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.     

Nonthermal pasteurization of liquid foods using high‐intensity pulsed electric fields

Processing foods with high‐intensity pulsed electric fields (PEF) is a new technology to inactivate microorganisms and enzymes with only a small increase in food temperature. The appearance and quality of fresh foods are not altered by the application of PEF, while microbial inactivation is caused by irreversible pore formation and destruction of the semipermeable barrier of the cell membrane. High‐intensity PEF provides an excellent alternative to conventional thermal methods, where the inactivation of the microorganisms implies the loss of valuable nutrients and sensory attributes. This article presents recent advances in the PEF technology, including microbial and enzyme inactivation, generation of pulsed high voltage, processing chambers, and batch and continuous systems, as well as the theory and its application to food pasteurization. PEF technology has the potential to improve economical and efficient use of energy, as well as provide consumers with minimally processed, microbiologically safe, nutritious and freshlike food products.     

Impact of PEF treatment inhomogeneity such as electric field distribution, flow characteristics and temperature effects on the inactivation of E. coli and milk alkaline phosphatase

High intensity pulsed electric field (PEF) treatment was investigated focusing on the alteration of electric field distribution, flow characteristics and temperature distribution due to the modification of the treatment chamber. The aim was the improvement of the effectiveness of microbial inactivation of E. coli and to reduce the PEF impact on alkaline phosphatase (ALP) activity in raw milk. Mathematical simulation of the PEF process conditions considering different treatment chamber setups was performed prior to experimental verification. Finally the impact of the treatment chamber modifications on microbial inactivation and enzyme activity was determined experimentally. Using a continuous flow-through PEF system and a co-linear treatment chamber configuration the insertion of stainless steel and polypropylene grids was performed to alter the field strength distribution, increase the turbulence kinetic energy and improve the temperature homogeneity. The Finite Element Method (FEM) analysis showed an improved electric field strength distribution with increased average electric field strength and a reduced standard deviation along the center line of the treatment zone indicating a more homogenous electric field. The velocity profile was improved resulting in an increase of turbulence kinetic energy due to the insertion of the grids. As revealed by mathematical modeling, the temperature of the liquid was decreased, and formation of temperature peaks was avoided. Measured inactivation of heat sensitive alkaline phosphatase (ALP) was reduced from 78% residual activity to 92% after PEF treatment and it could be shown that thermal effects and temperature peaks have been the main reason for enzyme inactivation due to PEF. At the same time, an increase of microbial inactivation of 0.6 log–cycles could be determined experimentally due to the modification of the treatment chamber design.

Industrial relevance

The application of pulsed electric field as a non-thermal pasteurization technology requires an accurately defined treatment intensity followed by a predictable microbial inactivation. Unavoidable thermal effects occurring during PEF treatment due to ohmic heating have to be minimized to assure the retention of heat-sensitive nutrients and bioactive compounds. The presented investigations contribute to the fulfilment of these requirements for further successful industrial implementation of the PEF technology such as the selective inactivation or retention of enzyme activity in liquid food systems.     

Pulsed electric field and mild heating for milk processing: a review on recent advances

Pulsed electric field (PEF) treatment consists of exposing food to electrical fields between electrodes within a treatment chamber, which can improve the preservation of fresh-like products such as milk. Although several studies support the use of PEF technology to process milk at low temperature, these studies reported microbial reductions of around 3 log₁₀ cycles and also indicated a limited impact of PEF on some endogenous and microbial enzymes. This scenario indicates that increasing the impact of PEF on both enzymes and microorganisms remains a major challenge for this technology in milk processing. More recently, combining PEF with mild heating (below pasteurization condition) has been explored as an alternative processing technology to enhance the safety and to preserve the quality of fresh milk and milk products. Mild heating with PEF enhanced the safety of milk and derived products (3 log₁₀–6 log₁₀ cycles reduction on microbial load and drastic impact on the activity enzymes related to quality decay). Moreover, with this approach, there was minimal impact on enzymes of technological and safety relevance, proteins, milk fat globules, and nutrients (particularly for vitamins) and improvements in the shelf-life of milk and selected derived products were obtained. Finally, further experiments should consider the use of milk processed by PEF with mild heating on cheese-making. The combined approach of PEF with mild heating to process milk and derived products is very promising. The characteristics of current PEF systems (which is being used at an industrial level in several countries) and their use in the liquid food industry, particularly for milk and some milk products, could advance towards this strategy. © 2019 Society of Chemical Industry     

