Use of phenolic compounds for sensitizing Listeria monocytogenes to high-pressure processing

Three Listeria monocytogenes strains (Scott A, OSY-8578, and OSY-328) that differ considerably in barotolerance were grown to stationary phase and suspended individually in phosphate buffer (pH 7.0). Twelve phenolic compounds, including commercially used food additives, were screened for the ability to sensitize L. monocytogenes to high-pressure processing (HPP). Each L. monocytogenes strain was exposed to each of the 12 phenolic compounds (100 ppm each) for 60 min; this was followed by a pressure treatment at 400 MPa for 5 min. Six phenolic compounds increased the efficacy of HPP against L. monocytogenes but tert-butylhydroquinone (TBHQ) was the most effective. The additives alone at 100 ppm were not lethal for L. monocytogenes. Subsequently, the three L. monocytogenes strains were exposed to TBHQ before or after pressure treatments at 400 or 500 MPa for 5 min. When TBHQ was added after the pressure treatment, the combined treatment was more lethal than was pressure alone. However, the lethality attributable to TBHQ was greater when the additive was applied before rather than after pressure treatment. The inactivation kinetics of the L. monocytogenes strains at 300, 500, and 700 MPa, in the presence or absence of TBHQ, was investigated. All survivor plots showed non-linear inactivation kinetics, but tailing behavior was most pronounced when HPP was used alone. Combinations of TBHQ and HPP eliminated tailing behavior when survivors were monitored by direct plating or an enrichment procedure. Pressure and phenolic additives are apparently a potent bactericidal combination against L. monocytogenes.     

Pressure death and tailing behavior of Listeria monocytogenes strains having different barotolerances

The objectives of this study were to investigate the variability among Listeria monocytogenes strains in response to high-pressure processing, identify the most resistant strain as a potential target of pressure processing, and compare the inactivation kinetics of pressure-resistant and pressure-sensitive strains under a wide range (350 to 800 MPa) of pressure treatments. The pressure resistance of Listeria innocua and nine strains of L. monocytogenes was compared at 400 or 500 MPa and 30 degrees C. Significant variability among strains was observed. The decrease in log CFU/ml during the pressure treatment was from 1.4 to 4.3 at 400 MPa and from 3.9 to >8 at 500 MPa. L. monocytogenes OSY-8578 exhibited the greatest pressure resistance, Scott A showed the greatest pressure sensitivity, and L. innocua had intermediate resistance. On the basis of these findings, L. monocytogenes OSY-8578 is a potential target strain for high-pressure processing efficacy studies. The death kinetics of L. monocytogenes Scott A and OSY-8578 were investigated at 350 and 800 MPa. Survivors at 350 MPa were enumerated by direct plating, and survivors at 800 MPa were enumerated by the most-probable-number technique. Both pressure-resistant and pressure-sensitive strains exhibited non-first-order death behavior, and excessive pressure treatment did not eliminate the tailing phenomenon.     

Inhibition of Listeria monocytogenes in cooked ham through active packaging with natural antimicrobials and high-pressure processing

Enterocins A and B and sakacin K at 200 and 2,000 activity units (AU)/cm2, nisin at 200 AU/cm2, 1.8% potassium lactate, and a combination of 200 AU/cm2 of nisin and 1.8% lactate were incorporated into interleavers, and their effectiveness against Listeria monocytogenes spiked in sliced, cooked ham was evaluated. Antimicrobial-packaged cooked ham was then subjected to high-pressure processing (HPP) at 400 MPa. In nonpressurized samples, nisin plus lactate-containing interleavers were the most effective, inhibiting L. monocytogenes growth for 30 days at 6 degrees C, with counts that were 1.9 log CFU/g lower than in the control after 3 months. In the other antimicrobial-containing interleavers, L. monocytogenes did not exhibit a lag phase and progressively grew to levels of about 8 log CFU/g. HPP of actively packaged ham slices reduced Listeria populations about 4 log CFU/g in all batches containing bacteriocins (i.e., nisin, sakacin, and enterocins). At the end of storage, L. monocytogenes levels in the bacteriocin-containing batches were the lowest, with counts below 1.51 log CFU/g. In contrast, HPP moderately reduced L. monocytogenes counts in the control and lactate batches, with populations gradually increasing to about 6.5 log CFU/g at the end of storage.     

