Novel Food Processing Technologies

23 Advances in Ohmic Heating and Moderate Electric Field (MEF) Processing

Sudhir K. Sastry The Ohio State University, Columbus, Ohio, U.S.A.

I. INTRODUCTION

The technology of heating materials by passing a current through them has been variously termed Ohmic or Joule heating, in honor of Georg Ohm (who elucidated the Ohm’s law) and James Prescott Joule (who showed that passage of electricity resulted in heating effects).In the latter part of the 19th century, a number of inventions were developed for the heating of flowable materials.The technology saw periodic revival with industrial applications, for example, in milk pasteurization in the 1930s, before being discontinued. In the 1980s, the technology was again revived, and it has achieved some industrial applications that include the pasteurization of liquid eggs and processing of fruit products. New developments in various areas have improved the prospects for future use of this technology.Developments in power supply technology in the past 5 years have shown promise in significantly decreasing costs.The control of electrolytic effects has been greatly advanced by recent developments.Further, the emergence of new manufacturers of low-cost ohmic heaters on the market suggests that the technology is undergoing improvement and refinement.Moreover, research on ohmic heating has identified new applications in the area of moderate electric field (MEF) processing. The basic principle of ohmic heating is the dissipation of electrical energy into heat, which is proportional to the square of the electric field strength and electrical conductivity (Sastry and Palaniappan, 1992a; Sastry and Li, 1996).

u ¼ AjVA2r ð1Þ where the electrical conductivity U is a function of temperature, the material, and the method of heating.For cellular materials, electrical conductivity undergoes a sharp increase in the temperature range of 70jC because of the breakdown of cell wall constituents.When an electric ﬁeld is applied, cell wall breakdown occurs at lower temperatures, and the increase of electrical conductivity occurs over a wider range of temperatures (Palaniappan and Sastry, 1991a; Fig.1). 491

Figure 1 Electrical conductivity of carrot subjected to various electric ﬁeld strengths.(From Palaniappan and Sastry, 1991a.)

Above a certain electric field strength, or when the material has been previously thermally treated (Wang and Sastry, 1997a), the electrical conductivity–temperature curve often becomes linear. Because electrical conductivity increases with temperature, ohmic heating becomes more effective at higher temperatures.The electrical conductivity of liquid foods tends to follow a linear trend, regardless of the mode of heating.Because no cellular structure exists, the properties remain the same (Palaniappan and Sastry, 1991b; Fig. 2). Because the rate of heating may be affected by varying the electric field strength or the product’s electrical conductivity, the technology provides opportunities for creativity on the part of the process engineer or product developer.It is possible to design heaters for materials of relatively low electrical conductivity if the electric field strength is made sufficiently large.It is also possible to heat materials at extremely rapid rates.Further, for

Figure 2 Electrical conductivity of orange juice subjected to various electric ﬁeld strengths.The decrease at high temperatures is due to evaporation and boiling within an unpressurized heater. (From Palaniappan and Sastry, 1991b.)

Copyright © 2005 by Marcel Dekker. Advances in Ohmic Heating and MEF Processing 493 materials with uniform electrical conductivity, the energy generation is far more uniform than in microwave heating.These basic principles have been addressed by Palaniappan and Sastry (1991a), de Alwis and Fryer (1990), Halden et al.(1990), and Sastry (1992), among others.

II. MICROBIAL DEATH KINETICS

The question of whether a nonthermal contribution to microbial lethality exists has been addressed in a number of studies in the literature. Early literature was inconclusive (Palaniappan et al., 1990), because most studies either did not specify sample temperatures or failed to eliminate it as a variable.It is essential that studies comparing conventional and ohmic heating be conducted under temperature histories that are as close as possible.Palaniappan et al.(1992) found no difference between ohmic and conventional heat treatments on the death kinetics of yeast cells (Zygosaccha- romyces bailii) under identical thermal histories.They found, however, that a mild electrical pretreatment of Escherichia coli decreased the subsequent inactivation requirement in certain cases. Recent studies suggest that mild electroporation might occur even under the relatively low field strengths encountered during ohmic heating.The presence of pore-forming mechanisms in cellular tissue has been confirmed by recent work (Imai et al., 1995; Wang, 1995; Kulshrestha and Sastry, 1999).Another recent study (Cho et al.,1999), which was conducted under near-identical temperature conditions, indicates that the inactivation

Table 1 Decimal Reduction Times (D Values) and Kinetic Reaction Rate Constants (k) for B. subtilis Spores Under Conventional and Ohmic Heating

