Accelerated inactivation of Geobacillus stearothermophilus spores by ohmic heating

Until recently, ohmic heating was commonly thought to kill microorganisms through a thermal effect. However a growing body of evidence suggests that non-thermal effects may occur. Our aim was to determine the kinetics of inactivation of Geobacillus stearothermophilus spores (ATCC 7953) under ohmic and conventional heating using a specially constructed test chamber with capillary sized cells to eliminate potential sources of error and ensure that identical thermal histories were experienced both by conventionally and ohmically heated samples. Ohmic treatments at frequencies of 60 Hz and 10 kHz were compared with conventional heating at 121, 125 and 130 °C for four different holding times. Both ohmic treatments showed a general trend of accelerated spore inactivation. It is hypothesized that vibration of polar dipicolinic acid molecules (DPA) and spore proteins to electric fields at high temperature conditions may result in the accelerated inactivation.     

微波加热对三种微生物致死的Weibull模型

探讨不同微波功率加热至不同温度下对沙门氏菌、大肠杆菌和金黄色葡萄球菌等三种对象菌的致死作用,运用Weibull模型描述其致死历程,由Weibull模型计算微波加热使微生物数量减少5-log所需要的时间,并验证其模型的精确度。结果表明,Weibull模型描述沙门氏菌、大肠杆菌和金黄色葡萄球菌的致死历程均有较高的拟合度,通过Weibull模型推算出来的沙门氏菌、大肠杆菌和金黄色葡萄球菌减少5-log的时间与实验结果相近。     

Application of ohmic heating for the inactivation of microbiological hazards in food products

Ohmic heating has long been used to inactivate pathogens in food products. Several research investigations on the use of ohmic heating technology in the inactivation of microbial hazards in food products are discussed in this review. These studies are discussed under the following sub-headings: (a) inactivation of microbiological hazards, (b) in combination treatments with other sanitizing technologies, and (c) mathematical modeling, all of which are of long-standing interest. In this review, we evaluate ohmic heating as a rapid and volumetric heating process that inactivates microbiological hazards in food products. We also examine ohmic heating-based combination treatments as promising methods to maximize microbial inactivation efficacy and minimize the quality deterioration of food products. We first highlight the fact that most researchers had an interest in the inactivation of vegetative pathogens, whereas only a few focused on the inactivation of bacterial spores. In general, significantly higher treatment conditions were needed to inactivate bacterial spores (>95°C) than vegetative pathogens (>50°C). Studies on the inactivation of viral pathogens by ohmic heating are limited, and further research is needed in this field. In the first part of this review, the nonthermal effects of ohmic heating are also discussed, which is a popular topic in the food industry. Cumulatively, research suggests that that these nonthermal effects are dependent on the treatment conditions and the electrical conductivity of different food samples. Therefore, we suggest that focus should be on the thermal rather than the nonthermal effects of ohmic heating when considering the application of this technology to inactivate pathogens. Finally, we introduced combination technology based on ohmic heating and mathematical modeling, which are of interest recently.     

Chlorine inactivation of Escherichia coli O157:H7 in water

Six human isolates of Escherichia coli O157:H7 and E. coli (ATCC 11229) were used to determine the concentrations of free chlorine and exposure times required for inactivation. Free chlorine concentrations of 0.25, 0.5, 1.0, and 2.0 ppm at 23 degrees C were evaluated, with sampling times at 0, 0.5, 1.0, and 2.0 min. Results revealed that five of six E. coli O157:H7 isolates and the E. coli control strain were highly susceptible to chlorine, with >7 log10 CFU/ml reduction of each of these strains by 0.25 ppm free chlorine within 1 min. However, comparatively, one of the seven strains was unusually tolerant to chlorine at 23 degrees C for 1 min, with a 4-, 5.5-, 5.8-, and >5.8-log CFU/ml reduction at free chlorine concentrations (ppm) of 0.25, 0.5, 1.0, and 2.0. respectively. Based on these studies most isolates of E. coli O157:H7 have no unusual tolerance to chlorine; however, one strain was exceptional in being recovered after 1-min of exposure of 10(7) CFU/ml to 2.0 ppm of free chlorine. This isolate may be a useful reference strain for future studies on chlorine tolerance of E. coli O157:H7.     

Influence of heat transfer with tube methods on measured thermal inactivation parameters for Escherichia coli

The purpose of this study was to investigate the influence of heat transfer on measured thermal inactivation kinetic parameters of bacteria in solid foods when using tube methods. The bacterial strain selected for this study, Escherichia coli K-12, had demonstrated typical first-order inactivation characteristics under isothermal test conditions. Three tubes of different sizes (3, 13, and 20 mm outer diameter) were used in the heat treatments at 57, 60, and 63 degrees C with mashed potato as the test food. A computer model was developed to evaluate the effect of transit heat transfer behavior on microbial inactivation in the test tubes. The results confirmed that the survival curves of E. coli K-12 obtained in 3-mm capillary tubes were log linear at the three tested temperatures. The survival curves observed under nonisothermal conditions in larger tubes were no longer log linear. Slow heat transfer alone could only partially account for the large departures from log-linear behavior. Tests with the same bacterial strain after 5 min of preconditioning at a sublethal temperature of 45 degrees C revealed significantly enhanced heat resistance. Confirmative tests revealed that the increased heat resistance of the test bacterium in the center of the large tubes during the warming-up periods resulted in significantly larger D-values than those obtained with capillary tube methods.     