Elimination of Listeria monocytogenes biofilms by ozone, chlorine, and hydrogen peroxide

This study evaluated the efficacy of ozone, chlorine, and hydrogen peroxide to destroy Listeria monocytogenes planktonic cells and biofilms of two test strains, Scott A and 10403S. L. monocytogenes was sensitive to ozone (O3), chlorine, and hydrogen peroxide (H2O2). Planktonic cells of strain Scott A were completely destroyed by exposure to 0.25 ppm O3 (8.29-log reduction, CFU per milliliter). Ozone's destruction of Scott A increased when the concentration was increased, with complete elimination at 4.00 ppm O3 (8.07-log reduction, CFU per chip). A 16-fold increase in sanitizer concentration was required to destroy biofilm cells of L. monocytogenes versus planktonic cells of strain Scott A. Strain 10403S required an ozone concentration of 1.00 ppm to eliminate planktonic cells (8.16-log reduction, CFU per milliliter). Attached cells of the same strain were eliminated at a concentration of 4.00 ppm O3 (7.47-log reduction, CFU per chip). At 100 ppm chlorine at 20 degrees C, the number of planktonic cells of L. monocytogenes 10403S was reduced by 5.77 log CFU/ml after 5 min of exposure and by 6.49 log CFU/ml after 10 min of exposure. Biofilm cells were reduced by 5.79 log CFU per chip following exposure to 100 ppm chlorine at 20 degrees C for 5 min, with complete elimination (6.27 log CFU per chip) after exposure to 150 ppm at 20 degrees C for 1 min. A 3% H2O2 solution reduced the initial concentration of L. monocytogenes Scott A planktonic cells by 6.0 log CFU/ml after 10 min of exposure at 20 degrees C, and a 3.5% H2O2 solution reduced the planktonic population by 5.4 and 8.7 log CFU/ml (complete elimination) after 5 and 10 min of exposure at 20 degrees C, respectively. Exposure of cells grown as biofilms to 5% H2O2 resulted in a 4.14-log CFU per chip reduction after 10 min of exposure at 20 degrees C and in a 5.58-log CFU per chip reduction (complete elimination) after 15 min of exposure.     

Screening for Listeria monocytogenes surrogate strains applicable to food processing by ultrahigh pressure and pulsed electric field

Ultrahigh pressure (UHP) and pulsed electric field (PEF) are emerging processing technologies developed to enhance the safety while maintaining the fresh-like quality of food. For each food and process combination, a pathogen of concern (i.e., target pathogen) must be determined, and a low-risk microorganism that serves as the pathogen surrogate for process validation must be identified. The objective of this study was to identify a surrogate for Listeria monocytogenes for UHP and PEF process validation. Potential surrogates tested include four Lactobacillus spp., a Pediococcus sp., and a Listeria innocua strain. These were compared with nine L. monocytogenes strains, with regard to sensitivity to UHP and PEF processing. For UHP treatment, the strains were suspended in citrate-phosphate buffer (pH 7.0 or 4.5), sweet whey, or acidified whey and pressure processed at 500 MPa for 1 min. For PEF treatment, the strains were suspended in NaCl solution, acid whey, or sweet whey and processed at 25 kV/cm. The lethality of UHP or PEF treatment varied considerably, depending on medium types and pH and the treated strain. Treating the tested microorganisms with UHP inactivated 0.3 to 6.9 log CFU/ml for L. monocytogenes strains and 0.0 to 4.7 log CFU/ml for the potential surrogates. When PEF was employed, populations of tested microorganisms decreased < 1.0 to 5.3 log CFU/ml. L. monocytogenes V7 and OSY-8578 were among the most resistant strains to UHP and PEF treatments, and thus are candidate target strains. Lactobacillus plantarum ATCC 8014 demonstrated similar or greater resistance compared with the target organisms; therefore, the bacterium is proposed as a surrogate of L. monocytogenes for both processes under the conditions specified in the food matrices tested in this study.     

