Patent Publication Number: US-2020281053-A1

Title: Applicator system for heating with radio frequency energy

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application No. 62/812,603, filed Mar. 1, 2019, and entitled RADIO FREQUENCY HEATING SYSTEM AND METHOD FOR HEATING ARTICLES USING RADIO FREQUENCY ENERGY, the entire disclosure of which is hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to systems that use radio frequency (300 kHz to 300 MHz) energy to heat articles. 
     BACKGROUND 
     Electromagnetic radiation is a known mechanism for delivering energy to an object. The ability of electromagnetic energy to penetrate and heat an object in a rapid and effective manner has proven advantageous for a number of chemical and industrial processes. In the past, radio frequency (RF) energy has been used to heat articles by, for example, induction heating or dielectric heating. However, the use of RF energy to heat articles can have drawbacks. For example, the wavelength of RF energy can make it difficult to transmit and launch RF energy in an efficient manner. 
     The present invention involves discoveries for minimizing and/or eliminating many of the drawbacks conventionally associated with the use of RF energy to heat articles. For example, conventional RF heating processes that simultaneously convey and heat articles can have problems with nonuniform heating of the articles. This lack of uniform heating can be particularly problematic when the packages are relatively thick. Also, when RF heating is carried out under water (e.g., for pasteurizing or sterilizing packaged foodstuffs), the complexity of conveying articles increases due to the buoyancy of the articles. One cannot simply use a simple conveyor belt under water because the articles would tend to shift on the convey belt or even float off. 
     SUMMARY 
     One aspect of the present invention concerns a process for heating a plurality of articles using radio frequency (RF) energy, the process comprising: (a) passing RF energy through a waveguide toward an RF applicator; (b) introducing RF energy into the RF applicator filled with liquid, wherein the RF applicator has a substantially round cross-sectional shape and extends along a central axis of elongation; (c) passing a plurality of articles through the RF applicator in a convey direction using a convey system; and (d) during at least a portion of the passing, heating the articles using RF energy, wherein at least one of the RF waveguide and the RF applicator are substantially filled with liquid. 
     Another embodiment of the present invention concerns a radio frequency heating system for heating a plurality of articles. The RF heating system comprises an RF generator for generating RF energy, an RF waveguide configured to be substantially filled with liquid and, when filled with liquid, capable of transmitting RF energy produced by the RF generator; a cylindrical RF applicator configured to be substantially filled with liquid, and when filled with liquid, capable of receiving RF energy transmitted through the RF waveguide, wherein the RF applicator has a substantially round cross-sectional shape and extends along a central axis of elongation; and a convey system for transporting a plurality of articles through the RF applicator in a convey direction while the articles are being heated by RF energy. At least one of the RF waveguide and the RF applicator are configured to be substantially filled with liquid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the typical zones or steps of an RF heating system or process configured according to various embodiments of the present invention; 
         FIG. 2  is a block diagram of the typical zones or steps of an RF heating system according to various embodiments of the present invention, particularly where the system is used to pasteurize articles; 
         FIG. 3  is a block diagram of typical zones or steps of an RF heating system according to various embodiments of the present invention, particularly where the system is used to sterilize articles; 
         FIG. 4 a    is a schematic cross-sectional view of a vessel configured according to embodiments of the present invention, particularly illustrating use of a carrier to transport articles through the vessel; 
         FIG. 4 b    is a schematic cross-sectional view of a vessel configured according to embodiments of the present invention, particularly illustrating transport of articles through the vessel without a carrier; 
         FIG. 5 a    is a schematic cross-sectional view of a portion of an RF heating section configured according to embodiments of the present invention, particularly illustrating select elements of an RF transmission system; 
         FIG. 5 b    is a schematic top view of a portion of an RF heating section configured according to embodiments of the present invention, particularly illustrating a possible configuration of RF waveguides; 
         FIG. 5 c    is a schematic cross-sectional view of a portion of an RF heating section configured according to embodiments of the present invention, particularly illustrating a possible configuration of RF waveguides; 
         FIG. 5 d    is a schematic cross-sectional view of a portion of an RF heating section configured according to embodiments of the present invention, particularly illustrating an RF launcher; 
         FIG. 5 e    is a schematic cross-sectional view of a portion of an RF heating section configured according to embodiments of the present invention, particularly illustrating a coaxial transmission line and an RF waveguide in the RF transmission system; 
         FIG. 6 a    is a cross-sectional view of an RF applicator and RF waveguide configured according to various embodiments of the present invention; 
         FIG. 6 b    is a perspective cut-away view of the RF applicator and RF waveguide shown in  FIG. 6   a;    
         FIG. 7 a    is a schematic cross-sectional view of an RF applicator, particularly illustrating the relative position of the RF waveguide and the central axis of elongation of the RF applicator according to various embodiments of the present invention; 
         FIG. 7 b    is another schematic cross-sectional view of an RF applicator, particularly illustrating the relative position of the RF waveguide and the central axis of elongation of the RF applicator according to various embodiments of the present invention; 
         FIG. 8 a    is a schematic cross-sectional view of an RF applicator including a tube convey system configured according to various embodiments of the present invention, particularly illustrating an example of a mechanical driver; 
         FIG. 8 b    is a schematic cross-sectional view of an RF applicator including a tube convey system configured according to various embodiments of the present invention, particularly illustrating an example of a hydraulic driver; 
         FIG. 8 c    is a schematic cross-sectional view of an RF applicator including a tube convey system configured according to various embodiments of the present invention, particularly illustrating another example of a mechanical driver; 
         FIG. 9 a    is a schematic cross-sectional view of an RF applicator including a tube convey system configured according to various embodiments of the present invention, particularly illustrating an example of the transport tube orientation; and 
         FIG. 9 b    is a schematic cross-sectional view of an RF applicator including a tube convey system configured according to various embodiments of the present invention, particularly illustrating another example of the transport tube orientation. 
     
    
    
