高压脉冲电场对绿茶饮料杀菌的研究

研究了高压脉冲电场对中性的绿茶饮料的杀菌效果，结果表明，当电场强度达到40kV／cm，处理时间达到120μs时，绿茶饮料中接种的大肠杆菌数量可降低5个对数级以上。此条件下生产的绿茶饮料样品，其茶多酚含量及色泽都无明显变化。     

高压脉冲电场对百合混汁灭菌的研究

研究了高压脉冲电场(PEF)对百合混汁的灭菌效果。操作过程中电场强度和处理时间是影响其灭菌效果的主要参数,随着电场强度的增大和处理时间的延长,灭菌效果越来越好。当电场强度达到30 kV/cm时,灭菌效果非常显著,处理时间为400μs时菌落总数已降低了4个对数值。经PEF处理后,百合混汁的理化指标没有明显的变化,可以保持其原有品质。     

Influence of Pulsed Electric Field on Escherichia coli and Saccharomyces cerevisiae

Heat sterilization destroys the color, smell, taste, and nutrients of food. Pulsed electric field can effectively kill microorganisms for many foods at room temperature without compromising sensory and nutritional quality. The effect of pulsed electric field on the sterilization rates of Escherichia coli and Saccharomyces cerevisiae was investigated. The fundamental mechanism of pulsed electric field sterilization on S. cerevisiae was briefly discussed. The inactivation of E. coli and S. cerevisiae increased as the electric field strength and treatment time increased. Pulsed electric field treatments at 35 kV/cm and 90 μs reduced S. cerevisiae and E. coli by 5.30 and 5.15 log numbers, respectively. The inactivation of E. coli and S. cerevisiae was not remarkable at increased conductivity and the same electric field strength. Concaves and holes in S. cerevisiae cell surface, protoplast deletion, overflow of substances from cells, and denatured DNA were observed. These results supported the hypothesis of “membrane perforation” to explain the mechanism underlying pulsed electric field sterilization.     

Inactivation of Escherichia coli in a Tropical Fruit Smoothie by a Combination of Heat and Pulsed Electric Fields

ABSTRACT: Moderate heat in combination with pulsed electric fields (PEF) was investigated as a potential alternative to thermal pasteurization of a tropical fruit smoothie based on pineapple, banana, and coconut milk, inoculated with Escherichia coli K12. The smoothie was heated from 25 °C to either 45 or 55 °C over 60 s and subsequently cooled to 10 °C. PEF was applied at electric field strengths of 24 and 34 kV/cm with specific energy inputs of 350, 500, and 650 kJ/L. Both processing technologies were combined using heat (45 or 55 °C) and the most effective set of PEF conditions. Bacterial inactivation was estimated on standard and NaCl‐supplemented tryptone soy agar (TSA) to enumerate sublethally injured cells. By increasing the temperature from 45 to 55 °C, a higher reduction in E. coli numbers (1 compared with 1.7 log₁₀ colony forming units {CFU} per milliliter, P < 0.05) was achieved. Similarly, as the field strength was increased during stand‐alone PEF treatment from 24 to 34 kV/cm, a greater number of E. coli cells were inactivated (2.8 compared with 4.2 log₁₀ CFU/mL, P < 0.05). An increase in heating temperature from 45 to 55 °C during a combined heat/PEF hurdle approach induced a higher inactivation (5.1 compared with 6.9 log₁₀ CFU/mL, respectively {P < 0.05}) with the latter value comparable to the bacterial reduction of 6.3 log₁₀ CFU/mL (P≥ 0.05) achieved by thermal pasteurization (72 °C, 15 s). A reversed hurdle processing sequence did not affect bacterial inactivation (P≥ 0.05). No differences were observed (P≥ 0.05) between the bacterial counts estimated on nonselective and selective TSA, suggesting that sublethal cell injury did not occur during single PEF treatments or combined heat/PEF treatments.     

Phage adsorption on Enteropathogenic and Shiga Toxin-Producing Escherichia coli strains: Influence of physicochemical and physiological factors