Survival of Listeria monocytogenes Scott A on vacuum-packaged raw beef treated with polylactic acid, lactic acid, and nisin

Low-molecular-weight polylactic acid (LMW-PLA) and lactic acid (LA) were used to inhibit growth of Listeria monocytogenes Scott A on vacuum-packaged beef. Nisin was also used simultaneously as an additional hurdle to the growth of this pathogen. Inoculated beef cubes were immersed in a solution of 2% LMW-PLA, 2% LA, 400 IU/ml of nisin, or combinations of each acid and nisin for 5 min and drip-dried for 15 min. The cubes were then vacuum-packaged and stored at 4 degrees C for up to 42 days. Surface pH values of beef cubes treated with 2% LMW-PLA, the combination of 400 IU/ml of nisin and 2% LMW-PLA (2% NPLA), or 400 IU/ml of nisin alone were significantly reduced from 5.59 to 5.18, 5.01, and 5.19, respectively, whereas those decontaminated with 2% LA or 400 IU/ml of nisin and 2% LA (2% NLA) were significantly decreased from 5.59 to 4.92 and 4.83, respectively, at day 0 (P < or = 0.05). The 2% LMW-PLA, 2% LA, 2% NPLA, 2% NLA, and 400 IU/ml of nisin showed immediate bactericidal effects on L. monocytogenes Scott A (1.22-, 1.56-, 1.57-, 1.94-, and 1.64-log10 reduction, respectively) compared with the initial number of 5.33 log10 CFU/cm2 of the untreated control at day 0 (P < or = 0.05). These treatments, combined with vacuum-packaging and refrigeration temperature, succeeded to inhibit growth of L. monocytogenes during storage up to 42 days. At the end of 42 days, the numbers of L. monocytogenes Scott A remaining viable on these samples were 1.21, 0.36, 2.21, 0.84, and 0.89 log10 CFU/cm2, respectively.     

Synergy between nisin and select lactates against Listeria monocytogenes is due to the metal cations

Listeria monocytogenes, a major foodborne pathogen, has been responsible for many outbreaks and recalls. Organic acids and antimicrobial peptides (bacteriocins) such as nisin are produced by lactic acid bacteria and are commercially used to control pathogens in some foods. This study examined the effects of lactic acid (LA) and its salts in combination with a commercial nisin preparation on the growth of L. monocytogenes Scott A and its nisin-resistant mutant. Because of an increase in its activity at a lower pH, nisin was more active against L. monocytogenes when used in combination with LA. Most of the salts of LA, including potassium lactate, at up to 5% partially inhibited the growth of L. monocytogenes and had no synergy with nisin. Zinc and aluminum lactate, as well as zinc and aluminum chloride (0.1%), worked synergistically with 100 IU of nisin per ml to control the growth of L. monocytogenes Scott A. No synergy was observed when zinc or aluminum lactate was used with nisin against nisin-resistant L. monocytogenes. The nisin-resistant strain was more sensitive to Zn lactate than was wild-type L. monocytogenes Scott A; however, the cellular ATP levels of the nisin-resistant strain were not significantly affected. Changes in the intracellular ATP levels of the wild-type strain support our hypothesis that pretreatment with zinc lactate sensitizes cells to nisin. The similar effects of thesalts of hydrochloric and lactic acids support the hypothesis that metal cations are responsible for synergy with nisin.     

Fate of Listeria monocytogenes inoculated onto the surface of model Turkey frankfurter pieces treated with zein coatings containing nisin, sodium diacetate, and sodium lactate at 4 degrees C

The antimicrobial effects of zein coatings containing nisin, sodium lactate, and sodium diacetate against Listeria monocytogenes on turkey frankfurters at 4 degrees C were determined. Our objectives were to determine whether zein, nisin, lactate, and diacetate alone or in combination could control the growth of L. monocytogenes on full-fat turkey frankfurters at 4 degrees C and to determine whether lactate or diacetate had any synergistic effect on the activity of nisin. Turkey frankfurter pieces surface inoculated with L. monocytogenes strain V7 were treated with zein-ethanol-glycerol (ZEG), zein-propylene-glycol (ZPR), ethanol-glycerol (EG), propylene glycol (PR), nisin (N), sodium lactate (L), or sodium diacetate (D) alone or in combination. Over 28 days, treatment with N or D alone reduced L. monocytogenes counts on frankfurters by 6.6 or 6.3 log CFU/g, respectively. N-D treatment reduced L. monocytogenes by 6 log CFU/g. The zein solvents EG and PR reduced L. monocytogenes by about 5.6 and 5.2 log CFU/g, respectively, similar to the results obtained with ZEG and ZPR, which suggests that zein powder per se had no antimicrobial activity. After 28 days, ZEG-N-D, ZEG-N-D-L, ZPR-N-D, and ZPR-N-D-L yielded no detectable CFU. L alone was ineffective. No synergies were observed. N and D when used singly and the combinations of N-D, ZEG-N-D, ZEG-N-D-L, ZPR-N-D, ZPR-N-D-L, EG, and PR were effective as inhibitors of the growth of recontaminating L. monocytogenes cells on full-fat turkey frankfurters.     

