Efficacy of disinfectants to reduce Listeria monocytogenes on precut iceberg lettuce

The efficacy of water, chlorinated water (100 ppm), peracetic acid solution (0.05%), and commercial citric acid-based produce wash (0.25%) to reduce the population of Listeria monocytogenes on precut lettuce was tested. Samples were inoculated with a mixture of equal amounts of five L. monocytogenes strains at a level of 4.7 log CFU/g, and analyzed on the day of washing and after 3 and 6 days of storage at 6 degrees C. Sanitizer reduced the number of L. monocytogenes at maximum 1.7 log CFU/g and number of L. monocytogenes reached the inoculation level during 6 days of storage. Thus, disinfectants do not eliminate L. monocytogenes on precut lettuce and cannot be solely relied on in producing precut lettuce safely. The inoculated L. monocytogenes strains were recovered at different rates after 6 days of storage; one of these strains was not recovered at all. Thus, strain-specific differences exist in the ability of L. monocytogenes to survive the washing treatments of the lettuce.     

Enhancing the bactericidal effect of electrolyzed water on Listeria monocytogenes biofilms formed on stainless steel

Biofilms are potential sources of contamination to food in processing plants, because they frequently survive sanitizer treatments during cleaning. The objective of this research was to investigate the combined use of alkaline and acidic electrolyzed (EO) water in the inactivation of Listeria monocytogenes biofilms on stainless steel surfaces. Biofilms were grown on rectangular stainless steel (type 304, no. 4 finish) coupons (2 by 5 cm) in a 1:10 dilution of tryptic soy broth that contained a five-strain mixture of L. monocytogenes for 48 h at 25 degrees C. The coupons with biofilms were then treated with acidic EO water or alkaline EO water or with alkaline EO water followed by acidic EO water produced at 14 and 20 A for 30, 60, and 120 s. Alkaline EO water alone did not produce significant reductions in L. monocytogenes biofilms when compared with the control. Treatment with acidic EO water only for 30 to 120 s, on the other hand, reduced the viable bacterial populations in the biofilms by 4.3 to 5.2 log CFU per coupon, whereas the combined treatment of alkaline EO water followed by acidic EO water produced an additional 0.3- to 1.2-log CFU per coupon reduction. The population of L. monocytogenes reduced by treatments with acidic EO water increased significantly with increasing time of exposure. However, no significant differences occurred between treatments with EO water produced at 14 and 20 A. Results suggest that alkaline and acidic EO water can be used together to achieve a better inactivation of biofilms than when applied individually.     

茉莉酸甲酯处理对鲜切莴苣和甘蓝苯丙烷代谢的影响

本文以鲜切莴苣和鲜切甘蓝为实验材料,采用10μmol/L茉莉酸甲酯(以10％甲醇为溶剂)浸泡处理,研究了处理后鲜切莴苣和鲜切甘蓝苯丙烷代谢的变化.结果表明,MeJA处理可有效提高鲜切莴苣和鲜切甘蓝苯丙氨酸解氨酶(PAL)活性,同时,次生代谢产物如总酚、类黄酮以及木质素的含量也有所增加 由此可见,茉莉酸甲酯作为外界信号分子可诱导并激活苯丙烷代谢途径运行,合成多酚类化合物用以修复伤口,提高鲜切莴苣和甘蓝的自身修复能力与抗性.     

外源乙烯处理对鲜切莴苣和甘蓝苯丙烷代谢的影响

为阐明外源乙烯对鲜切叶菜类蔬菜苯丙烷代谢的影响,本实验以莴苣和甘蓝为材料,经清洗与切割后,采用外源乙烯处理鲜切莴苣和鲜切甘蓝,研究了乙烯对苯丙烷代谢过程中关键酶—苯丙氨酸解氨酶(PAL)活性的影响,同时,还分析了主要次生代谢物质,总酚、类黄酮和木质素的含量变化。结果表明:外源乙烯处理对鲜切莴苣和鲜切甘蓝组织中PAL活性具有明显抑制作用,降低了总酚和类黄酮含量,促进了木质素合成而使含量有所上升,鲜切莴苣对乙烯作用的敏感性显著大于鲜切甘蓝(p<0.05)。      