Bacteriophages have proved to be useful tools against pathogenic Escherichia coli strains. The first step in the phage life cycle is the adsorption to the host cell surface. In the present work, three Myoviridae phages (DT1, DT5 and DT6) were used to characterize the adsorption process on three pathogenic E. coli strains, namely two Shiga-toxin producing E. coli (STEC) and one enteropathogenic E. coli (EPEC), in several conditions found on food. The influence of Na⁺, Mg^2 +, temperature, pH, periodate, proteinase K and physiological cell state on phage adsorption was investigated. The three phages evaluated showed high adsorption rates at pH 7.5 and 5.7 while they were moderate at the lowest pH evaluated (4.5). Sodium or magnesium ions were not indispensable for the adsorption of the three phages evaluated. Specifically, phage particles were adsorbed either in the presence or absence of Mg^2 +, while increasing Na⁺ concentration resulted in lower adsorption values for all the systems evaluated. Regarding temperature, phage adsorption was slightly affected at 4 °C and 50 °C, while it reached its maximum at 37 °C. Adsorption rates decreased after the thermal inactivation of cells, though, when chloramphenicol (as protein-synthesis inhibitor) was used, adsorption values on treated and untreated cells were similar. In addition, periodate was able to decrease phage adsorption, thus suggesting the receptors were carbohydrates in nature. All these results showed that the adsorption process was only partially affected and most conditions are suitable for the completion of the first step in the phage life cycle. Therefore, phages evaluated in this study can be used to prevent foodborne diseases on several food matrices since they are active in a wide range of environmental conditions.     

高压脉冲电场对蓝莓汁杀菌效果及品质的影响

研究了高压脉冲电场(PEF)对蓝莓汁的杀菌效果及其对蓝莓汁理化性质的影响。结果显示:随电场强度、处理时间的增大,PEF对蓝莓汁中大肠杆菌的杀灭效果增强。当电场强度为35 kV/cm,处理时间为82μs时,蓝莓汁中大肠杆菌致死率为5.12个对数。电场强度一定时,电导率增大,PEF对大肠杆菌的钝化效果略微下降。PEF对蓝莓汁VC、花青素、总酚、可滴定酸、还原糖、可溶性固形物及色泽等理化性质没有影响。     

高压脉冲电场对酵母和大肠杆菌的杀灭效果

研究了高压脉冲电场的电场强度和脉冲时间对胡萝卜汁中接种的啤酒酵母 (CGM CC 2 60 4)和大肠杆菌 (CGMCC 1 90 )的杀灭效果。结果表明 ,啤酒酵母和大肠杆菌的残活率都随着电场强度的增大和脉冲时间的延长而逐渐下降 ,同轴式电极中 ,在 2 3kV的电压和1 2 42ms的脉冲处理时间下 ,酵母达到最小残活率 -4 113个对数 ;平板电极中 ,在 17kV/cm的电场强度和 0 45ms的脉冲处理时间下 ,大肠杆菌达到最小残活率 -2 3 97个对数。同时发现 ,酵母对电场强度和脉冲时间的敏感程度大于大肠杆菌。     

高压脉冲电场对大肠杆菌胞内蛋白的影响

通过聚丙烯酰胺凝胶电泳和傅里叶变换红外光谱研究了高压脉冲电场(PEF)处理对大肠杆菌(E.coli)胞内蛋白的影响,并应用去卷积和曲线拟合等数学方法和圆二(CD)色谱进一步分析PEF处理前后蛋白二级结构中α-螺旋、β-转角、β-折叠和无规卷曲相对含量的变化。结果表明,经过PEF(30 kV/cm,400μs)处理后,在分离胶的顶端出现了蛋白质的聚集。蛋白二级结构中β-折叠相当含量从45.40%降至21.50%,β-转角相对含量从23.8%降至8.81%,而无规卷曲结构从0.13%升至36.95%,α-螺旋结构没有明显的变化。     

Inactivation of E. coli 8739 in Enriched Soymilk Using Pulsed Electric Fields

ABSTRACT: Soymilk enriched with dairy proteins was subjected to pulsed electric fields (PEF) to evaluate the inactivation of Escherichia coli 8739 and the extension of microbial shelf-life. The maximum thermal exposure level of sample was 60 °C for 1.6 s during a PEF treatment. A 5.7-log reduction was achieved using PEF at 41.1 kV/cm for 54 μs. PEF inactivation of E. coli 8739 followed a 1st-order kinetic model. D-values of E. coli 8739 were 31.9,18.6, and 11.0 μs at 30,35, and 40 kV/cm, respectively. PEF treatment at 41.1 kV/cm for 54 μs significantly extended the microbial shelf-life at 4 °C ( P < 0.05). No significant change in brightness and viscosity of PEF-treated samples was observed during a 30-d storage at 4 °C. PEF was found effective in inactivation of E. coli in and extension of microbial shelf-life of enriched soymilk.     

