Potential antimicrobials to control Listeria monocytogenes in vacuum-packaged cold-smoked salmon pâté and fillets

Bioprotective cultures and bacteriocins as food preservatives

Food preservation technologies face the challenge of extending product shelf life applying different factors to prevent the microbiological spoilage of food and inhibit/inactivate food borne pathogens maintaining or even enhancing its quality. One such preservation strategy is the application of bacteriocins or bacteriocin-producer cultures as a kind of food biopreservation. Bacteriocins are ribosomally synthesized small polypeptide molecules that exert antagonistic activity against closely related and unrelated bacteria without harming the producing strain by specific immunity proteins. This chapter aims to contribute to current knowledge about innovative natural preservative agents and their application in the food industry. Specifically, its purpose is to analyze the classification of bacteriocins from lactic acid bacteria (LAB), desirable characteristics of bacteriocins that position them in a privileged place in food biopreservation technology, their success story as well as the bacteriocinogenic LAB in various food systems. Finally, challenges and barrier strategies used to enhance the efficiency of the bacteriocins antimicrobial effect are presented in this chapter.

Combined antimicrobial effect of bacteriocins with other hurdles of physicochemic and microbiome to prolong shelf life of food: A review

Bacteriocins are ribosomally synthesized peptides to inhibit food spoilage bacteria, which are widely used as a kind of food biopreservation. The role of bacteriocins in therapeutics and food industries has received increasing attention across a number of disciplines in recent years. Despite their advantages as alternative therapeutics over existing strategies, the application of bacteriocins suffers from shortcomings such as the high isolation and purification cost, narrow spectrum of activity, low stability and solubility and easy enzymatic degradation. Previous studies have studied the synergistic or additive effects of bacteriocins when used in combination with other hurdles including physics, chemicals, and microbes. These combined treatments reduce the adverse effects of chemical additives, extending the shelf life of food products while guaranteeing food quality. This review highlights the advantages and disadvantages of bacteriocins in food preservation. It then reviews the combined effect and mechanism of different hurdles and bacteriocins in enhancing food preservation in detail. The combination of bacterioncins and other hurdles provide potential approaches for maintaining food quality and food safety.

Microbial Safety and Sensory Analyses of Cold-Smoked Salmon Produced with Sodium-Reduced Mineral Salts and Organic Acid Salts

Cold-smoked (CS) salmon contains high levels of sodium salts, and excess dietary sodium intake is associated with an array of health complications. CS salmon may also represent a food safety risk due to possible presence and growth of the foodborne pathogen Listeria monocytogenes which may cause fatal human infections. Here we determine how reformulated CS salmon using commercial sodium-reduced salt replacers containing KCl (e.g., Nutek, Smart Salt, SOLO-LITE) and acetate-based preservative salts (Provian K, proviant NDV) affect sensory properties, quality, and microbial safety. Initial sensory screening of sodium-reduced CS salmon was followed by L. monocytogenes growth analyses in selected variants of reformulated CS salmon, and finally by analyses of CS salmon variants produced in an industrial smokehouse. Projective mapping indicated overall minor sensory changes in sodium-replaced samples compared with a conventional product with NaCl. Growth of L. monocytogenes was temperature-dependent (4 °C vs. 8 °C storage) with similar growth in sodium-reduced and conventional CS salmon. The addition of 0.9% of the preservative salts Provian K or Provian NDV gave up to 4 log lower L. monocytogenes counts in both sodium-reduced and conventional cold-smoked salmon after 29 days of chilled storage. No changes in pH (range 6.20−6.33), aw levels (range 0.960−0.973), or weight yield (96.8 ± 0.2%) were evident in CS salmon with salt replacers or Provian preservative salts. Analyses of CS salmon produced with selected mineral salt and preservative salt combinations in an industrial salmon smokery indicated marginal differences in sensory properties. Samples with the preservative salt Provian NDV provided L. monocytogenes growth inhibition and low-level total viable counts (<2.8 log/g) dominated by Photobacterium and Carnobacterium during storage. Production of sodium-reduced CS salmon with inhibiting salts provides a simple method to achieve a healthier food product with increased food safety.

Improved microbial and sensory quality of chicken meat by treatment with lactic acid, organic acid salts and modified atmosphere packaging

Microbial contamination and growth play important roles in spoilage and quality loss of raw poultry products. We evaluated the suitability of three commercially available organic acid based antimicrobial compounds, Purac FCC80 (l-lactic acid), Verdad N6 (buffered vinegar fermentate) and Provian K (blend of potassium acetate and diacetate) to prevent growth of the innate microbiota, reduce spoilage and enhance the sensory quality of raw chicken under vacuum, high CO₂ (60/40% CO₂/N₂), and high O₂ (75/25% O₂/CO₂) modified atmosphere (MA) storage conditions. Solutions were applied warm (50 °C) or cold (4 °C) to reflect treatments prior to (Prechill) or after (Postchill) cooling of chicken carcasses, respectively. Single postchill treatments of raw chicken wings with 5% Verdad N6 or Provian K solutions and MA storage enabled complete growth inhibition during the first seven days of storage before growth resumed. Enhanced bacterial control was obtained by combining Prechill lactic acid and Postchill Verdad N6 or Provian K treatments which indicated initial reductions up to 1.1 log and where total bacterial increase after 20 days storage was limited to 1.8-2.1 log. Antibacterial effects were dependent on the concentration of the inhibiting salts used, pH and the storage conditions. Bacterial community analyses showed increased relative levels of Gram-positive bacteria and with reductions of potential spoilage organisms in samples treated with the organic acid salts Verdad N6 and Provian K. Sensory analyses of raw, treated wings showed prominent lower scores in several spoilage associated odour attributes when compared with untreated chicken wings after 13 days storage. For heat-treated chicken, only minor differences for 22 tested attributes were detected between seven antimicrobial treatments and untreated control chicken. Immersion in commercially available organic acid/salt solutions combined with MA storage can reduce bacterial levels, improve microbial and sensory quality, and potentially improve shelf life and reduce food waste of chicken products.

