Listeria monocytogenes inhibition by whey protein films and coatings incorporating lysozyme

The effects of whey protein isolate (WPI) films and coatings incorporating lysozyme (LZ) on the inhibition of Listeria monocytogenes both in and on microbial media, as well as on cold-smoked salmon, were studied. The antimicrobial effects of LZ were examined using various growth media by turbidity and plate counting tests. Disc-covering and disc-surface-spreading tests were also used to evaluate the effects of WPI films incorporating LZ. Smoked salmon was used as a model food to test the antimicrobial effects of WPI coatings incorporating LZ, both initially and during storage at 4 and 10 degrees C for 35 days. Tensile properties (elastic modulus, tensile strength, and percentage of elongation), oxygen permeability, and color (Hunter L, a, and b) of WPI films with and without LZ were also compared. LZ inhibited L. monocytogenes in broth and on agar media. The number of cells surviving after LZ treatments depended on the type of media. WPI films incorporating 204 mg of LZ per g of film (dry basis) inhibited the growth of a preparation of 4.4 log CFU/cm2 L. monocytogenes. WPI coatings prepared with 25 mg of LZ per g of coating solution initially inactivated more than 2.4, 4.5, and 3.0 log CFU/g of L. monocytogenes, total aerobes, and yeasts and molds in smoked salmon samples, respectively. The WPI coatings incorporating LZ efficiently retarded the growth of L. monocytogenes at both 4 and 10 degrees C. The anti-L. monocytogenes effect of LZ-WPI coating was more noticeable when the coating was applied before inoculation than when the coating was applied after inoculation. Significantly higher elastic modulus values and lower percentage of elongation and oxygen permeability values were measured with the WPI films incorporating LZ than with the plain WPI films.     

Development of a defatted mustard meal-based composite film and its application to smoked salmon to retard lipid oxidation

An antioxidant composite film was developed from defatted mustard meal (DMM) without incorporating external antioxidants and its applicability as a coating to smoked salmon has been investigated. Film-forming variables included the concentration of xanthan, heat treatment, and high pressure homogenisation. Tensile strength, elongation, water vapour permeability, and water solubility of composite films of DMM and xanthan were 5.1–8.8 MPa, 2.9–5.0%, 1.5–2.4 g mm/kPa/h/m², and 1.1–1.6%, respectively. The composite film with xanthan at 5% (w/w) demonstrated antioxidant properties and the coating with the solution forming the composite film (5% xanthan) retarded lipid oxidation and significantly reduced volatile changes in smoked salmon during storage at 4, 10, and 20 °C for 21 days. Coated salmon was preferred to uncoated salmon in glossiness and fish smell. The composite coating improved the stability of smoked salmon against lipid oxidation without imparting a negative sensory quality to the salmon.     

Effectiveness and stability of plastic films coated with nisin for inhibition of Listeria monocytogenes

Plastic films were coated with a cellulose-based carrier solution containing nisin, a natural antimicrobial peptide with the potential to inhibit growth of food spoilage and pathogenic microorganisms such as Listeria monocytogenes. Five commercial plastic films with different chemical compositions and surface properties were compared in this study: low-density polyethylene, ethylene-vinyl acetate copolymer, and three types of ethylene-methacrylic acid copolymers: Surlyn 1601, Nucrel 0403, and Nucrel 0903. The films were coated with nisin at a concentration of 1000 IU/cm2. Nisin-coated films were stored at room temperature (21 degrees C) and at 4 degrees C and analyzed weekly for 12 weeks. Antimicrobial activity of the different nisin-coated films against a nisin indicator strain, Lactococcus lactis subsp. cremoris ATCC 14365, and against L. monocytogenes ATCC 19115 was assessed using an inhibition zone assay. Nisin incorporated into the films was recovered by a boiling and extraction procedure, and its activity was quantified using an agar well diffusion assay. Film type did not have any significant effect on the antimicrobial activity of the nisin-coated films (P < 0.05); all five film types had comparable inhibition zones on both assays. The films maintained stable activity for the duration of the study, both at room temperature and refrigeration. The results of this study demonstrate that commercially available packaging films can be coated with nisin and the resulting antimicrobial films can be conveniently stored at room temperature with no adverse effect on nisin activity.     

