Unit Operations in Food Engineering (Food Preservation Technology)

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The salient features of food processing which has some relevance to bioprocess engineering and technology are given here. For appropriate processing of food, many criteria need to be taken into account. These include the ability of the microorganisms and pests to invade and grow on foods, and the chemical instability and biological activity of foods.



Among the many food processing techniques, food preservation is the most important one. Selected food preservation processes are briefly described.

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Exposure of foods to sunlight and drying is a natural, and an earliest method of food preservation. Drying involves the removal of water. Consequently, the moisture content of the food falls, and the invading microorganisms cannot grow. Hot air is most frequently used to remove moisture. The technique of chilling is commonly used to preserve foods without freezing.

The shelf-life of fresh foods like meat, fish and dairy products can be extended by chilling usually less than a month. It is reported that the if chilling is done under low oxygen conditions, the shelf-life of foods increases. Chilling is frequently used for long distance shipping of meat, apples, vegetables etc.

A low-energy PEF system, which consists of a high voltage pulse generator Fig. The details are given by Ho and Mittal The system consists of a 30 kV d. The V a. The d. The pulse generator emits a train of 5V pulses, and the trigger circuit serves to convert that to V pulses using a silicon control rectifier SCR. The generation of high voltage pulses relies on the discharge of the 0.

The batch unit can generate short duration pulses 2 ms width, 0. One of the most important and complicated components in the processing system is the treatment chamber. The basic idea of the treatment chamber is to keep the treated product inside during pulsing, although the uniformity of the process is highly dependent on the characteristic design of the treatment chamber. When the strength of applied electric fields exceeds the electric field strength of the food product treated in the chamber, breakdown of food occurs as a spark.

Treatment chambers are mainly grouped together to operate in either a batch or continuous manner; batch systems are generally found in early designs for handling of static volumes of solid or semi-solid foods. Several treatment chambers have been designed. Common electrode configurations in pulsed electric field treatment chambers parallel-plate coaxial. Schematic diagram of a pulsed electric fields operation design of treatment chambers for pulsed electric fields equipment: a static chamber, b side view of a basic continuous design, c coaxial chamber, and d co-linear chamber.

PEF investigators studying inactivation and preservation effects have been highly inventive in treatment chamber design Fig. Several different designs have been developed through the years for this key component, wherein high voltage delivered by the power supply is applied to the product located between a pair of electrodes. When the strength of applied electric fields exceeds the electric field strength of the food product treated in the chamber, break down of food occurs as a spark.

Known as the dielectric breakdown of food, this is one of the most important concepts to be considered in treatment chamber design. Dielectric breakdown of the food is generally characterized as causing damage on the electrode surfaces in the form of pits, a result of arching and increased pressure, leading to treatment chamber explosions and evolution of gas bubbles. Dunn and Pearlman designed a chamber consisting o f two parallel plate electrodes and a dielectric space insulator Fig.

The electrodes are separated from the food by ion conductive membranes made of sulfonated polystyrene and acrylic acid copolymers, but fluorinated hydrocarbon polymers with pendant groups would also be suitable. An electrolyte is used to facilitate electrical conduction between electrodes and ion permeable membranes. Suitable electrolyte solutions include sodium carbonate, sodium hydroxide, potassium carbonate, and potassium hydroxide. These are circulated continuously to remove the products of electrolysis and replaced in the event of excess concentration or depletion of ionic components.

From the electrical point of view, the PEF treatment chamber represents the electrical load consisting of two or more electrodes filled with the liquid substance to be treated. The chamber has to be constructed in such a way that the electrical field acting on the liquid is more or less homogeneous across the entire active region. Planar electrode configurations consist of two parallel electrodes fixed by insulators.

The insulators and the electrodes form a channel for the streaming liquid. Coaxial electrode configurations consist of two coaxial electrodes. The liquid streams between these electrodes that are fixed by insulators not shown in the Fig.. Axial electrode configurations consist of several electrode rings on alternating potentials separated by insulating rings. Electrode materials also play an essential role. If monopolar voltage waveforms are applied, electrode corrosion can become critical and the substance to be treated can be contaminated. The static parallel plate electrode chamber was modified by adding baffled flow channels inside to make it operate as a continuous chamber Fig.