Applications of Lactic Acid Bacteria and Their Bacteriocins against Food Spoilage Microorganisms and Foodborne Pathogens

Lactic acid bacteria (LAB) are Gram-positive and catalase-negative microorganisms used to produce fermented foods. They appear morphologically as cocci or rods and they do not form spores. LAB used in food fermentation are from the Lactobacillus and Bifidobacterium genera and are useful in controlling spoilage and pathogenic microbes, due to the bacteriocins and acids that they produce. Consequently, LAB and their bacteriocins have emerged as viable alternatives to chemical food preservatives, curtesy of their qualified presumption of safety (QPS) status. There is growing interest regarding updated literature on the applications of LAB and their products in food safety, inhibition of the proliferation of food spoilage microbes and foodborne pathogens, and the mitigation of viral infections associated with food, as well as in the development of creative food packaging materials. Therefore, this review explores empirical studies, documenting applications and the extent to which LAB isolates and their bacteriocins have been used in the food industry against food spoilage microorganisms and foodborne pathogens including viruses; as well as to highlight the prospects of their numerous novel applications as components of hurdle technology to provide safe and quality food products.

Application of Whey Protein-Based Edible Films and Coatings in Food Industries: An Updated Overview

The recent surge in environmental awareness and consumer demand for stable, healthy, and safe foods has led the packaging and food sectors to focus on developing edible packaging materials to reduce waste. Edible films and coatings as a modern sustainable packaging solution offer significant potential to serve as a functional barrier between the food and environment ensuring food safety and quality. Whey protein is one of the most promising edible biopolymers in the food packaging industry that has recently gained much attention for its abundant nature, safety, and biodegradability and as an ecofriendly alternative of synthetic polymers. Whey protein isolate and whey protein concentrate are the two major forms of whey protein involved in the formation of edible films and coatings. An edible whey film is a dry, highly interacting polymer network with a three-dimensional gel-type structure. Films/coatings made from whey proteins are colorless, odorless, flexible, and transparent with outstanding mechanical and barrier properties compared with polysaccharide and other-protein polymers. They have high water vapor permeability, low tensile strength, and excellent oxygen permeability compared with other protein films. Whey protein-based films/coatings have been successfully demonstrated in certain foods as vehicles of active ingredients (antimicrobials, antioxidants, probiotics, etc.), without considerably altering the desired properties of packaging films that adds value for subsequent industrial applications. This review provides an overview of the recent advances on the formation and processing technologies of whey protein-based edible films/coatings, the incorporation of additives/active ingredients for improvement, their technological properties, and potential applications in food packaging.

Bacteriocins of Lactic Acid Bacteria as Potent Antimicrobial Peptides against Food Pathogens

An ever-growing demand for food products with minimal chemical additives has generated a necessity for exploring new alternatives for food preservation. In this context, more recently, bacteriocins, the peptides having antimicrobial property, synthesized ribosomally by numerous bacteria have been attracting a lot of attention. They are known to possess the potential to restrict the growth of microorganisms causing food spoilage without causing any harm to the bacteria themselves owing to the presence of self-defensive proteins. In particular, the bacteriocins of lactic acid bacteria have been considered harmless and safe for consumption and are indicated to evade the development of unwanted bacteria. Use of bacteriocins as biopreservatives has been studied in various food industries, and they have been established to elevate the shelf life of minimally processed food items by exerting killing mechanism. They restrict the growth of undesirable bacteria by breaking the target cell membrane and finally resulting into pore formation. The current article provides an insight on bacteriocins of lactic acid bacteria, their biosynthesis, mechanism of action, and promising applications of these antimicrobial peptides in the food sector.

Antimicrobial effect of nisin in processed cheese - Quantification of residual nisin by LC-MS/MS and development of new growth and growth boundary model for Listeria monocytogenes

This study tested the hypothesis that growth of Listeria monocytogenes in processed cheese with added nisin can be predicted from residual nisin A concentrations in the final product after processing. A LC-MS/MS method and a bioassay were studied to quantify residual nisin A concentrations and a growth and growth boundary model was developed to predict the antilisterial effect in processed cheese. 278 growth rates were determined in broth for 11 L. monocytogenes isolates and used to determine 13 minimum inhibitory concentration (MIC) values for nisin between pH 5.5 and 6.5. To supplement these data, 67 MIC-values at different pH-values were collected from the scientific literature. A MIC-term was developed to describe the effect of pH on nisin MIC-values. An available growth and growth boundary model (doi: https://doi.org/10.1016/j.fm.2019.103255) was expanded with the new MIC-term for nisin to predict growth in processed cheese. To generate data for model evaluation and further model development, challenge tests with a total of 45 growth curves, were performed using processed cheese. Cheeses were formulated with 11.2 or 12.0 ppm of nisin A and heat treated to obtain residual nisin A concentrations ranging from 0.56 to 5.28 ppm. Below 15 °C, nisin resulted in extended lag times. A global regression approach was used to fit all growth curves determined in challenge tests. This was obtained by combining the secondary growth and growth boundary model including the new term for the inhibiting effect of nisin on μ_max with the primary logistic growth model with delay. This model appropriately described the growth inhibiting effect of residual nisin A and showed that relative lag times depended on storage temperatures. With residual nisin A concentrations, other product characteristics and storage temperature as input the new model correctly predicted all observed growth and no-growth responses for L. monocytogenes. This model can support development of nisin A containing recipes for processed cheese that prevent growth of L. monocytogenes. Residual nisin A concentrations in processed cheese were accurately quantified by the developed LC-MS/MS method with recoveries of 83 to 110% and limits of detection and quantification being 0.04 and 0.13 ppm, respectively. The tested bioassay was less precise and nisin A recoveries varied for 53% to 94%.