Principles and recent applications of novel non-thermal processing technologies for the fish industry—a review

Thermal treatment is a traditional method for food processing, which can kill microorganisms but also lead to physicochemical and sensory quality damage, especially to temperature-sensitive foods. Nowadays consumers’ increasing interest in microbial safety products with premium appearance, flavor, great nutritional value and extended shelf-life has promoted the development of emerging non-thermal food processing technologies as alternative or substitution to traditional thermal methods. Fish is an important and world-favored food but has a short shelf-life due to its extremely perishable characteristic, and the microbial spoilage and oxidative process happen rapidly just from the moment of capture, making it dependent heavily on post-harvest preservation. The applications of novel non-thermal food processing technologies, including high pressure processing (HPP), ultrasound (US), pulsed electric fields (PEF), pulsed light (PL), cold plasma (CP) and ozone can extend the shelf-life by microbial inactivation and also keep good sensory quality attributes of fish, which is of high interest for the fish industry. This review presents the principles, developments of emerging non-thermal food processing technologies, and also their applications in fish industry, with the main focus on microbial inactivation and sensory quality. The promising results showed great potential to keep microbial safety while maintaining organoleptic attributes of fish products. What’s more, the strengths and weaknesses of these technologies are also discussed. The combination of different food processing technologies or with advanced packaging methods can improve antimicrobial efficacy while not significantly affect other quality properties under optimized treatment.     

Cold plasma for the preservation of aquatic food products: An overview

Cold plasma (CP) is an upcoming technology implemented for the preservation of highly perishable foods, especially aquatic food products (AFPs). The high moisture content, high-quality protein with all essential amino acids and unsaturated fatty acids makes AFP more susceptible to microbial spoilage and oxidation of lipids and proteins. Spoilage lowers the nutritive value and could generate toxic components, making it unsafe for consumption. In recent times, the rising demand for food products of aquatic origin with preserved quality and extended shelf-life has been recorded. In addition, minimally or nonthermally processed and preserved foods are gaining great attention. CP technology has demonstrated an excellent ability to inactivate microorganisms without promoting their resistance and triggering some deteriorative enzymes, which are typical factors responsible for the spoilage of AFP. Consequently, CP could be recommended as a minimal processing intervention for preserving the quality of AFP. This review focuses on different mechanisms of fish spoilage, that is, by microorganisms and oxidation, their inhibition via the application of CP, and the retention of quality and shelf-life extension of AFP.     

Quality-Related Enzymes in Plant-Based Products: Effects of Novel Food Processing Technologies Part 2: Pulsed Electric Field Processing

Pulsed electric field (PEF) processing is an effective technique for the preservation of pumpable food products as it inactivates vegetative microbial cells at ambient to moderate temperature without significantly affecting the nutritional and sensorial quality of the product. However, conflicting views are expressed about the effect of PEF on enzymes. In this review, which is part 2 of a series of reviews dealing with the effectiveness of novel food preservation technologies for controlling enzymes, the scientific literature over the last decade on the effect of PEF on plant enzymes is critically reviewed to shed more light on the issue. The existing evidence indicates that PEF can result in substantial inactivation of most enzymes, although a much more intense process is required compared to microbial inactivation. Depending on the processing condition and the origin of the enzyme, up to 97% inactivation of pectin methylesterase, polyphenol oxidase, and peroxidase as well as no inactivation have been reported following PEF treatment. Both electrochemical effects and Ohmic heating appear to contribute to the observed inactivation, although the relative contribution depends on a number of factors including the origin of the enzyme, the design of the PEF treatment chamber, the processing condition, and the composition of the medium.     

Opportunities and challenges in pulsed electric field processing of dairy products

Consumer demand and current market conditions warrant investigation of dairy processing technologies that can deliver improved product quality and stability and reduced energy use during processing, without compromising product and process safety. One candidate technology for the extension of shelf-life in dairy products is pulsed electric field (PEF) processing. PEF is considered to be an effective, non-thermal intervention that appears to hold some promise. Research on the application of PEF to control spoilage and pathogenic microorganisms and enzyme systems in dairy products spans a wide array of processing equipment and reaction conditions. PEF has been reported to effectively reduce the numbers of both pathogens and spoilage organisms in milk; however, there is a high degree of variability between studies. The application of PEF in combination with lower temperature thermal processing can deliver comparable reductions in microbial load without significant detrimental effects to the sensory and physico-chemical properties of food products.     

