Growth of Listeria monocytogenes in vacuum-packed, smoked salmon, during storage at 4 degrees C

Mild processing of seafood-A review

Recent years have shown a tremendous increase in consumer demands for healthy, natural, high-quality convenience foods, especially within the fish and seafood sector. Traditional processing technologies such as drying or extensive heating can cause deterioration of nutrients and sensory quality uncompilable with these demands. This has led to development of many novel processing technologies, which include several mild technologies. The present review highlights the potential of mild thermal, and nonthermal physical, and chemical technologies, either used alone or in combination, to obtain safe seafood products with good shelf life and preference among consumers. Moreover, applications and limitations are discussed to provide a clear view of the potential for future development and applications. Some of the reviewed technologies, or combinations thereof, have shown great potential for non-seafood products, yet data are missing for fish and seafood in general. The present paper visualizes these knowledge gaps and the potential for new technology developments in the seafood sector. Among identified gaps, the combination of mild heating (e.g., sous vide or microwave) with more novel technologies such as pulsed electric field, pulsed light, soluble gas stabilization, cold plasma, or Ohmic heat must be highlighted. However, before industrial applications are available, more research is needed.

Microbiome Population Dynamics of Cold-Smoked Sockeye Salmon during Refrigerated Storage and after Culture Enrichment

ABSTRACT

Cold-smoked salmon is a ready-to-eat seafood product of high commercial importance. The processing and storage steps facilitate the introduction, growth, and persistence of foodborne pathogens and spoilage bacteria. The growth of commensal bacteria during storage and once the product is opened also influence the quality and safety of cold-smoked salmon. Here we investigated the microbial community through targeted 16S rRNA gene and shotgun metagenomic sequencing as means to better understand the interactions among bacteria in cold-smoked salmon. Cold-smoked salmon samples were tested over 30 days of aerobic storage at 4°C and cultured at each time point in a buffered Listeria enrichment broth (BLEB) commonly used to detect Listeria in foods. The microbiomes were composed of Firmicutes and Proteobacteria, namely, Carnobacterium, Brochothrix, Pseudomonas, Serratia, and Psychrobacter. Pseudomonas species were the most diverse species, with 181 taxa identified. In addition, we identified potential homologs to 10 classes of bacteriocins in microbiomes of cold-smoked salmon stored at 4°C and corresponding BLEB culture enrichments. The findings presented here contribute to our understanding of microbiome population dynamics in cold-smoked salmon, including changes in bacterial taxa during aerobic cold storage and after culture enrichment. This may facilitate improvements to pathogen detection and quality preservation of this food.

HIGHLIGHTS

Antilisterial activity of Cymbopogon citratus on crabsticks

Listeria monocytogenes is a gram positive, psychrotrophic, facultative anaerobic bacterium and it is the etiological agent of listeriosis, a severe foodborne disease of major public health concern. There is a rising concern about the cross-contamination of surimi-based products with L. monocytogenes during handling and storage. Lemon grass (Cymbopogon citratus) is known to exhibit strong antimicrobial activity against bacteria due to the presence of citral. The objectives of this research were: (i) to develop a water-based extraction procedure for the antimicrobial component(s) in lemon grass and (ii) to evaluate the antimicrobial effect of a concentrated water-based extract and commercial essential oil (EO) of lemon grass against L. innocua (ATCC 33090), a surrogate strain of L. monocytogenes, in vitro and on crabsticks. Briefly, antilisterial activity of concentrated extract and commercial EO of lemon grass was tested using the agar well diffusion technique. Crabsticks were subsequently inoculated with L. innocua to a final density of ca. 4 log cfu/g and then coated with 500 μl of either concentrated extract or 0.5% commercial EO and stored at 4 °C for up to 15 days. Samples were then subjected to microbiological analysis every 5 days to enumerate counts of Listeria. Following the agar well diffusion assay, inhibition zones with mean diameters of 18.3 and 21.0 mm were obtained with the concentrated extract and commercial EO respectively. The population of L. innocua in WBE-coated (4.2 log cfu/g) and 0.5% EO-coated (2.7 log cfu/g) samples were significantly lower (P < 0.05) after 15 days than their untreated control counterpart (5.2 log cfu/g). Lemon grass extract and essential oil have the potential to control growth of L. monocytogenes in seafood surimi products with minimal adverse effect on the organoleptic characteristics of the product and thus can possibly be used as a natural food preservative.

Risks Associated with Fish and Seafood

Fresh fish and seafood are highly perishable, and microbiological spoilage is one of the important factors that limit their shelf life and safety. Fresh seafood can be contaminated at any point from rearing or harvesting to processing to transport or due to cross-contamination by consumer mishandling at home. With the increase in the demand for fish and seafood, aquaculture production is increasing, which could lead to new risks that will need to be addressed in the future to control foodborne pathogens.