茉莉酸甲酯处理对鲜切莴苣和甘蓝苯丙烷代谢的影响

本文以鲜切莴苣和鲜切甘蓝为实验材料,采用10μmol/L茉莉酸甲酯(以10%甲醇为溶剂)浸泡处理,研究了处理后鲜切莴苣和鲜切甘蓝苯丙烷代谢的变化。结果表明,MeJA处理可有效提高鲜切莴苣和鲜切甘蓝苯丙氨酸解氨酶(PAL)活性,同时,次生代谢产物如总酚、类黄酮以及木质素的含量也有所增加。由此可见,茉莉酸甲酯作为外界信号分子可诱导并激活苯丙烷代谢途径运行,合成多酚类化合物用以修复伤口,提高鲜切莴苣和甘蓝的自身修复能力与抗性。      

Electron beam and gamma irradiation effectively reduce Listeria monocytogenes populations on chopped romaine lettuce

Fresh, chopped romaine lettuce contaminated with a seven-strain cocktail of Listeria monocytogenes (in a solution containing approximately 10(8) organisms per ml) that had attained a level of contamination of between 7 and 8 log CFU/g was packaged in 15-g samples. The lettuce was irradiated with a Co60 source at 1.15 or 0.51 kGy and then stored at 4 degrees C. In addition, samples contaminated with isolated strains 16397, 0733, and 1992 were subjected to either electron beam irradiation at doses ranging from 0.3 to 1.2 kGy or gamma irradiation at 0.56 kGy without subsequent refrigerated storage. All postirradiation and control samples were diluted with Butterfield's phosphate buffer and plated in duplicate on modified Oxford media. Samples that received electron beam or gamma irradiation without subsequent refrigerated storage were also plated in duplicate on modified Oxford media plates coated with two 7-ml layers of basal yeast extract agar. Electron beam irradiation yielded D10-values (the dose required to eliminate 90% of the microbial population) of 0.16, 0.17, and 0.19 kGy for strains 16397, 0733, and 1992, respectively. The corresponding log reductions obtained for these same three strains at 0.56 kGy of gamma irradiation were 2.91, 2.62, and 2.66 log, respectively. Gamma irradiation at 1.15 and 0.51 kGy with subsequent refrigerated storage (4 degrees C) reduced populations by > 5 and > 2 log, respectively, compared with controls. Neither the irradiated samples nor the control samples showed increases in population during the storage periods. Our results indicate that low-dose irradiation can effectively reduce or eliminate L. monocytogenes on chopped romaine lettuce, improving the safety of ready-to-eat salads.     

两种抗菌剂对不同温度下形成的单增李斯特菌生物膜的清除作用

该研究考察单增李斯特菌在32℃和10℃下生物膜的形成,并以1/2、1、2、4 MIC(minimum inbibitory concentration,最低抑菌浓度)的薰衣草精油和84消毒液处理单增李斯特菌的生物膜,以结晶紫染色法、二甲氧唑黄法(XTT法)及叠氮溴化丙锭-荧光定量PCR(PMA-qPCR)等方法评估其清除效果。研究结果表明,单增李斯特菌在两种温度下均能形成稳定的生物膜,32℃下形成的生物膜量显著高于10℃; 4种浓度薰衣草精油处理3 h对32℃下形成的生物膜的清除率为62%～82%,对10℃的生物膜的清除率为25%～56%; 4种浓度84消毒液处理3 h对32℃和10℃下形成的生物膜的清除率分别为47%～78%和36%～56%; 4 MIC(6. 4%体积分数)的薰衣草精油处理3 h后10℃生物膜的残余活菌数为32℃生物膜的2倍以上。该研究阐明了单增李斯特菌的低温生物膜抗性更强,薰衣草精油对生物膜的清除效果优于84消毒液,对植物精油的应用以及控制单增李斯特菌残留具有积极的意义。     

Efficiency of different sanitation methods on Listeria monocytogenes biofilms formed under various environmental conditions

