Effect of combining nisin and/or lysozyme with in-package pasteurization for control of Listeria monocytogenes in ready-to-eat turkey bologna during refrigerated storage

Nisin: From a structural and meat preservation perspective

Nisin is a posttranslationally modified antimicrobial peptide that is widely used as a food preservative. It contains five cyclic thioethers of varying sizes. Nisin activity and stability are closely related to its primary and three dimensional structures. It has nine reported natural variants. Nisin A is the most studied nisin as it was the first one purified. Here, we review the sequence feature of nisin A and its natural variants, and their biosynthesis pathway, mode of action and application as a meat preservative. We systematically illustrate the functional domains of the main enzymes (NisB, NisC, and NisP) involved in nisin synthesis. NisB was shown to dehydrate its substrate NisA via a tRNA associated glutamylation mechanism. NisC catalysed the cyclization of the didehydro amino acids with the neighboring cysteine residues. After cyclization, the leader peptide is removed by the protease NisP. According to multiple sequence alignments, we detected five conserved sites Dha⁵, Pro⁹, Gly¹⁴, Leu¹⁶, and Lys²². These residues are probably the structural and functional important ones that can be modified to produce peptides versions with enhanced antimicrobial activity. Through comparing various application methods of nisin in different meats, the antimicrobial effects of nisin used individually or in combination with other natural substances were clarified.

Lysozyme and Its Application as Antibacterial Agent in Food Industry

Lysozymes are hydrolytic enzymes characterized by their ability to cleave the β-(1,4)-glycosidic bonds in peptidoglycan, a major structural component of the bacterial cell wall. This hydrolysis action compromises the integrity of the cell wall, causing the lysis of bacteria. For more than 80 years, its role of antibacterial defense in animals has been renowned, and it is also used as a preservative in foods and pharmaceuticals. In order to improve the antimicrobial efficacy of lysozyme, extensive research has been intended for its modifications. This manuscript reviews the natural antibiotic compound lysozyme with reference to its catalytic and non-catalytic mode of antibacterial action, lysozyme types, susceptibility and resistance of bacteria, modification of lysozyme molecules, and its applications in the food industry.

The Effect of Modified Lysozyme Treatment on the Microflora, Physicochemical and Sensory Characteristics of Pork Packaged in Preservative Gas Atmospheres

The aim of the study was to investigate the effect of modified lysozyme on the microflora, physicochemical and sensory characteristics of pork loin packaged in modified atmospheres and stored at 4 ± 1 °C. Different gas compositions (M1 65:25:10 O2:CO2:N2; M2 50:40:10 O2:CO2:N2; M3 80:20 O2:CO2) were used. The microbiological parameters (APC, Enterobacteriaceae, Pseudomonas spp., lactic acid bacteria), physicochemical indexes (pH, colour) as well as a sensory attribute, i.e., aroma were analysed. Meat samples were tested after five, 12, 19, 23, and 28 days of storage. Changes in the qualities of pork were determined throughout the storage. The proportions of polymeric forms, hydrolytic activity and hydrophobicity were determined in the lysozyme preparation. Modified lysozyme exhibited higher hydrophobicity and lower hydrolytic activity than lysozyme monomer. The colour parameters L* and a* were not considerably affected by the addition of modified lysozyme. The sample with the modified lysozyme was given the highest score for aroma. In comparison with the monomer, the modified lysozyme exhibited greater antibacterial effect, especially against Pseudomonas and Enterobacteriaceae. Microbial growth rates in the sample with modified lysozyme, packaged in an atmosphere with the highest content of CO2 (total plate count 4.59 log CFU/cm2; moulds and yeasts 2.17 log CFU/cm2) were lower than those observed in the sample without lysozyme packed under M1 and M3 (20−25% CO2). The use of an atmosphere with gas composition and modified lysozyme considerably extended the shelf life of pork. The combination of the atmosphere with the highest content of carbon dioxide (50% O2, 40% CO2, 10% N2) and modified lysozyme resulted in the best effect. This strategy extended the shelf-life by more than 20%, as compared with the control sample without lysozyme, packaged in an atmosphere of 50:40:10 O2:CO2:N2.