Two stainless-steel disk-shaped electrodes separated by a polysulfone spacer form the chamber. The designed operating conditions are: chamber volume, 20 or 8 ml; electrode gap, 0. Jemai and Vorobiev, mentioned that In most runs, one or two chambers were manually filled with grated cossettes see Fig. In a few runs, up to six chambers were used at the same time.

Each chamber consists of a plate covered by a filter cloth and a flexible electrode metallic grid on one side and a rigid electrode on the other. The pressure compressed air is applied to the membrane of the plate, which in turn exerts and distributes the pressure over the cossettes placed between the plate and the rigid electrode Fig.

Juice is drained through channels leading to the outlet, where juice accumulation is monitored by a weighing balance connected to a data acquisition system Bouzrara, Application of pulsed electric fields technology has been successfully demonstrated for the pasteurization of foods such as juices, milk, yogurt, soups, and liquid eggs. Application of PEF processing is restricted to food products with no air bubbles and with low electrical conductivity.

The maximum particle size in the liquid must be smaller than the gap of the treatment region in the chamber in order to ensure proper treatment. PEF is a continuous processing method, which is not suitable for solid food products that are not pump able. PEF is also applied to enhance extraction of sugars and other cellular content from plant cells, such as sugar beets. PEF also found application in reducing the solid volume sludge of wastewater.


PEF processing has been successful in a variety of fruit juices with low viscosity and electrical conductivity such as orange, apple, and cranberry juice. Recent studies reported more than a g reduction in orange juice Qin et al.

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Considering the effectiveness of PEF treatment on liquid products, such as milk, fruit juices, liquid egg, and any other pumpable food products, extensive research has been done to implement the process at an industrial level. Flavor freshness, economic feasibility, improvements in functional and textural attributes and extended shelf life are some of the main points of interest besides achievement of microbiological safety of food products Dunn, Among all liquid products, PEF technology has been most widely applied to apple juice, orange juice, milk, liquid egg, and brine solutions Qin et al.

Each of the nonthermal technologies has specific applications in terms of the types of foods that can be processed. Among these, pulsed electric fields PEF is one of the most promising nonthermal processing methods for inactivation of microorganisms, with the potential of being an alternative for pasteurization of liquid foods. Induction of membrane potentials exceeding a threshold value often result in cell damage and death Zimmermann, PEF technology has recently been used in alternative applications including drying enhancement, enzyme activity modification, preservation of solid and semisolid food products, and waste water treatment, besides pretreatment applications for improvement of metabolite extraction.

The ability of PEF to increase permeabilization means it can be successfully used to enhance mass and heat transfer to assist drying of plant tissues. Studies conducted on different plant tissues such as potato tissue Angersbach and Knorr, , coconut Ade-Omowaye et al. Considering the effectiveness of PEF treatment on liquid products, such as milk, fruit juices, liquid egg, and any other pump able food products, extensive research has been done to implement the process at an industrial level.

Application of PEF is especially promising for the citrus industry, which is concerned with the spoilage microorganisms and resultant production of off-flavor compounds such as lactic acid bacteria Hendrix and Red, Jemai and Vorobiev stated the enhancing effect of a PEF treatment on the diffusion coefficients of soluble substances in apple slices. The results available in literature clearly indicate that PEF can be successfully applied to disintegrate biological tissue and to improve the release of intracellular compounds, though an industrial application has not been achieved up to now.

The power supply and the treatment chamber are shown in Fig. It is noteworthy that avoiding an enzymatic maceration the pectin fractions will remain in a native, highly esterified structure. This provides a potential to extract high quality pectin from the pomace after juice winning and therefore a step toward a more economic and sustainable processing.

PEF treatments are applied in the form of short pulses to avoid excessive heating or undesirable electrolytic reactions. In general, a continuous PEF treatment system is composed of treatment chambers, a pulse generator, a fluid-handling system, and monitoring systems Fig.

Six co-field chambers with a diameter of 2. Power supply and treatment chamber of a pilot scale system for fruit mashes. The treatment chamber is a co-linear configuration of 3 cylindrical electrodes separated by insulators. The treatment cell has a polypropylene frame with a cylindrical cavity compartment 20 mm thick, 56 mm in diameter , which should be initially filled with gratings and then closed from both sides by steel covers.