Gelation characteristics of tropical surimi under water bath and ohmic heating

Gelation characteristics of tropical surimi, namely threadfin bream (TB), bigeye snapper (BS), goatfish (GF) and lizardfish (LF) prepared in the absence and presence of 10 g kg^?1 egg white proteins were evaluated using either ohmic (OH) or water bath (WB) heating. LF and GF surimi exhibited higher endogenous proteolytic activity than BS and TB. Ohmic heating markedly minimized proteolysis of LF and GF surimi as evidenced by a reduction of trichloroacetic acid (TCA)-soluble oligopeptide content of gels and more retention of myosin heavy chain (MHC). Ohmic heating increased breaking force and deformation of TB and BS surimi by 1.3 and 1.6 times, respectively, as compared to water bath heating. However, TB surimi gels heated by a higher applied voltage gradient of 16.7 V cm^?1 exhibited lower breaking force than those heated at 6.7 V cm^?1. Gels heated ohmically contained lower total sulfhydryl concentration, indicating the greater extent of disulfide bond formation as compared to gels heated in a 90 °C water bath. The rapid heating method with shorter heating time could improve water holding capacity and preserve color of tropical surimi gels when compared to water bath heating.     

UV inactivation, liquid-holding recovery, and photoreactivation of Escherichia coli O157 and other pathogenic Escherichia coli strains in water

Drinking water, water used in food production and for irrigation, water for fish farming, waste water, surface water, and recreational water have been recently recognized as a vector for the transmission of pathogenic Escherichia coli, especially serotype O157:H7. We investigated the UV (253.7 nm) inactivation behavior and the capability of dark repair (liquid-holding recovery) and photoreactivation of seven pathogenic (including three enterohemorrhagic E. coli) strains and one nonpathogenic strain of E. coli (ATCC 11229) with respect to the use of UV light for water disinfection purposes. Because most bacteria and yeast are known to be able to repair UV damage in their nucleic acids, repair mechanisms have to be considered to ensure safe water disinfection. We found a wide divergence in the UV susceptibility within the strains tested. A 6-log reduction of bacteria that fulfills the requirement for safe water disinfection was reached for the very most susceptible strain O157:H7 (CCUG 29199) at a UV fluence of 12 J/m2, whereas for the most resistant strain, O25:K98:NM, a UV fluence of about 125 J/m2 was needed. Except for one strain (O50:H7) liquid-holding recovery did not play an important role in recovery after UV irradiation. By contrast, all strains, particularly strains O25:K98:NM, O78:K80:H12, and O157:H7 (CCUG 29193), demonstrated photorepair ability. For a 6-log reduction of these strains, a UV fluence (253.7 nm) up to 300 J/m2 is required. The results reveal that the minimum fluence of 400 J/m2 demanded in the Austrian standard for water disinfection is sufficient to inactivate pathogenic E. coli. A fluence of 160 J/m2 (recommendation in Norway) or 250 J/m2 (recommendation in Switzerland) cannot be regarded as safe in that respect.     

猕猴桃浆欧姆加热特性及酶失活率数学模型的建立

利用欧姆加热技术对猕猴桃浆进行加热,探讨了加热过程中加热速率和电导率的变化规律,建立了多酚氧化酶和过氧化物酶失活率的数学模型。结果表明,加热速率随着电场强度的升高而增大,电场强度对试样的电导率影响不大,随着温度的升高,电导率呈线性关系增大;回归方程在α=0.05水平显著,可用于欧姆加热工艺参数对多酚氧化酶和过氧化物失活影响的预测。研究结果可为猕猴桃浆的深加工提供理论基础。     

Evaluation of key parameters during construction and operation of an ohmic heating apparatus

This study aimed to design, build and validate an ohmic heating apparatus in bench scale and evaluate its performance on liquid food processing. The equipment developed showed adequate performance, monitoring data and heating products satisfactorily. Three ohmic cells were developed and tested in the device. When acerola and blueberry pulps were heated in the ohmic heater, a nonlinear behavior of electrical conductivity with temperature was observed. This behavior is associated with bubble formation caused by water boiling due to temperature gradients inside the cells. This phenomenon is influenced by a number of factors, such as solids content, electric field strength, ohmic cell size and agitation. Two of the ohmic cells developed, when placed on agitating devices, are suitable for a uniform heat treatment of liquid foods. The ohmic heating apparatus developed is adequate to conduct studies to better understand this technology and its applications on food processing.Industrial relevanceThe apparatus of ohmic heating for liquid foods designed and tested in this work can be used to evaluate fundamental parameters, such as electrical conductivity of the product, heating time and process homogeneity. It also provides a tool to monitor processing effects on the quality of end products in order to find the best conditions for a continuous ohmic heating process in an industrial scale.     