Minimization of thermal impact by application of electrode cooling in a co-linear PEF treatment chamber

A co-linear pulsed electric field (PEF) treatment chamber was analyzed and optimized considering electrical process conditions, temperature, and retention of heat-sensitive compounds during a continuous PEF treatment of peach juice. The applicability of a jacket heat-exchanger device surrounding the ground electrode was studied in order to provide active cooling and to avoid temperature peaks within the treatment chamber thus reducing the total thermal load to which the product is exposed. Simulation of the PEF process was performed using a finite element method prior to experimental verification. Inactivation of polyphenoloxydase (PPO) and peroxidase (POD) as well as the degradation of ascorbic acid (AA) in peach juice was quantified and used as indirect indicators for the temperature distribution. Peaks of product temperature within the treatment chamber were reduced, that is, from 98 to 75 °C and retention of the indicators PPO, POD, and AA increased by more than 10% after application of the active electrode cooling device. Practical Application: The co-linear PEF treatment chamber is widely used for continuous PEF treatment of liquid products and also suitable for industrial scale application; however, Joule heating in combination with nonuniform electric field distribution may lead to unwanted thermal effects. The proposed design showed potential to reduce the thermal load, to which the food is exposed, allowing the retention of heat-sensitive components. The design is applicable at laboratory or industrial scale to perform PEF trials avoiding temperature peaks, which is also the basis for obtaining inactivation kinetic models with minimized thermal impact on the kinetic variables.     

Antimicrobial food packaging in meat industry

Antimicrobial packaging, an active packaging concept, can be considered an extremely challenging technology that could have a significant impact on shelf-life extension and food safety of meat and meat products. Use of antimicrobial substances can control the microbial population and target specific microorganisms to provide higher safety and quality products. Many classes of antimicrobial compounds have been evaluated in film structures, both synthetic polymers and edible films: organic acids and their salts, enzymes, bacteriocins, and miscellaneous compounds such as triclosan, silver zeolites, and fungicides. The characteristics of some antimicrobial packaging systems are reviewed in this article. The regulatory status of antimicrobial packaging in EU is also examined.     

脉冲电场在固体食品加工中的应用

脉冲电场(pulsed electric field, PEF)技术被视为21世纪食品非热加工技术发展史上的里程碑之一。迄今为止, PEF已广泛应用于果汁、牛奶和液态蛋等液体食品的杀菌和钝酶,并朝着商业化道路前进。然而,与PEF在液体食品中的应用相比,其在固体食品中的应用还处于起步阶段。固体食品的表面虽然也富含微生物,但PEF处理这类食品对微生物的影响较小,因此不能将其应用于固体食品的杀菌保鲜。仅管如此, PEF诱导的细胞电穿孔使其可作为一种预处理方法 ,通过增加质量和能量传递效率的方式来进行辅助固体食品的干燥、冻融、烹饪等。因此,本文重点介绍基于PEF细胞响应的高品质食品加工应用,总结PEF处理室的特点及PEF预处理固体食品的相关机制。最后,本文探讨了PEF在固体食品加工中的主要障碍和前景,为PEF未来在食品行业的发展拓宽研究方向。     

Critical factors determining inactivation kinetics by pulsed electric field food processing

The potential to commercialize the non-thermal pulsed electric field (PEF) technology as a new method to preserve food products has caught the attention of the food industry that wishes to fulfil consumers demands for fresh products. In recent years, numerous research groups have demonstrated the possibility to inactivate a range of microorganisms both in buffer systems and in food products using different PEF systems. In this review, we survey the critical process factors and main characteristics of food products that determine the microbial inactivation kinetics.     

Review: Pulsed Electric Fields Processing of Protein-Based Foods

Pulsed electric fields (PEF) processing is a promising nonthermal food preservation technology, which is ongoing from laboratory and pilot plant scale level to the industrial level. Application of PEF processing may be a good alternative treatment to thermal methods in protein-based foods. A large number of literatures have fully demonstrated that small molecule compounds in plant-based foods, mainly aroma compounds and health-related phytochemicals, were not significantly affected by PEF. However, there was a lack of knowledge on the effects of PEF on proteins and qualities of protein-based foods. This review focuses on effects of PEF processing on endogenous enzymes, safety, and quality of protein-based foods. Finally, the ways to achieve food quality assurance and food safety in PEF processing of protein-based foods are proposed.     