Efficacy of nisin in combination with protective cultures against Listeria monocytogenes Scott A in tofu.

Nisin can be used as a biopreservative to control growth of Listeria monocytogenes in various minimally processed foods. Tofu is an example of a non-fermented soybean product, which may allow growth of Listeria at refrigeration temperatures and in which nisin may be applied to prevent multiplication of Listeria. The efficacy of nisin against Listeria may be compromised by the emergence of spontaneous nisin-resistant mutants. Exposure of L. monocytogenes Scott A to nisin in a culture medium or in a food product results in an initial reduction of Listeria population which is followed by regrowth of survivors to nisin during further incubation. In vitro studies using Standard I Nutrient broth showed that Enterococcus faecium BFE 900-6a and Lactobacillus sakei Lb 706-1a used as protective cultures in combination with nisin were able to suppress proliferation of Listeria cells not killed by nisin at 10 degrees C. Growth and bacteriocin production of these two strains and a third protective culture, Lactococcus lactis BFE 902 was also observed in soymilk and tofu at 10 degrees C. Inoculation studies with tofu prepared with nisin and protective cultures showed that lower amounts of nisin are required for an effective inhibition of L. monocytogenes Scott A when either E. faecium BFE 900-6a or Lc. lactis BFE 902 are used in addition. The combination of nisin with these bacteriocinogenic lactic acid bacteria (LAB) resulted in a complete suppression of listerial growth in homemade tofu stored at 10 degrees C for 1 week. Lb. sakei Lb 706-1a was less effective and did not prevent a slight increase of L. monocytogenes Scott A numbers during storage.     

Transmission electron microscopy study of Listeria monocytogenes treated with nisin in combination with either grape seed or green tea extract

The objective of this study was to use transmission electron microscopy to investigate the morphological changes that occurred in Listeria monocytogenes cells treated with grape seed extract (GSE), green tea extract (GTE), nisin, and combinations of nisin with either GSE or GTE. The test solutions were prepared with (i) 1% GSE, 1% GTE, 6,400 IU of nisin, and the combination of these dilutions with nisin or with (ii) the pure major phenolic constituents of GSE (0.02% epicatechin plus 0.02% catechin) or GTE (0.02% epicatechin plus 0.02% caffeic acid) and their combinations with 6,400 IU of nisin in tryptic soy broth with 0.6% yeast extract (TSBYE). Test solutions were inoculated with L. monocytogenes at approximately 10(6) CFU/ml and incubated for 3 or 24 h at 37 degrees C. After 3 h of incubation, cells were harvested and evaluated under a transmission electron microscope (JEOL-100 CX) operating at 80 kV (50,000X). Microscopic examination revealed an altered cell membrane and condensed cytoplasm when L. monocytogenes cells were exposed to a combination of nisin with either GSE or GTE or to pure compounds of the major phenolic constituents in combination. After 24 h of incubation at 37 degrees C, the combinations of nisin with GSE and nisin with GTE reduced the L. monocytogenes population to undetectable levels and 3.7 log CFU/ml, respectively. These observations indicate that the combination of nisin with either GSE or GTE had a synergistic effect, and the combinations of nisin with the major phenolic constituents were most likely associated with the L. monocytogenes cell damage during inactivation in TSBYE at 37 degrees C.     

Encapsulation of nisin and lysozyme in liposomes enhances efficacy against Listeria monocytogenes