Sensorial and Physicochemical Characteristics of Salmon (Salmo salar) Treated by Different Smoking Processes during Storage

This study compares the effects of four different smoking processes on the sensorial and physicochemical characteristics of Atlantic salmon (Salmo salar) fillets. Two commercial liquid smoke flavourings (FA and FB) and two types of wood used for cold-smoking wood smoke generation (BS: Beech wood and OS: Oak wood) are investigated. Comparisons were made over a 45 day storage period. Control salmon samples (Cn) (i.e., samples that underwent no smoke treatment of any kind) were characterised by their low colour intensity, firmness, fat release and fibrousness and high adhesiveness, pH, TBARS and TMA values, and a strong amine odour. Fillets treated with smoke flavouring FA showed characteristics very similar to those of the Cn samples, however their colour and smoke odour are more intense and their adhesiveness, amine odour and TMA values lower. Fish treated with smoke flavouring FB show low brightness and pH, but high firmness, elasticity, colour intensity and a high expressible water content. Salmon samples smoked with BS or OS smoke had a low intensity amine odour as well as low brightness, pH, TBARS and TMA values, high colour and smoke odour intensities, and a high fat release value. The effect of storage time on sensorial and physicochemical parameters was evident: the values recorded for certain characteristics decrease, such as, colour intensity, elasticity or firmness, and others increase, such as, amine odour, expressible water content or TMA, and some showing a linear relationship with time (elasticity or amine odour intensity).

Listeria monocytogenes persistence in food-associated environments: epidemiology, strain characteristics, and implications for public health

Over the last 10 to 15 years, increasing evidence suggests that persistence of Listeria monocytogenes in food processing plants for years or even decades is an important factor in the transmission of this foodborne pathogen and the root cause of a number of human listeriosis outbreaks. L. monocytogenes persistence in other food-associated environments (e.g., farms and retail establishments) may also contribute to food contamination and transmission of the pathogen to humans. Although L. monocytogenes persistence is typically identified through isolation of a specific molecular subtype from samples collected in a given environment over time, formal (statistical) criteria for identification of persistence are undefined. Environmental factors (e.g., facilities and equipment that are difficult to clean) have been identified as key contributors to persistence; however, the mechanisms are less well understood. Although some researchers have reported that persistent strains possess specific characteristics that may facilitate persistence (e.g., biofilm formation and better adaptation to stress conditions), other researchers have not found significant differences between persistent and nonpersistent strains in the phenotypic characteristics that might facilitate persistence. This review includes a discussion of our current knowledge concerning some key issues associated with the persistence of L. monocytogenes, with special focus on (i) persistence in food processing plants and other food-associated environments, (ii) persistence in the general environment, (iii) phenotypic and genetic characteristics of persistent strains, (iv) niches, and (v) public health and economic implications of persistence. Although the available data clearly indicate that L. monocytogenes persistence at various stages of the food chain contributes to contamination of finished products, continued efforts to quantitatively integrate data on L. monocytogenes persistence (e.g., meta-analysis or quantitative microbial risk assessment) will be needed to advance our understanding of persistence of this pathogen and its economic and public health impacts.

Refined liquid smoke: a potential antilisterial additive to cold-smoked sockeye salmon (Oncorhynchus nerka)

Cold-smoked salmon (CSS) is a potentially hazardous ready-to-eat food product due to the high risk of contamination with Listeria monocytogenes and lack of a listericidal step. We investigated the antilisterial property of liquid smokes (LS) against Listeria innocua ATCC 33090 (surrogate to L. monocytogenes) as a potential supplement to vacuum-packaged CSS. A full-strength LS (Code 10-Poly), and three commercially refined fractions (AM-3, AM-10, and 1291) having less color and flavor (lower content of phenols and carbonyl-containing compounds) were tested. In vitro assays showed strong inhibition for all LS except for 1291. The CSS strips were surface coated with AM-3 and AM-10 at 1% LS (vol/wt) with an L-shaped glass rod and then inoculated with L. innocua at 3.5 log CFU/g, vacuum packaged, and stored at 4°C. The LS did not completely eliminate L. innocua but provided a 2-log reduction by day 14, with no growth up to 35 days of refrigerated storage. A simple difference sensory test by 180 untrained panelists showed the application of AM-3 did not significantly influence the overall sensorial quality of CSS. In essence, the application of the refined LS as an antilisterial additive to CSS is recommended.

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

Smoked salmon can be contaminated with Listeria monocytogenes. It is important to identify the factors that are capable of controlling the growth of L. monocytogenes in smoked salmon so that control measures can be developed. The objective of this study was to determine the effect of salt, a smoke compound, storage temperature, and their interactions on L. monocytogenes in simulated smoked salmon. A six-strain mixture of L. monocytogenes (10(2) to 10(3) CFU/g) was inoculated into minced, cooked salmon containing 0 to 10% NaCl and 0 to 0.4% liquid smoke (0 to 34 ppm of phenol), and the samples were stored at temperatures from 0 to 25 degrees C. Lag-phase duration (LPD; hour), growth rate (GR; log CFU per hour), and maximum population density (MPD; log CFU per gram) of L. monocytogenes in salmon, as affected by the concentrations of salt and phenol, storage temperature, and their interactions, were analyzed. Results showed that L. monocytogenes was able to grow in salmon containing the concentrations of salt and phenol commonly found in smoked salmon at the prevailing storage temperatures. The growth of L. monocytogenes was affected significantly (P < 0.05) by salt, phenol, storage temperature, and their interactions. As expected, higher concentrations of salt or lower storage temperatures extended the LPD and reduced the GR. Higher concentrations of phenol extended the LPD of L. monocytogenes, particularly at lower storage temperatures. However, its effect on reducing the GR of L. monocytogenes was observed only at higher salt concentrations (>6%) at refrigerated and mild abuse temperatures (< 10 degrees C). The MPD, which generally reached 7 to 8 log CFU/g in salmon that supported L. monocytogenes growth, was not affected by the salt, phenol, and storage temperature. Two models were developed to describe the LPD and GR of L. monocytogenes in salmon containing 0 to 8% salt, 0 to 34 ppm of phenol, and storage temperatures of 4 to 25 degrees C. The data and models obtained from this study would be useful for estimating the behavior of L. monocytogenes in smoked salmon.