Reduction and inhibition of Listeria monocytogenes in cold-smoked salmon by Verdad N6, a buffered vinegar fermentate, and UV-C treatments

Contamination, survival and growth of Listeria monocytogenes in cold-smoked salmon represent serious health hazards to consumers and major challenges for salmon processors. Verdad N6, a commercially available buffered vinegar, was evaluated as an ingredient in cold-smoked salmon with regard to anti-listerial effects under processing and storage, sensory quality and consumer preference, effects on background microbiota and yield during production. Cold-smoked salmon with Verdad N6 added in the dry-salting process was produced. Salmon fillets were surface contaminated with a mix of L. monocytogenes. Levels of L. monocytogenes were determined during vacuum pack refrigerated storage for 29 days. The use of Verdad N6 resulted in increased lag times and reduced growth rates of L. monocytogenes. The inhibitory effects were dependent on Verdad N6 levels (0-2%), storage time and temperature (4 or 8 °C), type of contamination (between slices or on non-sliced salmon) and degree of smoking. The presence of dextrose (1%) in the recipe had no significant effects on L. monocytogenes levels after storage. On sliced salmon, complete growth inhibition at 4 °C storage could be obtained using 1% Verdad N6 compared to a 3 log increase in L. monocytogenes counts in control salmon. At abuse temperatures (8 °C), corresponding L. monocytogenes levels increased <2 log and 5-6 log during 29 days storage. On non-sliced salmon, 1% Verdad N6 provided complete growth reductions at 4 and 8 °C storage while L. monocytogenes in control salmon increased 2.3 and 4.6 log, respectively, in the same period. The use of Verdad N6 in combination with bactericidal UV-C treatments (fluence 50 mJ/cm²) provided an initial 0.8 log reduction and complete L. monocytogenes growth inhibition on subsequent storage at 4 and 8 °C. Salmon with Verdad N6 showed reduced levels of total counts during storage and a shift in the dominating bacteria with reduced and increased relative levels of Photobacterium and lactic acid bacteria, respectively. A consumer test showed no consistent differences in liking of salmon with and without Verdad N6. In summary, Verdad N6 is an option for the production of high quality cold-smoked salmon with enhanced food safety through its robust listeriostatic effects. The application of Verdad N6 in combination with listericidal UV-C light treatment can further reduce the listeria-risks of this ready-to-eat food product category.

Antibacterial efficacy of nisin, bacteriophage P100 and sodium lactate against Listeria monocytogenes in ready-to-eat sliced pork ham

The effectiveness of bacteriophage P100, nisin and sodium lactate, individually and in combination, in inhibiting Listeria monocytogenes in ready-to-eat pork ham slices was assessed. The antimicrobials were applied to the surfaces of ready-to-eat pork ham slices, which were inoculated with a mixture of L. monocytogenes. Among the individual antimicrobial treatments, bacteriophage P100 was the most effective, decreasing L. monocytogenes to undetectable levels at zero and 72h post-infection. Sodium lactate was the least effective treatment. Treatment with nisin at zeroh significantly reduced initial cell density (p<0.05). However, this pattern was not observed at 72h of storage. A significant difference (p<0.05) existed between the results of separate bacteriophage and nisin treatments after refrigerated storage, but not immediately upon inoculation of the bacteria. The results showed that the use of bacteriophage P100 is the method of choice for the control of bacteria.

Listeria monocytogenes in Smoked Salmon and Other Smoked Fish at Retail in Italy: Frequency of Contamination and Strain Characterization in Products from Different Manufacturers

Seven hundred seventy-eight samples of packaged smoked fish (774 smoked salmon and 4 smoked swordfish) on sale in Italy, from 50 different manufacturers located in 12 European Union countries, were purchased from the Italian market between May and December 2011. The surface temperatures of the samples on sale ranged from 0 to 13°C (3.4 ± 1.5°C, mean ± SD). Six hundred eighty (87.4%) of 778 samples were stored at ≤4°C. One hundred fifty-seven samples (20.2%, 95% confidence interval 17.5 to 23.1%) were contaminated by Listeria monocytogenes , with 26 samples (3.3%, 95% confidence interval 2.3 to 4.8%) at levels >100 CFU/g. The maximum level of contamination was 1.3 ×10⁶ CFU/g. The differences in the level of contamination of smoked fish between countries (χ² = 91.54, P < 0.05) and manufacturers (χ² = 193.22, P < 0.05) were significant. The frequency of detection for products from different manufacturing premises ranged from 0 to 76.9%. Serotyping by serological agglutination revealed that the main serotypes detected were 1/2a (65.3%) and 1/2b (22.4%). Pulsed-field gel electrophoresis typing with restriction enzymes AscI and ApaI yielded 36 pulsotypes from 144 isolates, clustering into 17 groups. Eight main pulsotypes accounted for 70.8% of the isolates. Three of the main pulsotypes were exclusively from products of a single manufacturer. In general, products from the same manufacturer showed genetic homogeneity, with one strongly prevalent pulsotype. Different manufacturers usually showed very different levels of contamination of the final product, confirming the importance of the management of process hygiene for controlling L. monocytogenes contamination.