Characterization of resistance patterns and detection of apramycin resistance genes in Escherichia coli isolated from swine exposed to various environmental conditions

Weaned pigs were separated into eight treatments including a control without exposure to apramycin; a control with exposure to apramycin; and apramycin plus either cold stress, heat stress, overcrowding, intermingling, poor sanitation, or intervention with oxytetracycline, to determine the effects of management and environmental conditions on antibiotic resistance among indigenous Escherichia coli. Pigs exposed to apramycin sulfate received that antibiotic in the feed at a concentration of 150 g/ton for 14 days. Environmental treatments were applied 5 days following initial antibiotic administration and maintained throughout the study. Fecal samples were obtained on day 0 (prior to antibiotic treatment) and on days 2, 7, 14, 28, 64, 148, and 149. E. coli were isolated and tested for resistance to apramycin using a minimum inhibitory concentration (MIC) broth microdilution method. Macrorestriction profiling, arbitrarily primed PCR, PCR targeting a gene coding for apramycin resistance, and DNA hybridization were used to characterize genetic elements of resistance. Increased (P<0.0001) resistance to apramycin was noted in E. coli from all treatment groups administered apramycin. MICs of isolates from control pigs receiving apramycin returned to pretreatment levels following removal of the antibiotic, whereas isolates from cold stress, overcrowding, and oxytetracycline groups expressed greater (P<0.05) MICs through day 64, before returning to pretreatment levels. Genetic analysis indicated that all resistant isolates carried the aac(3)IV gene sequence and this sequence was found in a variety of E. coli isotypes. Our data indicate that E. coli resistance to apramycin is increased upon exposure to various stressors.     

高强脉冲电场对维生素C的影响研究

采用自行研制的高强脉冲电场处理设备研究了脉冲场强、流速和电导率对橙汁中维生素C含量的影响情况.结果表明:304μs/cm电导率的橙汁中Vc(维生素C)的含量随着流速的降低而降低,25kV/cm场强下,流速为10mL/min,Vc降到76.47%:Vc含量随着脉冲场强和电导率的增大而减小,25kV/cm场强下,流速为20mL/min,1 054μs/cm电导率的橙汁中Vc降到最低:47.60%,橙汁温度上升到最高:68.5℃.     

高压脉冲电场对乳铁蛋白抑菌性能的影响

研究了高压脉冲电场(PEF)对乳铁蛋白(LF)的抑菌性能的影响。结果表明,LF质量浓度、电场强度、脉冲频率、脉冲数的增加,有利于LF抑菌能力的增强,而温度的升高会使之减弱。电场强度为35 kV/cm时,LF的抑菌能力达到最大值。LF的质量浓度越高,对电场强度的变化越敏感。当脉冲个数达到744时,LF的相对抑菌能力提高了34%。处理温度为15～55℃时,PEF处理的LF的相对抑菌性能增长了5%左右。当温度升高至65℃时,LF的相对抑菌能力下降显著,降低了约22%。     

Heat resistance kinetics variation among various isolates of Escherichia coli

This paper reports an investigation of serotype-specific differences in heat resistance kinetics of clinical and food isolates of Escherichia coli. Heat resistance kinetics for 5 serotypes of E. coli at 60 °C were estimated in beef gravy using a submerged coil heating apparatus. The observed survival curves were sigmoidal and there were significant differences (p=0.05) of the survival curves among the serotypes. Consequently, a model was developed that accounted for the sigmoidal shape of the survival curves and the serotype effects. Specifically, variance components for serotypes and replicates within serotypes were estimated using mixed effect nonlinear modeling. If it is assumed that the studied serotypes represent a random sample from a population of E. coli strains or serotypes, then, from the derived estimates, probability intervals of the expected lethality for random selected serotypes can be computed. For example, expected serotype-specific lethalities at 60 °C for 10 min are estimated to range between 5 and 9 log₁₀ with 95% probability. On the other hand, to obtain a 6-log₁₀ lethality, the expected minutes range, with 95% probability, from 6 to 12 min. The results from this study show that serotypes of E. coli display a wide range of heat resistance with nonlinear survival curves.Industrial relevanceThis paper is of high current interest since it deals with the ongoing international debate on log linear vs. non-log linear microbial inactivation curves observed during thermal and non-thermal processing. The data on 5 serotypes of E. coli indicate a clear need for further studies with more strains to fully characterize the heat resistance kinetics for E. coli.     

高压脉冲电场对橙汁大肠杆菌和理化性质的影响效果

高压脉冲电场(pulsedelectricfield)技术是目前研究人员关注的非热加工技术之一。本文研究了高压脉冲电场技术对橙汁中E.coli和理化性质的影响效果。研究表明,高压脉冲电场对橙汁确实有杀菌钝酶的效果。当电场强度为12kV/cm、脉冲时间为1200个脉冲时,橙汁中E.coli的数量减少了1.73个对数;10kV/cm、400个脉冲时过氧化物酶(POD)活性下降了60%。对于橙汁理化性质总酸、0Brix、pH、浊度和色差等指标的影响不大。     

Antimicrobial resistance and resistance genes in Escherichia coli strains isolated from commercial fish and seafood