A mobile electrode is attached to the elastic rubber diaphragm. A stationary wire gauze electrode is installed between the filter cloth and the layer of gratings. Both electrodes are connected to the PEF generator, which can provide the monopolar or bipolar pulses of near-rectangular shape. Studies conducted on the effects of PEF on dairy products such as skim milk, whole milk, and yogurt compromise a major section of PEF applications Alvarez and Ji, Milk is very susceptible to both spoilage and pathogenic microorganisms requiring the application of thermal pasteurization under current regulations, which ensures safety but generally results in a cooked flavor Wirjantoro and Lewis, In order to use PEF technology as a pasteurization process it is necessary to estimate its efficacy against pathogenic and spoilage food-borne microorganisms.

To obtain this objective there is a need to accumulate knowledge on the critical factors affecting microbial inactivation, to describe the PEF inactivation kinetics and to understand the mechanisms involved in microbial PEF inactivation. The lethality factors contributing to the effectiveness of pulsed electric field technology can be grouped as technological, biological, and media factors.

Each group of determinant factors is related to type of equipment, processing parameters, target microorganism, and type and condition of media used.

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  4. A number of other factors during PEF processing can affect specific microbial inactivation as well. Some of these critical factors include the field strength, treatment time, treatment temperature, pulse shape, type of microorganism, growth stage of microorganism, and characteristics of the treatment substrate. Microbial inactivation increases with an increase in the electric field intensity, above a critical trans membrane potential Qin et al. It is important that the electric field intensity should be evenly distributed in the treatment chamber to achieve an efficient treatment.

    Electric field intensities of smaller than kV cm"' usually do not affect microbial inactivation Peleg, In general, the electric field intensity required to inactivate microorganisms in foods in the range of kV cm"1. The fact that microbial inactivation increases with increases in the applied electric field intensity and can be attributed to the high energy supplied to the cell suspension in a liquid product Wouters et al.

    An important aspect that differentiates between PEF processing and other microbial inactivation technologies is that the PEF treatment is delivered by pulsing. The pulses commonly used in PEF treatments are usually either exponential or square wave pulses Jeyamkondan et aI. There is some controversy with respect to the influence of the pulse width on the PEF microbial lethality. Treatment time could be defined as the effective time during which range microorganisms are subjected to the field strength.

    It depends on the number of pulses and the width of the pulses applied. This parameter and the electric field strength are the main factors determining the lethal effect of PEF treatments Sale and Hamilton, ; Jayaram et al. Studies on microbial inactivation by PEF have been conducted at frequencies ranged from 1 to Hz.

    PEF treatment time is calculated by multiplying the pulse number by the pulse duration. An increase in any of these variables increases microbial inactivation Sale and Hamilton A good understanding of the electrical principles behind PEF technology is essential for a comprehensive analysis of the PEF system. The electrical field concept, introduced by Faraday, explains the electrical field force acting between two charges. When unit positive charge q located at a certain point within the electric field is generated in the treatment gap Er , it experiences force F identified by position vector r Blatt, The electrical field per unit charge is then defined as shown in Eq.

    The electrical potential difference V between voltage across two points, separated by a nonconductive material, results in generation of an electric field between these points, with an electrical intensity E directly proportional to the magnitude of potential difference V and inversely proportional to the distance d between points, as given in Eq. The type of electrical field waveform applied is one of the important descriptive characteristics of a pulsed electric field treatment system.

    The exponentially decaying or square waves are among the most common waveforms used. To generate an exponentially decaying voltage wave, a DC power supply charges the bank of capacitors that are connected in series with a charging resistor. When a trigger signal is applied, the charge stored in the capacitor flows through the food in the treatment chamber. Exponential waveforms are easier to generate from the generator point of view.

    Generation of square waveform generally requires a pulse forming network PFN consisting of an array of capacitors and inductors. It is more challenging to design a square waveform system compared to an exponential waveform system. However, square waveforms may be more lethal and energy efficient than exponentially decaying pulses since square pulses have longer peak voltage duration compared to exponential pulses Amiali et al. The electric field should be evenly distributed in the treatment chamber in order to achieve an efficient treatment.

    Biological factors that include the individual characteristics of target microorganisms and their physiological and growth states are determinant factors affecting PEF application. The susceptibility of a microorganism to PEF inactivation is highly related to the intrinsic parameters of the microorganism such as size, shape, species or growth state.

    Generally, Gram-positive vegetative cells are more resistant to PEF than Gram-negative bacteria, while yeasts show a higher sensitivity than bacteria. Induction of electric fields into cell membranes is greater when larger cells are exposed to PEF treatment Sale and Hamilton, ; Htilsheger et al. Most of the research focuses on the inactivation of vegetative cells of bacteria, while only a few reports are available on the inactivation of spores describ ing a limited effect of PEF.