Inactivation of foodborne pathogens by the synergistic combinations of food processing technologies and food-grade compounds

There is a need to develop food processing technologies with enhanced antimicrobial capacity against foodborne pathogens. While considering the challenges of adequate inactivation of pathogenic microorganisms in different food matrices, the emerging technologies are also expected to be sustainable and have a minimum impact on food quality and nutrients. Synergistic combinations of food processing technologies and food-grade compounds have a great potential to address these needs. During these combined treatments, food processes directly or indirectly interact with added chemicals, intensifying the overall antimicrobial effect. This review provides an overview of the combinations of different thermal or nonthermal processes with a variety of food-grade compounds that show synergistic antimicrobial effect against pathogenic microorganisms in foods and model systems. Further, we summarize the underlying mechanisms for representative combined treatments that are responsible for the enhanced microbial inactivation. Finally, regulatory issues and challenges for further development and technical transfer of these new approaches at the industrial level are also discussed.

Development of an antimicrobial bioactive paper made from bacterial cellulose

Bacterial cellulose (BC) has emerged as an attractive adsorptive material for antimicrobial agents due to its fine network structure, its large surface area, and its high porosity. In the present study, BC paper was first produced and then lysozyme was immobilized onto it by physical adsorption, obtaining a composite of lysozyme-BC paper. The morphology and the crystalline structure of the composite were similar to that of BC paper as examined by scanning electron microscopy and X-ray diffraction, respectively. Regarding operational properties, specific activities of immobilized and free lysozyme were similar. Moreover, immobilized enzyme showed a broader working temperature and higher thermal stability. The composites maintained its activity for at least 80 days without any special storage. Lysozyme-BC paper displayed antimicrobial activity against Gram-positive and Gram-negative bacteria, inhibiting their growth by 82% and 68%, respectively. Additionally, the presence of lysozyme increased the antioxidant activity of BC paper by 30%. The results indicated that BC is a suitable material to produce bioactive paper as it provides a biocompatible environment without compromising the activity of the immobilized protein. BC paper with antimicrobial and antioxidant properties may have application in the field of active packaging.

Development and evaluation of pullulan-based composite antimicrobial films (CAF) incorporated with nisin, thymol and lauric arginate to reduce foodborne pathogens associated with muscle foods

A novel composite antimicrobial film (CAF), made from a pullulan-based biopolymer and polyethylene (PE) was developed and evaluated for controlling pathogens associated with muscle foods. Initially, CAFs were developed by incorporating thymol (T), nisin (N) and/or lauric arginate (LAE) into the pullulan layer and layering it on top of PE. The antimicrobial activity of the resulting CAFs was evaluated against cocktails of Shiga toxin-producing E. coli (STEC), Salmonella spp., Listeria monocytogenes (L. monocytogenes) and Staphylococcus aureus (S. aureus) in disk diffusion assays (DDAs). CAFs containing N were ineffective, while those containing T were effective for inhibiting the pathogens in DDAs. However, CAFs made with them did not exhibit desirable physical and mechanical properties since solvents (HCl and ethanol, respectively) interfered with the binding of pullulan to PE. Conversely, CAFs made with 0.5, 1 and 2.5% LAE maintained proper physical and mechanical characteristics and inhibited the four bacterial pathogens in DDAs. Based on these preliminary results, cocktails consisting of approximately 8 log₁₀ CFU/ml of STEC, Salmonella, L. monocytogenes, or S. aureus were experimentally-inoculated onto raw beef, raw chicken breast, or ready-to-eat (RTE) turkey breast to obtain approximately 6.6 log₁₀ CFU/cm², aseptically transferred to CAFs containing 0.5, 1, or 2.5% LAE that were made into sachets/bags, vacuum packaged, sealed, and remaining microbial populations determined up to 28 days of refrigerated storage (4 °C). By day 28, CAFs containing 0.5, 1, and 2.5% LAE reduced: STEC by 1.13, 1.33 and 2.88 log₁₀ CFU/cm² respectively, on raw beef; Salmonella by 2.03, 2.12 and 3.01 log₁₀ CFU/cm² respectively, on raw chicken breast; L. monocytogenes by 1.12, 1.81 and 3.56 log₁₀ CFU/cm² respectively, on RTE turkey breast; and S. aureus by 0.68, 2.02 and 3.43 log₁₀ CFU/cm², respectively, on RTE turkey breast. CAFs may be of interest to the meat and poultry industry to control foodborne pathogens associated with these food products.