Could modifications of processing parameters enhance the growth and selection of lactic acid bacteria in cold-smoked salmon to improve preservation by natural means?

Several smoking conditions were examined with the objective of enhancing the numbers of lactic acid bacteria (LAB) by natural means in vacuum-packaged cold-smoked salmon during 21 days of storage at 5 degrees C. Three combinations of salting, drying, and smoking were used: (i) dry salting x time of salting (2 or 6 h); (ii) wet salting (6 h) x dry salting (6 h) x with or without sugar; and (iii) wet salting (6 h) x dry salting (6 h) x different times of smoking (2 or 6 h of drying and 2 or 6 h of smoking). Two batches were processed for each set of conditions. Determinations of pH and salt content in the water phase were carried out for products in each treatment. Microbiological analyses (total viable count, total LAB, Lactobacillus spp., and Enterobacteriaceae) also were conducted at the beginning of storage (t0) and after 21 days of refrigerated storage (tl). There were differential increases in total LAB and lactobacilli during the storage period according to the treatment performed. The most effective treatment to enhance LAB growth was 6 h of dry salting with sugar, 6 h of drying, and 2 h of smoking. These salting-drying-smoking conditions also selected the LAB as the dominant flora at the end of the storage period. The LAB promoted by these processing parameters seem to be potentially useful protective cultures because of their anti-Listeria activity. From the results of this research, we conclude that it is possible to enhance the growth of LAB in general and that of inhibitory strains in particular by suitable choices of processing parameters.

Low prevalence of Listeria monocytogenes in foods from Italy

Listeria monocytogenes is an important foodborne pathogen that causes gastrointestinal disorders, and, especially in immunocompromised people, serious extraintestinal diseases, such as septicemia and meningitis, as well as abortion in pregnant women. Many foods, from both plant and animal origin, have been involved in listeriosis outbreaks. This article reports the results of a 12-year survey (1993 through 2004) on the presence of L. monocytogenes in several kinds of food marketed in Italy. Of 5,788 analyzed samples, 121 (2.1%) were contaminated with L. monocytogenes. The highest prevalence was found in smoked salmon (10.6%) and in poultry meat samples (8.5%) and the lowest in red meat (0.3%). L. monocytogenes was not found in 154 samples of fresh seafood products. Fifty-two isolates were also serotyped by the agglutination method. The most common serotypes detected in the 52 strains tested were 1/2a (36.5%), followed by 1/2c (32.8%), 1/2b (13.5%), 4b (11.5%), 3a (3.8%), and 3b (1.9%). The results of the present study showed low levels of L. monocytogenes in the analyzed samples. A total of 61.5% of the 52 L. monocytogenes strains analyzed belonged to serotypes 1/2a, 1/2b, and 4b, namely the serovars that are most commonly involved in extraintestinal human listeriosis outbreaks. In the ready-to-eat samples, these three serotypes were 40.0% (1/2a), 17.1% (1/2b), and 14.3% (4b). This finding highlights the need to implement strict hygienic measures during the production, distribution, and sale of foods to reduce the risk of foodborne listeriosis in humans to an acceptable level.

Influence of salt, smoke, and high pressure on growth of Listeria monocytogenes and spoilage microflora in cold-smoked dolphinfish (Coryphaena hippurus)

The effects of different salting and smoking conditions on the growth of Listeria monocytogenes in cold-smoked dolphinfish (Coryphaena hippurus) fillets were evaluated. High concentrations of phenol (72.47 ppm) and salt (3.25%) in muscle inhibited L. monocytogenes growth in smoked fish stored at 20 degrees C for 4 days. The antibacterial effect of high pressure in cold-smoked dolphinfish during long-term chilled (5 degrees C) storage was evaluated in fillets prepared according to two different sets of salting and smoking conditions. Combining the milder salting and smoking conditions (1.97% salt and 42 ppm phenol) with a high pressure treatment of 300 MPa at 20 degrees C for 15 min sufficed to exert a bacteriostatic effect on the total viable bacteria, total lactic acid bacteria, and L. monocytogenes. However, in fillets prepared using the more severe salting and smoking conditions (2.93% salt and 82 ppm phenol), pressurization kept L. monocytogenes counts under the detection limit throughout 100 days of storage. A similar effect was obtained by dosing the fillets with nisin. No luminescent bacteria, hydrogen sulfide-producing bacteria, or Enterobacteriaceae were found in any of the fillets produced using either of the two sets of processing conditions.