     DETAILED DESCRIPTION 
     In many commercial processes, it is often desirable to heat large numbers of individually packaged articles in a rapid and uniform manner. The present invention relates to systems and processes for such heating that use radio frequency (RF) energy to heat, or assist in heating, a plurality of articles. Examples of the types of articles that can be processed according to the present invention include, but are not limited to, packaged foodstuffs and beverages, as well as packaged pharmaceuticals, and packaged medical or veterinary fluids. The systems described herein may be configured for pasteurization, for sterilization, or for both pasteurization and sterilization. In general, pasteurization involves the rapid heating of an article or articles to a minimum temperature between about 60° C. and 100° C., about 65° C. to about 100° C., or about 70° C. and 100° C., while sterilization involves heating articles to a minimum temperature between about 100° C. and about 145° C., about 110° C. and about 140° C., or about 120° C. and about 135° C. 
       FIGS. 1-3  are overall diagrams of various embodiments of an RF heating system  10 . As shown in  FIGS. 1-3 , articles introduced into the RF heating system  10  can pass from a loading zone  12  into an optional initial thermal regulation section  16 , wherein the articles can be thermally treated to achieve a substantially uniform temperature. Next, the articles can be introduced into an RF heating section  18 , wherein the articles can be rapidly heated using RF energy, as described in further detail below. The heated articles are then passed through a subsequent thermal regulation section  20 , wherein the temperature of the articles can again be regulated. In some embodiments, as shown in  FIG. 3 , the subsequent thermal regulation section  20  may also include a thermal hold zone  30  in which the articles can be maintained at a constant temperature for a specified amount of time. Additionally, as shown in  FIGS. 2 and 3 , the subsequent thermal regulation section  20  may also include a high-pressure cooling zone  32  and a low-pressure cooling zone  34  for reducing the maximum surface temperature of the articles to a suitable handling temperature (e.g., 20° C. to 80° C.). Additional details regarding RF heating systems and sections thereof suitable for use in embodiments of the present invention are provided in co-pending U.S. patent application Ser. No. 16/163,481, the entire disclosure of which is incorporated herein by reference to the extent not inconsistent with the present disclosure. 
     In some embodiments, each of the initial thermal regulation section  16 , RF heating section  18 , and subsequent thermal regulation section  20  may be defined in a single vessel, while in other embodiments, at least one of these stages may be defined within two or more separate vessels. Additionally, one or more transition zones between individual processing stages or steps may also be defined in one or more separate vessels, or one or more of those transition zones may be defined within the same vessel as at least one preceding (e.g., upline) or subsequent (e.g., downline) stage or zone. 
     One or more of the vessels defining the initial thermal regulation section  16 , the RF heating section  18  and/or the subsequent thermal regulation section  20  may be configured to be at least partially liquid-filled. As used herein, the terms “liquid-filled,” or “filled with liquid,” denote a configuration in which at least 50 percent of the total internal volume of a vessel is filled with liquid. In certain embodiments, at least about 60, at least about 70, at least about 80, at least about 90, at least about 95, or at least about 99 percent of the total internal volume of one or more vessels may be filled with liquid. While being passed through a liquid-filled vessel, the articles may be at least partially, or completely, submerged in liquid during the processing step performed in that vessel. When two or more vessels are at least partially liquid-filled, the liquid in one vessel may be the same as, or different than, the liquid in another adjacent vessel. Thus, articles that are at least partially submerged in one type of liquid during the processing step performed in one vessel may be at least partially submerged in the same or in a different type of liquid during the processing step performed in a previous or subsequent vessel. 
     The liquid used in the vessel or vessels of RF heating system can be any suitable non-compressible fluid that exists in a liquid state at the operating conditions within the vessel. The liquid may have a dielectric constant greater than the dielectric constant of air. In some cases, the liquid may have a dielectric constant similar to the dielectric constant of the packaged substance being processed. For example, the dielectric constant of the liquid may be at least about 20, at least about 25, at least about 30, at least about 35, or at least about 40 and/or not more than about 120, not more than about 110, not more than about 100, not more than about 80, or not more than about 70, measured at a temperature of 80° C. and a frequency of 100 MHz. Water (or a liquid comprising water) may be particularly suitable for systems used to heat ingestible substances such as foodstuffs and medical or pharmaceutical fluids. Additives such as, for example, oils, alcohols, glycols, or salts, may be optionally be added to the liquid medium to alter or enhance its physical properties (e.g., boiling point) during processing, if needed. 
     The articles passing through the RF heating system may be contacted with liquid during at least a portion, or substantially all, of the travel path through the initial thermal regulation section  16 , the RF heating section  18 , and/or the subsequent thermal regulation section  20 . For example, in some embodiments, the initial thermal regulation section  16 , the RF heating section  18 , and the subsequent thermal regulation section  20  can be configured to maintain the articles in substantially continuous contact with liquid, thereby defining a liquid contact zone between the point at which the articles are initially contacted with liquid, such as, for example by spraying or submersion, and the point at which the articles are removed from contact with the liquid. In some embodiments, the liquid contact zone may include all or a portion of initial thermal regulation section  16 , RF heating section  18 , and/or subsequent thermal regulation section  20 . In other embodiments, the articles may not be submerged in, or even in contact with, liquid in all or a portion of initial thermal regulation section  16 , RF heating section  18 , and/or subsequent thermal regulation section  20 . 
     The RF heating system described herein may be configured to heat many different types of articles. Each article may include, for example, a sealed package surrounding at least one ingestible substance. Examples of ingestible substances can include, but are not limited to, food, beverages, medical, or pharmaceutical items suitable for human and/or animal consumption or use. A packaged article may include a single type of foodstuff (or other ingestible substance), or it may include two or more different ingestible substances, which may be in contact with each other or separated from one another within the package. The total volume of foodstuff (or other ingestible substance) within each sealed package can be at least about 4, at least about 6, at least about 8, at least about 10, at least about 20, at least about 25, or at least about 50 cubic inches and/or not more than about 500, not more than about 400, not more than about 300, not more than about 200, or not more than about 100 cubic inches. 
     In certain embodiments, the foodstuff or other ingestible substance being heated may have a dielectric constant of at least about 20 and not more than about 150. Additionally, or in the alternative, the foodstuff or other ingestible substance may have a dielectric loss factor of at least about 10 and not more than about 1500. Unless otherwise noted, the dielectric constant and dielectric loss factors provided herein are measured at a frequency of 100 MHz and a temperature of 80° C. In other embodiments, the foodstuff or other ingestible substance can have a dielectric constant of at least about 25, at least about 30, at least about 35, or at least about 40 and/or not more than about 140, not more than about 130, not more than about 120, not more than about 110, not more than about 100, not more than about 90, not more than about 80, not more than about 70, or not more than about 60, or it can be in the range of from about 20 to about 150, about 30 to about 100, or about 40 to about 60. Additionally, the foodstuff or other ingestible substance can have a dielectric loss factor of at least about 10, at least about 25, at least about 50, at least about 100, at least about 150, or at least about 200 and/or not more than about 1500, not more than about 1250, not more than about 1000, or not more than about 800, or it can be in the range of from about 10 to about 1500, about 100 to about 1250, or about 200 to about 800. 
     Examples of suitable ingestible substances can include solid foodstuffs such as, for example, fruits, vegetables, meats, soups, pastas, and pre-made meals. In other embodiments, the articles heated in the RF heating system can comprise packaged medical or pharmaceutical fluids, or medical or dental instruments. In still other embodiments, the ingestible substance within the package can comprise a liquid or semi-liquid. As used herein, the term “semi-liquid” refers to a liquid that also includes a gas, another liquid, or a solid, such as, for example, an emulsion, a suspension, a gel, or a solution. Semi-liquids can also include larger pieces of solid material, such as chunks of meat and vegetables in a soup or stew or pieces of fruit in a jam. Examples of suitable semi-liquids can include, but are not limited to, soups, stews, jams, sauces, gravy, or beverages. 
     The articles processed within the RF heating system can include packages of any suitable size and shape. In one embodiment, each package can have a length (longest dimension) of at least about 2 inches, at least about 4 inches, at least about 6 inches, or at least about 8 inches and/or not more than about 30, not more than about 20, not more than about 18, not more than about 15, not more than about 12, or not more than about 10 inches and a width (second longest dimension) of at least about 1 inch, at least about 2 inches, at least about 4 inches and/or not more than about 12 inches, not more than about 10 inches, or not more than about 8 inches. 
     In some embodiments, the packages can have a substantially round cross-sectional shape. For example, the packages may have a generally cylindrical shape and may include, for example, cans, jars, bottles, and/or other containers. When the packages have a cylindrical shape, the width of the package may be its diameter. In other embodiments, the package can have a generally rectangular or prism-like shape or may be a pouch. Such packages may also have a depth (shortest dimension) of at least about 0.10, at least about 0.25, at least about 0.5 inches, at least about 1 inch, at least about 2 inches and/or not more than about 8 inches, not more than about 6 inches, not more than about 4 inches, not more than about 2 inches, or not more than about 1 inch. 