脉冲电场技术通过氧化应激途径诱导营养物质合成和杀灭微生物的研究进展

脉冲电场（Pulsed Electric Field，PEF）技术是一种新兴非热食品加工技术，由于PEF对食品质量损伤小且能杀灭食品中有害微生物所以被广泛应用。PEF通过氧化应激途径激活植物类食品的营养物质合成途径，诱导营养物质合成、提高食品质量，然而关于这一领域的研究较少。PEF发生了氧化应激反应造成了营养物质合成与积累，植物类食品受到PEF刺激后产生大量活性氧（Reactive Oxygen Species，ROS），ROS会激活代谢物合成途径最终合成蛋白、多酚、硫代葡萄糖苷和胡萝卜素等物质；ROS聚集在微生物的细胞膜上会造成蛋白表达异常，损害脂质层和脱氧核糖核酸（Deoxyribonucleic Acid，DNA），最终导致微生物失活。     

高压脉冲电场对食品微生物、酶及成分的影响

高压脉冲电场加工是一种非热杀菌技术,是指在高电场强度、短脉冲和温和的温度下处理食品。与传统的热杀菌比较,它具有很多优点,不仅能杀死微生物钝化酶类,而且能保持食品的营养成分和新鲜度。本文综述了高压脉冲电场对食品的杀菌效果、杀菌机制、影响杀菌的因素以及对食品中酶类和成分的影响。     

Inactivating effect of pulsed electric field on lipase in brown rice

Pulsed electric field (PEF) treatment was applied to brown rice grains in a treatment chamber which was surrounded by organic glass as walls around a pair of horizontally paralleled plate electrodes to investigate the feasibility of PEF on low moisture food materials. Based on a monolayer of brown rice grains the results showed that the lipase activity could be significantly inactivated by PEF. Among the PEF parameters, the voltage was the most important to the inactivating efficiency, followed by frequency and pulse width; while the time was less dominant. The interactions between voltage and pulse width and between frequency and pulse width also contributed to the lipase inactivation significantly. By using Box–Behnken design, response surface methodology was applied to optimize the process and a well fitting model was obtained with PEF parameters, voltage, frequency, pulse width, and residence time.Industrial relevancePulsed electric field (PEF) is a low temperature and environment friendly technology in food processing. It is promising and has received considerable attention over the years in the past. PEF has been applied to inactivate microorganisms or enzymes. However, research work regarding PEF focused almost only on liquid food processing so far. There has been no report of PEF on solid food materials. Rice bran is abundant and nutritious, but it could not be stored for a long time because enzymolysis takes place soon after it is scraped off from rice grains. If the PEF could be used for lipase inactivation in brown rice grains, the stabilized rice bran should be obtained after milling during the material convey. It would be another effective and in-line rice bran stabilization technique potential in the rice industry. Furthermore, the application scope of PEF in food industry could be widened.     

Effects of combined pressure and temperature on enzymes related to quality of fruits and vegetables: from kinetic information to process engineering aspects

Throughout the last decade, high pressure technology has been shown to offer great potential to the food processing and preservation industry in delivering safe and high quality products. Implementation of this new technology will be largely facilitated when a scientific basis to assess quantitatively the impact of high pressure processes on food safety and quality becomes available. Besides, quantitative data on the effects of pressure and temperature on safety and quality aspects of foods are indispensable for design and evaluation of optimal high pressure processes, i.e., processes resulting in maximal quality retention within the constraints of the required reduction of microbial load and enzyme activity. Indeed it has to be stressed that new technologies should deliver, apart from the promised quality improvement, an equivalent or preferably enhanced level of safety. The present paper will give an overview from a quantitative point of view of the combined effects of pressure and temperature on enzymes related to quality of fruits and vegetables. Complete kinetic characterization of the inactivation of the individual enzymes will be discussed, as well as the use of integrated kinetic information in process engineering.     