A Bioengineered Nisin Derivative, M21A, in Combination with Food Grade Additives Eradicates Biofilms of Listeria monocytogenes

The burden of foodborne disease has large economic and social consequences worldwide. Despite strict regulations, a number of pathogens persist within the food environment, which is greatly contributed to by a build-up of resistance mechanisms and also through the formation of biofilms. Biofilms have been shown to be highly resistant to a number of antimicrobials and can be extremely difficult to remove once they are established. In parallel, the growing concern of consumers regarding the use of chemically derived antimicrobials within food has led to a drive toward more natural products. As a consequence, the use of naturally derived antimicrobials has become of particular interest. In this study we investigated the efficacy of nisin A and its bioengineered derivative M21A in combination with food grade additives to treat biofilms of a representative foodborne disease isolate of Listeria monocytogenes. Investigations revealed the enhanced antimicrobial effects, in liquid culture, of M21A in combination with citric acid or cinnamaldehyde over its wild type nisin A counterpart. Subsequently, an investigation was conducted into the effects of these combinations on an established biofilm of the same strain. Nisin M21A (0.1 μg/ml) alone or in combination with cinnamaldehyde (35 μg/ml) or citric acid (175 μg/ml) performed significantly better than combinations involving nisin A. All combinations of M21A with either citric acid or cinnamaldehyde eradicated the L. monocytogenes biofilm (in relation to a non-biofilm control). We conclude that M21A in combination with available food additives could further enhance the antimicrobial treatment of biofilms within the food industry, simply by substituting nisin A with M21A in current commercial products such as Nisaplin® (Danisco, DuPont).

An Evaluation of Alternatives to Nitrites and Sulfites to Inhibit the Growth of Salmonella enterica and Listeria monocytogenes in Meat Products

In recent years, the use of nitrites and sulfites as food preservatives has been a cause for concern due to the health problems that these additives can cause in humans. Natural products have been studied as an alternative, but most of them have hardly been applied in the food industry for technological and economic reasons. In this sense, organic salts such as sodium acetate are a good alternative due to their affordability. Thus, this study evaluated the capacity of sodium nitrite, sodium sulfite, a sodium acetate product (TQI C-6000), and chitosan to inhibit two important foodborne pathogens, Salmonella enterica and Listeria monocytogenes. The MIC of each chemical was in vitro evaluated and their antibacterial action was subsequently checked in situ using minced meat as a food model. MIC values of sodium nitrite (10,000 mg/L) and sodium sulfite (50,000 mg/L) for Salmonella enterica were higher than the values allowed by legislation (450 mg/L for sulfites and 150 mg/L for nitrites). Additionally, the sodium acetate product caused the inhibition of Salmonella enterica and Listeria at a relative low quantity. The two foodborne pathogens were inhibited in the food model with 1% of the sodium acetate product. Additionally, there were no significant differences between sodium nitrite, sodium sulfite, and sodium acetate products in the inhibition of Salmonella enterica and Listeria monocytogenes in the food model. Thus, products based on sodium acetate can be an alternative to traditional preservatives in food products.

Extracts containing CLPs of Bacillus amyloliquefaciens JN68 isolated from chicken intestines exert antimicrobial effects, particularly on methicillin-resistant Staphylococcus aureus and Listeria monocytogenes

Bacillus amyloliquefaciens JN68, which has been discussed with regards to its antimicrobial activities, was successfully isolated from healthy chicken intestines in the present study. Using the spot-on-the-lawn antagonism method, the preliminary study indicated that a suspension culture of the B. amyloliquefaciens JN68 strain can inhibit the growth of Aspergillus niger and Penicillium pinophilum. Furthermore, the cyclic lipopeptides (CLPs) produced by the B. amyloliquefaciens JN68 strain were further purified through acid precipitation and Bond Elut®C18 chromatography, and their structures were identified using the liquid chromatography‑electrospray ionization‑mass spectrometry (MS)/MS method. Purified CLPs exerted broad spectrum antimicrobial activities on various pathogenic and foodborne bacteria and fungi, as determined using the agar well diffusion method. Listeria monocytogenes can induce listeriosis, which is associated with a high mortality rate. Methicillin‑resistant Staphylococcus aureus (MRSA) is a major pathogenic bacteria that causes nosocomial infections. Therefore, L. monocytogenes and MRSA are currently of great concern. The present study aimed to determine whether B. amyloliquefaciens JN68 extracts could inhibit L. monocytogenes and MRSA. The results indicated that extracts of B. amyloliquefaciens JN68 have CLP components, and can successfully inhibit the growth of L. monocytogenes and MRSA.

Prokaryotic expression and mechanism of action of α-helical antimicrobial peptide A20L using fusion tags

BACKGROUND

Antimicrobial peptides have become important candidates as new antibiotics against resistant bacterial strains. However, the major industrial manufacture of antimicrobial peptides is chemical synthesis with high costs and in relatively small scale. The Ub-tag and SUMO-tag are useful for increasing the yield of enzymes and other proteins in expression system. In this study, antimicrobial peptide A20L (KWKSFLKTFKSAKKTVLHTLLKAISS), a derivative of V13K in the previous study is used as a template to be expressed in different Ub-tag and human SUMO tag systems to compare the prokaryotic expression approaches of antimicrobial peptide. The antibacterial mechanism of action and membrane specificity of A20L was further studied.

METHODS

We fused the Ub and SUMO1/2/3/4 with A20L to construct expression plasmids. Ub-A20L and SUMO1/2/34 gene sequences were inserted into the pHUE plasmids and pET-28b+ plasmids, respectively, to construct pHUE-A20L plasmids and pET-28b+-SUMO1/2/3/4-A20L plasmids. These plasmids were transformed into E. coli Rosetta (DE3) and induced with IPTG to express Ub-A20L and SUMO1/2/3/4 fusion proteins. The recombinant proteins were found in the soluble fraction after being over expressed in E. coli Rosetta (DE3). Antibacterial and hemolytic activities and membrane permeabilization ability of A20L were determined. Peptide structure was also studied by circular dichroism experiments.