     The articles can be in the form of separated individual items or packages or can be in the form of a continuous web of connected items or packages passed through the RF heating system. The items or packages may be constructed of any material, including plastics, cellulosics, and other substantially RF-transparent materials. The articles can be flexible, rigid, or semi-rigid. 
     The RF heating system can also include at least one conveyance system for transporting the articles through one or more of the processing zones described above. Examples of suitable conveyance systems can include, but are not limited to, plastic or rubber belt conveyors, chain conveyors, roller conveyors, flexible or multi-flexing conveyors, wire mesh conveyors, bucket conveyors, pneumatic conveyors, screw conveyors, trough or vibrating conveyors, helical conveyors, and combinations thereof. The conveyance system can include any number of individual convey lines and can be arranged in any suitable manner within the process vessels. The conveyance system utilized by the RF heating system can be configured in a generally fixed position within the vessel or at least a portion of the system can be adjustable in a lateral or vertical direction. 
     When the articles comprise individual packages, the articles may be transported through all or a portion of the RF heating system in a carrier. When used, the carrier can include an outer frame, upper and lower retention grids, and optionally a dielectric nest. The dielectric nest can include a plurality of openings for receiving the individual articles being heated. In some embodiments, the carrier may not include a dielectric nest, so that the individual articles are placed between and in contact with the upper and lower retention grids. Additional details of suitable carriers and other details about an RF heating system are described in U.S. Patent Application Publication No. 2016/0119984, which is incorporated herein by reference to the extent not inconsistent with the present disclosure. 
     In some embodiments, as shown in  FIG. 4 a   , the articles  98  loaded in a carrier  90  can be moved through one or more vessels  88  using a convey system  80 , that can include, for example, a chain drive  82 . Alternatively, no carrier is used to transport the articles through all or a portion of the RF heating system, so that the individual articles  98  are in contact with a portion of the convey line  84  as they are transported through the vessel  88 , as generally illustrated in  FIG. 4   b.    
     Turning back to  FIGS. 1-3 , the articles may be initially introduced into a loading zone  12 . In some embodiments, the loading zone  12  may be configured to initially contact the articles with liquid. This contacting may include, for example, spraying the articles with and/or at least partially submerging the articles in liquid. The articles introduced into the loading zone  12  may have an average temperature, measured at the geometric center of each article, of at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, or at least about 30° C. and/or not more than about 70, not more than about 60, not more than about 50, not more than about 40, or not more than about 30° C. As used herein, the “geometric center” of an article is the common point of intersection of planes passing through the midpoints of the article&#39;s length, width, and height. The loading zone may be operated at approximately ambient temperature and/or pressure. 
     As shown in  FIGS. 1-3 , the articles may be passed from a loading zone  12  into the initial thermal regulation zone  16 , when present. When introduced into the initial thermal regulation section  16 , the average temperature at the geometric center of the articles can be at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, or at least about 30° C. and/or not more than about 90, not more than about 80, not more than about 70, not more than about 60, not more than about 50, or not more than about 40° C. For pasteurization systems, the temperature at the geometric center of the articles introduced into initial thermal regulation section  16  may be in the range of from about 5° C. to about 70° C. or about 25° C. to about 40° C., while it may be in the range of from about 15° C. to about 90° C. or about 30° C. to about 60° C. for sterilization systems. 
     In certain embodiments, when present, the initial thermal regulation section  16  may be configured to change the temperature of each article, measured at its geometric center, by at least about 1, at least about 5, at least about 10, at least about 15, or at least about 20° C. and/or not more than about 60, not more than about 55, not more than about 50, not more than about 45, not more than about 40, not more than about 35, or not more than about 30° C., or it can be changed by an amount in the range of from about 1° C. to about 60° C. or about 10° C. to about 30° C. Depending on the temperature of the articles introduced into the initial thermal regulation section  16 , the temperature change may be an increase or decrease in an amount within the above ranges. 
     In certain embodiments, the average temperature at the geometric center of the articles exiting the initial thermal regulation section  16  may be at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, at least about 50, at least about 55, or at least about 60° C. and/or not more than about 90, not more than about 85, not more than about 80, not more than about 75, not more than about 70, or not more than about 65° C. During pasteurization, the average temperature at the geometric center of the articles exiting the initial thermal regulation section  16  can be in the range of from about 25° C. to about 90° C. or about 40° C. to about 70° C., while it may be in the range of from about 40° C. to about 90° C., or about 60° C. to about 80° C. during sterilization. When no initial thermal regulation section  16  is present, the articles introduced into the RF heating section  18  from the loading zone  12  can have temperatures within the above ranges. 
     Additionally, the initial thermal regulation section  16  may be configured to regulate the temperature of the articles passing therethrough to promote temperature uniformity amongst the articles. For example, in certain embodiments, the temperature of the articles may be regulated within the initial thermal regulation section  16  so that the average difference between the maximum temperature (i.e., hottest portion) and the minimum temperature (i.e., coldest portion) of each article exiting the initial thermal regulation section  16  can be not more than about 5, not more than about 2.5, not more than about 2, not more than about 1.5, not more than about 1, or not more than about 0.5° C. Similar differences can be achieved between the temperatures of adjacent articles removed from the initial temperature regulation section  16 , measured at the geometric center of each article. 
     In certain embodiments, the articles can have an average residence time in the initial thermal regulation section  16  of at least about 10, at least about 15, at least about 20, or at least about 25 minutes and/or not more than about 70, not more than about 65, not more than about 60, not more than about 55, not more than about 50, not more than about 45, or not more than about 40 minutes, or it can be in the range of from about 10 to about 70 minutes, or about 25 to about 40 minutes. 
     As shown in  FIGS. 2 and 3 , whether the RF heating system is configured for pasteurization or sterilization, the initial thermal regulation section  16  may include a thermal equilibration zone  24  followed by an optional pressure lock  26 . The thermal equilibration zone  24  may be configured to change the temperature of the articles passing therethrough in order to promote temperature uniformity within each article and amongst the articles passing therethrough, as described previously. In certain embodiments, articles passing through the thermal equilibration zone  24  may be contacted with liquid during at least a portion of the thermal equilibration step. The liquid may comprise or be water and can have a temperature within about 25, within about 20, within about 15, or within about 10° C. of the average temperature at the geometric center of the articles introduced into the thermal equilibration zone  24 . 
     The contacting may be performed by any suitable method including, but not limited to, by spraying the articles with and/or by submerging, or partially submerging, the articles in liquid. In some embodiments, the thermal equilibration zone  24  may further include one or more liquid jets for discharging streams of pressurized liquid toward the articles. Such pressurization may increase the Reynolds number of the liquid surrounding the article to values above, for example, 4500, thereby enhancing heat transfer. When present, the liquid jets may be positioned along or more walls of the vessel in which the thermal equilibration step is performed and may be used whether or not the articles are additionally submerged in liquid. 
     The articles exiting the thermal equilibration zone  24  shown in  FIGS. 2 and 3  can have an average temperature, measured at the geometric center of the articles, of at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, at least about 50, at least about 55, or at least about 60° C. and/or not more than about 90, not more than about 85, not more than about 80, not more than about 75, not more than about 70, or not more than about 65° C. When the articles are being pasteurized, the average temperature at the geometric center of the articles exiting the thermal equilibration zone  24  can be in the range of from about 25° C. to about 90° C. or about 40° C. to about 70° C., while it may be in the range of from about 40° C. to about 90° C., or about 60° C. to about 80° C. when the articles are being sterilized. The heated articles may also have a substantially uniform temperature such that, for example, the temperature at the geometric center of adjacent articles exiting the thermal equilibration zone  24  can be within about 2, within about 1.5, within about 1, or within about 0.5° C. of one another. 
     As shown in  FIGS. 2 and 3 , after exiting the thermal equilibration zone  24  of the initial thermal regulation section  16 , the articles may then be passed through a pressure lock  26   a  before entering the RF heating section  18 . In general, a pressure lock can be any device suitable for transitioning the articles between two environments having different pressures. Pressure locks may transition the articles from a higher-pressure environment to a lower-pressure environment or from a lower-pressure environment to a higher-pressure environment. In certain embodiments, pressure lock  26   a  may be configured to transition the articles from the lower-pressure thermal equilibration zone  24  when present or from the ambient-pressure loading zone  12  to the higher-pressure RF heating section  18 . In certain embodiments, the RF heating section  18  can have a pressure that is at least about 2, at least about 5, at least about 10, or at least about 15 psig and/or not more than about 50, not more than about 40, not more than about 30, not more than about 20, or not more than about 10 psig higher than the pressure in the thermal equilibration zone  24 , when present, or loading zone  12 . 