The resistance of Listeria monocytogenes biofilms formed under food processing conditions, against various sanitizing agents and disinfection procedures was evaluated in the present study. The first sanitation procedure included biofilm formation on stainless steel coupons (SS) placed in tryptic soy broth supplemented with 0.6% yeast extract (TSBYE) of various concentrations of NaCl (0.5, 7.5 and 9.5%) at different temperatures (5 and 20 °C). The biofilms formed were exposed to warm (60 °C) water for 20 min, or to peroxyacetic acid (2% PAA) for 1, 2, 3 and 6 min. Treatment with warm water caused no significant (P ≥ 0.05) reductions in the attached populations. Conversely, surviving bacteria on SS coupons decreased as the exposure time to 2% PAA increased and could not be detected by culture after 6 min of exposure. Biofilms formed at 20°C were more resistant to PAA than biofilms formed at 5 °C. Salt concentration in the growth medium had no marked impact on the resistance to PAA. The second sanitation procedure included biofilm formation of nonadapted (NA) and acid-adapted (AA) cells in TSBYE of pH 5.0 and 7.0 (i.e., NA-5.0, NA-7.0 and AA-5.0, AA-7.0) at 4 °C. Coupons bearing attached cells of L. monocytogenes were periodically exposed to chlorine (0.465% Cl(-)), quaternary ammonium compound (1% QAC) and 2% PAA. The resistance of attached cells to QAC, PAA and Cl(-) followed the order: AA-5.0>NA-7.0 ≥ AA-7.0>NA-5.0. The most effective sanitizer was QAC followed by PAA and Cl(-). The results can lead to the development of efficient sanitation strategies in order to eliminate L. monocytogenes from the processing environment. Furthermore, such results may explain the presence of L. monocytogenes after sanitation as a result of cell attachment history.     

The effect of physical and chemical treatments on the esterase activity from Pseudomonas fragi CRDA 037

The effect of different treatments on the esterase activity of the cellular debris of Pseudomonas fragi, responsible for the production of natural fruity flavors in processed products, was investigated. Glass bead homogenization (GBH) decreased activity by 50%, French press homogenization (FPH) produced only a slight decrease, while ultrasonication (US) alone or with GBH increased activity by 25 and 50%, respectively. Treatment with (i) GBH (4 min) and Triton increased activity two-fold, (ii) GBH or US (4 min), Triton and EDTA produced little effect, (iii) FPH (three passes), Triton and EDTA produced a decrease of 30 to 50%, (iv) FP (three passes), US (4 min), Triton and EDTA had no effect, while (v) GBH (4 min), US (4 min), Triton and EDTA showed a 35% decrease in activity. The esterase activity of the cellular debris stored in buffer (4°C) and hexane (−80°C) decreased by 80% after 3 days, while that of whole cells stored in hexane (−80°C), and buffer at 4 and −80°C remained preserved for 3 days whereas that of the lyophilized whole cells greatly decreased. The esterase activity of whole cells stored in buffer containing 15 and 30% glycerol was preserved for three weeks.     

组合处理对双菌种生物被膜的清除和杀灭效果

该研究建立了食源性病原菌（液化沙雷菌S1和腐生葡萄球菌S2）双菌种生物被膜研究模型,采用微孔板法和MTT法检测单一、组合处理方式对双菌种生物被膜的清除和杀灭效果。研究发现,采用24孔板37 ℃于TSB培养基中培养24 h可获得稳定的液化沙雷菌-腐生葡萄球菌双菌种生物被膜,组合处理方式优于单一处理效果,处理顺序的不同也会影响生物被膜的清除和杀灭效果,1% NaOH处理20 min再用0.3% NaClO处理20 min清除效果最好,双菌种生物被膜的量由1.11（OD595 nm）减少至0.08（OD595 nm）,杀灭率达95.73%。     

Influence of drying treatments on the physical and chemical properties of cucumber

This work aimed at determining the properties of cucumber exposed to convective air drying and freeze-drying. The samples were analysed in terms of colour, texture, chemical properties (moisture content, fibre, ash, vitamin C and sugars), phenolic compounds and antioxidant activity in fresh and after drying. The trials in the convective chamber were done at 40 and 60 °C, in the drying tunnel at 60 °C and in the freeze dryer at ?50 °C. The results showed that the antioxidant activity and the phenolic compounds were not affected by any of the drying treatments tested, since the values were quite similar in the fresh product as compared to the dried cucumbers. With respect to colour, the freeze drying treatment was identified as the one originating less colour change, when compared with the colour of the fresh product. Finally, texture was less affected by drying in the chamber at 40 °C and freeze drying.     