The purpose of this study was to investigate the antimicrobial resistance and to characterize the implicated genes in Escherichia coli isolated from commercial fish and seafood. Fish and seafood samples (n=2663) were collected from wholesale and retail markets in Seoul, Korea between 2005 and 2008. A total of 179 E. coli isolates (6.7%) from those samples were tested for resistance to a range of antimicrobial agents. High rates of resistance to the following drugs were observed: tetracycline (30.7%), streptomycin (12.8%), cephalothin (11.7%), ampicillin (6.7%) and ticarcillin (6.1%). No resistances to amikacin, amoxicillin/clavulanic acid and cefoxitin were observed. Seventy out of 179 isolates which were resistant to one or more drugs were investigated by PCR for the presence of 3 classes of antimicrobial resistance genes (tetracycline, aminoglycosides and beta-lactams), class 1, 2 and 3 integrons. Gene cassettes of classes 1 and 2 integrons were further characterized by amplicon sequencing. The tetracycline resistance genes tetB and tetD were found in 29 (41.4%) isolates and 14 (20%) isolates, respectively. The beta-lactam resistance gene, bla(TEM) was found in 15 (21.4%) isolates. The aminoglycoside resistance gene, aadA was found in 18 (25.7%) isolates. Class 1 integron was detected in 41.4% (n=29) of the isolates, while only 2.9% (n=2) of the isolates were positive for the presence of class 2 integron. Two different gene cassettes arrangements were identified in class 1 integron-positive isolates: dfrA12-aadA2 (1.8 kb, five isolates) and aadB-aadA2 (1.6 kb, four isolates). One isolate containing class 2 integron presented the dfrA1-sat-aadA1 gene cassette array. These data suggest that commercial fish and seafood may act as the reservoir for multi-resistant bacteria and facilitate the dissemination of the resistance genes.     

高压脉冲电场处理室多物理场仿真研究

高压脉冲电场杀菌技术是一种新型的高效非热杀菌技术。高压脉冲电场处理室是杀菌处理的关键部件,其结构形式决定了处理室内部电场分布、温度场分布、流场分布,并影响杀菌处理的效果,同时电场强度和温度分布影响被处理食品的口味。以不同结构共场杀菌处理室为研究对象,利用COMSOL Multiphysics软件对共场杀菌处理室内部电场分布、温度场分布、流场分布对处理参数均匀性的影响进行了仿真。仿真结果表明,绝缘体无内嵌的处理室电场强度分布不均匀,对绝缘体倒椭圆角后的处理室可有效改善电场及流场分布,进而有效降低局部温升,改善处理效果。     

Growth and survival of various strains of enterohemorrhagic Escherichia coli in hydrochloric and acetic acid

Nineteen strains of enterohemorrhagic Escherichia coli isolated from humans and foods were examined for their ability to grow and survive at low pH in organic (acetic) and mineral (HCl) acids. Strains were subcultured in tryptic soy broth adjusted to various pH values (3.75 to 4.75 for HCl and 4.75 to 5.75 for acetic acid) and incubated for 72 h at 37 degrees C to determine the minimum growth pH value. Minimum pH values for growth of 4.25 and 5.5 were found for HCl and acetic acid, respectively. Strains were also exposed to pH 2.0 (HCl) and pH 4.0 (acetic acid) for up to 24 h at 37 degrees C to assess their ability to survive. HCl was a more effective inhibitor after 6 h of exposure, whereas acetic acid was more effective after 24 h. Outbreak strains survived acid treatment significantly (P < or = 0.05) better than strains isolated from fermented or high-pH foods or animal or human isolates. Significant (P < or = 0.05) differences among serotypes and between O157:H7 and other serotypes were apparent after 3 or 6 h of exposure to acids.     

高压脉冲电场应用于液蛋杀菌的研究

本文研究了高压脉冲电场对液蛋的杀菌效果及其对液蛋功能性质的影响。结果表明,当电场强度为40kV/cm,处理时间为1660μs时,液蛋中接种的大肠杆菌、沙门氏菌、金黄色葡萄球菌细菌数量可分别降低4．9、5．4和3．8个1g值。在此条件下,PEF处理过的液蛋的黏度、色泽、起泡性、泡沫稳定性、乳化能力和乳化稳定性均没有发生明显变化。     

高压脉冲电场(PEF)对蛋清蛋白功能特性的影响

蛋清蛋白不仅具有重要的营养价值,而且还具有重要的功能特性如乳化性、起泡性等,作为一种重要的蛋白原料,本实验研究了高压脉冲电场对蛋清蛋白功能性质的影响。结果表明:蛋清蛋白的溶解度在脉冲电场强度大于35kV/cm时下降;蛋清蛋白的乳化性、起泡能力、泡沫稳定性及疏水性先随脉冲电场强度增加而增大,但当脉冲电场强度大于30kV/cm后,蛋清蛋白的乳化性、起泡能力、泡沫稳定性及疏水性下降;随着脉冲电场数的增加,蛋清蛋白的溶解度、乳化性、起泡能力、泡沫稳定性及疏水性变化不显著。