    Another study conducted by Pagan et al. On the other hand, Marquez et al. Additionally, mold Condi spores were reported to be sensitive to PEF in fruit juices whereas Neosartorya fischeri asco spores were resistant to PEF treatments Raso et al. Compared to the number of studies reported for enzyme inactivation by PEF, little information is available on the mechanism of inactivation, which may be due to the lack of analysis of enzyme structural data Yeom et al.

    The most important factors in the PEF system are the electric field intensity, number of pulses, pulse waveform, pulse width, treatment time and treatment temperature. But enzymes and proteins are generally more resistant to electric field intensity and pulses than microorganisms. The physical and chemical characteristics of food products are known to strongly influence the effectiveness of microbial inactivation during PEF application Wouters et al. These factors most likely influence the recovery of injured microbial cells and their subsequent growth following PEF exposure, since the presence of food components, such as fats and proteins, has reportedly had a preventive effect on microorganisms against PEF treatment Ho et aI.

    Similar to the intrinsic parameters of microorganisms, treated media has its own intrinsic factor s such as conductivity, resistivity, dielectric properties, ionic strength, pH, and composition. Each of these parameter sinfluences the PEF treatment either alone or in combination. Temperature is one factor proposed that has been correlated with microbial inactivation, and although PEF application is strictly a nonthermal processing technology, the synergistic effect of temperature on foods due to changes in the properties of cell membranes becomes greater when foods are subjected to high intensity pulse electric fields Jayaram et al.

    Food Processing: Objectives and Preservation Processes

    In general, the lethality of PEF treatments increases with an increase in processing temperature; therefore, a proper cooling device is necessary to maintain temperatures below levels. The influence of pH and water activity aw on microbial growth was documented by Jay, Sepulveda proposed that a PEF treatment time between 0. Typically, increasing the number of pulses causes an increase in treatment time, as the pulse width is fixed by the impulse generation setup. Dunn and Pearlman found that a combination of PEF and heat was more efficient than conventional heat treatment alone.

    The objective of food preservation technologies used by the food industry is to control microorganisms once they are contaminating foods. Pulsed electric field PEF is a potential non-thermal food preservation technique to replace conventional thermal processing. When exposed to high electrical field pulses, cell membranes develop pores either by enlargement of existing pores or by creation of new ones.

    These pores may be permanent or temporary, depending on the condition of treatment. The pulsed electric processing system is composed basically of a high power pulse generator, a treatment cell, voltage and current measuring devices. A traditional treatment cell consists of two electrodes held in parallel by insulating material that form an enclosure containing the food to be treated. The application of high intensity pulsed electric fields consists of the generation of short time pulses of electric fields between two parallel plate electrodes enclosing a dielectric material.

    Research of pulsed electric fields technology is ongoing around the world. Most of the research conducted up until now has been in the laboratory and on a pilot plant scale level, and has shown promising results. According to the intensity of the field strength, electroporation can be either reversible cell membrane discharge. The present chapter reviews the current state of the art in microbial inactivation by PEE after discussing critical factors determining microbial inactivation by PEF and mathematical kinetic models used for describing PEF death, the most successful combinations of PEF with other preservation techniques for enhancing the safety of minimally processed foods are presented.

    The chapter concludes with some aspects that need further investigation for the development of PEF processes to supply safe food products of high organoleptic and nutritional quality. The chapters focus on the electrical bases, various equipment configurations, and principal components of pulsed electric field systems, including various types of electric circuits and processing chambers,By explaining the following points.

    Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications. Edited by Ayman Amer Eissa. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals.

    Mohamed and Ayman H. Downloaded: Introduction Pulsed electric fields PEF is a non-thermal method of food preservation that uses short pulses of electricity for microbial inactivation and causes minimal detrimental effect on food quality attributes. Nonthermal technologies for food processing Nonthermal technologies represent a novel area of food processing and are currently being explored on a global scale; research has grown rapidly in the last few years in particular.

    System components A pulsed Electric Field processing system consists of a high-voltage power source, an energy storage capacitor bank, a charging current limiting resistor, a switch to discharge energy from the capacitor across the food and a treatment chamber.

    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)
    Unit Operations in Food Engineering (Food Preservation Technology) Unit Operations in Food Engineering (Food Preservation Technology)

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