RESULTS

A20L (KWKSFLKTFKSAKKTVLHTLLKAISS) was successfully expressed by fusion with an ubiquitin tag (Ub-tag) and human SUMO tags (SUMO1/2/3/4-tags). A20L exhibited antimicrobial activity against various Gram-negative and Gram-positive bacteria. Based on the hemolytic activity against human red blood cells, A20L showed good specificity against bacteria. The circular dichroism experiments illustrated that A20L was transferred into an α-helical structure in the presence of hydrophobic environment. The antibacterial mechanism of action and membrane specificity of A20L was further studied using membrane permeabilization experiments and tryptophan fluorescence and quenching experiments in liposomes.

CONCLUSIONS

The Ub-tag and human SUMO-tags represent good alternatives to chemical synthesis for the industrial production of antimicrobial peptides with low costs and high yields. The antibacterial mechanism of action of A20L was proved as membrane disruption. A20L showed stronger specificity on liposomes mimicking bacterial membrane than those mimicking eukaryotic cell membrane, which is consistent with the biological activity studies.

VirR-Mediated Resistance of Listeria monocytogenes against Food Antimicrobials and Cross-Protection Induced by Exposure to Organic Acid Salts

Formulations of ready-to-eat (RTE) foods with antimicrobial compounds constitute an important safety measure against foodborne pathogens such as Listeria monocytogenes. While the efficacy of many commercially available antimicrobial compounds has been demonstrated in a variety of foods, the current understanding of the resistance mechanisms employed by L. monocytogenes to counteract these stresses is limited. In this study, we screened in-frame deletion mutants of two-component system response regulators associated with the cell envelope stress response for increased sensitivity to commercially available antimicrobial compounds (nisin, lauric arginate, ε-polylysine, and chitosan). A virR deletion mutant showed increased sensitivity to all antimicrobials and significantly greater loss of membrane integrity when exposed to nisin, lauric arginate, or ε-polylysine (P < 0.05). The VirR-regulated operon, dltABCD, was shown to be the key contributor to resistance against these antimicrobial compounds, whereas another VirR-regulated gene, mprF, displayed an antimicrobial-specific contribution to resistance. An experiment with a β-glucuronidase (GUS) reporter fusion with the dlt promoter indicated that nisin does not specifically induce VirR-dependent upregulation of dltABCD. Lastly, prior exposure of L. monocytogenes parent strain H7858 and the ΔvirR mutant to 2% potassium lactate enhanced subsequent resistance against nisin and ε-polylysine (P < 0.05). These data demonstrate that VirRS-mediated regulation of dltABCD is the major resistance mechanism used by L. monocytogenes against cell envelope-damaging food antimicrobials. Further, the potential for cross-protection induced by other food-related stresses (e.g., organic acids) needs to be considered when applying these novel food antimicrobials as a hurdle strategy for RTE foods.

Effect of Acidified Sorbate Solutions on the Lag-Phase Durations and Growth Rates of Listeria monocytogenes on Meat Surfaces

The surfaces of ready-to-eat meats are susceptible to postprocessing contamination by Listeria monocytogenes. This study quantified the lag-phase durations (LPD) and growth rates (GR) of L. monocytogenes on the surfaces of cooked ham as affected by sorbate solutions of different concentrations and pH levels. Slices of cooked ham inoculated with a four-strain mixture of L. monocytogenes (ca. 10(3) CFU/g) were surface treated with sorbate solutions of 0 to 4% (wt/vol) at pH 4.0 to 6.5, vacuum packaged, and stored at 4 to 12 °C for up to 45 days. The LPD and GR of L. monocytogenes were used to develop response surface models. The models estimated that the LPD of L. monocytogenes in samples treated with solutions of pH 4.0 to 5.5 (no sorbate) were 0 to 11 days and the GR were 0.25 to 0.36 log CFU/day, respectively, at 4 °C. With the treatments of 2 and 4% (wt/vol) sorbate solutions, the LPD were estimated to be extended to 2 to 26 days and 34 to >45 days, and the GR were reduced to 0.15 to 0.30 and 0 to 0.19 log CFU/day, respectively. At 4 °C, increasing sorbate concentrations by 1% (wt/vol) to 2, 3, and 4% (wt/vol) at pH 5.5 to 4.0 led to an extension of LPD by 2 to 11, 10 to 19, and 18 to 27 days, whereas the GR were reduced by 0.037 to 0.055, 0.048 to 0.066, and 0.060 to 0.078 log CFU/day, respectively. Sorbate also extended the LPD and reduced the GR of L. monocytogenes at 8 and 12 °C. Results indicated that sorbate concentration and pH level were significant factors affecting the LPD and GR of L. monocytogenes and that the combination of sorbate and low pH has potential for use as a surface treatment to control L. monocytogenes on meat surfaces.

Use of antimicrobial films and edible coatings incorporating chemical and biological preservatives to control growth of Listeria monocytogenes on cold smoked salmon

The relatively high incidence of Listeria monocytogenes in cold smoked salmon (CSS) is of concern as it is a refrigerated processed food of extended durability (REPFED). The objectives of this study were to compare and optimize the antimicrobial effectiveness of films and coatings incorporating nisin (Nis) and sodium lactate (SL), sodium diacetate (SD), potassium sorbate (PS), and/or sodium benzoate (SB) in binary or ternary combinations on CSS. Surface treatments incorporating Nis (25000 IU/mL) in combination with PS (0.3%) and SB (0.1%) had the highest inhibitory activity, reducing the population of L. monocytogenes by a maximum of 3.3 log CFU/cm(2) (films) and 2.9 log CFU/cm(2) (coatings) relative to control samples after 10 days of storage at 21°C. During refrigerated storage, coatings were more effective in inhibiting growth of L. monocytogenes than their film counterparts. Cellulose-based coatings incorporating Nis, PS, and SB reduced the population of L. monocytogenes, and anaerobic and aerobic spoilage flora by a maximum of 4.2, 4.8, and 4.9 log CFU/cm(2), respectively, after 4 weeks of refrigerated storage. This study highlights the effectiveness of cellulose-based edible coatings incorporating generally regarded as safe (GRAS) natural and chemical antimicrobials to inhibit the development of L. monocytogenes and spoilage microflora thus enhancing the safety and quality of CSS.