Effect of vacuum-thermosonication on the inactivation of Escherichia coli,Staphylococcus aureus,polyphenol oxidase and the quality parameters of soursop puree

The combined effect of vacuum, heat and ultrasound (vacuum-thermosonication, VTS) on soursop puree was investigated with regard to the viability of Escherichia coli and Staphylococcus aureus, inactivation of polyphenol oxidase and sensory quality. The VTS conditions were: vacuum (8.46, 11 and 16.93 kPa), heat (40, 45 and 50 °C), 1–3 intermittent vacuum pulses and ultrasound (24 kHz and 0.34 W/g of acoustic energy density) during 10 min. According to response surface methodology, the best conditions to obtain the highest microbial inactivation of ≥7 log CFU of inoculated E. coli and S. aureus, and reduction in polyphenol oxidase activity (94%) were 16.5 kPa vacuum, 50 °C, and three intermittent vacuum pulses for 10 min with ultrasound. The best VTS conditions did not negatively affect the quality parameters, and there were no significant changes in sensory attributes of the puree (a panel of 50 untrained judges). Therefore, we conclude that VTS appears a viable option in the processing of soursop.     

Synergistic effect of ozonated and electrolyzed water on the inactivation kinetics of Escherichia coli on goat meat

Ozonated and electrolyzed water have been reported to have bactericidal activities against most pathogenic and spoilage microorganisms associated with fresh meat and contact surfaces in meat processing facilities at room temperature. However, antimicrobial effects of these two treatments combined are not known. Therefore, the objective of this study was to evaluate the effectiveness of ozonated and electrolyzed water in combination for inactivating Escherichia coli K12 on goat meat. The combination of ozonated water and alkaline electrolyzed water resulted in higher log reductions (1.03 CFU/ml) compared to ozonated water alone (0.53 CFU/ml). Regression analysis performed using the GInaFiT tool showed that nonlinear Weibull models were more effective than log-linear models for describing the inactivation kinetics of E. coli K12 on goat meat.     

Electro activated water mechanism on Escherichia coli and Enterococcus faecalis and optimization of operating parameters

In this study, the efficacy of electro activated water (EAW) and its mechanism on Escherichia coli and Enterococcus faecalis were investigated. The effects of different operating parameters of EAW on inactivation of bacteria were evaluated by response surface methodology (RSM). The interaction of free chlorine concentration and treatment time at 25 °C had a synergetic effect on E. coli and E. faecalis inactivation. The increase in crystal violet uptake showed the deterioration of cell membrane permeability. The release of 260 nm absorbing materials from both bacteria increased after EAW treatment. Protein degeneration increased with deformation of E. coli and E. faecalis cell membranes, which in turn was followed by the release of proteins together with other intracellular components such as DNA, RNA, potassium, and phosphate.

Practical applications

An electrode module was setup to produce EAW from a mixture of salt and tap water. Freshly prepared EAW was added to the bacterial pellets, mixing thoroughly and immediately initiating a timer. The mixtures were set at different treatment times. The effects of three operating parameters (treatment time, free chlorine concentration (FCC) and temperature) on bacteria inactivation were investigated using Response surface methodology. Crystal violet uptake assay, 260 nm release study and FT‐IR spectroscopy analysis were conducted to determine the mechanism of action of EAW.     

Novel inactivation methods of Doenjang (fermented soybean paste) by high pressure and ohmic heating

The changes of the total cell number in doenjang (Korean traditional fermented soybean paste) by conventional conduction heating, high pressure non-thermal treatment, and ohmic heating were compared. A total of (10¹–10²) CFU/g cells were decreased by heating at (100–105) °C for 10 min. The inactivation rate was improved when heated to a temperature higher than 110 °C, but the taste, color of doenjang were severely changed. Inactivation by high pressure at (200–800) MPa was not achieved, because the total cell did not reach a reduction of 10¹ CFU/g. The total bacterial counts of 10³ CFU/g were decreased during ohmic heating at 15 V and 60 Hz for 10 min, and showed the most effective inactivation. Therefore, application of the ohmic heating in doenjang with high viscosity can kill target microorganisms related to quality deterioration, and rapid and uniform ohmic heating leads to reduction in sensory quality damage.     

苹果法的欧姆加热研究

研究了苹果汁的欧姆加热规律及其对果汁品质的影响,考察了温度(T)和可溶性固形物(SS)含量对电导率σ的影响,以及电场强度(E)、可溶性固形物(SS)和加热体积(液高 h)对加热速率的影响、同时考察了欧姆加热对果汁糖度、pH、透光率等的影响.结果表明,苹果汁的电导率随温度的升高而增大,且呈线性关系,随着可溶性固形物含量的增加,苹果汁的电导率和加热速率都是先增大然后减小,在26°Bx左右时最大.苹果汁的加热速率随电场强度的增加而增大,而在相同电场强度下,液高对加热速率没有影响.并且,欧姆加热对果汁品质影响很小,因此极具开发前景.