Inhibition of Listeria innocua growth by antimicrobial-producing lactic acid cultures in vacuum-packed cold-smoked salmon

The biopreservative potential of three antimicrobial-producing lactic acid bacteria strains was evaluated on cold-smoked salmon. Lactobacillus casei, Lactobacillus plantarum and Carnobacterium piscicola were added singly or in association to cold-smoked salmon, artificially contaminated with Listeria innocua and stored under vacuum for 30 days at 4 degrees C. All the lactic cultures were able to inhibit Listeria innocua growth, showing a bacteriostatic or bactericidal effect, without affecting negatively the sensory quality of the product. Lactobacillus casei was bacteriostatic when inoculated at 6 log cfu/g, but bactericidal at 8 log cfu/g, reducing Listeria innocua of 3.3 log cfu/g in comparison with the test at the end of storage. Lactobacillus plantarum and C. piscicola strains, inoculated singly at 6 log cfu/g reduced Listeria innocua counts of 2.8 and 2.7 log cfu/g, respectively, compared with the test. The association Lactobacillus casei-Lactobacillus plantarum was the most effective among the treatments with 6 log cfu/g inoculum, as Listeria innocua counts decreased of 3.2 log cfu/g compared with the test. The treatment with Lactobacillus casei-C. piscicola association was less effective than C. piscicola alone.

Growth rate and growth probability of Listeria monocytogenes in dairy, meat and seafood products in suboptimal conditions

AIMS

To evaluate the performances of models predicting the growth rate or the growth probability of Listeria monocytogenes in food.

METHODS AND RESULTS

Cardinal and square root type models including or not interactions between environmental factors and probability models were evaluated for their ability to describe the behaviour of L. monocytogenes in liquid dairy products, cheese, meat and seafood products. Models excluding interactions seemed sufficient to predict the growth rate of L. monocytogenes. However, the accurate prediction of growth/no-growth limits needed to take interactions into account. A complete and a simplified form (preservatives deducted) of a new cardinal model including interactions and parameter values were suggested to predict confidence limits for the growth rate of L. monocytogenes in food. This model could also be used for the growth probability prediction.

CONCLUSIONS

The new cardinal model including interactions was efficient to predict confidence limits for the growth rate of L. monocytogenes and its growth probability in liquid dairy products, meat and seafood products. In cheese, the model was efficient to predict the absence of growth of the pathogen.

SIGNIFICANCE AND IMPACT OF THE STUDY

The suggested model can be used for risk assessment and risk management concerning L. monocytogenes in dairy, meat and seafood products.

Use of Bayesian modelling in risk assessment: application to growth of Listeria monocytogenes and food flora in cold-smoked salmon

An attempt to use a Bayesian approach to model variability and uncertainty separately in microbial growth in a risk assessment is presented. It was conducted within the framework of a French project aiming at assessing the exposure to Listeria monocytogenes in cold-smoked salmon. The chosen model describes the effect of time and temperature on bacterial growth. A Bayesian approach close to the one proposed by Pouillot et al. [Int. J. Food Microbiol. 81 (2003) 87] is used to estimate the variability and uncertainty of growth parameters from both literature data and data experimentally acquired during the project. Variability between strains and between products is taken into account. The growth of the food flora of cold-smoked salmon is also modelled by the same method. The results obtained for both models are used to predict the simultaneous growth of L. monocytogenes and food flora in cold-smoked salmon with a competitive model, expressing variability and uncertainty through a second-order Monte Carlo simulation.

Control of growth and survival of Listeria monocytogenes on smoked salmon by combined potassium lactate and sodium diacetate and freezing stress during refrigeration and frozen storage

In this study, we evaluated the antimicrobial effects of different levels of a potassium lactate (PL) plus sodium diacetate (SDA) mixture against the growth and survival of Listeria monocytogenes Scott A inoculated onto smoked salmon stored at 4, 10, and -20 degrees C. The effect of freezing stress on the growth kinetics of L. monocytogenes Scott A on smoked salmon at 4 and 10 degrees C was also investigated. The use of PL+SDA at all tested levels (1.5, 3.3, and 5% of a 60% commercial solution of PURASAL P Opti. Form 4) completely inhibited the growth of L. monocytogenes Scott A on smoked salmon stored at 4 degrees C during 32 days of storage. It also delayed the growth of L. monocytogenes Scott A on smoked salmon stored at 10 degrees C for up to 11 days, but a listeriostatic effect was observed only with 5% PURASAL P Opti. Form 4 at 10 degrees C after 11 days. Addition of PL+SDA at all tested levels decreased the surviving populations of L monocytogenes Scott A on smoked salmon during 10 months of frozen storage at -20 degrees C. Freezing stress significantly (P < 0.001) extended the lag time and delayed the growth of L. monocytogenes Scott A at both 4 and 10 degrees C. However, the effect of freezing stress was more significant at 4 degrees C than at 10 degrees C, indicating the importance of temperature control of smoked salmon during the retail storage period.