     Referring again to  FIGS. 2 and 3 , articles exiting the pressure lock  26   a  may be introduced into the RF heating section  18 . In RF heating section  18 , the articles may be rapidly heated via exposure to RF energy. As used herein, the term “RF energy” or “radio frequency energy” refers to electromagnetic energy having a frequency of greater than 300 kHz and less than 300 MHz. In certain embodiments, the RF heating section  18  can utilize RF energy having a frequency of at least about 500 kHz, at least about 1 MHz, at least about 5 MHz, at least about 10 MHz, at least about 20 MHz, at least about 30 MHz, at least about 40 MHz, or at least about 50 MHz. Additionally, or in the alternative, the RF heating section  18  may utilize RF energy having a frequency of not more than about 250 MHz, not more than about 200 MHz, or not more than about 150 MHz. The frequency of the RF energy utilized in the RF heating section  18  can be in the range of from 50 to 150 MHz. 
     In addition to RF energy, the RF heating section  18  may optionally utilize one or more other types of heat sources such as, for example, conductive or convective heat sources, or other conventional heating methods or devices. However, at least about 35, at least about 45, at least about 55, at least about 65, at least about 75, at least about 85, at least about 95 percent, or substantially all, of the energy used to heat the articles within the RF heating section  18  can be derived from an RF energy source. In some embodiments, not more than about 50, not more than about 40, not more than about 30, not more than about 20, not more than about 10, or not more than about 5 percent or substantially none of the energy used to heat the articles in the RF heating section  18  may be provided by other heat sources, including non-RF electromagnetic radiation having a frequency greater than 300 MHz. The RF energy is not microwave energy and does not have a frequency in the microwave energy band. 
     According to one embodiment, the RF heating section  18  can be configured to increase the temperature of the articles above a minimum threshold temperature. In embodiments where the RF heating system is configured to sterilize a plurality of articles, the minimum threshold temperature can be at least about 120° C., at least about 121° C., at least about 122° C. and/or not more than about 130° C., not more than about 128° C., or not more than about 126° C. The RF heating section  18  can be operated at approximately ambient pressure, or it can include one or more pressurized RF chambers operated at a pressure of at least about 5 psig, at least about 10 psig, at least about 15 psig and/or not more than about 80 psig, not more than about 60 psig, or not more than about 40 psig. In one embodiment, the pressurized RF chamber can have an operating pressure such that the articles being heated can reach a temperature above the normal boiling point of the liquid medium employed therein. 
     In some embodiments, the articles passing through the RF heating section  18  may be at least partially submerged in liquid while being heated with RF energy. In some embodiments, the liquid may be the same liquid in which the articles were submerged while passing through the initial thermal regulation section  16 . The RF heating section  18  may be at least partially defined within a pressurized vessel so that the pressure in the RF heating zone or within the RF applicator is maintained at a pressure of at least about 2, at least about 5, at least about 10, or at least about 15 psig and/or not more than about 80, not more than about 75, not more than about 70, not more than about 65, not more than about 60, not more than about 55, not more than about 50, not more than about 45, not more than about 40, not more than about 35, not more than about 30, not more than about 25, not more than about 20 psig during the heating step. When the articles passing through the RF heating section  18  are being pasteurized, the pressure in the RF heating section  18  may be in the range of from about 1 psig to about 40 psig or about 2 psig to about 20 psig. When the articles passing through the RF heating section  18  are being sterilized, the pressure in the RF heating section  18  may be in the range of from about 5 psig to about 80 psig, or about 15 psig to about 40 psig. When pressurized, the RF heating section  18  may or may not be at least partially filled with liquid and the articles may or may not be least partially submerged in liquid during the heating. 
     The temperature at the geometric center of the articles introduced into the RF heating section  18  can be at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, at least about 50, at least about 55, or at least about 60° C. and/or not more than about 110, not more than about 105, not more than about 100, not more than about 95, not more than about 90, not more than about 85, not more than about 80, not more than about 75, not more than about 70° C. When the articles are being pasteurized, the temperature at the geometric center of the articles introduced into the RF heating section  18  can be in the range of from about 25° C. to about 90° C. or about 40° C. to about 70° C., while articles being sterilized may have a temperature at the geometric center of the articles in the range of from about 40° C. to about 110° C. or about 60° C. to about 90° C. when entering the RF heating section  18 . 
     In certain embodiments, the RF heating section  18  may be configured to heat the articles passing therethrough so that the temperature of the geometric center of the articles increases by at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, or at least about 50° C. and/or not more than about 120, not more than about 110, not more than about 100, not more than about 90, not more than about 85, not more than about 80, not more than about 75, not more than about 70, not more than about 65, not more than about 60, not more than about 55, not more than about 50, not more than about 45, or not more than about 40° C. 
     When the articles are being pasteurized, the RF heating section  18  may be configured to increase the temperature of the geometric center of the articles by an amount in the range of from about 10° C. to about 60° C. or about 20° C. to about 40° C. When the articles are being sterilized, the RF heating zone may be configured to increase the temperature of the geometric center of the articles by an amount in the range of from about 20° C. to about 120° C. or about 35° C. to about 65° C. The RF heating section  18  can be configured to heat the articles at a heating rate of at least about 5° C. per minute (° C./min), at least about 10° C./min, at least about 15° C./min, at least about 20° C./min, at least about 25° C./min, at least about 35° C./min and/or not more than about 75° C./min, not more than about 50° C./min, not more than about 40° C./min, not more than about 35° C./min, not more than about 30° C./min, not more than about 25° C./min, not more than about 20° C./min, or not more than about 15° C./min. 
     The articles introduced into the RF heating section  18  may be heated to the desired temperature in a relatively short period of time. In some cases, this may help minimize damage or degradation of the foodstuff or other ingestible substance being heated. In certain embodiments, the articles passed through RF heating section  18  may have an average residence time in the RF heating section  18  (also called an RF heating period) of at least about 0.1, at least about 0.25, at least about 0.5, at least about 0.75, at least about 1, at least about 1.25, or at least about 1.5 minutes and/or not more than about 10, not more than about 8, not more than about 6, not more than about 5.5, not more than about 5, not more than about 4.5, not more than about 4, not more than about 3.5, not more than about 3, not more than about 2.5, not more than about 2, not more than about 1.5, or not more than about 1 minute. When the articles are being pasteurized, the average residence time of each article in the RF heating section  18  may be in the range of from about 0.1 minutes to 3 minutes, or 0.5 minutes to 1.5 minutes. When the articles are being sterilized, each article may have an average residence time in the range of from about 0.5 minutes to about 6 minutes, or about 1.5 minutes to about 3 minutes. 
     In some embodiments, the temperature at the geometric center of the articles exiting the RF heating section  18  can be at least about 60, at least about 65, at least about 70, at least about 75, at least about 80, at least about 85, at least about 90, at least about 95, at least about 100, at least about 105, or at least about 110° C. and/or not more than about 135, not more than about 130, not more than about 125, not more than about 120, not more than about 115, not more than about 110, or not more than about 105° C. When being pasteurized, the temperature at the geometric center of the articles exiting the RF heating section  18  can be in the range of from about 65° C. to about 115° C. or about 80° C. to about 105° C. When being sterilized, the temperature at the geometric center of articles exiting the RF heating section  18  can be in the range of from about 95° C. to about 135° C., or about 110° C. to about 125° C. 
     Turning now to  FIGS. 5 a -6 b   , various views of an RF heating section  118  configured according to embodiments of the present invention are shown. The RF heating section  118  may include an RF generator  120 , an RF energy transmission system  122 , and an RF applicator  124 , which can define the RF heating zone  126  therein. RF energy from the RF generator  120  may be passed by the RF energy transmission system  122  and discharged into the RF heating zone  126 , which is generally defined within the RF applicator  124 . Once in the RF heating zone  126 , the RF energy may be used to heat articles passing therethrough via at least one convey system  130 . 
     The RF generator  120  may be any device suitable for producing RF energy. In certain embodiments, the RF generator  120  can generate power in an amount of at least about 10, at least about 20, at least about 25, at least about 30, at least about 35 kW and/or not more than about 500, not more than about 250, not more than about 200, not more than about 150, not more than about 100, or not more than about 50 kW. RF heating systems of the present invention may use a single RF generator, or two or more RF generators to provide sufficient energy to the RF heating zone  126 . 
     The RF applicator  124  can be configured to act as a resonant cavity for the RF energy. In some embodiments, the RF applicator  124  can be a cylindrical RF applicator comprising continuous sidewalls that have a substantially round cross-sectional shape. As used herein, the term “substantially round” refers to a shape having a roundness factor of 2 or less. Roundness factor is defined by the Perimeter following formula: 
     
