Effect of salt, smoke compound, and storage temperature on the growth of Listeria monocytogenes in simulated smoked salmon

The efficacy of nisin against Listeria monocytogenes on cold-smoked salmon at natural contamination levels is concentration-dependent and varies by serotype

Cold-smoked salmon is a ready-to-eat food product capable of supporting Listeria monocytogenes growth at refrigeration temperatures. While the FDA-approved antimicrobial nisin can be used to mitigate L. monocytogenes contamination, stresses associated with cold-smoked salmon and the associated processing environments may reduce nisin efficacy. A previous study in our laboratory showed that, at high inoculation levels, pre-exposure of L. monocytogenes to sublethal concentrations of quaternary ammonium compounds had an overall detrimental effect on nisin efficacy. The objective of this study was to investigate the impact of nisin concentration and storage temperature on nisin efficacy against L. monocytogenes inoculated on salmon at natural contamination levels. Three L. monocytogenes strains were pre-grown in the presence of sublethal levels of benzalkonium chloride prior to inoculation at ~10² CFU/g on salmon slices that were pre-treated with either 0, 25, or 250 ppm nisin, followed by vacuum-packing and incubation at 4 or 7°C for up to 30 days. L. monocytogenes was enumerated on days 1, 15, and 30 using direct plating and/or most probable number methods. A hurdle model was constructed to describe the odds of complete elimination of L. monocytogenes on salmon and the level of L. monocytogenes when complete elimination was not achieved. Our data showed that (i) nisin efficacy (defined as L. monocytogenes reduction relative to the untreated control) was concentration-dependent with increased efficacy at 250 ppm nisin, and that (ii) 250 ppm nisin treatments led to a reduction in L. monocytogenes prevalence, independent of storage temperature and serotype; this effect of nisin could only be identified since low inoculation levels were used. While lower storage temperatures (i.e., 4°C) yielded lowered absolute L. monocytogenes counts on days 15 and 30 (as compared to 7°C), nisin efficacy did not differ between these two temperatures. Finally, the serotype 1/2b strain was found to be more susceptible to nisin compared with serotype 1/2a and 4b strains on samples incubated at 7°C or treated with 25 ppm nisin. This variation of nisin susceptibility across serotypes, which is affected by both the storage temperature and nisin concentration, needs to be considered while evaluating the efficacy of nisin.

Quantifying the bioprotective effect of Lactobacillus sakei CTC494 against Listeria monocytogenes on vacuum packaged hot-smoked sea bream

In this study, the bioprotective potential of Lactobacillus sakei CTC494 against Listeria monocytogenes CTC1034 was evaluated on vacuum packaged hot-smoked sea bream at 5 °C and dynamic temperatures ranging from 3 to 12 °C. The capacity of three microbial competition interaction models to describe the inhibitory effect of L. sakei CTC494 on L. monocytogenes was assessed based on the Jameson effect and Lotka-Volterra approaches. A sensory analysis was performed to evaluate the spoiling capacity of L. sakei CTC494 on the smoked fish product at 5 °C. Based on the sensory results, the bioprotection strategy against the pathogen was established by inoculating the product at a 1:2 ratio (pathogen:bioprotector, log CFU/g). The kinetic growth parameters of both microorganisms were estimated in mono-culture at constant storage (5 °C). In addition, the inhibition function parameters of the tested interaction models were estimated in co-culture at constant and dynamic temperature storage using as input the mono-culture kinetic parameters. The growth potential (δ log) of L. monocytogenes, in mono-culture, was 3.5 log on smoked sea bream during the experimental period (20 days). In co-culture, L. sakei CTC494 significantly reduced the capability of L. monocytogenes to grow, although its effectiveness was temperature dependent. The LAB strain limited the growth of the pathogen under storage at 5 °C (<1 log increase) and at dynamic profile 2 (<2 log increase). Besides, under storage at dynamic profile 1, the growth of L. monocytogenes was inhibited (<0.5 log increase). These results confirmed the efficacy of L. sakei CTC494 for controlling the pathogen growth on the studied fish product. The Lotka-Volterra competition model showed slightly better fit to the observed L. monocytogenes growth response than the Jameson-based models according to the statistical performance. The proposed modelling approach could support the assessment and establishment of bioprotective culture-based strategies aimed at reducing the risk of listeriosis linked to the consumption of RTE hot-smoked sea bream.