Combined effect of ultrasound and organic acids to reduce Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on organic fresh lettuce

This study was performed to compare the effectiveness of individual treatments (ultrasound and organic acids) and their combination on reducing foodborne pathogens on organic fresh lettuce. Lettuce leaves were inoculated with a cocktail of three strains each of Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes and treated with ultrasound (40 kHz) alone, organic acids (0.3, 0.5, 0.7, 1.0, and 2.0% — malic acid, lactic acid, and citric acid) alone and combined with ultrasound and organic acids for 5 min. For all 3 pathogens, the combined treatment of ultrasound and organic acids resulted in additional 0.8 to 1.0 log reduction compared to individual treatments, without causing significant quality change (color and texture) on lettuce during 7 day storage. The maximum reductions of E. coli O157:H7, S. Typhimurium, and L. monocytogenes were 2.75, 3.18, and 2.87 log CFU/g observed after combined treatment with ultrasound and 2% organic acid for 5 min, respectively. Our results suggest that the combined treatment of ultrasound with organic acids was effective at increasing pathogen reduction compared to individual treatments without significantly affecting quality, and demonstrates its potential as a novel method to increase the microbial safety on organic fresh lettuce.     

Reduction of Campylobacter jejuni on chicken wings by chemical treatments

Eight chemicals, including glycerol monolaurate, hydrogen peroxide, acetic acid, lactic acid, sodium benzoate, sodium chlorate, sodium carbonate, and sodium hydroxide, were tested individually or in combination for their ability to inactivate Campylobacter jejuni at 4 degrees C in suspension. Results showed that treatment for up to 20 min with 0.01% glycerol monolaurate, 0.1% sodium benzoate, 50 or 100 mM sodium chlorate, or 1% lactic acid did not substantially (< or = 0.5 log CFU/ml) reduce C. jejuni populations but that 0.1 and 0.2% hydrogen peroxide for 20 min reduced C. jejuni populations by ca. 2.0 and 4.5 log CFU/ml, respectively. By contrast, treatments with 0.5, 1.0, 1.5, and 2.0% acetic acid, 25, 50, and 100 mM sodium carbonate, and 0.05 and 0.1 N sodium hydroxide reduced C. jejuni populations by >5 log CFU/ml within 2 min. A combination of 0.5% acetic acid plus 0.05% potassium sorbate or 0.5% acetic acid plus 0.05% sodium benzoate reduced C. jejuni populations by >5 log CFU/ml within 1 min; however, substituting 0.5% lactic acid for 0.5% acetic acid was not effective, with a reduction of C. jejuni of <0.5 log CFU/ml. A combination of acidic calcium sulfate, lactic acid, ethanol, sodium dodecyl sulfate, and polypropylene glycol (ACS-LA) also reduced C. jejuni in suspension by >5 log CFU/ml within 1 min. All chemicals or chemical combinations for which there was a >5-log/ml reduction of C. jejuni in suspension were further evaluated for C. jejuni inactivation on chicken wings. Treatments at 4 degrees C of 2% acetic acid, 100 mM sodium carbonate, or 0.1 N sodium hydroxide for up to 45 s reduced C. jejuni populations by ca. 1.4, 1.6, or 3.5 log CFU/g, respectively. Treatment with ACS-LA at 4 degrees C for 15 s reduced C. jejuni by >5 log CFU/g to an undetectable level. The ACS-LA treatment was highly effective in chilled water at killing C. jejuni on chicken and, if recycled, may be a useful treatment in chill water tanks for poultry processors to reduce campylobacters on poultry skin after slaughter.     