       
         
           
             
               Roundness 
                
               
                   
               
                
               Factor 
             
             = 
             
               
                 Perimeter 
                 
                   4 
                    
                   π 
                   × 
                   
                     Area 
                     
                       0 
                       . 
                       5 
                     
                   
                 
               
               . 
             
           
         
       
     
     In some embodiments, the RF applicator  124  can have a cross-sectional shape with a roundness factor of not more than 1.9, not more than about 1.8, not more than about 1.75, not more than about 1.7, not more than about 1.6, not more than about 1.5, not more than about 1.4, not more than about 1.2, not more than about 1.2, or not more than about 1.1. In some embodiments, the RF applicator  124  can have a round cross-sectional shape, such that its roundness factor is not more than 1.05. 
     The RF applicator  124  can also extend along a central axis of elongation shown, for example, by dashed line  250  in  FIG. 5 b   . In some embodiments, the RF applicator  124  can have a length-to-diameter ratio (L:D) of at least about 1:2, at least about 1:3, at least about 1:4, or at least about 1:5 and/or not more than about 15:1, not more than about 12:1, not more than about 10:1, not more than about 8:1, or not more than about 6:1. The cross-sectional shape of the RF applicator  124  can be substantially constant, such that the roundness factor varies by not more than about 10, not more than about 8, not more than about 5, not more than about 3, not more than about 2, or not more than about 1 percent along the entire length of the central axis of elongation  250 . In some cases, the exterior walls of the RF applicator  124  are the exterior walls of the vessel itself such that, for example, no additional pressure vessel surrounds the RF applicator  124 . 
     In some embodiments, the RF applicator  124  can be pressurized so that the pressure within the interior of the applicator  124  during operation is at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, or at least about 35 psig. As used herein, the unit of “psig” in reference to the pressure within the RF applicator  124  (or any other process vessel) is measured as the pressure above ambient fluid pressure within the vessel. Alternatively, or in addition, the RF applicator  124  can be at least partially filled with liquid, as discussed previously. Any suitable liquid may be used, including, for example, liquid that comprises or is water. The liquid may have a conductivity within one or more of the ranges provided herein. In some embodiments, the RF applicator can be both pressurized and at least partially liquid-filled. 
     The RF energy transmission system  122  is configured to transport RF energy from the RF generator  120  toward the RF applicator  124 . Several components of an RF energy transmission system  122  configured according to various embodiments of the present invention are shown in  FIGS. 5 a - e   . For example, the RF energy transmission system  122  can include at least one RF waveguide  132  for transporting RF energy from the RF generator  120  toward the RF applicator  124 . Additionally, in some embodiments, the RF energy transmission system  122  can include at least two waveguides  132   a,b  configured to pass RF energy into the same (as shown in  FIG. 5 b   ) or different (as shown in  FIG. 5 c   ) sides of the RF applicator  124 . When configured to pass RF energy into the same side of the RF applicator  124 , the waveguides  132   a,b  may be spaced apart from one another along the length of the RF applicator  124  in a direction generally parallel to the central axis of elongation  250  of the RF applicator  124  as shown in  FIG. 5 b   . When configured to pass RF energy into generally opposite sides of the RF applicator  124  (as shown in  FIG. 5 c   ), the waveguides  132   a,b  may be oppositely facing, or may be staggered from one another in a direction parallel to the central axis of elongation  250  of the RF applicator  124 . 
     As generally shown in  FIGS. 5 a -6 b   , at least one RF waveguide  132  (or when present RF launcher  138 ) may be configured to introduce RF energy into the side of the RF applicator  124 . For example, the launch axis of the RF waveguide  132  (or when present RF launcher), shown as dashed line  252  in  FIG. 5 a   , can lie in a plane substantially parallel to the horizontal plane containing the central axis of elongation  250  of the RF applicator  124 . The launch axis  252  of the RF waveguide  132  (or RF launcher) is defined by a straight line between the center point of the outlet of the waveguide  132  (or RF launcher) and the center point of the RF applicator  124 . As used herein, the term “substantially parallel” means within about 30° of being parallel. In some embodiments, the launch axis  252  of the RF waveguide  132  can lie in a plane that is within about 25°, within about 20°, within about 15°, within about 10°, within about 5°, within about 2°, or within 1° of being parallel to the horizontal plane containing the axis of elongation  250  of the RF applicator  124 . In some embodiments, the launch axis  252  of the RF waveguide  132  (or RF launcher, when present) lies in the same horizontal plane  260  as the central axis of elongation  250  of the RF applicator  124 , as generally depicted in  FIG. 7   a.    
     In other embodiments, the RF waveguide  132  (or when present, RF launcher) may be oriented such that RF energy is introduced into the upper and/or lower portions of the RF applicator  124 . In these embodiments, the launch axis  252  of the RF waveguide  132  (or RF launcher) may lie in a plane substantially perpendicular to the horizontal plane  260  containing the central axis of elongation  250  of the RF applicator  124 . As used herein, the term “substantially perpendicular” means within about 30° of being perpendicular. In some embodiments, the launch axis  252  of the RF waveguide  132  (or RF launcher) can lie in a plane that is within about 25°, within about 20°, within about 15°, within about 10°, within about 5°, within about 2°, or within 1° of being perpendicular to the horizontal plane containing the axis of elongation  250  of the RF applicator  124 . In some embodiments, the launch axis  252  of the RF waveguide  132  lies in a plane that is perpendicular to the horizontal plane containing the central axis of elongation  250  of the RF applicator  124 , as generally depicted in  FIG. 7   b.    
     In some embodiments, shown for example in  FIG. 5 e   , the RF energy transmission system  122  can include at least one coaxial conductor  134 , at least one waveguide  132 , and at least one coax-to-waveguide transition  136 . In these embodiments, RF energy produced by the RF generator  120  may be transferred by the coaxial conductor  134  and into the waveguide  132 . The coax-to-waveguide transition  136  may be configured to transition the RF energy from the coaxial conductor  134  into the waveguide  132 , which guides the RF energy into the RF applicator  124 . As generally shown in  FIG. 5 e   , the coaxial conductor  134  may include an inner conductor and an outer conductor that extend coaxially from the RF energy generator  120  to the inlet of the waveguide  132 . As shown in  FIG. 5 e   , the outer conductor may terminate at the wall of the waveguide  132 , while the inner conductor may extend through one wall of the waveguide  132  and into its interior to form the coax-to-waveguide transition  136 . Optionally, the inner conductor may extend through the opposite wall of the waveguide. A dielectric sleeve may surround the inner conductor where the inner conductor penetrates the wall or walls of the waveguide  132  in order to prevent fluid from flowing into the coaxial conductor  134 . The dielectric sleeve may be formed from any material capable of being sealed with the waveguide and that is substantially transparent to RF energy. One example of a suitable material includes, but is not limited to, glass fiber filled polytetrafluoroethylene (PTFE). 
     In addition, as shown in  FIG. 5 d    for example, the RF energy transmission system  122  may also include at least one RF launcher  138  located between the waveguide  132  and the RF applicator  124  for emitting RF energy into the RF applicator  124 . Each RF launcher  138  is configured to discharge energy from the waveguide  132  into the RF applicator  124  and may include, for example, a narrow end  137  and a broad end  139 . As shown in  FIG. 5 d   , the narrow end  137  can be coupled to the waveguide  132 , while the broad end  139  can be coupled to the RF applicator  124 . Although shown only shown in the embodiment depicted in  FIG. 5 d   , it should be understood that one or more of the configurations shown in  FIGS. 5 a -5 c  and 5 e    could also include one or more RF launchers between the waveguides  132  and the RF applicator  124 . 
     In certain embodiments, at least one of the RF applicator  124  and the RF waveguide  132  may be configured to be or be substantially filled with liquid, such as, for example, water. In some embodiments, each of the RF applicator  124  and RF waveguide  132  may be configured to be or be substantially filled with liquid. 
     When the waveguide  132  is at least partially filled with liquid, it may be capable of transmitting RF energy produced by said RF generator  120  towards the RF applicator  124 . The dimensions of the waveguide may be much smaller than if the waveguide were filled with air. For example, in certain embodiments, the waveguide  132  can have a generally rectangular cross-section with the dimension of the widest waveguide wall being in the range of from about 5 inches to about 40 inches or about 12 inches to about 20 inches, and the dimension of the narrowest waveguide wall being in the range of from about 2 inches to about 20 inches, about 4 inches to about 12 inches, or about 6 inches to about 10 inches. In one or more embodiments, the narrowest wall of the generally rectangular waveguide is substantially parallel to the central axis of elongation  250  of the RF applicator  124  and the widest wall is substantially perpendicular to the central axis of elongation  250  of the RF applicator  124 . 
     As shown in  FIGS. 6 a  and 6 b   , the waveguide  132  can include an inductive iris  140  disposed within the interior of the waveguide  132 . When present, the inductive iris  140  in the waveguide  132  can be formed by a pair of inductive iris panels  142   a,b  coupled to and extending from opposing walls of the RF waveguide  132 . In some embodiments, the iris panels  142   a,b  can be fixed to the upper walls of the waveguide  132 , as shown in  FIGS. 6 a  and 6 b   . When the RF waveguide  132  has a generally rectangular cross-section, the waveguide  132  can be oriented such that iris panels  142   a,b  are coupled to and extend downwardly and upwardly, respectively, from the narrower walls of the waveguide, thereby forming inductive iris  140 . As a result, the longest dimension of inductive iris  140  can extend parallel to the direction of extension of the RF applicator  124 . In some cases, the iris panels  142   a,b  may obstruct at least about 35, at least about 40, at least about 45, at least about 50, at least about 55, at least about 60, at least about 65, or at least about 70 percent of the total cross-section of the waveguide  132  at the location of the iris panels  142   a,b . Alternatively, or in addition, the iris panels  142   a,b  may obstruct not more than about 95, not more than about 90, not more than about 85, not more than about 80, not more than about 75, or not more than about 70 percent of the total cross-sectional area of the waveguide  132  at the location of the iris panels  142   a,b.    