Prevalence and growth of Listeria on naturally contaminated smoked salmon over 28 days of storage at 4 degrees C

Only limited data are available on the growth characteristics of Listeria in naturally contaminated ready-to-eat foods. To evaluate Listeria contamination patterns and growth in smoked salmon, 72 smoked salmon product samples from two processing plants were tested for Listeria spp. and L. monocytogenes. Samples were divided into four approximately equal portions: one portion was tested on receipt, and the other three were vacuum sealed and stored at 4 degrees C for 7, 14, and 28 days. Listeria testing was performed using both an enrichment procedure and direct plating to enumerate Listeria in samples that contained >2 to 10 CFU/g. Five samples were positive for Listeria spp., including one sample that was positive for L. monocytogenes. Most samples yielded only sporadic positive results among the portions tested on days 0, 7, 14, and 28. Only one sample contained Listeria spp. in numbers above the detection limit for enumeration. For this sample, the portions tested on days 7 and 28 contained 46 and 52 CFU/g, respectively, whereas the portion tested on day 14 was negative. Overall, our data indicate that there is considerable heterogeneity in Listeria spp. distribution within a single positive smoked fish sample. Even with refrigerated storage for 28 days, none of the naturally contaminated samples reached Listeria spp. numbers >100 CFU/g, which indicates that Listeria growth was limited within a 4-week storage period. However, because of the apparent heterogeneity of Listeria distribution within samples, the interpretation of growth data collected on naturally contaminated samples is difficult.

Occurrence of Listeria monocytogenes in fresh and processed foods in Navarra (Spain)

The presence of Listeria spp. was investigated in a total of 3685 food samples obtained from different industries and markets of Northern Spain in the last 4 years. The samples analyzed include fresh raw products (meat, milk and poultry) and treated products (cooked and cured meats, frozen vegetables and smoked salmon). Occurrence of Listeria spp. varied from 8.1% in soft cheese to 76.3% in raw poultry samples. The highest incidence of L. monocytogenes also occurred in raw poultry (36.1% positive samples). Despite this high incidence of contamination, these kinds of products carry a low risk of listeriosis transmission because of the heat treatment prior to consumption. On the other hand, the ready-to-eat products (RTE) tested in this study showed incidences that could pose serious health problems, taking into account that the storage conditions may allow for rapid growth of the pathogen. It was also found that up to 75.5% of the L. monocytogenes strains isolated in this study belonged to serogroup 1, mainly serotype 1/2a, while the clinical cases observed in Navarra in the same period of time belonged mainly to serotype 4b/4bx.

Bacteriocin-producing Enterococcus casseliflavus IM 416K1, a natural antagonist for control of Listeria monocytogenes in Italian sausages ("cacciatore")

The bacteriocinogenic Enterococcus casseliflavus IM 416K1 (Bac+) isolated from Italian sausages or its bacteriocin Enterocin 416K1, with strong anti-listerial activity, were used in trials to evaluate the effect on Listeria monocytogenes NCTC 10888 in artificially inoculated Italian sausages ("cacciatore"). In trials with Enterocin 416K1 added, L. monocytogenes showed a significant reduction as compared to the control inoculated with L. monocytogenes alone. The elimination of L. monocytogenes was only obtained in sausages added with E. casseliflavus IM 416K1 Bac+.

Factors affecting production of an antilisterial bacteriocin by Carnobacterium piscicola strain A9b in laboratory media and model fish systems

AIMS

To investigate factors influencing bacteriocin production and bacteriocin stability of the bioprotective culture Carnobacterium piscicola strain A9b.

METHODS AND RESULTS

Maximum activity was obtained in MRS7 broth (MRS adjusted to pH 7.2), with or without glucose. No bacteriocin was produced in APT broth when a low inoculum level (0.001%) was used. In contrast, inoculum level did not influence bacteriocin production in BHI and MRS7 without glucose. Bacteriocin production in APT was induced by the presence of an extracellular compound present in the sterile, filtered, cell-free supernatant fluid of a stationary-phase culture. Increasing concentrations of NaCl (2-7%) reduced bacteriocin production and maximum cell density of C. piscicola A9b when grown in cooked fish juice at 4 degrees C.

CONCLUSION

Media composition, inoculum level and sodium chloride concentration affected production.

SIGNIFICANCE AND IMPACT OF THE STUDY

The influence of NaCl on bacteriocin production may negate the inhibitory effect of C. piscicola A9b against Listeria monocytogenes in salty foods.

Modelling the growth rate of Listeria monocytogenes with a multiplicative type model including interactions between environmental factors

A multiplicative secondary model previously published to describe independently the effects of environmental factors on the growth rate of Listeria monocytogenes (Augustin and Carlier, 2000) was improved by taking into account interactions between these environmental factors. The proposed model allowed to decrease the rate of fail-safe growth predicted from 13.5% to 12.1% and the rate of fail-dangerous no growth predicted from 16.1% to 7.1%.