Fumigating broiler hatching eggs with lysozyme product (Inovapure) to reduce eggshell microbial load

Experiments were conducted to evaluate the effectiveness of a lysozyme product (InovapureTM) (LP) against E. coli penetrating eggshells. In the first microbiological experiment, 60 agar-filled eggs were inoculated with E. coli suspension, then fumigated with distilled water, 1.5% or 3.0% LP or a quaternary ammonium product (QA) at 0.125% for 10 min. In the second microbiological experiment, another 60 agar-filled eggs were fumigated with the same sanitizer treatments first, then inoculated with the E. coli suspension. Eggshells were candled and visual colonies were counted after 48 h incubation. An animal experiment was conducted to evaluate LP applied to the surface of 2080 broiler hatching eggs on hatching and growth performance. Hatching eggs were submerged in an E. coli suspension. After drip drying, eggs were randomly divided into four fumigation treatments, each with four subsets of 150 eggs. Fumigation treatments were the same as in the microbiological experiments. Eggs were incubated in 8 incubators (2 replicate incubators per treatment) and the broilers were grown to 33 d of age. In the microbiological experiments, inoculated eggs fumigated with 3.0% LP and 0.125% QA reduced (P < 0.05) the total amount of E. coli to 11 cfu/egg and 10 cfu/egg, respectively. When eggs were sanitized prior to inoculation, 3.0% LP demonstrated (P < 0.05) ongoing bactericidal action to prevent E. coli penetration. No differences in hatchability, fertility rate or egg weight loss percent were found among sanitation treatments. At hatch, body weight or the ratio of yolk sac weight to yolk-free body weight were not affected by the sanitation treatments. However, the application of sanitizers decreased (P < 0.05) the presence of E. coli in the yolk sac of newly hatched chicks. Feed consumption, body weight and feed conversion ratio were not affected by sanitation treatments. However, average daily body weight gain was lower (P < 0.05) following QA. Overall, 3.0% LP demonstrated acceptable activity against E. coli on eggshells, and provided ongoing bactericidal action to prevent E. coli penetration without negatively affecting growth performance.

Trends in microbial control techniques for poultry products

Fresh poultry meat and poultry products are highly perishable foods and high potential sources of human infection due to the presence of several foodborne pathogens. Focusing on the microbial control of poultry products, the food industry generally implements numerous preventive measures based on the Hazard Analysis and Critical Control Points (HACCP) food safety management system certification together with technological steps, such as refrigeration coupled to modified atmosphere packaging that are able to control identified potential microbial hazards during food processing. However, in recent years, to meet the demand of consumers for minimally processed, high-quality, and additive-free foods, technologies are emerging associated with nonthermal microbial inactivation, such as high hydrostatic pressure, irradiation, and natural alternatives, such as biopreservation or the incorporation of natural preservatives in packaging materials. These technologies are discussed throughout this article, emphasizing their pros and cons regarding the control of poultry microbiota and their effects on poultry sensory properties. The discussion for each of the preservation techniques mentioned will be provided with as much detail as the data and studies provided in the literature for poultry meat and products allow. These new approaches, on their own, have proved to be effective against a wide range of microorganisms in poultry meat. However, since some of these emergent technologies still do not have full consumer's acceptability and, taking into consideration the hurdle technology concept for poultry processing, it is suggested that they will be used as combined treatments or, more frequently, in combination with modified atmosphere packaging.

Recombinant Expression of a Putative Amidase Cloned from the Genome of Listeria monocytogenes that Lyses the Bacterium and its Monolayer in Conjunction with a Protease

Listeria monocytogenes is a Gram-positive, non-spore forming, catalase-positive rod that is a major bacterial food-borne disease agent associated with uncooked meats, including poultry, uncooked vegetables, soft cheeses, and unpasteurized milk. The bacterium may be carried by animals without signs of disease, can replicate at refrigeration temperatures, and is frequently associated with biofilms. There is a need to discover innovative pathogen intervention technologies for this bacterium. Consequently, bioinformatic analyses were used to identify genes encoding lytic protein sequences in the genomes of L. monocytogenes isolates. PCR primers were designed that amplified nucleotide sequences of a putative N-acetylmuramoyl-L-alanine amidase gene from L. monocytogenes strain 4b. The resultant amplification product was cloned into an expression vector, propagated in Escherichia coli Rosetta strains, and the recombinant protein was purified to homogeneity. Gene and protein sequencing confirmed that the predicted and chemically determined amino acid sequence of the recombinant protein designated PlyLM was a putative N-acetylmuramoyl-L-alanine amidase. The recombinant lytic protein was capable of lysing both the parental L. monocytogenes strain as well as other strains of the bacterium in spot and MIC/MIB assays, but was not active against other bacteria beyond the genus. A microtiter plate assay was utilized to assay for the ability of the recombinant lysin protein to potentially aid with digestion of a L. monocytogenes biofilm. Protease or lysozyme digestion alone did not significantly reduce the L. monocytogenes biofilm. Although the recombinant protein alone reduced the biofilm by only 20%, complete digestion of the bacterial monolayer was accomplished in conjunction with a protease.