     In certain embodiments, the RF applicator  124  can be in open communication with the interior of at least one RF waveguide  132 . As used herein, the terms “open communication” or “open to” mean that a fluid present in the RF applicator  124  and a fluid within the waveguide  132  may be permitted to flow therebetween with little or no restriction. When the interior of the RF applicator  124  is in open communication with the interior of the waveguide  132 , each can have a similar pressure. In some embodiments, the pressure within the RF applicator  124  and the interior of the waveguide  132  can be at least about 5, at least about 10, at least about 15, at least about 20, or at least about 25 psig and/or not more than about 80, not more than about 70, not more than about 60, not more than about 50, not more than about 40, or not more than about 35 psig. When the articles passing through the RF applicator  124  are being pasteurized, the pressure in the RF applicator and/or RF waveguide  132  can be in the range of from about 1 psig to about 40 psig or about 2 psig to about 20 psig. When the articles passing through the RF applicator  124  are being sterilized, the pressure can be in the range of from about 5 psig to about 80 psig, or about 15 psig to about 40 psig. 
     In certain embodiments, the interior of the RF applicator  124  and the interior of the waveguide  132  may be filled with a common liquid. The liquid can act as a transfer medium through which RF energy is passed as it is directed toward to the articles in the RF applicator  124 . The liquid can comprise, or be, any of the aforementioned types of liquid and, in some embodiments, may be pretreated in order to minimize its conductivity. For example, in some embodiments, the liquid may be treated so that it has a conductivity of not more than about 100, not more than about 90, not more than about 80, not more than about 70, not more than about 60, not more than about 50, not more than about 40, not more than about 30, not more than about 20, not more than about 10, not more than about 5, not more than about 1, not more than about 0.5, not more than about 0.1, or not more than about 0.05, or not more than about 0.01 milli-Siemens per minute (mS/m). In some embodiments, the liquid can comprise or be deionized or distilled water. 
     Alternatively, the interior of the RF applicator  124  and the RF waveguide  132  can be filled with different fluids. For example, the RF applicator  124  may be filled with one liquid, and the waveguide  132  may be filled with another, different liquid. In some embodiments, the RF heating section  118  may further include at least one window  148  positioned in the RF waveguide  132  between the RF generator  120  and the interior of the RF applicator  124 . The window  148  may be substantially transparent to RF energy, while still being capable of fluidly sealing the waveguide  132  from the interior of the RF applicator  124 . Thus, the window  148  may permit RF energy to pass through while preventing fluid flow between the RF waveguide  132  and the interior of the RF applicator  124 . In some embodiments, when present, the window  148  may form at least a portion of the sidewall of the RF applicator  124 . 
     The RF heating section may include at least one convey system  130  for transporting the articles in a convey direction through the RF heating zone  126  and into and out of the RF applicator  124 . The convey system  130  may include at least one conveyor and at least one driver for moving the conveyor in the convey direction. In some cases, the convey direction can be substantially horizontal, while in other cases, it can be substantially vertical. Any suitable type of conveyor can be used, including, for example, plastic or rubber belt conveyors, chain conveyors, roller conveyors, flexible or multi-flexing conveyors, wire mesh conveyors, bucket conveyors, pneumatic conveyors, trough conveyors, vibrating conveyors, helical conveyors, and combinations thereof. The conveyor may comprise a single convey segment, or it may include two or more convey segments arranged in parallel or in series. In some embodiments, the convey system is oriented within the RF applicator  124  so that the convey direction is substantially parallel to the central axis of elongation  250  of the RF applicator  124 . 
     In some embodiments, the convey system  130  in RF applicator  124  can comprise a tube convey system  230 , as generally shown in  FIGS. 6 a  and 6 b   . Tube convey system  230  can include at least one transport tube  232  for holding the articles and a drive mechanism for moving the articles through the transport tube  232 . As shown in  FIG. 6 b   , the transport tube  232  may be cylindrical and may have a substantially round cross-sectional shape. In some embodiments, the transport tube  232  may have a roundness factor of not more than about 1.9, not more than about 1.8, not more than about 1.7, not more than about 1.6, not more than about 1.5, not more than about 1.4, not more than about 1.3, not more than about 1.2, or not more than about 1.1, or it may have a round cross-sectional shape with a roundness factor of not more than 1.05. The tube conveyor  230  may include a single transport tube  232 , as shown in  FIGS. 6 a  and 6 b   , or it may include two or more transport tubes positioned within the RF applicator  124 . 
     In some embodiments, both the RF applicator  124  and the transport tube  232  may have a substantially round cross-sectional shape. The ratio of the diameter of the transport tube  232 , shown as D 2  in  FIG. 6 a   , to the diameter of the RF applicator  124 , shown as D 1  in  FIG. 6 a   , can be not more than about 0.95:1, not more than about 0.90:1, not more than about 0.85:1, not more than about 0.80:1, not more than about 0.75:1, not more than about 0.70:1, not more than about 0.65:1, not more than about 0.60:1, not more than about 0.55:1, not more than about 0.50:1, not more than about 0.45:1, or not more than about 0.40:1. The ratio of the diameters D 2 :D 1  can be at least about 0.10:1, at least about 0.15:1, at least about 0.20:1, at least about 0.25:1, or at least about 0.30:1. 
     Transport tube  232  may be formed from any suitable RF-transparent material, such as, for example, plastics like PTFE. In some cases, transport tube  232  may be fluidly isolated from the RF applicator  124 , such that, for example, the pressure in the transport tube  232  or the RF applicator  124  is at least about 1, at least about 2, at least about 5, at least about 10, at least about 15, or at least about 20 psig different than the other. In some cases, the transport tube  232  may be filled with liquid, while in other cases, the transport tube  232  may not be filled with liquid. As the articles pass through transport tube  232 , they may or may not be submerged in liquid during the heating step. When transport tube  232  is filled with liquid, it may be the same liquid as is present in the RF applicator  124  or it may be a different liquid. Transport tube  232  may be configured to permit fluid to flow between the interior of the RF applicator  124  and the interior of the transport tube  232 , or it may be configured to prevent such flow. The pressure within transport tube  232  may be the same as or different than, such as, for example, higher or lower by at least 2, at least 5, or at least 10 psig, the pressure in the RF applicator  124 . In some embodiments, each of the RF waveguide  132 , the RF applicator  124 , and the transport tube  232  may be substantially filled with liquid. 
     Transport tube  232  may extend along a central axis of elongation, shown in  FIG. 6 b    as numeral  260 . As shown in  FIG. 6 b   , the transport tube  232  may be positioned within the RF applicator  124  such that the central axis of elongation  250  of the RF applicator  124  and at least a portion of the central axis of elongation  260  of the transport tube  232  are substantially parallel to one another. In some embodiments, the central axis of elongation  260  of the transport tube  232  and the central axis of elongation  250  of the RF applicator  124  may overlap over a portion, or all, of the length of the RF applicator  124 . In some embodiments, at least about 50, at least about 55, at least about 60, at least about 65, at least about 70, at least about 75, at least about 80, at least about 85, at least about 90, or at least about 95 percent of the central axis of elongation  260  of the transport tube  232  is substantially parallel to, or overlaps, the central axis of elongation  250  of the RF applicator  124 . When the tube convey system  230  includes two or more transport tubes, the tubes may be centered or grouped around the central axis of elongation  250  of the RF applicator  124 . 
     Tube convey system  230  further comprises at least one drive mechanism for moving the articles through the transport tube  232 . In some embodiments, the drive mechanism can be a mechanical mechanism. One example of a mechanical mechanism is shown in  FIG. 8 a    and includes a plurality of pusher tabs  236  for contacting and pushing the articles  100  through the transport tube  232 . In such embodiments, the lower portion of the transport tube  132  may include a slit (not shown) through which the pusher tabs  236  are inserted in order to contact the articles  100 . Each pusher tab  236  is coupled to a drive member  238 , such as a belt, chain, or rod, that moves in the convey direction  262  to thereby move the pusher tabs  236  and articles  100  through the transport tube  232 . 
     Another example of a mechanical mechanism is shown in  FIG. 8 c    and includes a pusher arm  238  for contacting an article  100  (for example on its back surface as shown) and moving the article  100  into the transport tube  232 . As the front surface of the article  100  introduced into the transport tube  232  contacts the back surface of the adjacent article  100 , the pusher arm  238  continues to push the last article  100 , thereby causing all of the articles to move through the transport tube  232  and, in some cases, causing an article  100  to exit the tube  232  in a “one-in, one-out” type of movement. In some embodiments, such a mechanical driver can be used alone, or in combination with one or more other types of drivers. 
     In another embodiment, the drive mechanism of the tube convey system  230  may be a hydraulic drive mechanism. When the transport tube  232  is at least partially filled with liquid, at least a portion of the liquid may be pressurized by a liquid pressurization mechanism and the resulting jet of pressurized liquid can be used to push the articles through the transport tube  232 . One example of this is illustrated in  FIG. 8 b   , where the pressurization mechanism is shown as pump  240 . Jets of fluid discharged from pump  240  are used to push the articles  100  through the tube  232 . In some embodiments, the pressurized liquid can originate from within, by for example being recycled from the end of transport tube  232 , as shown in  FIG. 8 b   , or it can originate from another source (not shown). 