Modelling the interaction of the sakacin-producing Lactobacillus sakei CTC494 and Listeria monocytogenes in filleted gilthead sea bream (Sparus aurata) under modified atmosphere packaging at isothermal and non-isothermal conditions

The objective of this work was to quantitatively evaluate the effect of Lactobacillus sakei CTC494 (sakacin-producing bioprotective strain) against Listeria monocytogenes in fish juice and to apply and validate three microbial interaction models (Jameson, modified Jameson and Lotka Volterra models) through challenge tests with gilthead sea bream (Sparus aurata) fillets under modified atmosphere packaging stored at isothermal and non-isothermal conditions. L. sakei CTC494 inhibited L. monocytogenes growth when simultaneously present in the matrix (fish juice and fish fillets) at different inoculation ratios pathogen:bioprotector (i.e. 1:1, 1:2 and 1:3). The higher the inoculation ratio, the stronger the inhibition of L. monocytogenes growth, with the ratio 1:3 yielding no growth of the pathogen. The maximum population density (N_max) was the most affected parameter for L. monocytogenes at all inoculation ratios. According to the microbiological and sensory analysis outcomes, an initial inoculation level of 4 log cfu/g for L. sakei CTC494 would be a suitable bioprotective strategy without compromising the sensory quality of the fish product. The performance of the tested interaction models was evaluated using the Acceptable Simulation Zone approach. The Lotka Volterra model showed slightly better fit than the Jameson-based models with 75-92% out of the observed counts falling into the Acceptable Simulation Zone, indicating a satisfactory model performance. The evaluated interaction models could be used as predictive modelling tool to simulate the simultaneous behaviour of bacteriocin-producing Lactobacillus strains and L. monocytogenes; thus, supporting the design and optimization of bioprotective culture-based strategies against L. monocytogenes in minimally processed fish products.

Growth Kinetics of Listeria monocytogenes in Cut Produce

Cut produce continues to constitute a significant portion of the fresh fruit and vegetables sold directly to consumers. As such, the safety of these items during storage, handling, and display remains a concern. Cut tomatoes, cut leafy greens, and cut melons, which have been studied in relation to their ability to support pathogen growth, have been specifically identified as needing temperature control for safety. Data are needed on the growth behavior of foodborne pathogens in other types of cut produce items that are commonly offered for retail purchase and are potentially held without temperature control. This study assessed the survival and growth of Listeria monocytogenes in cut produce items that are commonly offered for retail purchase, specifically broccoli, green and red bell peppers, yellow onions, canned green and black olives, fresh green olives, cantaloupe flesh and rind, avocado pulp, cucumbers, and button mushrooms. The survival of L. monocytogenes strains representing serotypes 1/2a, 1/2b, and 4b was determined on the cut produce items for each strain individually at 5, 10, and 25°C for up to 720 h. The modified Baranyi model was used to determine the growth kinetics (the maximum growth rates and maximum population increases) in the L. monocytogenes populations. The products that supported the most rapid growth of L. monocytogenes, considering the fastest growth and resulting population levels, were cantaloupe flesh and avocado pulp. When stored at 25°C, the maximum growth rates for these products were 0.093 to 0.138 log CFU/g/h and 0.130 to 0.193 log CFU/g/h, respectively, depending on the strain. Green olives and broccoli did not support growth at any temperature. These results can be used to inform discussions surrounding whether specific time and temperature storage conditions should be recommended for additional cut produce items.