Antimicrobial activity of reuterin produced by Lactobacillus reuteri on Listeria monocytogenes in cold-smoked salmon

Lactobacillus reuteri INIA P579 was used for the production and purification of reuterin. The purity of reuterin was assessed by high resolution electrospray ionization mass spectrometry (HRESIMS) and nuclear magnetic resonance (NMR) spectroscopy. After purification, reuterin concentration obtained was 1.3 M. The inhibitory activity using Escherichia coli K12 as indicator strain was estimated to be 510 AU/ml. Survival curves in tryptic soy broth revealed that reuterin required to inhibit the growth of three Listeria monocytogenes strains was in the range of 2-4 AU/ml. Purified reuterin (10 AU/g) significantly reduced the growth of L. monocytogenes in cold-smoked salmon kept under moderate or strong temperature abuse conditions. After 15 d at 8 °C, cold-smoked salmon with added reuterin exhibited L. monocytogenes counts 2.0 log CFU/g lower than control smoked salmon with no reuterin added. At 30 °C, reuterin also controlled the growth of the pathogen, with counts 1.4 and 0.9 log CFU/g lower than those observed in control smoked salmon after 24 and 48 h, respectively. The addition of purified reuterin might be used as a hurdle technology to improve the safety and extend the shelf-life of lightly preserved seafood products such as cold-smoked salmon.

Listeria monocytogenes in Vacuum-Packed Smoked Fish Products: Occurrence, Routes of Contamination, and Potential Intervention Measures

The occurrence of Listeria monocytogenes in ready-to-eat (RTE) fish products is well documented and represents an important food safety concern. Contamination of this pathogen in vacuum-packed (VP) smoked fish products at levels greater than the RTE food limit (100 CFU/g) has been traced to factors such as poor sanitary practices, contaminated processing environments, and temperature abuse during prolonged storage in retail outlets. Intervention technologies including physical, biological, and chemical techniques have been studied to control transmission of L. monocytogenes to these products. High-pressure processing, irradiation, and pulsed UV-light treatment have shown promising results. Potential antilisterial effects of some sanitizers and combined chemical preservatives have also been demonstrated. Moreover, the concept of biopreservation, use of bioactive packaging, and a combination of different intervention technologies, as in the hurdle concept, are also under consideration. In this review, the prevalence, routes of contamination, and potential intervention technologies to control transmission of L. monocytogenes in VP smoked fish products are discussed.

Characterization of a new bacteriocin from Lactobacillus plantarum LE5 and LE27 isolated from ensiled corn

Bacteriocins are low molecular peptides with antimicrobial activity, which are of great interest as food bio-preservatives and for treating diseases caused by pathogenic bacteria. In this study, we present the characterization of bacteriocins produced by Lactobacillus plantarum LE5 and LE27 isolated from ensiled corn. Bacteriocins were purified through ammonium sulfate precipitation and double dialysis by using 12- and 1-kDa membranes. Bacteriocins showed activity against Listeria innocua, Listeria monocytogenes, and Enteroccocus faecalis. Molecular weight was estimated through Tricine-SDS-PAGE and overloading the gel onto Mueller-Hinton agar seeded with L. monocytogenes, showing an inhibition zone between 5 and 10 kDa. NanoLC-MS/MS analysis allowed the identification of UPF0291 protein (UniProtKB/Swiss-Prot Q88VI7), which is also presented in other lactic acid bacteria without assigned function. Ab initio modeling showed it has an α-helix-rich structure and a large positive-charged region. Bacteriocins were stable between 4 and 121 °C and pH 2 and 12, and the activity was inhibited by SDS and proteases. Mode of action assay suggests that the bacteriocin causes of target microorganism. Taken together, these results describe a possible new class IIa bacteriocin produced by L. plantarum, which has a wide stability to physicochemical conditions, and that could be used as an alternative for the control of foodborne diseases.

Effect of Curing Method and Freeze-Thawing on Subsequent Growth of Listeria monocytogenes on Cold-Smoked Salmon

The presence of the foodborne pathogen Listeria monocytogenes on cold-smoked salmon is a major concern for the seafood industry. Understanding processing and postprocessing handling factors that affect the ability of this pathogen to grow on cold-smoked salmon is critical for developing effective control strategies. In this study, we investigated the effect of curing method and freeze-thawing of cold-smoked salmon on (i) physicochemical properties and (ii) subsequent growth of genetically diverse strains of L. monocytogenes (inoculated after freeze-thawing) and endogenous lactic acid bacteria. The majority of the measured physicochemical properties were unaffected by freezing and thawing. Overall, wet-cured cold-smoked salmon had higher pH, water activity, and moisture, as well as lower fat, water-phase salt, and phenolic content compared with dry-cured cold-smoked salmon. The curing method and freeze-thawing did not affect growth of endogenous lactic acid bacteria. Freeze-thawing cold-smoked salmon prior to inoculation led to pronounced growth of L. monocytogenes at 7°C. The increase in cell density between days 0 and 30 was significantly (P = 0.0078) greater for cold-smoked salmon that was frozen and thawed prior to inoculation compared with nonfrozen cold-smoked salmon. On dry-cured, freeze-thawed cold-smoked salmon, L. monocytogenes had a lag phase ranging from 3.7 ± 0.1 to 11.2 ± 1.4 days compared with salmon that was wet cured and freeze-thawed, on which L. monocytogenes began to grow within 24 h. Variation in growth among L. monocytogenes strains was also observed, indicating the significance of assessing multiple strains. Further efforts to understand the impact of processing and postprocessing handling steps of cold-smoked salmon on the growth of genetically diverse L. monocytogenes will contribute to improved challenge study designs and data. This, in turn, will likely lead to more reliable and unbiased risk assessments and control measures.