     In other embodiments, at least a portion of the transport tube  232  can be oriented vertically, or within about 45° from the vertical, so that the articles move through at least a portion of the transport tube  232  by, or with the assistance of, gravity. In some embodiments, at least a portion of the transport tube  232  can be oriented within about 40, within about 35, within about 30, within about 25, within about 20, within about 15, within about 10, within about 5, or within about 2° of the vertical, while in other embodiments, the transport tube can be oriented vertically. Examples of this are shown in  FIGS. 9 a  and 9 b   . In embodiments as depicted in  FIG. 9 a   , the entire transport tube  232  is oriented vertically, while only a portion of the transport tube  232  shown in  FIG. 9 b    is oriented from the vertical  280 . In each embodiment, the articles  100  introduced into the upper end of the tube  232  move downwardly through the tube by gravity before (as shown in  FIG. 9 b   ) or while (as shown in  FIG. 9 a   ) passing through the RF heating zone  126 . In some embodiments, even if it includes a vertically oriented transport tube  232 , the tube convey system  230  can further include least one driver for moving the articles through the tube  232 , while, in other embodiments, the articles move by gravity only. As shown in  FIGS. 9 a  and 9 b   , the central axis of elongation of the transport tube  232  may be aligned substantially parallel to the central axis of elongation of the RF applicator  124  when the articles pass by the RF waveguide  132  and, when present, RF launcher  138 . 
     As the articles pass through the RF heating zone  126 , at least a portion of the RF energy discharged into the RF applicator  124  may be used to heat the articles. It was found that when the applicator  124  has a substantially round cross-sectional shape, the RF energy field can become concentrated in the centermost region of the vessel near the central axis of elongation. This field can be denser and more stable than the RF field in other regions of the applicator, so positioning the articles to be heated within this concentrated RF energy field results in more rapid and uniform heating than if all, or part, of the articles were positioned outside this field. Thus, in some embodiments when the articles being heated also have a substantially round cross-sectional shape, the ratio of the maximum diameter of an article to the diameter of the RF applicator  124  can be not more than about 0.60:1, not more than about 0.55:1, not more than about 0.50:1, not more than about 0.45:1, not more than about 0.40:1, not more than about 0.35:1, not more than about 0.30:1, not more than about 0.25:1, not more than about 0.20:1, not more than about 0.15:1, not more than about 0.10:1, or not more than about 0.05:1. This may help ensure that at least about 80, at least about 85, at least about 90, at least about 95, or all of the volume of the article is located within the concentrated RF energy field. 
     Returning again to  FIG. 1 , the articles exiting the RF heating section  18  may be introduced into a subsequent thermal regulation section  20 , wherein, ultimately, the average temperature at the geometric center of the articles can be reduced by at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, or at least about 50° C. Thus, the average temperature at the geometric center of the articles withdrawn from the last stage of the subsequent thermal regulation section  20  can be at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, or at least about 50° C. cooler than the average temperature at the geometric center of the articles introduced into the first stage of the subsequent thermal regulation section  20 . 
     The average temperature at the geometric center of the articles withdrawn from the last stage of the subsequent thermal regulation section  20  can be not more than about 120, not more than about 110, not more than about 100, not more than about 90, not more than about 80, not more than about 70, not more than about 60, not more than about 50, not more than about 40° C. lower than the average temperature at the geometric center of the articles entering the subsequent thermal regulation section  20 . When the articles are being pasteurized, the temperature of the articles passed through the subsequent thermal regulation section  20  can be reduced by about 10° C. to about 60° C., or about 20° C. to about 40° C. When the articles are being sterilized, the average temperature at the geometric center of the articles passed through the subsequent thermal regulation section  20  can be reduced by about 20° C. to about 120° C. or about 40° C. to about 60° C. 
     In certain embodiments, the articles can have an average residence time in the subsequent thermal regulation section  20  of at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, or at least about 50 minutes and/or not more than about 120, not more than about 110, not more than about 100, not more than about 90, not more than about 80, not more than about 70, not more than about 60, not more than about 50, or not more than about 40 minutes. When the articles are being pasteurized, the average residence time of the articles in the subsequent thermal regulation section  20  can be in the range of from about 5 minutes to about 60 minutes or about 25 minutes to about 40 minutes. When the articles are being sterilized, the average residence time of the articles in subsequent thermal regulation section  20  can be in the range of from about 15 minutes to about 120 minutes, or about 50 minutes to about 80 minutes. 
     Referring again to  FIG. 2 , when the articles passing through the RF heating system are being pasteurized, the subsequent thermal regulation section  20  can include a high-pressure cooling zone  32 , a pressure lock  26   b , and a low-pressure cooling zone  34 . Articles being pasteurized are not passed through a hold zone (as shown in  FIG. 3 ), but are instead transitioned directly from the RF heating section  18  into the high-pressure cooling zone  32 , as shown in  FIG. 2 . In the case that the RF heating system includes a hold zone (such, as for example, in the case that the same system is used for both pasteurization and sterilization), the articles being pasteurized can have an average residence time in a hold zone of not more than about 10, not more than about 8, not more than about 6, not more than about 4, not more than about 2, or not more than about 1 minute. Additionally, or in the alternative, not more than about 15, not more than about 12, not more than about 10, not more than about 8, not more than about 5, not more than about 2, or not more than about 1 percent of the total travel path of the articles through the RF heating system may be defined in the hold zone when the articles are being pasteurized. In such embodiments, the average temperature at the geometric center of the articles being pasteurized changes by not more than about 15, not more than about 10, not more than about 5, not more than about 2, or not more than about 1° C. as the articles pass through a hold zone. The temperature of at least about 95, at least about 98, or at least about 99 percent of the total volume of the articles being pasteurized withdrawn from the hold zone, if present, can be within a temperature range of about 2.5, about 2, about 1.5, about 1, about 0.75, about 0.50, or about 0.25° C. 
     Turning now to  FIG. 3 , when the articles passed through the RF heating system  10  are being sterilized, the subsequent thermal regulation section  20  includes a thermal isolation zone  28 , a hold zone  30 , a high-pressure cooling zone  32 , a pressure lock  26   b , and a low-pressure cooling zone  34 . Articles exiting the RF heating section  18  may be passed through a thermal isolation zone  28  before entering the hold zone  30 . In certain embodiments, the temperature of the fluid (e.g., liquid medium if liquid-filled) in the hold zone  30  may be at least about 2, at least about 5, at least about 8, at least about 10, at least about 12, at least about 15, at least about 18, or at least about 20° C. higher than the average temperature of the fluid (e.g., liquid medium if liquid-filled) in the RF heating section  18 . The thermal isolation zone  28  may be configured to transition the articles from the RF heating section  18  to the hold zone  30  while maintaining the difference in temperature between the two zones. 
     In the hold zone  30 , the temperature of each article being sterilized is maintained at or above a specified minimum temperature for a certain amount of time. In certain embodiments, the temperature at the geometric center of each article passing through the hold zone  30  can be maintained at a temperature at or above the average temperature at the geometric center of the articles exiting the RF heating section  18 . As a result, the articles exiting the hold zone  30  may be sufficiently and uniformly sterilized. 
     In certain embodiments, articles passing through hold zone  30  may be contacted with liquid during at least a portion of the hold step. The liquid may comprise or be water and can have a temperature within about 25, within about 20, within about 15, or within about 10° C. of the average temperature at the geometric center of the articles introduced into the hold zone  30 . The step of contacting may include submerging the articles in liquid and/or contacting at least a portion of the articles with a jet of liquid emitted from one or more spray nozzles within the hold zone  30 . 
     Overall, the average temperature at the geometric center of the articles passing through the hold zone  30  may increase by at least about 2, at least about 4, at least about 5, at least about 8, at least about 10, or at least about 12° C. and/or not more than about 40, not more than about 35, not more than about 30, not more than about 25, or not more than about 20° C., or it may increase by about 4° C. to about 40° C., or about 10° C. to about 20° C. In certain embodiments, the articles withdrawn from the hold zone  30  can be uniformly heated so that, for example, the temperature of at least about 95, at least about 98, or at least about 99 percent of the total volume of the articles can be within a temperature range of about 2.5, about 2, about 1.5, about 1, about 0.75, about 0.50, or about 0.25° C. 
     In certain embodiments, the average residence time of each article passed through the hold zone  30  (e.g., the hold time) can be at least about 1, at least about 2, at least about 5, at least about 6, or at least about 8 minutes and/or not more than about 40, not more than about 35, not more than about 30, not more than about 25, not more than about 20, not more than about 15, or not more than about 10 minutes, or it can be in the range of from 2 minutes to 40 minutes or 6 minutes to 20 minutes. 
     As shown in  FIGS. 2 and 3 , articles being pasteurized removed from the RF heating section  18 , and articles being sterilized removed from the hold zone  30  may be introduced into the high-pressure cooling zone  32 . In the high-pressure cooling zone  32  the average temperature at the geometric center of the articles can be reduced by at least about 5, at least about 10, at least about 15, or at least about 20° C. and/or not more than about 60, not more than about 55, not more than about 50, not more than about 45, not more than about 40, not more than about 35, or not more than about 30° C. When the articles are being pasteurized, the average temperature at the geometric center of the articles can be reduced by about 5° C. to about 40° C. or about 10° C. to about 30° C. When the articles are being sterilized, the average temperature at the geometric center of the articles can be reduced by about 10° C. to about 60° C., or about 20° C. to about 40° C. as the articles pass through the high-pressure cooling zone  32 . 