Behavior of Listeria monocytogenes inoculated on cantaloupe surfaces and efficacy of washing treatments to reduce transfer from rind to fresh-cut pieces

Attachment and survival of Listeria monocytogenes on external surfaces (rind) of inoculated cantaloupe, resistance of the surviving bacteria to chlorine or hydrogen peroxide treatments, transfer of the pathogen from unsanitized and sanitized rinds to fresh-cut tissues during cutting and growth, and survival of L. monocytogenes on fresh-cut pieces of cantaloupe were investigated. Surface treatment with 70% ethanol to reduce the native microflora on treated melon, followed by immersion in a four-strain cocktail of L monocytogenes (10(8) CFU/ml) for 10 min, deposited 4.2 log10 CFU/cm2 and 3.5 log10 CFU/cm2 of L monocytogenes on treated and untreated cantaloupe rinds, respectively. L. monocytogenes survived on the treated or untreated cantaloupe rinds for up to 15 days during storage at 4 and 20 degrees C, but populations declined by approximately 1 to 2 log10 CFU/cm2. Fresh-cut pieces prepared from inoculated whole cantaloupes stored at 4 degrees C for 24 h after inoculation were positive for L. monocytogenes. Washing inoculated whole cantaloupes in solutions containing 1,000 ppm of chlorine or 5% hydrogen peroxide for 2 min at 1 to 15 days of storage at 4 degrees C after inoculation resulted in a 2.0- to 3.5-log reduction in L. monocytogenes on the melon surface. Fresh-cut pieces prepared from the sanitized melons were negative for L. monocytogenes. After direct inoculation onto fresh-cut pieces, L. monocytogenes survived, but did not grow, during 15 days of storage at 4 degrees C. Growth was evident by 4 h of storage at 8 and 20 degrees C. It is concluded that sanitizing with chlorine or hydrogen peroxide has the potential to reduce or eliminate the transfer of L. monocytogenes on melon surfaces to fresh-cut pieces during cutting.     

Considerations for post-lethality treatments to reduce Listeria monocytogenes from fully cooked bologna using ambient and pressurized steam

During processing of ready-to-eat (RTE) deli meats, any secondary processing procedures such as peeling and cutting introduce the distinct possibility of cross-contamination between equipment, personnel, and food. To eliminate or reduce pathogens such as Listeria monocytogenes and ensure food safety, RTE deli meats can be pasteurized prior to or after packaging. In this study, ambient steam in-package pasteurization was compared with pressurized steam prepackaging pasteurization to reduce L. monocytogenes from fully cooked RTE bologna. The bologna (14 cm diameter×1.5 cm thickness) samples were surface-inoculated to contain about 8 log₁₀ of L. monocytogenes. To achieve 2 log reductions for L. monocytogenes, the bologna samples needed to be treated for about 10 s in pressurized steam at 131 °C or for about 2.5 min in ambient steam at 100 °C. The pasteurization time using pressurized steam treatment was about 75–90% shorter than using ambient steam treatment. Pressurized steam treatment may be integrated into a vacuum packaging unit to effectively eradicate L. monocytogenes from RTE meats just prior to sealing the retail packages to further reduce the treatment time, avoid post-treatment recontaminations by pathogens, and improve food safety without detrimentally affecting meat quality.     

The effect of physical and chemical treatments on the degradation of wheat and barley straws by rumen liquor-pepsin and pepsin-cellulase systems

Samples of wheat and barley straw, milled through a 1 mm sieve, were ball milled to <0.25 mm as a representative physical treatment, or delignified by the acidchlorite procedure as a representative chemical treatment. The original and treated samples were examined using in-vitro digestion systems based on rumen liquor-pepsin and pepsin-cellulase. The digestibility was determined as well as the extent of digestion of the cellulose, hemicelluloses and lignin. The rumen liquor-pepsin system was able to digest a greater proportion of the original and treated straws than the pepsin-cellulase system while ball milling improved the digestibility more than delignification, irrespective of the digestion system. Core lignin was not digested by either system. Ball milling improved the digestibility of the cellulose fraction to a greater extent than the hemicellulose fraction, whereas delignification had a greater effect on hemicellulose digestion than on cellulose digestion. There were also differences in the rate of removal of the different sugar residues present in the hemicellulose fraction.     