Nisin and ALTA 2341 inhibit the growth of Listeria monocytogenes on smoked salmon packaged under vacuum or 100% CO2

The effects of nisin and ALTA 2341 on the growth of Listeria monocytogenes were assessed on smoked salmon packaged under vacuum or 100% CO2. Smoked salmon slices (pH 6.3) were inoculated with a cocktail of seven L. monocytogenes isolates at a level of approximately 2.5 log10 colony forming units (cfu) g-1. After inoculation, the surface of the smoked salmon slices was treated with either nisin (400 or 1250 IU g-1) or ALTA 2341 (0.1 or 1%). The smoked salmon was packaged and stored at 4 degrees C (28 d) or 10 degrees C (9 d). On untreated vacuum-packaged smoked salmon, L. monocytogenes grew by 3.8 log10 cfu g-1 at 4 degrees C and 5.1 log10 cfu g-1 at 10 degrees C. Growth was reduced on nisin- and ALTA 2341-treated vacuum-packaged smoked salmon. On the nisin-treated samples, L. monocytogenes increased by 2.5 (400 IU g-1) and 1.5 (1250 IU g-1) log10 cfu g-1 at 4 degrees C, and by 4.3 (400 IU g-1) and 2.7 (1250 IU g-1) log10 cfu g-1 at 10 degrees C. With the ALTA 2341-treated samples, L. monocytogenes increased by 2.8 (0.1%) or 1.6 (1.0%) log10 cfu g-1 at 4 degrees C, and 3.3 (0.1%) or 3.6 (1.0%) log10 cfu g-1 at 10 degrees C. The growth of L. monocytogenes was retarded by packaging the smoked salmon in 100% CO2. On untreated smoked salmon, only a 0.8 log10 cycle increase was observed at 10 degrees C. Under all the other conditions tested with 100% CO2, L. monocytogenes was detected but growth was prevented.

Occurrence and typing of Listeria monocytogenes strains in retail vacuum-packed fish products and in a production plant

One hundred and ten samples of ready-to-eat, vacuum-packed, smoked and cold-salted fish products were collected from retail outlets in southern Finland during 1996 for examination of the occurrence and level of Listeria monocytogenes. The samples originated from 12 producers. Positive samples with levels exceeding 100 CFU/g were encountered mainly in one of the producers (no. 8). Therefore, 200 samples from the plant and the products of this producer were studied during August-September 1996 and May-September 1997, as well as 55 samples from the six fish farms providing raw material fish to this plant, during September 1997-January 1998. The isolates were characterised by serotyping and pulsed-field gel electrophoresis (PFGE). L. monocytogenes was isolated in 20% (22/110) of the samples from the retail market, originating from 6 producers. Ten of these positive samples contained L. monocytogenes at > 100 CFU/g (maximum 1.37 X 10(4) CFU/g). Seventeen percent (5/30) of cold-smoked and 50% (16/32) of cold-salted rainbow trout samples were contaminated. Only one hot-smoked fish product (2%) was found to be positive by enrichment. Nineteen (86%) of the strains isolated from the retail samples belonged to serovar 1/2a and three (14%) to serovar 4b. In further studies the production line of plant no. 8 was found to be contaminated. All of isolates from up until autumn, 1997 both the products and the production plant were serovar 1/2a; thereafter one strain of 4b and one of 1/2 (H-antigen untypeable) were isolated from the plant. The samples from raw material fish were all negative for L. monocytogenes. The samples from retail market fell into seven PFGE types. Five and nine PFGE types, respectively, were found from the products and the plant of producer no. 8. PFGE type A was detected from the retail products of four producers and was also dominant among the isolates from production plant no. 8. PFGE type A was the only one found repeatedly from skinning, salting and slicing units as well as from products throughout the whole period. PFGE proved to be a powerful tool for studying contamination points and routes in the production plant. The measures based on hazard analysis critical control points (HACCP) program resulted in L. monocytogenes negative samples at production plant no. 8 from the beginning of January 1998.

Predicted and observed growth of Listeria monocytogenes in seafood challenge tests and in naturally contaminated cold-smoked salmon

The performance of the Pathogen Modelling Program, the Food MicroModel, the Murphy-model and the Ross-model for growth of L. monocytogenes was evaluated by comparison with data from 100 seafood challenge tests and data from 13 storage trials with naturally contaminated sliced vacuum-packed cold-smoked salmon. Challenge tests with both cured and noncured products were studied, and graphs as well as the bias- and the accuracy factors were used for comparison of the observed and predicted growth. The Pathogen Modelling Program could not be successfully validated in seafood challenge tests. Growth rates were markedly overestimated and the mu(max)-bias factor was as high as 3.9 in challenge tests with cured products. On the basis of the effect of temperature, NaCl/a(w) and pH, the mu(max)-bias factor of the other three models studied, varied between 1.0 and 2.3 in the challenge tests with cured and noncured seafoods. None of the models accurately predicted the growth in both cured and noncured seafoods. However, the results indicated that a new expanded model, including the additional effect of lactate and phenol, may provide accurate predictions of the growth of L. monocytogenes in challenge tests with various types of seafoods. Storage trials clearly showed the growth of L. monocytogenes in naturally contaminated cold-smoked salmon to be markedly slower than growth in inoculated challenge tests. Consequently, all four models substantially overestimated growth in the naturally contaminated products. Temperature, pH, NaCl/a(w) and lactate were measured in the storage trials and on the basis of these parameters, the Food MicroModel mu(max)-bias factor was 5.2. Clearly, the model could not be successfully validated with naturally contaminated cold-smoked salmon. To improve the applicability of predictive models to fish products, it is suggested to include studies with naturally contaminated products in the development and validation of models with seafood pathogens.