     Articles introduced into the high-pressure cooling zone  32  can have an average temperature at the geometric center of at least about 80, at least about 85, at least about 90, at least about 95, at least about 100, at least about 105, at least about 110, at least about 115, or at least about 120° C. and/or not more than about 135, not more than about 130, not more than about 125, not more than about 120, not more than about 115, not more than about 110, or not more than about 105° C. When the articles are being pasteurized and are introduced into the high-pressure cooling zone  32  from the RF heating section  18 , the average temperature at the geometric center of the articles can be in the range of from about 80° C. to about 115° C., or about 95° C. to about 105° C. When the articles are being sterilized and are introduced into the high-pressure cooling zone  32  from the hold zone  30 , the average temperature at the geometric center of the articles can be in the range of from about 110° C. to about 135° C. or about 120° C. to about 130° C. The average difference between the maximum temperature (i.e., hottest portion) and the minimum temperature (i.e., coldest portion) of each article exiting the hold zone  30  can be not more than about 5, not more than about 2.5, not more than about 2, not more than about 1.5, not more than about 1, or not more than about 0.5° C. 
     In certain embodiments, the hold zone  30  can have a pressure of at least about 2, at least about 5, at least about 10, or at least about 15 psig and/or not more than about 80, not more than about 75, not more than about 70, not more than about 65, not more than about 60, not more than about 55, not more than about 50, not more than about 45, not more than about 40, not more than about 35, not more than about 30, not more than about 25, not more than about 20 psig. 
     The average residence time of the articles passing through the high-pressure cooling zone  32  can be at least about 1, at least about 2, at least about 5, or at least about 10 minutes and/or not more than about 60, not more than about 55, not more than about 50, not more than about 45, not more than about 40, not more than about 35, not more than about 30, not more than about 25, not more than about 20, not more than about 15, or not more than about 10 minutes. When the articles passed through the high-pressure cooling zone  32  are being pasteurized, the average residence time of the articles in high-pressure cooling zone  32  can be in the range of from about 1 minute to about 30 minutes, or about 5 minutes to about 10 minutes. When the articles are being sterilized, the average residence time of the articles passing through the high-pressure cooling zone  32  can be in the range of from about 2 to about 60 minutes, or about 10 to about 20 minutes. 
     When the articles heated in the RF heating system are being sterilized, the residence time of the articles in the hold zone  30  can be less than, similar to, or greater than the residence time of the articles in the high-pressure cooling zone  32 . For example, in certain embodiments, the average residence time of the articles passing through the hold zone  30  can be at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, or at least about 50 percent and/or not more than about 400, not more than about 300, not more than about 200, not more than about 150 percent of the average residence time of the articles passing through the high-pressure cooling zone  32 . 
     When the articles are being pasteurized (and are not passed through a hold zone), the residence time of articles passing through the hold zone can be not more than about 25, not more than about 20, not more than about 15, not more than about 10, or not more than about 5 percent of the residence time of the articles passing through the high-pressure cooling zone  32 . When the articles are being sterilized, the residence time of the articles passing through the hold zone  30  can be in the range of from about 25 percent to about 400 percent, or about 50 percent to about 150 percent of the average residence time of the articles passing through the high-pressure cooling zone  32 . 
     Articles passing through the high-pressure cooling zone  32  may be contacted with liquid during at least a portion of the cooling step. The liquid may comprise or be water and can have a temperature within about 25, within about 20, within about 15, or within about 10° C. of the average temperature at the geometric center of the articles withdrawn from the outlet of the high-pressure cooling zone  32 . The step of contacting may include submerging the articles in liquid and/or contacting at least a portion of the articles with a jet of liquid emitted from one or more spray nozzles within the high-pressure cooling zone  32 . 
     In certain embodiments, when the hold zone  30  and the high-pressure cooling zone  32  are at least partially liquid filled, the average temperature of the liquid in the hold zone  30  can be at least about 20, at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, at least about 50, at least about 55, at least about 60, at least about 65, at least about 70, at least about 75, at least about 80, at least about 85, at least about 90, at least about 95, or at least about 100° C. and/or not more than about 200, not more than about 190, not more than about 180, not more than about 170, not more than about 160, not more than about 150, not more than about 140, not more than about 130, not more than about 120, not more than about 110, not more than about 100, or not more than about 90° C. higher than the average temperature of the liquid in the high-pressure cooling zone  32 . Additionally, or in the alternative, the pressures of the hold zone  30  and the high-pressure cooling zone  32  may be within about 10, within about 5, within about 2, or within about 1 psig of one another. 
     As shown in  FIGS. 2 and 3 , the articles exiting the high-pressure cooling zone  32  can be passed through another pressure lock  26   b  before entering the low-pressure cooling zone  34 . Similarly to pressure lock  26   a  described previously with respect to  FIGS. 11 and 12 , the pressure lock  26   b  can be configured to transition the articles between two environments having different pressures. Pressure lock  26   b  shown in  FIGS. 2 and 3  may be configured to transition the articles from a higher-pressure environment to a lower-pressure environment, such as, for example, from the high-pressure cooling zone  32  to the low-pressure cooling zone  34 . In certain embodiments, the high-pressure cooling zone  32  can have a pressure that is at least about 2, at least about 5, at least about 10, or at least about 15 psig and/or not more than about 50, not more than about 40, not more than about 30, not more than about 20, or not more than about 10 psig higher than the pressure in high-pressure cooling zone  32 . 
     Low-pressure cooling zone  34  may be configured to reduce the temperature at the geometric center of the articles by at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, at least about 35, or at least about 40° C. and/or not more than about 100, not more than about 95, not more than about 90, not more than about 85, not more than about 80, not more than about 75, not more than about 70, not more than about 65, not more than about 60, or not more than about 55° C. When the articles are being pasteurized, the low-pressure cooling zone  34  may reduce the temperature at the geometric center of the articles passing therethrough by about 5° C. to about 100° C. or about 50° C. to about 80° C. When the articles are being sterilized, the low-pressure cooling zone  34  may reduce the temperature at the geometric center of the articles by about 10° C. to about 75° C. or about 40° C. to about 60° C. 
     When removed from the low-pressure cooling zone  34 , the articles may be at a suitable handling temperature. For example, the temperature at the geometric center of the articles exiting the low-pressure cooling zone  34  can be at least about 50, at least about 55, at least about 60, at least about 65, at least about 70, at least about 75, or at least about 80° C. and/or not more than about 100, not more than about 97, not more than about 95, not more than about 90, or not more than about 85° C. When being pasteurized, the articles withdrawn from the low-pressure cooling zone  34  can have an average temperature at the geometric center in the range of from about 50° C. to about 97° C. or about 80° C. to about 95° C. When being sterilized, the average temperature at the geometric center of the articles exiting the low-pressure cooling zone  34  can be about 50° C. to about 100° C. or about 80° C. to about 97° C. The average difference between the maximum temperature (i.e., hottest portion) and the minimum temperature (i.e., coldest portion) of each article exiting the low-pressure cooling zone can be not more than about 5, not more than about 2.5, not more than about 2, not more than about 1.5, not more than about 1, or not more than about 0.5° C. 
     The average residence time of the articles passing through the low-pressure cooling zone  34  can be at least about 1, at least about 2, at least about 5, at least about 8, at least about 10, at least about 12, or at least about 15 minutes and/or not more than about 80, not more than about 70, not more than about 60, not more than about 50, not more than about 40, not more than about 30, or not more than about 20 minutes. When the articles are being pasteurized, the average residence time of the articles in the low-pressure cooling zone  34  can be in the range of from about 1 minute to about 80 minutes, or about 5 minutes to about 20 minutes. When the articles are being sterilized, the average residence time of the articles in the low-pressure cooling zone  34  can be in the range of from about 2 minutes to about 80 minutes or about 15 minutes to about 40 minutes. 
     As shown in  FIG. 1 , the cooled articles exiting the low-pressure cooling zone  34  may be removed from the RF heating system  10  via an unloading zone  22 . Any suitable method or device may be used to remove the articles from contact with liquid in unloading zone  22 . The temperature at the geometric center of the articles removed from the unloading zone  22  can be at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, or at least about 50° C. and/or not more than about 80, not more than about 75, not more than about 70, not more than about 65, or not more than about 60° C. The unloading zone may be operated at approximately ambient temperature and/or pressure. Once removed from the unloading zone  22 , the articles may be transported for further processing, storage, shipment, or use. 
     The RF heating systems as described herein may be configured to achieve an overall production rate of at least about 2, at least about 5, at least about 10, at least about 15, at least about 20, at least about 25, at least about 30, at least about 35, at least about 40, at least about 45, at least about 50 articles per minute (articles/min) and/or not more than about 500, not more than about 450, not more than about 400, not more than about 350, not more than about 300, not more than about 250, not more than about 200 articles/min. In other embodiments, the mass convey rate of the food (or other edible substance) passing through the RF heating system can be at least about 1, at least about 5, at least about 10, at least about 15, at least about 20, or at least about 25 pounds of food (or other edible substance) per minute (lb/min) and/or not more than about 500, not more than about 450, not more than about 400, not more than about 350, not more than about 300, not more than about 250, not more than about 200, not more than about 150 lb/min. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventor hereby states his intention to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any method or apparatus departing from but not outside the literal scope of the invention as set forth in the following claims.