D values for k for D values for conventional conventional ohmic heating k for ohmic Temperature (jC) heating (min1) heating (sec1) (min1) heating (sec1)

88 32.8 0.00117 30.2 0.001271 90 (stage 1 of 17.1 0.002245 14.2 0.002703 two-stage heating) (stage 2 of 9.2 0.004172 8.5 0.004516 two-stage heating) 92.3 9.87 0.003889 8.55 0.004489 95 5.06 0.007586 95.5 4.38 0.008763 97 3.05 0.012585 99.1 1.76 0.021809 Z value (C) 8.74a 70.0b 9.16a 67.5b or activation energy (Ea) (kcal/mol) a Z value. b Activation energy. Source: Cho et al.(1999).

Table 2 Reaction Rate Constants (k) for Z. bailii Under Conventional and Ohmic Heating

D values for D values for conventional k for conventional ohmic heating k for ohmic Temperature (jC) heating (min1) heating (sec1) (min1) heating (sec1)

49.8 294.6 0.008 274.0 0.009 52.3 149.7 0.016 113.0 0.021 55.8 47.21 0.049 43.11 0.054 58.8 16.88 0.137 17.84 0.130 Z values (C) or 7.19a 29.63b 7.68a 27.77b activation energy (Ea) (kcal/mol) a Z value. b Activation energy. Source: Palaniappan et al.(1992).

kinetics of Bacillus subtilis spores can be accelerated by ohmic treatment.A two-stage ohmic treatment (ohmic treatment followed by a holding time, before second heat treatment) accelerated the death rates further.Lee and Yoon (1999) have indicated that leakage of intracellular constituents of Saccharomyces cerevisiae was found to be enhanced under ohmic heating, as compared to conventional heating in boiling water. The additional effect of ohmic treatment may be because of the low frequency used (50–60 Hz), which allows cell walls to build up charges and form pores.This effect is less apparent in high-frequency methods such as radiofrequency or microwave heating.In such cases, the electric field is reversed before sufficient charge buildup occurs, so the cell membrane does not break down because of electrical effects (Tables 1 and 2).

III. ELECTROLYTIC EFFECTS

It is necessary to consider any electrolytic reactions that may occur at the electrode–solution interface.When direct current is used, diﬀerent reactions occur at the cathode and anode; under alternating currents, however, the cathode and anode are periodically reversed, and both cathodic and anodic products may occur at either electrode.Electrolytic processes can be prevented if the potential drop at the electrode–solution interface can be kept below the critical electrode potential for the system.This implies that either the frequency is suﬃciently increased, or the electrode capacitance is increased for such minimization to occur.A detailed analysis of these phenomena has been presented by Amatore et al.(1998) and Bazhal et al.(1983a).An important consideration in developing ohmic technology is the need to address such issues economically.

IV. OHMIC AND MODERATE ELECTRIC FIELD (MEF) PROCESSING APPLICATIONS

While a wealth of potential applications exist for ohmic heating, most are still awaiting commercial exploitation.These include sterilization, pasteurization, processing of fouling-

Copyright © 2005 by Marcel Dekker. Advances in Ohmic Heating and MEF Processing 495 sensitive material, blanching, thawing, on-line detection of starch gelatinization, and ohmic heating as a pretreatment for drying and extraction.Additionally, the presence of an electric field has various interesting effects on bacterial cells.The effects of the electric field are worthy of note with or without any attendant heating effects.Thus we may loosely define the area of moderate electric field processing as processes conducted using an electric field in the range from about 1–1000 V/cm, or arbitrary waveform, and with or without ohmic heating effects.Processes in this category have shown many significant results.

A. Sterilization Uniform heating has resulted in great expectations for sterilization technology for partic- ulate foods.Typical systems for this purpose involve a material flowing through a suitably designed set of electrodes.One common commercial embodiment is the APV ohmic heater, where the field is aligned with the flow (Fig.3a).A coaxial design is shown in Fig.3b. Commercial sterilization requires that all parts of the flowing material be exposed to a sufficient temperature for a time adequate to destroy pathogenic spore-formers.When the material contains solids of significant size (about 1–2 cm), the problem is considerably more difficult, because it is not possible to noninvasively measure temperatures within the flowing solids.A mathematical model is necessary to characterize the worst-case situation. The approaches to modeling have involved the static system models of de Alwis and Fryer (1990) for a limited number of particles, and Zhang and Fryer (1993) for a set of static particles in a lattice.Sastry and Palaniappan (1992b) have analyzed cases involving high solids concentration in a mixed fluid.For a flow system, models have been developed by Sastry (1992) for a plug-flow system and by Orangi et al.(1998) for a system involving a velocity profile. Models for ohmic heating have yielded results on the heating of solid–liquid mixtures, which have been subsequently verified.It has been shown (Sastry and Palaniappan, 1992b) that if a single solid particle is of a lower electrical conductivity than the surroundings, its heating rate tends to lag behind that of the fluid.However, as the concentration of low- electrical conductivity solids increases, the solids heating rate bypasses that of the fluid. For sterilization, the crucial items of interest are the worst-case heating scenario and the location of cold zones.If the solid and liquid phases are of equal electrical conductivity, the heating is relatively uniform.When an individual inclusion particle of electrical conductivity significantly different from its surroundings enters the system, one of the phases will lag behind the other.