Optimising the inactivation of grape juice spoilage organisms by pulse electric fields

The effect of some pulsed electric field (PEF) processing parameters (electric field strength, pulse frequency and treatment time), on a mixture of microorganisms (Kloeckera apiculata, Saccharomyces cerevisiae, Lactobacillus plantarum, Lactobacillus hilgardii and Gluconobacter oxydans) typically present in grape juice and wine were evaluated. An experimental design based on response surface methodology (RSM) was used and results were also compared with those of a factorially designed experiment. The relationship between the levels of inactivation of microorganisms and the energy applied to the grape juice was analysed. Yeast and bacteria were inactivated by the PEF treatments, with reductions that ranged from 2.24 to 3.94 log units. All PEF parameters affected microbial inactivation. Optimal inactivation of the mixture of spoilage microorganisms was predicted by the RSM models at 35.0 kV cm^− 1 with 303 Hz pulse width for 1 ms. Inactivation was greater for yeasts than for bacteria, as was predicted by the RSM. The maximum efficacy of the PEF treatment for inactivation of microorganisms in grape juice was observed around 1500 MJ L^− 1 for all the microorganisms investigated. The RSM could be used in the fruit juice industry to optimise the inactivation of spoilage microorganisms by PEF.     

Experimental and computational analysis of microbial inactivation in a solid by ohmic heating using pulsed electric fields

Pulsed electric field technology (PEF) has traditionally been used as a technique to inactivate microorganisms in liquid foods at temperatures below those used in heat treatments; however, application of high-intensity PEF (E>1 kV/cm) at high frequencies (>10 Hz) can allow rapid and volumetric solid food electrical heating in order to replace traditional convection/conduction heating that progresses from the heating medium to the inside of the product. This investigation is the first one to evaluate the inactivation of Salmonella Typhimurium 878 in a solid product (cylinder of technical agar used as reference solid) by applying PEF treatments (2.5 and 3.75 kV/cm, and up to 9000 microseconds) at 50 Hz. The evolution of temperature in different locations of the agar cylinder was measured by observing the variability of heating rates depending on location and PEF intensity. Microbial inactivation was determined and compared with isothermal heat treatments that predicted similar inactivation values, but did not detect additional inactivation. Computational analysis enabled us to predict temperature and microbial inactivation for any spatial and temporal distribution of the cylinder agar by detecting the coldest point in the transition zone between the high-voltage electrode, the agar, and the plastic container of the treatment chamber. In order to evaluate the variability of the temperature, computational predictions were done each 0.5-mm. The difference between the coldest and the hottest point (e.g. at the center of the cylinder) resulted in around 10 ^∘C and 10 second variation in temperature and processing time, respectively. In any case, it was possible to obtain 5-log₁₀-reductions after 60 s of PEF treatments when using 2.5 kV/cm and 50% reduction for 3.75 kV/cm. These results suggested the potential of PEF technology as a rapid heating system based on ohmic heating for microbial inactivation in solid food products.     

Evaluation of pulsed electric field and minimal heat treatments for inactivation of pseudomonads and enhancement of milk shelf-life

Pulsed Electric Field (PEF) treatment of milk provides the opportunity to increase the shelf-life of fresh milk for distribution to distant markets. PEF treatments were evaluated in sterile (UHT) milk to determine the inactivation of added spoilage Pseudomonas isolates and the subsequent gains in microbial shelf-life (time taken to reach 10⁷ CFU mL^− 1). Little inactivation of Pseudomonas was achieved at 15 or 40 °C compared with 50 or 55 °C. The greatest inactivation (> 5 logs) was achieved by processing at 55 °C with 31 kV cm^− 1 (139.4 kJ L^− 1). Heat treatment at the application temperature without PEF treatment caused minimal inactivation of Pseudomonas (only 0.2 logs), demonstrating that the inactivation of the Pseudomonas was due to the PEF treatment rather than the heat applied to the milk. At added Pseudomonas levels of 10³ and 10⁵ CFU mL^− 1, the microbial shelf-life of PEF-treated milk was extended by at least 8 days at 4 °C compared with untreated milk. The total microbial shelf-life of the PEF-treated milk was 13 and 11 days for inoculation levels of 10³ and 10⁵ CFU mL^− 1 respectively. The results indicate that PEF treatment is useful for the reduction of pseudomonads, the major spoilage bacteria of milk.Industrial relevancePseudomonads are the major psychrotrophic spoilage microflora of refrigerated, stored HTST pasteurised milk. Long-life (UHT) products are an important component of milk sales in South-East Asia, but in recent years there has been an increasing demand for less processed milk products with extended shelf-life. The recent practice of shipping fresh bulk milk from Australia to South-East Asian countries has necessitated additional heat treatment prior to export and on arrival, to achieve the required shelf-life. Pulsed electric field treatment of HTST milk, applied alone or in combination with mild heat under optimised conditions, offers the opportunity of shelf-life extension, while limiting the reduction in quality attributes of milk associated with more severe additional heat treatments.