WO2008144499A1

WO2008144499A1 - System and methods for pasteurizing food using ultrashort electrical pulses

Info

Publication number: WO2008144499A1
Application number: PCT/US2008/063883
Authority: WO
Inventors: Karl H. Schoenbach
Original assignee: Old Dominion Univesity Research Foundation
Priority date: 2007-05-16
Filing date: 2008-05-16
Publication date: 2008-11-27

Abstract

An apparatus for pasteurizing packaged food is provided. The apparatus includes a pair of electrical conducting members, and a pulse generator for supplying electric pulses to the pair of electrical conducting members. Additionally, the apparatus includes a chamber positioned between the pair of electrical conducting members, the chamber being configured to receive a packaged food. The apparatus further includes a conductive fluid flowing between the conducting members and the packaged food for effecting electrical contact between the pair of conducting members and a surface area of the packaged food.

Description

SYSTEM AND METHODS FOR PASTEURIZING FOOD USING ULTRASHORT ELECTRICAL PULSES

FIELD OF THE INVENTION

[0001] The present invention is related to the field of food processing, and more particularly, to techniques for pasteurizing packaged foods.

BACKGROUND OF THE INVENTION

[0002] In many situations involving a large variety of liquids it is desirable to significantly reduce, if not eliminate entirely, the quantity of bacteria contained in a liquid. The various liquids for which this is desirable range from drinking water and liquid food to metal working fluids.

[0003] Various techniques have been developed for reducing the bacteria content of these various liquids. Conventional techniques include filtration, chemical treatment, low and high radiation, and heating. The last technique is often employed while the liquid is subjected to increased pressure.

[0004] A more recent technique for bacterial decontamination of liquids involves the use of pulsed electric fields (PEFs). The decontamination effect is assumed to stem from the charging, and resultant breakdown, of the cell wall of the bacteria. Based on this hypothesis, it is expected that optimum efficiency is achieved for pulses with a duration slightly longer than the charging time of the cell membrane. One possible effect of ultra-short electric pulses is that a PEF with relatively high frequency components is capable of penetrating the cell membrane and reaching the internal structures of the cell.

[0005] The use of electrical pulses for bacterial decontamination of liquids has been studied fairly extensively. It appears, however, that to date there do not exist devices or techniques for effectively and efficiently pasteurizing packaged solid foods using such pulses.

SUMMARY OF THE INVENTION

[0006] The invention is directed to a system, apparatus, and methods for pasteurizing packaged foods. Pasteurizing, according to the invention, is performed using electrical pulses. [0007] One embodiment of the invention is a system for pasteurizing packaged food. The system can include a pair of electrical conducting members, and a pulse generator for supplying electric pulses to the electrical conducting members. The system also can include a chamber positioned between the electrical conducting members, the chamber being configured to receive a packaged food. A conductive fluid can be disposed between the electrical conducting members and the packaged food so as to effect electrical contact between the electrical conducting members and a surface portion of the packaged food. Additionally, the system can include a fluid flow system for supplying the fluid between the electrical conducting members and the packaged food.

[0008] Another embodiment of the invention is a method of pasteurizing a packaged food. The method can include positioning a packaged food between two electrical conducting members. The method also can include supplying a conductive fluid in gaps between the packaged food and two conducting members. The method further can include generating and conveying through the two electrical conducting members high- voltage electrical pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] There are shown in the drawings, embodiments which are presently preferred. It is expressly noted, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

[0010] FIG. 1 is a schematic view of an apparatus for pasteurizing packaged foods, according to one embodiment of the invention.

[0011] FIGS. 2A-C are schematic views of a system for pasteurizing packaged foods, according to another embodiment of the invention.

[0012] FIG. 3 is a flowchart of exemplary steps of a method for pasteurizing packaged foods, according yet another embodiment of the invention.

DETAILED DESCRIPTION

[0013] The invention is directed to systems and methods for pasteurizing packaged foods. One aspect of the invention is the recognition that with packaged foods the packaging material typically acts as a dielectric. For example, a thin plastic wrapping acts as an insulator. Therefore, even when the packaged food is subjected to an applied electric field, no voltage is generated across the food. The electric power, determined by the product of conduction current times voltage, is thus zero. Accordingly, no bacterial decontamination effect is produced. [0014] The invention overcomes this obstacle by treating packaging wrapping or film as a capacitor, which is charged to the voltage that is externally applied. There is a short period at the beginning of the pulse when the displacement current, as understood by one of ordinary skill in the art, is flowing. Another aspect of the invention is the determination of the characteristic charging time constant of the packaging film. This time constant is the product of the resistance of the food times the capacitance of the packaging film. Only during this time is a current flowing through the food owing to an applied voltage. Accordingly, the characteristic charging time determines the maximum duration of an electric pulse. Because the typical charging time constants in the context of packaged foods are in the nanosecond range, only nanosecond pulsed power technology provides an adequate solution to the problem of pasteurizing packaged foods using pulsed electric fields (PEFs).