Edible films and coatings from whey proteins: a review on formulation, and on mechanical and bioactive properties

The latest decade has witnessed joint efforts by the packaging and the food industries to reduce the amount of residues and wastes associated with food consumption. The recent increase in environmental awareness has also contributed toward development of edible packaging materials. Viable edible films and coatings have been successfully produced from whey proteins; their ability to serve other functions, viz. carrier of antimicrobials, antioxidants, or other nutraceuticals, without significantly compromising the desirable primary barrier and mechanical properties as packaging films, will add value for eventual commercial applications. These points are tackled in this review, in a critical manner. The supply of whey protein-based films and coatings, formulated to specifically address end-user needs, is also considered.

Growth of enterotoxigenic Bacillus cereus on salmon (Oncorhynchus nerka)

We previously demonstrated the widespread presence of enterotoxigenic Bacillus cereus in marine foods. In view of the widespread consumption of raw fish, we sought to determine the ability of this organism to grow on the surface of wild Alaskan salmon at abusive temperatures (12, 16, and 20°C), using an isolate able to produce elevated levels of hemolysin BL enterotoxin and nonhemolytic enterotoxin. An incubation temperature of 37°C for colony formation was found to be selective for B. cereus grown on salmon held for up to 24 h at each temperature. A fivefold increase in log CFU per gram was observed after 26 and 22 h at 16 and 20°C, respectively, while a >4-log CFU/g increase occurred on salmon held at 12°C for 48 h. Generation times of 169.7, 53.5, and 45.6 min were observed at 12, 16, and 20°C. Nonhemolytic enterotoxin was detected when levels of B. cereus were in excess of 10(8) CFU/g. Nisin, at concentrations of 1 and 15 m g/g of salmon, reduced levels of B. cereus 2.5- and 25-fold, respectively. Our results indicate that fresh salmon can serve as an excellent substrate for enterotoxigenic B. cereus and that this organism can reach levels associated with foodborne illness following moderate temperature abuse.

Initial adhesion of Listeria monocytogenes to solid surfaces under liquid flow

Some strains of the food borne pathogen Listeria monocytogenes persist in food processing environments. The exact reason behind this phenomenon is not known, but strain differences in the ability to adhere to solid surfaces could offer an explanation. In the present work, initial adhesion of nine strains of L. monocytogenes was investigated under liquid flow at two levels of shear stress on six different surfaces using a flow chamber set-up with microscopy measurements. The surfaces tested were glass and PVC, and glass coated with beef extract, casein, and homogenised and unhomogenised milk. In addition, the effect of prior environmental stress (5% NaCl, low nutrient availability) on initial adhesion was investigated. The hydrophobicity of the investigated surfaces was determined by contact angle measurements and the surface properties of the investigated L. monocytogenes strains were determined using Microbial Adhesion To Solvents (MATS). All surfaces with the exception of PVC were found to be hydrophilic. Strain differences were found to significantly influence the initial adhesion rate (IAR) of all nine strains to all the surfaces (p<0.05) at both low and high shear stress. Furthermore, there was a significant effect of the surfaces tested (p<0.05) in the adhesion ability of almost all strains. The IAR was affected by flow rate (shear stress) as seen by a decrease in adhesion at high shear stress for most strains. A significant effect of interactions between strain-surface and strain-shear stress (p<0.001) was observed but not of interactions between surface-shear stress. No correlation between surface hydrophobicity and IAR was observed. Addition of 5% NaCl during propagation resulted in a decrease in IAR whilst propagation in low nutrient media caused an increase indicating a general change in surface characteristics under these conditions. Known persisting strains did not display general better adherence.

Prior frozen storage enhances the effect of edible coatings against Listeria monocytogenes on cold-smoked salmon during subsequent refrigerated storage

AIMS

Listeria monocytogenes is a major safety concern for ready-to-eat foods. The overall objective of this study was to investigate whether prior frozen storage could enhance the efficacy of edible coatings against L. monocytogenes on cold-smoked salmon during subsequent refrigerated storage.

METHODS AND RESULTS

A formulation consisting of sodium lactate (SL, 1·2-2·4%) and sodium diacetate (SD, 0·125-0·25%) or 2·5% Opti.Form (a commercial formulation of SL and SD) was incorporated into each of five edible coatings: alginate, κ-carrageenan, pectin, gelatin and starch. The coatings were applied onto the surface of cold-smoked salmon slices inoculated with L. monocytogenes at a level of 500 CFU cm⁻². In the first phase, the slices were first frozen at -18°C for 6 days and stored at 22°C for 6 days. Alginate, gelatin and starch appeared to be the most effective carriers. In the second phase, cold-smoked salmon slices were inoculated with L. monocytogenes, coated with alginate, gelatin or starch with or without the antimicrobials and stored frozen at -18°C for 12 months. Every 2 months, samples were removed from the freezer and kept at 4°C for 30 days. Prior frozen storage at -18°C substantially enhanced the antilisterial efficacy of the edible coatings with or without antimicrobials during the subsequent refrigerated storage.