The efficacy and stability against Listeria monocytogenes of nisin and lysozyme encapsulated in phospholipid liposomes was evaluated. Antimicrobial-containing liposomes were prepared by hydrating dried lipids with buffer containing nisin, nisin plus the fluorescence probe calcein, or calcein and lysozyme. Mixtures were then centrifuged and sonicated, and encapsulated liposomes were collected using size-exclusion chromatography. Antimicrobial concentration in liposomes was determined by bicinchoninic acid assay prior to determination of antimicrobial activity against strains of L. monocytogenes. When nisin was encapsulated in liposomes, protein concentrations of 0.39, 0.27, and 0.23 mg/ml for phosphatidylcholine (PC), PC-cholesterol (7:3), and PC-phosphatidylglycerol (PG)-cholesterol (5:2:3), respectively, were obtained. Encapsulation of nisin with calcein yielded protein concentrations of 0.35, 0.39, and 0.28 mg/ml for PC, PC-cholesterol, and PC-PG-cholesterol, respectively. Encapsulation of calcein with lysozyme resulted in protein concentrations of 0.43, 0.26, and 0.19 mg/ml for PC, PC-cholesterol, and PC-PG-cholesterol, respectively. Encapsulated nisin in 100% PC and PC-cholesterol liposomes inhibited bacterial growth by >2 log CFU/ml compared with free nisin. Growth inhibition with liposomal lysozyme was strain dependent, with greater inhibition observed for strains 310 and Scott A with PC-cholesterol and PC-PG-cholesterol liposomes. Inhibition of L. monocytogenes indicated the potential of liposomes to serve as delivery vehicles for antimicrobials in foods while improving stability of antimicrobials.     

Destruction of Listeria monocytogenes in sturgeon (Acipenser transmontanus) caviar by a combination of nisin with chemical antimicrobials or moderate heat

The objective of this study was to investigate the effect of nisin in combination with heat or antimicrobial chemical treatments (such as lactic acid, chlorous acid, and sodium hypochlorite) on the inhibition of Listeria monocytogenes and total mesophiles in sturgeon (Acipenser transmontanus) caviar. The effects of nisin (250, 500, 750, and 1,000 IU/ml), lactic acid (1, 2, and 3%), chlorous acid (134 and 268 ppm), sodium hypochlorite (150 and 300 ppm), and heat at 60 degrees C for 3 min were evaluated for a five-strain mixture of L. monocytogenes and total mesophiles in sturgeon caviar containing 3.5% salt. Selected combinations of these antimicrobial treatments were also tested. Injured and viable L. monocytogenes cells were recovered using an overlay method. Treating caviar with > or =500 IU/ml nisin initially reduced L. monocytogenes by 2 to 2.5 log units. Chlorous acid (268 ppm) reduced L. monocytogenes from 7.7 log units to undetectable (<0.48 log units) after 4 days of storage at 4 degrees C. However, there were no synergistic effects observed for combinations of nisin (500 or 750 IU/ml) plus either lactic acid or chlorous acid. Lactic acid caused a slight reduction (approximately 1 log unit) in the microbial load during a 6-day period at 4 degrees C. Sodium hypochlorite was ineffective at the levels tested. Mild heating (60 degrees C for 3 min) with nisin synergistically reduced viable counts of L. monocytogenes and total mesophiles. No L. monocytogenes cells (<0.48 log units) were recovered from caviar treated with heat and nisin (750 IU/ml) after a storage period of 28 days at 4 degrees C.     

Effect of salting and cold-smoking process on the culturability, viability, and virulence of Listeria monocytogenes strain Scott A

The aim of the present study was to determine the effect of the different steps of the cold-smoking process and vacuum storage on the culturability and viability of Listeria monocytogenes strain Scott A inoculated in sterile salmon samples. Additionally, the virulence of L. monocytogenes cells was assessed by intravenous inoculation of immunocompetent mice. Salmon (Salmo salar) portions were inoculated with L. monocytogenes at a level of 6 log CFU/g and were then dry salted (5.9%), smoked (0.74 mg phenol per 100 g), partially frozen (-7 degrees C), vacuum packed, and stored for 10 days at 4 degrees C followed by 18 days at 8 degrees C. Salting represented the only step of the process with a weak but significant listericidal effect (0.6 log reduction). Although the other processing steps had no immediate reduction effect on L. monocytogenes, the combination of steps significantly lowered by 1.6 log CFU/g the number of L. monocytogenes. The culturable count remained less than 7 log CFU/g until the end of the storage period, whereas in unprocessed samples (control) the culturable counts reached values up to 9 log CFU/g. To mimic a postprocess contamination, salmon portions were also inoculated with L. monocytogenes after being cold-smoke processed. A reduction of the culturable count during the 2 first weeks of storage was observed, but then growth occurred and identical values observed for preprocess contamination were reached at the end of the storage. A viable but nonculturable state transition of strain Scott A was not observed, and the cold-smoking process did not affect the virulence of bacteria isolated at the beginning and end of the storage.     