Evaluation of novel micronized encapsulated essential oil-containing phosphate and lactate blends for growth inhibition of Listeria monocytogenes and Salmonella on poultry bologna, pork ham, and roast beef ready-to-eat deli loaves

Essential oils and their constituents are reported to possess potent antimicrobial activity, but their use in food processing is limited because of low solubility in aqueous systems and volatilization during processing. Two proprietary noncommercial essential oil-containing phosphate blends were evaluated for antimicrobial activity against Salmonella enterica cocktail (SC)-and Listeria monocytogenes (Lm)-inoculated deli meat products made from pork, poultry, or beef. Four treatments were tested on restructured cured pork ham, emulsified chicken bologna, and restructured beef loaf: nonencapsulated essential oil with phosphate version 1 at 0.45% of final batch (EOV145; chicken and pork, or EEOV245 beef), micronized encapsulated essential oil with phosphate version 2 at 0.60% of final batch (EEOV260), a 2.0% potassium lactate (PL) control, and a negative control (CN) with no applied antimicrobial agent. Compared with the CN, none of the antimicrobial agents (EEOV260, EOV145, PL) successfully limited Lm or SC growth to <2.0 log cycles over 49 days or 35 days of refrigerated storage, respectively. The PL and EEOV260-treated ham loaves did show Lm growth limiting ability of up to 1 log cycle by days 35 and 42. On formed roast beef, the EEOV260 was able to extend the lag phase and inhibited the growth of Lm in the same manner as the PL. For SC-treated samples, the following effects were observed: in poultry bologna treated with EEOV260, a lag-phase extension was observed through 35 days of storage compared with the other samples. For pork deli loaves, the EEOV260 inhibited growth of SC at days 21 and 28 to the same level of efficacy as PL (0.5 log cycle). In roast beef samples, on day 35, the SC growth was inhibited ca. 0.5 log CFU/g by EEOV260 when compared with the CN. In conclusion the EEOV260 can function to replace PL to limit Salmonella and Lm growth in ready-to-eat deli products. Further testing is needed to ensure consumer acceptability.

Microbiological Challenge Testing for Listeria Monocytogenes in Ready-to-Eat Food: A Practical Approach

Food business operators (FBOs) are the primary responsible for the safety of food they place on the market. The definition and validation of the product's shelf-life is an essential part for ensuring microbiological safety of food and health of consumers. In the frame of the Regulation (EC) No 2073/2005 on microbiological criteria for foodstuffs, FBOs shall conduct shelf-life studies in order to assure that their food does not exceed the food safety criteria throughout the defined shelf-life. In particular this is required for ready-to-eat (RTE) food that supports the growth of Listeria monocytogenes. Among other studies, FBOs can rely on the conclusion drawn by microbiological challenge tests. A microbiological challenge test consists in the artificial contamination of a food with a pathogen microorganism and aims at simulating its behaviour during processing and distribution under the foreseen storage and handling conditions. A number of documents published by international health authorities and research institutions describes how to conduct challenge studies. The authors reviewed the existing literature and described the methodology for implementing such laboratory studies. All the main aspects for the conduction of L. monocytogenes microbiological challenge tests were considered, from the selection of the strains, preparation and choice of the inoculum level and method of contamination, to the experimental design and data interpretation. The objective of the present document is to provide an exhaustive and practical guideline for laboratories that want to implement L. monocytogenes challenge testing on RTE food.

Genipin cross-linked nanocomposite films for the immobilization of antimicrobial agent

Cellulose nanocrystal (CNC) reinforced chitosan based antimicrobial films were prepared by immobilizing nisin on the surface of the films. Nanocomposite films containing 18.65 μg/cm(2) of nisin reduced the count of L. monocytogenes by 6.73 log CFU/g, compared to the control meat samples (8.54 log CFU/g) during storage at 4 °C in a Ready-To-Eat (RTE) meat system. Film formulations containing 9.33 μg/cm(2) of nisin increased the lag phase of L. monocytogenes on meat by more than 21 days, whereas formulations with 18.65 μg/cm(2) completely inhibited the growth of L. monocytogenes during storage. Genipin was used to cross-link and protect the activity of nisin during storage. Nanocomposite films cross-linked with 0.05% w/v genipin exhibited the highest bioactivity (10.89 μg/cm(2)) during the storage experiment, as compared to that of the un-cross-linked films (7.23 μg/cm(2)). Genipin cross-linked films were able to reduce the growth rate of L. monocytogenes on ham samples by 21% as compared to the un-cross-linked films. Spectroscopic analysis confirmed the formation of genipin-nisin-chitosan heterocyclic cross-linked network. Genipin cross-linked films also improved the swelling, water solubility, and mechanical properties of the nanocomposite films.