Inhibition of Listeria monocytogenes on cold-smoked salmon by nisin and carbon dioxide atmosphere

The bacteriostatic and bacteriocidal effect of nisin in combination with carbon dioxide, NaCl and low temperature on the survival of Listeria monocytogenes was investigated in in vitro model studies and in trials with cold-smoked salmon. Addition of nisin caused various degrees of inhibition and sometimes death of L. monocytogenes in model experiments performed at 10 degrees C. The antilisterial effect of nisin was improved in the presence of 100% CO2 and increasing NaCl concentrations (0.5 to 5.0% w/v). Minimal bactericidal concentrations (MBC) of nisin varied from 30 to more than 500 IU/ml. The most pronounced effect of nisin was found when 10(2) cfu/ml was grown in media with 5.0% NaCl and incubated in CO2 atmosphere (MBC = 30 IU/ml). The bactericidal effect of nisin was reduced in air and vacuum, and did not increase systematically with increasing NaCl concentrations. In general, nisin concentration < or = 50 IU/ml resulted in the survival and growth of L. monocytogenes in all combinations with other preservatives (NaCl, CO2). Addition of nisin (500 or 1000 IU/g) to cold-smoked salmon inoculated with L. monocytogenes and stored at 5 degrees C delayed, but did not prevent growth of L. monocytogenes in vacuum-packs. Numbers of L. monocytogenes increased to 10(8) cfu/g in vacuum packed cold-smoked salmon in 8 days, whereas CO2 packing of cold-smoked salmon resulted in an 8-day lag phase of L. monocytogenes, with numbers eventually reaching 10(6) cfu/g in 27 days. Addition of nisin to CO2 packed cold-smoked salmon resulted in a 1 to 2 log reduction of L. monocytogenes followed by a lag phase of 8 and 20 days in salmon with 500 and 1000 IU nisin/g, respectively. The levels of L. monocytogenes remained below 10(3) cfu/g during 27 days of storage at both concentrations of nisin.

Risk factors for contamination of smoked salmon with Listeria monocytogenes during processing

Forty smoked salmon processing plants were examined for the occurrence of Listeria monocytogenes and other Listeria spp. in the smoked salmon and the drains. L. monocytogenes was detected in smoked salmon from 13 (33%) and in the drains samples from 25 (63%) of the plants. Other Listeria spp. were found in smoked salmon samples from 16 (40%) and in the drains of 30 (75%) of the plants. Multivariate analyses of data on hygiene, management, production facilities of the plants and bacteriological results showed that job rotation was the strongest expressed risk factor for isolation of L. monocytogenes from the smoked salmon (hazard ratio, HR = 11.0, p = 0.002). Well-maintained facilities (HR = 0.31, p = 0.064) and use of vats for salting of the fillets (HR = 0.33, p = 0.109), showed a preventive effect. L. monocytogenes in the drains was found to be a sensitive predictor for the presence of L. monocytogenes in the smoked salmon. In general, detection of other Listeria spp. in the smoked salmon or the drains could not be demonstrated to have any association with detection of L. monocytogenes.

Distribution and behavior of Listeria monocytogenes in three lots of naturally-contaminated vacuum-packed smoked salmon stored at 2 and 10 degrees C

The incidence, the number and the behavior of L. monocytogenes in three lots of naturally-contaminated vacuum-packed sliced smoked salmon, processed in different plants, were investigated during storage at 2 and 10 degrees C. L. monocytogenes was isolated from 20 of the 100 packages stored at 2 degrees C (16 from lot 1, 1 from lot 2 and 3 from lot 3) and from 12 of the 65 packages stored at 10 degrees C (all from lot 1). The levels detected were 15, 20, 290, 1100 and > 1100 cfu/g in 5 packages, all belonging to lot 1, and < 10 cfu/g in the remaining samples. The high incidence of L. monocytogenes in lot 1 was assumed to be due mainly to the use of causal workers for the slicing and packing operations.

Predictive model of the effect of CO2, pH, temperature and NaCl on the growth of Listeria monocytogenes

The growth responses of L. monocytogenes as affected by CO2 concentration (0-100% v/v, balance nitrogen), NaCl concentration (0.5-8.0% w/v), pH (4.5-7.0) and temperature (4-20 degrees C) were studied in laboratory medium. Growth curves were fitted using the model of Baranyi and Roberts, and specific growth rates derived from the curve fit were modelled. Predictions for specific growth rate, doubling time and time to a 1000-fold increase could be made for any combination of conditions within the matrix. Predictions of growth from the model were compared with published data and this showed the model to be suitable for predicting growth of L. monocytogenes in a range of foods packaged under a modified atmosphere.