Enhancement of nisin, lysozyme, and monolaurin antimicrobial activities by ethylenediaminetetraacetic acid and lactoferrin

A microtiter plate assay was employed to systematically assess the interaction between ethylenediaminetetraacetic acid (EDTA) or lactoferrin and nisin, lysozyme, or monolaurin against strains of Listeria monocytogenes, Escherichia coli, Salmonella enteritidis, and Pseudomonas fluorescens. Low levels of EDTA acted synergistically with nisin and lysozyme against L. monocytogenes but EDTA and monolaurin interacted additively against this microorganism. EDTA synergistically enhanced the activity of nisin, monolaurin, and lysozyme in tryptic soy broth (TSB) against two enterohemorrhagic E. coli strains. In addition, various combinations of nisin, lysozyme, and monolaurin with EDTA were bactericidal to some gram-negative bacteria whereas none of the antimicrobials alone were bactericidal. Lactoferrin alone (2000 microg ml(-1)) did not inhibit any of the bacterial strains, but did enhance nisin activity against both L. monocytogenes strains. Lactoferrin in combination with monolaurin inhibited growth of E. coli O157:H7 but not E. coli O104:H21. While lactoferrin combined with nisin or monolaurin did not completely inhibit growth of the gram-negative bacteria, there was some growth inhibition. All combinations of EDTA or lactoferrin with antimicrobials were less effective in 2% fat UHT milk than in TSB. S. enteritidis and P. fluorescens strains were consistently more resistant to antimicrobial combinations. Resistance may be due to differences in the outer membrane and/or LPS structure.     

Reduction of Listeria monocytogenes and Escherichia coli O157:H7 numbers on vacuum-packaged fresh beef treated with nisin or nisin combined with EDTA.

Nisin or nisin combined with EDTA was used to treat fresh beef. Beef cubes (2.5 by 2.5 by 2.5 cm) that were inoculated with approximately 7 log CFU/ml of Listeria monocytogenes Scott A or Escherichia coli O157:H7 505 B were dipped in the following solutions: (i) H2O, (ii) HCl, (iii) nisin, (iv) EDTA, or (v) nisin combined with EDTA, respectively, for 10 min each, with an exception of one set of control beef samples without treatment. Beef samples were then drip-dried for 15 min, vacuum packaged, and stored at 4 degrees C for up to 30 days. The pH on beef after different treatments was not a key factor in preventing bacterial growth. Treatment with nisin or with nisin combined with EDTA reduced the population of L. monocytogenes by 2.01 and 0.99 log CFU/cm2 as compared to the control, respectively, under the conditions of vacuum package and storage at 4 degrees C for up to 30 days. However, the effect of nisin and nisin combined with EDTA against E. coli O157:H7 505 B was marginal at 1.02 log CFU/cm2 and 0.8 log CFU/cm2 reductions, respectively.     

Nisin treatment to enhance the efficacy of gamma radiation against Listeria monocytogenes on meat

Treatment of meat with gamma radiation for inactivation of foodborne pathogens might cause undesirable quality changes in the product. The objective of the present study was to use nisin for enhancing the lethality of gamma radiation against Listeria monocytogenes, so that moderate doses of radiation can effectively eliminate the pathogen on meat. Cubes of raw meat (10 g each) were inoculated with L. monocytogenes (10(7)CFU/g) and treated with nisin (10(3) IU/g), gamma radiation (0.25 to 1.5 kGy), or combinations of these treatments. Meat was analyzed for L. monocytogenes survivors immediately after treatment and during storage at 4 °C for up to 72 h. Nisin treatment alone inactivated L. monocytogenes by 1.2 log CFU/g. Gamma radiation caused dose-dependent inactivation of the pathogen. Treatment with combinations of nisin and gamma radiation resulted in an additive antimicrobial effect when inoculated meat was tested during the first 24 h and in a synergistic effect when tested after 72 h of storage at 4 °C. When L. monocytogenes was inoculated onto meat at low levels (4×10(3) CFU/g), treated with nisin (10(3) IU/g), and then irradiated (1.5 kGy) and stored at 4 °C for 72 h, the pathogen's most probable number was <0.03/g, indicating that such a combination is potentially effective in eliminating L. monocytogenes in meat.     