Synergistic inhibition of Listeria monocytogenes by nisin and garlic extract

Minimum Inhibitory Concentrations (MICs) were determined for nisin and an aqueous garlic extract (AGE) for six strains of Listeria monocytogenes, including a nisin-resistant mutant. When used in combination in broth a synergistic effect was observed between nisin and AGE in all strains. Increasing the pH reduced the activity of both nisin and garlic. During storage of the aqueous garlic extract at 4°C and at 20°C for 14 days, the MIC increased over the first 8 days but remained stable thereafter, indicating at least two antimicrobial components, one of which was relatively stable. Aqueous garlic extract was not appreciably bactericidal although there was a positive interaction with nisin in terms of its bactericidal effect in broth at 4°C. The effect of combining nisin and garlic in a food system, hummus, was also studied. Sub-MIC combinations of AGE and nisin were effective at preventing listerial growth and did enhance the slight bactericidal effect of nisin. The results indicate that combined use of nisin with garlic could help overcome problems of nisin resistance in Gram-positive organisms. Such interactions could also be a significant factor in traditional lactic fermented foods where garlic is an ingredient.     

Effectiveness of chitosan-coated plastic films incorporating antimicrobials in inhibition of Listeria monocytogenes on cold-smoked salmon

The objective of this study was to evaluate the efficacy of chitosan-coated plastic films incorporating five Generally Recognized as Safe (GRAS) antimicrobials (nisin, sodium lactate (SL), sodium diacetate (SD), potassium sorbate (PS) and sodium benzoate (SB)) against Listeria monocytogenes on cold-smoked salmon. Salmon samples were surface-inoculated with a five-strain cocktail of L. monocytogenes and packaged in chitosan-coated plastic films containing 500 IU/cm(2) of nisin, 9 mg/cm(2) of SL, 0.5 mg/cm(2) of SD, 0.6 mg/cm(2) of PS, or 0.2 mg/cm(2) of SB, and stored at room temperature (ca. 20 degrees C) for 10 days. The film incorporating SL was the most effective, completely inhibiting the growth of L. monocytogenes during 10 days of storage. L. monocytogenes in samples packaged in the other four antimicrobial films grew, but the increase in counts was lower than the control. The antilisterial efficacy of films containing lower concentrations of SL (2.3 mg/cm(2) and 4.5 mg/cm(2)) and binary combinations SL, PS, SD, SB and nisin were subsequently evaluated. Among all the treatments, chitosan-coated plastic films with 4.5 mg/cm(2) SL, 4.5 mg/cm(2) SL-0.6 mg/cm(2) PS and 2.3 mg/cm(2) SL-500 IU/cm(2) nisin were the most effective. These three most effective antimicrobial films were then tested at refrigerated temperature. They completely inhibited the growth of L. monocytogenes on smoked salmon for at least 6 weeks. Chitosan-coated plastic films containing 4.5 mg/cm(2) SL can potentially assist the smoked-salmon processing industry in their efforts to control L. monocytogenes.     

Defatted mustard seed meal-based biopolymer film development

Edible films were developed from a defatted mustard seed meal (Sinapis alba) (DMM), a byproduct from the bio-fuel industry. Films were formed by casting DMM suspensions (3-g DMM/100-g suspension) that were treated by high-pressure homogenization (HPH, 138 MPa), ultrasound (400 W, 30 min), or gamma irradiation (10 or 20 kGy), and mixed with glycerol and soy lecithin. Rheological properties, water vapor permeability, water solubility, tensile strength (TS), percentage elongation (%E), elastic modulus (EM), color, and structural properties of film-forming suspensions or films were determined. Films were successfully produced using the HPH-processed suspension with 0.6% glycerol. Rheology results indicated the polymer network structure of the film-forming suspension was loosened by HPH, but tightened by heating at 90 °C. The ranges for the properties of WVP, WS, TS, %E, and EM of the films were 3.4-5.0 g mm/kPa h m², 30.3-34.4%, 1.3-5.5 MPa, 0.9-18.1%, 33.2-294.7 MPa, respectively. L, a, and b by CIELAB coordinates were 73.3-77.9, 0.4-3.5, and 29.5-45.7, respectively. HPH increased TS and %E of the films and decreased EM, whereas the ultrasound and the 20 kGy-irradiation treatments increased %E and decreased EM. The TS and EM decreased and E% increased with increasing glycerol and soy lecithin. DMM is a promising material to produce edible biopolymer films and coatings for food packaging.     