Typing Listeria monocytogenes isolates from fish products and human listeriosis cases

Seventy-two Listeria monocytogenes isolates originating from 10 different fish products of 12 producers and 47 isolates from human listeriosis cases were typed by serotyping and multilocus enzyme electrophoresis. Seventy-five of these isolates were further subtyped by restriction analysis of genomic DNA with the enzyme XhoI and by pulsed-field gel electrophoresis using the enzymes ApaI and SmaI. The results show that several L. monocytogenes clones identified by multilocus enzyme electrophoresis are frequently found in fish products of different origins. One of these clones is the same as another previously shown to be frequently associated with meat and meat products. The epidemic-associated electrophoretic type 1 was only rarely found in fish products. No association was found between any type of fish product and a particular lineage of L. monocytogenes. Both long-term persistence of a strain and simultaneous presence of several clearly distinct strains in the products of single producers were observed. The comparison of L. monocytogenes isolates from human clinical listeriosis cases in Switzerland and those from imported fish products by use of multilocus enzyme electrophoresis showed that they do not form two clearly distinct lineages but nevertheless belong to two separate populations. None of the 48 subtypes distinguished by the combination of all four typing methods could be found in both populations of human origin and those of fish origin.

Contamination pattern of Listeria monocytogenes and other Listeria spp. in a salmon slaughterhouse and smoked salmon processing plant

A smoked salmon processing plant including a smokehouse and a slaughterhouse was examined for the occurrence of Listeria monocytogenes and other Listeria spp. From a total of 475 samples the overall frequency of L. monocytogenes was 16%, while other Listeria spp. were found in 22% of the samples. L. monocytogenes was most often detected in samples from the smokehouse, where 29% of the environmental and 26% of the fish samples during processing contained the bacteria. 17% of the fish raw material to the smokehouse were contaminated, while 11% of the samples from vacuum-packed smoked salmon were positive for L. monocytogenes. The slaughterhouse was sporadically contaminated, but L. monocytogenes was not found in 50 samples of slaughtered fish. L. monocytogenes was found in the seawater outside the slaughterhouse. Multilocus enzyme electrophoresis divided the isolated L. monocytogenes strains into 11 electrophoretic types (ETs). One ET, ET-6, which is the most common ET in Norway, seemed to have colonized the smokehouse. Isolates from the seawater, from the slaughterhouse and from fish coming into the smokehouse, before filleting, were other ETs.

Presence, detection and growth of Listeria monocytogenes in seafoods: a review

Listeria monocytogenes and other Listeria spp. have been isolated from seafoods on a regular basis since 1987. A relatively high incidence of the organism (6-36%) in ready-to-eat cold smoked salmon and cooked fish products has raised concern about the survival and growth potential of this organism in seafoods, as these products are not processed further before consumption. L. monocytogenes grows well at refrigeration temperature on most seafoods, but the sources of contamination in ready-to-eat fish products are still unknown. This paper reviews the knowledge available in order to make recommendations on control options and avenues for future research.

Heat resistance of Listeria monocytogenes in vacuum packaged pasteurized fish fillets

The heat resistance of two strains of Listeria monocytogenes in sous-vide cooked fillets of cod and salmon was investigated. Fish sticks of 5 g were inoculated, vacuum-packed and heated at different combinations of time and temperature (58-80 degrees C). Time-temperature combinations allowing survival and time-temperature combinations at which the bacteria were destroyed, were used to determine D- and z-values. D-values were in the range of what has been published for other food products. D60-values were between 1.95 and 4.48 min depending on the strain and the fish. Both strains were one-four-times more heat resistant in salmon than in cod, showing the importance of the heating menstruum. This difference may be due to the higher fat content in salmon as compared to cod. Z-values were calculated to be 5.65 and 6.4 degrees C, respectively, for the two strains. The suitability of methods for heat resistance experiments and the survival of L. monocytogenes in sous-vide cooked fish fillets are discussed.

Presence and coincidence of Listeria spp., motile aeromonads and Yersinia enterocolitica in a cold-smoked salmon processing and packing plant

Environmental samples and samples of partially processed fish from a cold-smoked salmon processing and packing plant, and product samples purchased from retail outlets, were examined for the presence of Listeria monocytogenes and other Listeria spp., motile aeromonads and Yersinia enterocolitica. Listeria spp. were not isolated from raw fish but were a contaminant of processed and partially processed fish. Listeria spp. were also detected in 18.8% of surfaces in contact with fish. Unlike Listeria spp., motile aeromonads were isolated from the raw fish but not from the finished product. They were more frequently isolated from environmental sources than Listeria spp. Yersinia enterocolitica was not isolated from any of the samples tested. Little evidence was found to show a coincidence of motile aeromonads and Listeria spp., since only one subset of samples showed such a link. It is concluded that contamination by Listeria spp. was from environmental sources at the processing plant at, or beyond, the slicing stage. Reducing the number of wet areas and special cleaning and sanitation considerations for a contaminated site (the freezer seal) are suggested as ways of reducing contamination of the product.