[0015] Referring initially to FIG. 1, a schematic view is provided of an apparatus 100 for pasteurizing packaged solid foods, according to one embodiment of the invention. The apparatus 100 illustratively includes two conducting members 102 A, 102B. A chamber 104 is illustratively disposed between the conducting members 102 A, 102B, the chamber being configured to hold packaged food. A pulse generator 106 is illustratively connected to the conducting members 102 A, 102B for supplying electric pulses to the conducting members. Gaps 108 between the conducting members 102 A, 102B is filled with a fluid conducting medium, such as saltwater or ionized gas.

[0016] Operatively, a packaged food is placed between the conducting members 102 A, 102B, held by the chamber 104. The gaps 108 between the conducting members 102A, 102B is filled with the fluid conducting medium. One condition for effective bacterial decontamination of the packaged food is that the duration of the electric pulses supplied by the pulse generator 106 is less than the characteristic charging time. Another condition is that the applied electric field, defined as the voltage of the pulse divided by the thickness of the package, exceeds a certain threshold. The threshold can be determined by the biological features of the bacteria that is known to be or suspected of being contained in the packaged food.

[0017] Most bacteria that causes food-borne illnesses have dimensions on the order of 0.1 to 1.0 micrometer. Since a voltage of approximately 1 V across a bacterium is required to damage the bacterium, the minimum electric field in a worst-case context is expected to be on the order of 100,000 V/cm. Alternatively, the required electric field intensity can be reduced by aligning typically rod-shaped bacteria through successive electric pulses. In the worst-case context, it is expected that for a packaged food, such as a package of hot dogs treated by successive pulsing, 10 nanosecond , 500,000 V pulses into a 5 Ohm load at high repetition rates are needed. [0018] Generally, the distance between the conducting members 102 A, 102B as well as the dimensions of their surface areas are determined by the dimensions of the food package that is being treated. For example, for a standard package of hotdogs, the gap typically is 5 cm and the areas of the conducting members 102A, 102B 100 cm². Although the gap distance is determined by the treated food, the area of the electrodes can be reduced depending on package shape. In the case of a square package, for example, the conducting members can have a rectangular shape, where one dimension is given by the length of the package, and the other dimension is a fraction of its width. By moving the package through the gap between the rectangular conducting members, it is possible to successively decontaminate the entire package. The conducting members, however, do not need to be square or rectangular, or even flat. The shape can be modified to fit the shape of the treated food package.

[0019] Regardless, though, it is typically necessary to create a good electrical contact between the conducting members 102 A, 102B and the package. The needed electrical contact is provided by the conductive fluid, such as saltwater or an ionizing the gas between the two surfaces, as described above. This can be obtained by utilizing so-called barrier discharges, which produces a cold plasma between a conducting electrode and a dielectric, in this case the plastic which is used for packaging.

[0020] One example of determining the characteristic charging time constant for a packaged food, is the determination made with respect to a package of hotdogs. First, the electrical characteristics of the hot dogs and the packaging material is measured. The resistivity of the hot dog filling is measured as 102 Ohm cm. For a package that is 5 cm thick having an area of 100 cm², the total resistance of the hotdogs in this package, across the 5 cm distance, is approximately 5 Ohm. The capacitance is calculated based on the measured thickness of the plastic film. It is approximately 40 micrometers on one side and 60 micrometers on the other side, totaling 100 micrometers, or 4 mil. The capacitance of the two films, assuming a relative dielectric constant of 3 (typical for many of the plastic films), yields a total capacitance of approximately 2.5 nanofarad. The product of both values is 12.5 nanoseconds. Accordingly, this is the characteristic charging time of a package of hot dogs. As long as the duration of the applied high voltage pulse is less than this time constant, most of the electrical power will be applied to the food. Longer pulses will be ineffective, and will only lower the efficiency and possibly cause electrical breakdown of the plastic film.

[0021] FIG. 2 is a schematic view of an exemplary system for pasteurizing packaged foods, according to another embodiment of the invention. The exemplary system described herein is a small-scale replica illustrative of an actual electrical system for performing food decontamination according to the invention. FIGS. 2A and 2B provide a cross-section and picture view a Blumlein line generator and the schematics of the water flow system. The Blumlein pulse-forming line (PFL) consists of three polished stainless steel bars (length: 17 cm, width: 2.5 cm, thickness: 1 cm) arranged as shown, enter bar is charged to a high voltage. Together with the other two bars, it forms two transmission lines. The stainless steel bars are polished to minimize breakdown between them. The length of the bars determines the pulse duration, which is twice the transit time of an electromagnetic wave in a single transmission line,

where 1 is the length of the transmission line, ε_r is the relative dielectric constant of water and c is the speed of light in vacuum. For a pulse time, T, of 10 ns, the calculated length, 1, is 17 cm. The matched load impedance is twice the characteristic impedance of the transmission line, Z_s

where w is the width (2.5 cm), and d is the gap between the transmission lines (3 mm), hi our system, R is 10 Ω.

[0022] One electrode configuration for the water switch illustrated in FIG. 2C one consists of two stainless steel pins with diameters of 2 mm and a gap distance that can be varied from 0.2-0.5 mm. The water flows in a transverse direction. The second configuration is a coaxial structure. The inner electrode has a diameter of 3.9 mm, and the inner diameter of the outer electrode is 4.3 mm, so the gap distance is 0.2 mm. The water flow is introduced via nine holes distributed around the inner electrode.