Figure 3 Designs of ohmic heaters: (a) in-line ﬁeld, (b) coaxial.

The question as to which phase lags is dependent on the electrical conductivity of the respective phases, and the extent of fluid motion (fluid–solid convective heat transfer coefficient).A number of other studies have attempted to address the use of chemical markers (Kim et al., 1996) or temperature measurement within ohmic heaters (Ruan et al., 1999).Clearly, model improvement and temperature measurements will be crucial in the future development of ohmic heating for sterilization.Studies by Salengke (2000) have looked at the heating of an inclusion particle in detail.

B. Fouling by Proteinaceous Materials One advantage claimed for ohmic heating has been its ability to heat proteinaceous materials, which represent a challenge to conventional processing.Protein deposits can adhere to electrode surfaces, however, and create a high-electrical-resistance ﬁlm at the interface.This may result in arcing, as discussed by Reznik (1996) in one of his patents. Studies by Wongsa-Ngasri (1999) have shown that arcing depends on the protein content, initial temperature, and ﬂow rate.This work also includes arcing diagrams, which show the range of conditions under which arcing may occur.

C. Seafood Processing Research at Oregon State University and in Japan indicate that ohmic heating is a useful method for protease inactivation in restructured seafood products, permitting rapid heating to inactivation temperatures (Yongsawatdigul et al., 1995). Work at the University of Florida has involved thawing of large blocks of shrimp using a specially designed ohmic heating process.

D. Pretreatments for Water Removal Wang (1995) showed that a short pretreatment of vegetable tissue with ohmic heating up to 80jC resulted in a significant acceleration of the drying process when compared to control (untreated), conventionally heat-pretreated, and microwave-pretreated samples.Wang also found that apple juice extraction could be made more efficient by such a permeabilization process, which is similar to an electroplasmolysis process developed in the former Soviet Union (Grishko et al., 1991). Further, the mechanical energy required for juice pressing was significantly reduced.Lima and Sastry (1999), following up on Wang’s work, showed that the waveform and frequency also had significant effects.

E. Ohmic Heating for Detection of Starch Gelatinization Wang (1995) and Wang and Sastry (1997b) showed that the electrical conductivity– temperature curve of starch solutions showed negative peaks at temperatures corresponding to similar peaks of a diﬀerential scanning calorimeter (DSC) thermograph (Fig.4). This led to the possibility of monitoring gelatinization from electrical conductivity measurements, and suggests that on-line techniques could be developed for this purpose.

F. Extraction Enhancement Bazhal and Guly (1983a,b), among others, have shown the eﬀectiveness of ohmic or moderate electric ﬁeld (MEF) pretreatment in the improvement of sugar beet extraction.

Figure 4 Superimposed DSC curve and electrical conductivity–temperature relationship for cornstarch suspension.(From Wang, 1995.)

Work in Korea (Kim and Pyun, 1995) has indicated that soymilk extraction can be improved by ohmic/MEF treatment.Schreier et al.(1993) reported the increase of diﬀusion by ohmic heating.Recent work by Kulshrestha and Sastry (1999) shows that mild treatment of beet tissue (temperature rise of 1–2jC) results in the leaching of dyes from cells.Lebovka et al. (2001) have developed models for breakage of cellular tissue.Work by Bazhal and Vorobiev (2000) has shown how electric pulses may be used to improve apple juice extraction.

V. CONCLUSIONS

The eﬀects of ohmic heating appear to have applications for a variety of processes.However, the exploitation of positive eﬀects, while minimizing and eliminating negative aspects, represents interesting engineering challenges for the future.

VI. NOMENCLATURE

D Time in minutes required for a log-cycle reduction in bacterial population u Internal energy generation rate V Voltage

Greek letters and other symbols U Electrical conductivity K Gradient

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