Edible films with anti‐Listeria monocytogenes activity

Edible films based on wheat gluten, gelatin and brea gum were prepared with incorporation of ca. 500 UA cm^?2 of enterocin with anti‐Listeria monocytogenes action, synthesised by Enterococcus faecium CRL1385. The analyses of the different edible films revealed that they did not show any significant changes in their functional and physicochemical features after the enterocin incorporation (P value < 0.05). Gelatin and brea gum films had a higher solubility in water than wheat gluten films. Listeria cell growth were not inhibited by edible matrices per se; while a similar inhibition to that of free enterocin was observed in gelatin films after 2 h and remained unchanged for the duration of the assay. Wheat gluten films showed a lower enterocin–matrix interaction and a limited bactericidal action was observed for 2 h, after which the activity disappeared. In contrast, brea gum was found to interact with the enterocin molecule inhibiting its anti‐Listeria activity. Edible gelatin and wheat gluten films with enterocin can be potentially used to control Listeria monocytogenes contamination in food products.     

Inhibition of Listeria monocytogenes on bologna sausages by an antimicrobial film containing mustard extract or sinigrin

The ability of Listeria (L.) monocytogenes to convert glucosinolates into antimicrobial isothiocyanates was investigated. Mustard glucosinolates in pure (sinigrin) or extract forms (sinigrin, oriental; sinalbin, yellow mustard) were used in broth media and in a polyvinyl polyethylene glycol graft copolymer (PPG) packaging film with bologna to examine their value as antimicrobial precursors for the control of L. monocytogenes viability and extension of bologna shelf-life at 4 °C. During broth tests with deodorized (myrosinase-inactivated) mustard extracts (10 d at 20 °C) or with purified sinigrin (21 d at 20 °C) L. monocytogenes was only inhibited when exogenous myrosinase was added. None the less, the organism was able to hydrolyze almost half the pure sinigrin by 21 d in tests without added enzyme. Reductions in sinigrin levels were measured by reversed-phase liquid chromatography, and in the absence of L. monocytogenes or added myrosinase the glucosinolate was stable. When pure sinigrin, oriental or yellow mustard extracts were incorporated in PPG films containing 3, 5 and 6% (w/w) of the corresponding glucosinolate and used to package bologna inoculated with 4 log CFU/g L. monocytogenes, the pathogen became undetectable in bologna packed with the oriental mustard extract at 52 d storage and remained undetectable at 70 d. The yellow mustard extract was less inhibitory and the pure sinigrin was not antimicrobial. L. monocytogenes numbers reached >7 log CFU/g in the film and untreated controls at 17 d storage. At 35 d storage, samples packed with control film contained sufficient numbers of lactic acid bacteria (LAB) (>7 log CFU/g) to be considered spoiled, whereas treatments containing mustard or sinigrin remained <7 log CFU/g LAB for ≤ 70 d. L. monocytogenes played a key role in exerting control over its own viability in bologna by hydrolysis of the glucosinolate in the oriental mustard film, but other antimicrobials in treatments may have contributed.     

Competitive inhibition of Listeria monocytogenes in ready-to-eat meat products by lactic acid bacteria

Forty-nine strains of lactic acid bacteria (LAB), isolated from commercially available ready-to-eat (RTE) meat products, were screened for their ability to inhibit the growth of Listeria monocytogenes at refrigeration (5 degrees C) temperatures on agar spot tests. The three most inhibitory strains were identified as Pediococcus acidilactici, Lactobacillus casei, and Lactobacillus paracasei by 16S rDNA sequence analysis. Their antilisterial activity was quantified in associative cultures in deMan Rogosa Sharpe (MRS) broth at 5 degrees C for 28 days, resulting in a pathogen reduction of 3.5 log10 cycles compared to its initial level. A combined culture of these strains was added to frankfurters and cooked ham coinoculated with L. monocytogenes, vacuum packaged, and stored at 5 degrees C for 28 days. Bacteriostatic activity was observed in cooked ham, whereas bactericidal activity was observed in frankfurters. Numbers of L. monocytogenes were 4.2 to 4.7 log10 and 2.6 log10 cycles lower than controls in frankfurters and cooked ham, respectively, after the 28-day refrigerated storage. In all cases, numbers of LAB increased by only 1 log10 cycle. The strain identified as P. acidilactici was possibly a bacteriocin producer, whereas the antilisterial activity of the other two strains was due to the production of organic acids. There was no significant difference (P > 0.05) in the antilisterial activity detected in frankfurters whether the LAB strains were used individually or as combined cultures. Further studies over a 56-day period indicated no impact on the quality of the product. This method represents a potential antilisterial intervention in RTE meats, because it inhibited the growth of the pathogen at refrigeration temperatures without causing sensory changes.     