Temperature effects on the antimicrobial efficacy of condensed smoke and lauric arginate against Listeria and Salmonella

Condensed smoke or liquid smoke (LS) and lauric arginate (LAE) are antimicrobials used in food preservation. They have demonstrated abilities to reduce or inhibit pathogenic and spoilage organisms. Few studies, however, have reported on the effectiveness of LS or LAE over the range of temperatures typically encountered in food marketing channels. Therefore, the effects of temperature on the antimicrobial properties of two commercial LS fractions, an LS derived from pecan shells, and LAE against two common foodborne pathogens, Listeria and Salmonella, were investigated. The MICs of the three LS samples and LAE were measured at 4, 10, and 37°C for Listeria monocytogenes strains 2045 (Scott A, serotype 4b) and 10403S (serotype 1/2a) and two strains of Listeria innocua, a well-established surrogate, and at 10, 25, and 37°C for Salmonella enterica serovar Typhimurium and Salmonella enterica serovar Heidelberg. The MICs for LS against Listeria ranged from 3 to 48% (vol/vol), with higher MICs seen with lower temperatures. The MICs for LS on Salmonella ranged from 3 to 24%. Values for LAE ranged between 0.004 and 0.07% for both pathogens, and like LS, higher MICs were always associated with lower incubation temperatures. Understanding how storage temperature affects the efficacy of antimicrobials is an important factor that can contribute to lowering the hurdles of use levels and costs of antimicrobials and ultimately improve food safety for the consumer.

Effect of salt, smoke compound, and temperature on the survival of Listeria monocytogenes in salmon during simulated smoking processes

The objectives of this study were to examine and develop a model to describe the survival of Listeria monocytogenes in salmon as affected by salt, smoke compound (phenol), and smoking process temperature. Cooked minced salmon containing selected levels of salt (0%, 2%, 4%, and 6%) and smoke compound (0, 5, 10, and 15 ppm phenol) were inoculated with a 6-strain mixture of L. monocytogenes to an inoculum level of 6.0 log(10) CFU/g. The populations of L. monocytogenes in salmon during processing at 40, 45, 50, and 55 degrees C that simulated cold- and hot-smoking process temperatures were determined, and the effects of salt, phenol, and temperature on the survival of L. monocytogenes in salmon were analyzed and described with an exponential regression. At 40 degrees C, the populations of L. monocytogenes in salmon decreased slightly with inactivation rates of <0.01 log(10) CFU/h, and at 45, 50, and 55 degrees C, the inactivation rates were 0.01 to 0.03, 0.15 to 0.30, and 2.8 to 3.5 log(10) CFU/h, respectively. An exponential regression model was developed and was shown to closely describe the inactivation rates of L. monocytogenes as affected by the individual and combined effects of salt, phenol, and smoking process temperature. Temperature was the main effector in inactivating L. monocytogenes while salt and phenol contributed additional inactivation effects. This study demonstrated the inactivation effects of salt, smoke compound, and temperature on L. monocytogenes in salmon under a smoking process. The data and model can be used by manufacturers of smoked seafood to select concentrations of salt and smoke compound and alternative smoking process temperatures at 40 to 55 degrees C to minimize the presence of L. monocytogenes in smoked seafood.

Impact of -2 degrees C superchilling before refrigerated storage (4 and 8 degrees C) on the microbiological and sensory qualities of cold-smoked salmon

Detection and enumeration of Listeria monocytogenes and total spoilage bacteria in 40 batches of cold-smoked salmon (one batch = 42 products from the same day of manufacture) straight from the factory were carried out. If L. monocytogenes was detected in at least one of the nine samples analyzed on receipt at the laboratory, 9 products of the same batch were stored for 10 days at 4 degrees C, which was followed by 18 days at 8 degrees C (control), 12 products were superchilled for 14 days at -2 degrees C, and 12 other products were superchilled for 28 days at -2 degrees C and then stored under the same conditions as the control was stored. L. monocytogenes was detected in 7% of the 40 batches analyzed immediately after receipt at the laboratory. L. monocytogenes prevalence was similar (approximately 25%) throughout the storage at 4 and 8 degrees C, both in control and super-chilled products at -2 degrees C for 14 days. After superchilling for 28 days at -2 degrees C, L. monocytogenes was found in 9% of products, and in 39% at the end of the storage above 0 degree C. Moreover, the L. monocytogenes count was higher after 3 and 4 weeks of storage at 4 and 8 degrees C in products superchilled 28 days at -2 degrees C than in control products or in products superchilled for 14 days. Serotype 1/2a-3a and nine genetic groups were identified and found throughout the storage scenario. At the end of shelf life, sensory characteristics of products superchilled for 28 days at -2 degrees C were slightly modified. A decrease in firmness associated with increased tearing of salmon slices was observed as well as a slight amine odor.