles. Freeze-thaw cycles were effective in reducing cell numbers at isoeugenol concentrations down to 25 ppm. Rapid freezing rates with liquid nitrogen were found to be less effective in reducing numbers of L. monocytogenes cells. Two rapid freeze-thaw cycles in the presence of 100 ppm isoeugenol and polysorbate 80 resulted in a reduction of 1.45 log10 MPN/ml. Two freezing (-20 degrees C) cycles involving slow freezing and thawing rates with samples being held frozen for 6 h for each cycle resulted in reductions larger than those obtained with faster freezing rates. It was found that complete thawing in freeze-thaw cycles was not necessary to achieve bactericidal action. The application of multiple freeze-thaw cycles in combination with low concentrations of isoeugenol could effectively reduce numbers of L. monocytogenes cells in bacteriological media.

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-1 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 regimens 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.

In New Zealand the product most frequently implicated in cases of scombroid poisoning is hot-smoked kahawai (Arripis trutta). Using a Hafnia alvei strain, previously isolated from a portion of hot-smoked kahawai with a histamine level of 1,659.4 mg/kg, thermal death trials were carried out in a model suspension (0.1% peptone) at 54, 55, 56, 57, and 58 degrees C. From the linear regression line (R2 = 0.98) fitted to observed D values plotted against temperature, calculated D values for 54, 55, 56, 57, and 58 degrees C were estimated to be 0.63, 0.36, 0.20, 0.11, and 0.06 min, respectively, giving a z value of 4.14 degrees C. Thermal death trials were also carried out for H. alvei associated with hot-smoked kahawai at 54, 55, 55.5, 56, and 57 degrees C. From the linear regression line (R2 = 0.93) fitted to the data, calculated D values for 54, 55, 56, and 57 degrees C were estimated to be 1.42, 0.74, 0.38, and 0.20 min, respectively, giving a z value of 3.57 degrees C. Results indicate that hot smoking has the potential to eliminate H. alvei from seafood products.

. SM I TH . 1998. Survival, recoverability and sublethal injury of two strains of Listeria monocytogenes, Scott A and an environmental strain KM, on exposure to sea water at 12·8 or 20·8°C was determined using in situ diffusion chambers. Plate counts were used to assess recoverability and injury while 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) reduction was used to determine respiratory activity. T 90 values (times for 10-fold decreases in numbers of recoverable cells) on non-selective medium (trypticase soya agar with 0·6% yeast extract) at 12·8 and 20·8°C were 61·7 and 69·2 h for L. monocytogenes Scott A, and 103·0 and 67·0 h for L. monocytogenes KM, respectively. On selective medium (Oxford agar), T 90 values at 12·8 and 20·8°C were 60·6 and 56·9 h for L. monocytogenes Scott A, and 83·0 and 65·9 h for L. monocytogenes KM, respectively. With Scott A, the percentage of sublethally injured cells at 12·8 and 20·8°C was 1·7 and 17·7%, respectively, while for KM the values were 19·0 and 1·6%, respectively. The fraction of cells reducing CTC but which were not recoverable on plating progressively increased on exposure to sea water. Listeria monocytogenes KM challenged at 58°C showed an apparent increase in heat resistance after exposure to sea water at 20·8°C for 7 d (D 58 2·64 min) compared with before exposure (D 58 1·24). This increase in thermal resistance was not apparent at temperatures greater than 63°C, and analysis of the best-fit regression lines fitted to the thermal data obtained from the two cell populations indicated that their thermal resistance was not significantly different (P × 0·05) over the temperature range tested (58-62°C).