PCR-焦磷酸测序检测单核细胞增生李斯特氏菌研究

目的 建立结合PCR-焦磷酸测序检测单核细胞增生李斯特氏菌的方法.方法 根据单核细胞增生李斯特氏菌的hly基因设计扩增引物和测序引物,特异地扩增目的片段,再制备单链模板,在测序引物引导下进行焦磷酸测序,通过测序结果与GenBank中的hly基因序列的比对进行鉴定.结果 扩增引物和测序引物表现出良好的特异性,16株单核细胞增生李斯特氏菌菌株均扩增出大小249 bp的DNA片段,焦磷酸测序结果与hly基因序列100％匹配,而阴性对照菌株均未扩增出DNA条带,焦磷酸测序结果为阴性.结论 建立的方法特异性高,是快速从DNA序列水平上检测单核细胞增生李斯特氏菌的有效手段.     

Control of Listeria monocytogenes on ham steaks by antimicrobials incorporated into chitosan-coated plastic films

Contamination of ready-to-eat (RTE) meat products such as ham steaks with Listeria monocytogenes has been a concern for the meat processing industry. The objective of this study was to evaluate the antilisterial efficacy of chitosan-coated plastic films alone or incorporating five generally recognized as safe (GRAS) antimicrobials. Effect of chitosan-coated plastic film on the growth of L. monocytogenes was first investigated in an aqueous system of culture medium broth and chitosan-coated films were able to inhibit the growth of L. monocytogenes in a concentration-dependent manner. However, chitosan-coated plastic films were not able to control the growth of L. monocytogenes on ham steaks. Therefore, five GRAS antimicrobials were subsequently incorporated into chitosan-coated plastic films to enhance their antilisterial effectiveness. Ham steaks were surface-inoculated with a five-strain cocktail of L. monocytogenes and then packaged in chitosan-coated plastic films containing 500 IU/cm(2) of nisin, 0.01 g/cm(2) of sodium lactate (SL), 0.0025 g/cm(2) of sodium diacetate, 0.003 g/cm(2) of potassium sorbate (PB), or 0.001 g/cm(2) of sodium benzoate (SB). The samples were stored at room temperature (ca. 20 degrees C) for 10 days. Incorporating antimicrobials into chitosan-coated plastic films slowed down or inhibited the growth of L. monocytogenes. The chitosan-coated plastic film containing SL was the most effective antimicrobial film and its efficacy against L. monocytogenes on ham steaks was evaluated during 12-week storage at 4 degrees C. The film showed excellent long-term antilisterial effect with the counts of L. monocytogenes being slightly lower than the initial inoculum. Chitosan-coated plastic films containing 0.001 g/cm(2) of SL have a potential to be used on ham steaks to control L. monocytogenes.     

Synergistic inhibition of Listeria monocytogenes on cold-smoked rainbow trout by nisin and sodium lactate

The inhibition of Listeria monocytogenes and mesophilic aerobic bacteria in cold-smoked rainbow trout by nisin, sodium lactate or their combination was studied. Nisin (4000-6000 IU/ml), sodium lactate (60%) or their combination (1:1) were injected into rainbow trout at an industrial scale before the smoking process, or injected into the finished smoked product. Both types of fish samples were smoked, sliced and vacuum-packed according to normal practice in the plant. Packages were opened and L. monocytogenes was inoculated (10(3)-10(4) log colony forming units (cfu)/g) onto the fish samples, which were then vacuum packed again. Samples were stored at 8 degrees C for 17 days or at 3 degrees C for 29 days. Listeria and mesophilic aerobic bacteria counts were measured once a week. The effects of treatments on sensory characteristics and storage stability were also analyzed. Both nisin and lactate inhibited the growth of L. monocytogenes in smoked fish, but the combination of the two compounds was even more effective. The combination of nisin and sodium lactate injected into smoked fish decreased the count of L. monocytogenes from 3.26 to 1.8 log cfu/g over 16 days of storage at 8 degrees C. The level of L. monocytogenes remained almost constant (4.66-4.92 log cfu/g) for 29 days at 3 degrees C in the samples injected before smoking and which contained both nisin and sodium lactate. The treatments did not affect the sensory characteristics of cold-smoked rainbow trout. Based on a triangle test, the sensory quality of all test samples remained unchanged for 23 days of storage at 3 degrees C, whereas the control fish prepared without additives or additional salt remained unchanged only for 16 days.     