Use of lysozyme, nisin, and EDTA combined treatments for maintaining quality of packed ostrich patties

The antimicrobial effectiveness of lysozyme, nisin, and ethylene diamine tetraacetic acid (EDTA) combination treatments (Mix(1): 250 ppm lysozyme, 250 ppm nisin, 5 mM EDTA; Mix(2): 500 ppm lysozyme, 500 ppm nisin, 5 mM EDTA) on bacterial growth of ostrich patties packaged in air, vacuum, and 2 different modified atmospheres (MAP(1): 80% O(2), 20% CO(2); MAP(2): 5% O(2), 30% CO(2), 65% N(2)) was evaluated. Moreover, the lipid oxidation was evaluated as well as color and sensory characteristics. The growth of total viable counts and lactic acid bacteria were strongly inhibited by the antimicrobial treatments in all the running time (Inhibition Index >97%) whereas for Enterobacteriaceae and Pseudomonas spp. lower inhibition indices from 12% to about 28% were observed. The lipid oxidation was more pronounced in the control respect to the treated meat patties. Moreover, the mixture at low concentration of lysozyme and nisin showed the best antioxidative effect. High concentrations of lysozyme and nisin showed the greatest color loss. Also, off-odors for the untreated patties developed faster than the treated samples.

PRACTICAL APPLICATION

Great interest is developing in food bio-preservation, because of the ever-increasing needs to protect consumers' health and to valorize the naturalness and safety of food products.

Control of Listeria monocytogenes on Cooked Cured Ham by Formulation with a Lactate-Diacetate Blend and Surface Treatment with Lauric Arginate

Ready-to-eat (RTE) meat products have been identified as a significant source of listeriosis in humans in the United States. Meat processors in the United States are required to use one of three alternatives to control L. monocytogenes in RTE meats: (i) a postlethality inactivation treatment along with a L. monocytogenes growth inhibitor; (ii) a postlethality inactivation treatment or a growth inhibitor; or (iii) sanitation measures and intensive testing. Lauric arginate (LAE) has been proposed as an effective postlethality inactivation treatment. The present study was conducted to investigate the antimicrobial effect of a lactate-diacetate blend in the formulation combined with surface application of LAE on cooked cured ham inoculated with L. monocytogenes, vacuum packaged, and stored at 4°C for up to 90 days. The treatments evaluated were (i) control ham with no added antimicrobials (control); (ii) ham formulated with 1.68% potassium lactate and 0.12% sodium diacetate (PLSD); (iii) control ham with 0.07% LAE as a surface treatment (LAE); and (iv) ham formulated with PLSD and LAE surface treatment (sprayed in bag and distributed across meat surface during vacuum packing) (PLSD+LAE). Use of only LAE as a surface treatment resulted in an initial 1-log CFU/g reduction in levels of L. monocytogenes on ham; however, this reduction only delayed the growth of the pathogen to 8 log CFU/g by 12 days when compared with the control ham without added antimicrobials. Use of PLSD in the formulation of ham resulted in a complete inhibition of L. monocytogenes throughout storage. The combination of PLSD in the formulation and a surface treatment with LAE resulted in an initial 0.7-log CFU/g reduction of the pathogen on ham and complete inhibition of the pathogen at the reduced level throughout storage. Formulation of ham with a lactate-diacetate blend combined with lauric arginate as a surface treatment will allow RTE meat processors to effectively achieve alternative 1 status, as designated by the U.S. Department of Agriculture Food Safety and Inspection Service, in their facilities.

Microbial antagonists to food-borne pathogens and biocontrol

Application of natural antimicrobial substances (such as bacteriocins) combined with novel technologies provides new opportunities for the control of pathogenic bacteria, improving food safety and quality. Bacteriocin-activated films and/or in combination with food processing technologies (high-hydrostatic pressure, high-pressure homogenization, in-package pasteurization, food irradiation, pulsed electric fields, or pulsed light) may increase microbial inactivation and avoid food cross-contamination. Bacteriocin variants developed by genetic engineering and novel bacteriocins with broader inhibitory spectra offer new biotechnological opportunities. In-farm application of bacteriocins, bacterial protective cultures, or bacteriophages, can decrease the incidence of food-borne pathogens in livestock, animal products and fresh produce items, reducing the risks for transmission through the food chain. Biocontrol of fungi, parasitic protozoa and viruses is still a pending issue.