Growth of Listeria monocytogenes on iceberg lettuce and solid media

The growth of pathogenic bacterium Listeria monocytogenes on fresh-cut iceberg lettuce under constant temperatures was modelled in order to investigate microbial safety during distribution of this vegetable. We examined the effects of several incubation temperatures, ranging from 5 to 25 degrees C, on bacterial growth. These data were fitted to the Baranyi model and the curves showed a high correlation coefficient at all temperature (R2 > 0.95). In addition, the native bacterial flora of the lettuce did not affect the growth rate of L. monocytogenes regardless of incubation temperature. However, the lag time was reduced at a ratio of native bacteria to inoculated L. monocytogenes (100:1) at low incubation temperatures (5 and 10 degrees C). Furthermore, the maximum population density (MPD) was increased at a low ratio of native to inoculated L. monocytogenes (1:1) at all incubation temperatures. These results were compared with the previous work published by [Buchanan, R.L., Stahl, H.G., Whiting, R.C., 1989. Effects and interactions of temperature, pH, atmosphere, sodium chloride, and sodium nitrite on the growth of Listeria monocytogenes. J. Food Prot. 52, 844-851] that is being developed at the US Department of Agriculture (USDA) Agricultural Research Service's Pathogen Modelling Program (PMP). The broth-based Buchanan model for L. monocytogenes was found to markedly deviate from the observed data. In order to investigate this discrepancy, we examined the effects of medium environment and nutrient content on L. monocytogenes growth using tryptic soy agar plates (TSAP) and agar plates (AP) containing 1.7% sucrose. The inoculated bacteria on both TSAP and AP showed slower growth rates than that predicted by the PMP. The MPD of bacteria grown on TSAP was similar to the PMP model ( approximately 9 log10 CFU/ml or plate (circle of diameter of 90 mm)) regardless of the incubation temperature. By contrast, the MPD observed on AP was approximately 4 log10 CFU lower than that observed on TSAP or predicted by the PMP. Both the growth rate and the MPD of L. monocytogenes on AP were similar to those on lettuce. These results suggest that the solid medium and poor nutrient content inhibited the growth of L. monocytogenes on lettuce. The growth rates of the inoculated L. monocytogenes on all media were described using Ratkowsky's simple square root model.     

Effects of different sanitizing treatments on biofilms and attachment of Escherichia coli and Listeria monocytogenes on green leaf lettuce

The effects of ozone (2 mg/L), chlorine (100 mg/L) and organic acid (0.25 g/100 g citric acid plus 0.50 g/100 g ascorbic acid) treatments at 10 °C for 2 min on the removal of Escherichia coli and Listeria monocytogenes cells embedded inside biofilms on the surface of lettuce leaves were studied. None of the sanitizing treatments were found effective in removing the bacterial biofilms. Initiation of biofilms was observed after 24 h of incubation. Bacterial cells appeared as individual cells, rather than clusters after 6 h incubation, thus 99.9% reductions in both E. coli and L. monocytogenes counts were achieved with all the three treatments. However, after 48 h incubation, none of the treatments resulted in higher than 90% reduction in microbial counts. Biofilm formation was demonstrated for the 48 h incubated samples with SEM images.     

Combined washing effect of noni extract and oregano essential oil on the decontamination of Listeria monocytogenes on romaine lettuce

Combined washing effect of noni extract (NE) and oregano essential oil (OE) on the decontamination of Listeria monocytogenes on romaine lettuce was examined. The fractional inhibitory concentration index (FICI) between NE and three different essential oils (cinnamon leaf, thyme and OE) was determined. Fractional inhibitory concentration index results indicated that OE only had an additive effect (FICI = 1). Noni extract/oregano essential oil combined washing resulted in the greatest reduction in viable cell numbers (3.42 log CFU g⁻¹) relative to non-washed samples. Furthermore, the combined washing was more effective than sodium hypochlorite washing in terms of reducing the microbial load (>1.44-log reduction). Washing treatment did not change the surface colour or total phenolic content in lettuce samples. These results suggest that NE/OE combined washing is applicable as a novel decontamination treatment to ensure the microbial safety of romaine lettuce without changing the quality.