[0023] In order to minimize the change in resistivity of the water caused by the formation of ions during charging and discharging, a filter can be used to de-ionize the circulated water in the transmission line during operation. A separate hydraulic system circulates water through the switch at a higher flow rate than through the transmission line. This allows for the removal of debris and vapor bubbles that are formed during each switching cycle and, as a result, can increase the repetition rate. The system operates in self-breakdown mode, but an electrical trigger can also be used. The water switch is designed as a plug-in unit and can be easily replaced as needed.

[0024] The open-loop-water-circulating systems with two paths share a common reservoir. One circulation path is for the dielectric of the Blumlein line, which occupies a volume of 30 ml, and the other circulation path is for the water gap. Water is circulated by pumps, such as a Gorman 14110, and de-ionized by filters, such as Mixed Bed Deionization Cartridges. The use of two distinct paths eliminates the possible spread of vapor bubbles generated by the water switch into the transmission line. The separate paths also allow separate adjustment of the flow rates to improve performance.

[0025] Due to the relatively high conductivity of water, the Blumlein line should be pulse- charged. A high voltage charging pulse is generated by means of a step-up pulse transformer. The primary stage has 12 resonant charging units that are closed by thyristors (MCR265) to increase the output charging current. Each thyristor is triggered simultaneously through a transformer by the same trigger module and controlled by a pulse generator, such as the Berkeley Nucleonics Corporation, Model 555. The load of this charging circuit consists mainly of the capacitance of the water Blumlein line (until the water switch closes). The ohmic contribution of the water to the load can be neglected for the microsecond long charging pulses. Using deionized water with a resistivity p > 10 MΩ-cm, the dielectric relaxation time (τ_r = ε_oε_rp) of the dielectric exceeds 70 μs, which is long compared to the charging time. The Blumlein capacitance and the output inductance of the HV transformer form a resonant charging circuit. [0026] Using a capacitor of 2 nF provides a load for which the output voltage of the charging system can reach 45 kV for a dc input voltage, V₀, of 150 V. The repetition rate of the charging system is mainly determined by the thyristors in the primary stages. In actual operation the illustrated load is replaced by two conducting members (e.g., plates) with a food package positioned between them, as described above.

[0027] FIG. 3 is a flowchart of exemplary steps of a method for pasteurizing packaged food, according to another embodiment of the invention. The method illustratively includes, at step 304, positioning a packaged food between two electrical conducting members. Additionally, the method includes supplying a conductive fluid in gaps between the packaged food and two conducting members at step 306. The method further includes, at step 308, generating and conveying through the two electrical conducting members high-voltage electrical pulses. The method illustratively concludes at step 310.

[0028] The foregoing description of preferred embodiments of the invention have been presented for the purposes of illustration. The description is not intended to limit the invention to the precise forms disclosed. Indeed, modifications and variations will be readily apparent from the foregoing description. Accordingly, it is intended that the scope of the invention not be limited by the detailed description provided herein.

Claims

CLAIMSWe claim:

1. An apparatus for pasteurizing packaged food, the apparatus comprising: a pair of electrical conducting members; a pulse generator for supplying electric pulses to the electrical conducting members; a chamber positioned between the electrical conducting members, the chamber being configured to receive a packaged food; and a conductive fluid disposed between the electrical conducting members and the packaged food for effecting electrical contact between the electrical conducting members and a surface portion of the packaged food.

2. The apparatus of Claim 1 , wherein the pair of conducting members comprises two rectangular plates each having a length and a width, wherein the length of each plate is equal to a length of the packaged food and wherein the width of each plate is equal to a predetermined fraction of a width of the packaged food.

3. The apparatus of Claim 1, wherein the conductive fluid is saltwater.

4. The apparatus of Claim 1 , wherein the conductive fluid is an ionized gas.

5. A system for pasteurizing packaged food, the system comprising: a pair of electrical conducting members; a pulse generator for supplying electric pulses to the electrical conducting members; a chamber positioned between the electrical conducting members, the chamber being configured to receive a packaged food; a fluid disposed between the electrical conducting members and the packaged food for effecting electrical contact between the pair of conducting members and a surface portion of the packaged food; and a fluid flow system for supplying the fluid between the electrical conducting members and the packaged food.

6. The system of Claim 5, wherein the pulse generator comprises a Blumlein line generator.

7. The system of Claim 6, wherein the system comprises a pulse charging system for generating a high-voltage charging pulse to charge the Blumlein line generator.

8. The system of Claim 7, wherein the pulse charging system comprises a step-up pulse transformer for generating the high-voltage charging pulse.

9. The system of Claim 5, wherein the fluid flow system comprises a fluid reservoir, a pump for pumping fluid from the reservoir, and at least one filter for filtering the fluid.

10. A method of pasteurizing packaged food, the method comprising: positioning a packaged food between two electrical conducting members; supplying a conductive fluid in gaps between the packaged food and two conducting members; generating and conveying through the two electrical conducting members high-voltage electrical pulses.