Inactivation of Listeria monocytogenes on ham and bologna using pectin-based apple, carrot, and hibiscus edible films containing carvacrol and cinnamaldehyde

Edible films can be used as wrapping material on food products to reduce surface contamination. The incorporation of antimicrobials into edible films could serve as an additional barrier against pathogenic and spoilage microorganisms that contaminate food surfaces. The objective of this study was to investigate the antimicrobial effects of carvacrol and cinnamaldehyde, incorporated into apple, carrot, and hibiscus-based edible films against Listeria monocytogenes on contaminated ham and bologna. Ham or bologna samples were inoculated with L. monocytogenes and dried for 30 min, then surface wrapped with edible films containing the antimicrobials at various concentrations. The inoculated, film-wrapped samples were stored at 4 °C. Samples were taken at day 0, 3, and 7 for enumeration of surviving L. monocytogenes by plating on appropriate media. Carvacrol films showed better antimicrobial activity than cinnamaldehyde films. Compared to control films without antimicrobials, films with 3% carvacrol induced 1 to 3, 2 to 3, and 2 to 3 log CFU/g reductions on ham and bologna at day 0, 3, and 7, respectively. Corresponding reductions with 1.5% carvacrol were 0.5 to 1, 1 to 1.5, and 1 to 2 logs, respectively. At day 7, films with 3% cinnamaldehyde reduced L. monocytogenes population by 0.5 to 1.5 and 0.5 to 1.0 logs on ham and bologna, respectively. Inactivation by apple films was greater than that by carrot or hibiscus films. Apple films containing 3% carvacrol reduced L. monocytogenes population on ham by 3 logs CFU/g on day 0 which was 1 to 2 logs greater than that by carrot and hibiscus films. Films were more effective on ham than on bologna. The food industry and consumers could use these films to control surface contamination by pathogenic microorganisms. PRACTICAL APPLICATION: Antimicrobial edible, food-compatible film wraps prepared from apples, carrots, and hibiscus calyces can be used by the food industry to inactivate Listeria monocytogenes on widely consumed ready to eat meat products such as bologna and ham. This study provides a scientific basis for large-scale application of edible fruit- and vegetable-based antimicrobial films on foods to improve microbial food safety.     

Inactivation of Listeria monocytogenes by Ultraviolet Energy

Short-wave UV-energy (100 μW/cm²) decreased the number of Listeria monocytogenes on Tryptose Agar (TA) ca. seven orders of magnitude in 4 min. Age of culture (48 vs 24 hr) did not alter the sensitivity of Listeria to this UV treatment. Increasing the intensity of irradiation (550 vs 100 μW/cm²) increased the rate at which L. monocytogenes was inactivated. Dry rather than moist cells of L. monocytogenes were most resistant to irradiation. In general, inactivation of Listeria with short-wave UV-energy yielded sigmoidal survivor curves. The population of Listeria on TA plates was not affected with long-wave UV-irradiation.     

Assessing biofilm formation by Listeria monocytogenes strains

When a microtitre plate assay was used to quantify biofilm production by Listeria monocytogenes strains following growth in Tryptone Soy Broth (TSB) for 48 h at 20 degrees C, 127 of 138 strains (92.0%) were classified as weak, 9 of 138 strains (6.5%) as moderate and only 2 of 138 strains (1.5%) as strong biofilm formers. The strains included environmental, animal, food (persistent and sporadic strains) and clinical isolates previously typed using esterase electrophoresis (ESE) and multi-locus enzyme electrophoresis (MEE). Strains from different sources produced similar quantities of biofilm, whereas biofilm production by ESE type II strains, irrespective of source, was greater than that observed for other ESE types. No correlation between MEE type and biofilm production was observed. A Petri dish assay which allowed parallel quantification and microscopic examination of biofilms was used to examine biofilm formation by selected L. monocytogenes strains during growth in TSB for 14 days at 20 degrees C. Results from these assays showed that following prolonged incubation, some L. monocytogenes strains categorized as weak biofilm formers by the 48 h microtitre assay, were able to form biofilms similar in terms of quantity and structure to those produced by strains classified as strong or medium biofilm formers. Results from 14-day Petri dish assays confirmed 48 h microtitre assays regarding greater biofilm production by ESE type II strains compared to other ESE types of L. monocytogenes. Biofilm production was similar for ESE type II persistent and sporadic food isolates but reduced for ESE type II clinical strains.     