CONCLUSIONS

Plain coatings with ≥ 2 months frozen storage and antimicrobial edible coatings represent an effective intervention to inhibit the growth of L. monocytogenes on cold-smoked salmon.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates the effectiveness of the conjunct application of frozen storage and edible coatings to control the growth of L. monocytogenes to enhance the microbiological safety of cold-smoked salmon.

The combination of lactate and diacetate synergistically reduces cold growth in brain heart infusion broth across Listeria monocytogenes lineages

Combinations of organic acids are often used in ready-to-eat foods to control the growth of Listeria monocytogenes during refrigerated storage. The purpose of this study was to quantitatively assess synergy between two organic acid growth inhibitors under conditions similar to those present in cold-smoked salmon, and to assess the effect of evolutionary lineage on response to those growth inhibitors. Thirteen strains of L. monocytogenes, representing lineages I and II, were grown at 7 degrees C in broth at pH 6.1 and 4.65% water-phase NaCl, which was supplemented with 2% potassium lactate, 0.14% sodium diacetate, or the combination of both at the same levels. Our data suggest that lineages adapt similarly to these inhibitors, as the only significant growth parameter difference between lineages was a minor effect (+/- 0.16 day, P = 0.0499) on lag phase (lambda). For all strains, lactate significantly extended lambda, from 2.6 +/- 0.4 to 3.8 +/- 0.5 days (P < 0.001), and lowered the maximum growth rate (mu(max)) from 0.54 +/- 0.06 to 0.49 +/- 0.04 log(CFU/ml)/day (P < 0.001), compared with the control. Diacetate was ineffective alone, but in combination with lactate, synergistically increased lambda to 6.6 +/- 1.6 days (P < 0.001) and decreased mu(max) to 0.34 +/- 0.05 log(CFU/ml)/day (P < 0.001). Monte Carlo simulations provided further evidence for synergy between diacetate and lactate by predicting signficantly slower growth to nominal endpoints for the combination of inhibitors. This study shows potassium lactate and sodium diacetate have significant synergistic effects on both lambda and mu(max) of L. monocytogenes at refrigeration temperature in broth, and justifies combining these inhibitors, at effective levels, in food product formulations.

Bacteriophage significantly reduces Listeria monocytogenes on raw salmon fillet tissue

We have demonstrated the antilisterial activity of generally recognized as safe (GRAS) bacteriophage LISTEX P100 (phage P100) on the surface of raw salmon fillet tissue against Listeria monocytogenes serotypes 1/2a and 4b. In a broth model system, phage P100 completely inhibited L. monocytogenes growth at 4 degrees Celsius for 12 days, at 10 degrees Celsius for 8 days, and at 30 degrees Celsius for 4 days, at all three phage concentrations of 10(4), 10(6), and 10(8) PFU/ml. On raw salmon fillet tissue, a higher phage concentration of 10(8) PFU/g was required to yield 1.8-, 2.5-, and 3.5-log CFU/g reductions of L. monocytogenes from its initial loads of 2, 3, and 4.5 log CFU/g at 4 or 22 degrees Celsius. Over the 10 days of storage at 4 degrees Celsius, L. monocytogenes growth was inhibited by phage P100 on the raw salmon fillet tissue to as low as 0.3 log CFU/g versus normal growth of 2.6 log CFU/g in the absence of phage. Phage P100 remained stable on the raw salmon fillet tissue over a 10-day storage period, with only a marginal loss of 0.6 log PFU/g from an initial phage treatment of 8 log PFU/g. These findings illustrate that the GRAS bacteriophage LISTEX P100 is listericidal on raw salmon fillets and is useful in quantitatively reducing L. monocytogenes.

Bioactive alginate coatings to control Listeria monocytogenes on cold-smoked salmon slices and fillets

The relatively high incidence of Listeria monocytogenes in cold smoked salmon (CSS) is of concern as CSS is a ready-to-eat product. No post-processing measures are currently available to control this pathogen in CSS. The objective of this study was to develop an effective antimicrobial edible coating containing organic salts to control the growth of L. monocytogenes in CSS slices and fillets. An in-house made formulation consisting of sodium lactate (SL, 0-2.4%) and sodium diacetate (SD, 0-0.25%) as well as 2.5% OptiForm (a commercial formulation of SL and SD) were incorporated into five edible coatings: alginate, kappa-carrageenan, pectin, gelatin or starch. The coatings were applied onto the surface of CSS slices inoculated with L. monocytogenes to an inoculum level of 500 CFU/cm(2) ( approximately 3 log CFU/g) and stored at room temperature (22 degrees C) for 6 days. Alginate coating was found to be the most effective carrier for the various antimicrobial treatments in inhibiting the growth of L. monocytogenes. In the second phase of the study, CSS slices and fillets inoculated with the pathogen at a level of 500 CFU/cm(2) were coated with alginate incorporating the in-house made and the commercial (OptiForm) SL/SD based formulations and stored for 30 days at 4 degrees C. When cold-smoked salmon slices and fillets were stored at 4 degrees C, alginate coatings supplemented with 2.4%SL/0.25%SD and the commercial product OptiForm significantly delayed the growth of L. monocytogenes during the 30-day storage with final counts reaching 4.1 and 3.3 log CFU/g (slices) and 4.4 and 3.8 log CFU/g (fillets), respectively, while the counts in their untreated counterparts were significantly higher (P<0.05) reaching 7.3 and 6.8 log CFU/g for slices and fillets, respectively. Therefore, this study demonstrates the effectiveness of using an alginate-based coating containing lactate and diacetate to control the growth of L. monocytogenes to enhance the microbiological safety of filleted and sliced smoked salmon.