Antimicrobial effects of corn zein films impregnated with nisin, lauric acid, and EDTA

Bacterial growth during food transport and storage is a problem that may be addressed with packaging materials that release antimicrobials during food contact. In a series of five experiments, EDTA, lauric acid (LA), nisin, and combinations of the three antimicrobial agents were incorporated into a corn zein film and exposed to broth cultures of Listeria monocytogenes and Salmonella Enteritidis for 48 h (sampled at 2, 4, 8, 12, 24, and 48 h). Four experiments used starting cultures of 10(8) CFU/ml in separate experiments tested against each bacterium; the fifth experiment examined the inhibitory effect of selected antimicrobial agents on Salmonella Enteritidis with an initial inoculum of 10(4) CFU/ml. L. monocytogenes cell numbers decreased by greater than 4 logs after 48 h of exposure to films containing LA and nisin alone. No cells were detected for L. monocytogenes (8-log reduction) after 24-h exposure to any film combination that included LA. Of all film agent combinations tested, none had greater than a 1-log reduction of Salmonella Enteritidis when a 10(8)-CFU/ml broth culture was used. When a 10(4) CFU/ml of Salmonella Enteritidis initial inoculum was used, the films with EDTA and LA and EDTA, LA, and nisin were bacteriostatic. However, there was a 5-log increase in cells exposed to control within 24 h. The results demonstrate bacteriocidal and bacteriostatic activity of films containing antimicrobial agents.     

The effect of nisin on L. monocytogenes in Turkish fermented sausages (sucuks)

Turkish fermented sausage (sucuk) is a traditional, well-known meat product in Turkey. The aim of this study was to evaluate the effect of different nisin concentrations on Listeria monocytogenes in experimentally contaminated sucuk. Analyses were performed on at 0, 1, 3, 5, 7, 10, 15, 20, 25 and 30days for L. monocytogenes and other microbiological parameters (Total mesophilic aerobic bacteria and lactic acid bacteria) and physico-chemical parameters (pH, a(w) and moisture content).The sucuk dough was contaminated with L. monocytogenes ATCC 7644 at a concentration of 10(6)cfu/g, and the dough was divided into six equal groups. Each group was treated separately with different nisin concentrations (0, 5, 10, 25, 50 and 100μg/g). No L. monocytogenes surviving cells were detected in groups which contained 100μg/g and 50μg/g nisin at day 20 and 25, respectively (p<0.001). In conclusion, the inhibition of L. monocytogenes in sucuk increases with the increasing concentrations of nisin.     

Use of nisin-coated plastic films to control Listeria monocytogenes on vacuum-packaged cold-smoked salmon

Cold-smoked (Salmo salar) salmon samples were surface-inoculated with a cocktail of three nisin-resistant strains of L. monocytogenes (PSU1, PSU2 and PSU21) to a level of approximately 5 x 10(2) or 5 x 10(5) CFU/cm2 of salmon surface. The inoculated smoked salmon samples were vacuum-packaged with control film (no nisin) or nisin-coated plastic films and stored at either 4 or 10 degrees C. When the inoculated smoked salmon samples were packaged with film coated with 2000 IU/cm2 of nisin, a reduction of 3.9 log CFU/cm2 (compared with control) was achieved at either temperature for samples inoculated with 5 x 10(2) CFU/cm(2 of L. monocytogenes after 56 (4 degrees C) and 49 (10 degrees C) days of storage while reductions of 2.4 and 0.7 log CFU/cm2 were achieved for samples inoculated with a high level of L. monocytogenes (5 x 10(5) CFU/cm2) after 58 (4 degrees C) and 43 (10 degrees C) days, respectively. For samples packaged in film coated with 500 IU/cm2 of nisin, reductions of 0.5 and 1.7 log CFU/cm2 were achieved for samples inoculated with a low level of L. monocytogenes (5 x 10(2) CFU/cm2) after 56 (4 degrees C) and 49 (10 degrees C) days of storage while reductions of 1.8 and 0.8 log CFU/cm2 were achieved for samples inoculated with high level of L. monocytogenes after 58(4 degrees C) and 43 (10 degrees C) days, respectively. In addition, nisin inhibited the proliferation of background microbiota on smoked salmon in a concentration-dependent manner at both storage temperatures although the bacteriostatic effect was more pronounced at refrigeration temperature. This work highlights the potential for incorporating nisin into plastic films for enhancing the microbial safety of smoked salmon as well as controlling its microbial spoilage.