Growth inhibition of Listeria monocytogenes by a bacteriocin of Pediococcus acidilactici M during fermentation of kimchi

Four lactic acid bacteria known to produce bacteriocins were evaluated for the inhibitory effect against Listeria monocytogenes strain Scott A. Lactococcus lactis subsp. lactis biovar diacetylactis 26-2 was the least antilisterial, whereas Pediococcus acidilactici M resulted in the largest zone of inhibition on an agar medium. Leuconostoc paramesenteroides OX and Lactobacillus sake Lb 706 were intermediate. Aqueous solutions of crude bacteriocin powders derived from supernatant fluids of L. sake Lb 706 and P. acidilactici M. cultures inhibited the growth of Lactobacillus plantarum UGA8 in MRS agar. The addition of 10 mg of crude bacteriocin powder from L. sake to 150 g of kimchi fermented at 14 ± 1°C for 16 days had no effect on the growth of L. monocytogenes. The same amount of crude bacteriocin derived from P. acidilactici, however, had an initial lethal effect on L. monocytogenes and controlled growth of the pathogen throughout the 16-day fermentation. The successful application of bacteriocins to control the growth of L. monocytogenes in lightly fermented foods such as kimchi would appear to be achievable.     

可食膜对裹衣花生氧化抑制作用的研究

采用大豆分离蛋白、壳聚糖和豌豆淀粉为成膜基材,制备出不同的可食膜。测定了涂抹不同可食膜对衰衣花生贮藏期间过氧化值含量的影响。研究表明,可食膜有较强的抑制裹衣花生贮藏周期内发生氧化的能力。65℃下贮藏25d后,与对照组相比,可食膜液处理的裹衣花生过氧化值含量均降低到0．2g／100g以下,可有效延长裹衣花生的贮藏期。     

单增李斯特氏菌MALDI-TOF-MS鉴定与分型研究

为建立单增李斯特氏菌的基质辅助激光解吸电离飞行时间质谱(MALDI-TOF-MS)快速鉴定与分型方法,实验收集37株单增李斯特氏菌分离株,应用MALDI-TOF-MS采集图谱,获取独特的蛋白质指纹图谱,汇总成标准图谱,建立单增李斯特氏菌鉴定数据库。采用单增李斯特氏菌标准菌株进行验证,表明鉴定结果的可信度很高。在数据库信息的基础上,对37株单增李斯特氏菌分离株进行聚类分型。分型结果表明,在蛋白质水平上,MALDI-TOF-MS可把37株单增李斯特氏菌分成9个型别。     

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.     

Edible chitosan films on ready-to-eat roast beef for the control of Listeria monocytogenes

The use of chitosan as an edible film was evaluated for its antimicrobial activity against Listeria monocytogenes (LM) on the surface of ready-to-eat (RTE) roast beef. L. monocytogenes, decimally diluted to give an initial inoculation of >6.50logCFU/g, was inoculated onto the surface of RTE roast beef cubes, and air-dried. The samples were dipped into chitosan (high or low molecular weights) solutions dissolved with acetic or lactic acid at 0.5% (w/v) or 1% (w/v) then bagged and refrigerated at 4 degrees C. The bacterial counts were determined on days 0, 7, 14, 21, and 28. The samples were spread plated onto modified Oxford agar plates and incubated at 37 degrees C for 48h. An initial 6.50logCFU/g of L. monocytogenes inoculated onto the surface of the non-coated RTE roast beef increased too >10logCFU/g by day 28. On day 14, L. monocytogenes counts were significantly different for all the chitosan-coated samples from the control counts by 2-3logCFU/g and remained significantly different on day 28. Our results have shown that the acetic acid chitosan coating were more effective in reducing L. monocytogenes counts than the lactic acid chitosan coating. Our study indicated that chitosan coatings could be used to control L. monocytogenes on the surface of RTE roast beef.