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Title:
METHOD AND APPARATUS FOR PASTEURIZING EGG PRODUCTS USING HIGH FREQUENCY
Document Type and Number:
WIPO Patent Application WO/1995/010943
Kind Code:
A1
Abstract:
Methods and apparatus for pasteurizing egg products utilizing high frequency radio waves generator (14) to heat the egg products being pasteurized. Suitable products to be treated include liquid egg products, packaged liquid egg products, and shell eggs. The methods may be used with either continuous processing systems or batch processing systems.

Inventors:
Hamid-samimi, Mohammad H.
Swartzel, Kenneth D.
Ball Jr., Hershell R.
Application Number:
PCT/US1994/011935
Publication Date:
April 27, 1995
Filing Date:
October 19, 1994
Export Citation:
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Assignee:
Michael
FOODS, INC.
Hamid-samimi, Mohammad H.
Swartzel, Kenneth D.
Ball Jr., Hershell R.
International Classes:
A23B5/01; (IPC1-7): A23B5/01
Domestic Patent References:
WO1993019620A11993-10-14
Foreign References:
EP0497099A11992-08-05
US3830945A1974-08-20
US4853238A1989-08-01
GB2265537A1993-10-06
GB364129A1931-12-28
Other References:
PATENT ABSTRACTS OF JAPAN vol. 11, no. 381 (C - 464) 12 December 1987 (1987-12-12)
PATENT ABSTRACTS OF JAPAN vol. 7, no. 151 (C - 174) 1 July 1958 (1958-07-01)
PATENT ABSTRACTS OF JAPAN vol. 8, no. 145 (C - 232) 6 July 1984 (1984-07-06)
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Claims:
CLAIMS :
1. A method of pasteurizing a flowable egg product while passing the product as a continuous stream through a pasteurizing apparatus, comprising: passing said flowable egg product continuously through a conduit transparent to high frequency radio waves; and heating said flowable egg product in said conduit to a predetermined temperature by subjecting said egg product to high frequency radio waves; and then holding said flowable egg product at said predetermined temperature for a predetermined time; wherein the total thermal treatment of said product is sufficient to pasteurize said egg product without coagulation thereof.
2. A method according to claim 1, further comprising the step of aseptically packaging said egg product following said holding step.
3. A method according to claim 1, further comprising the step of sterilizing said pasteurization apparatus before passing said flowable egg product therethrough.
4. A method according to claim 1, further comprising the step of preheating said liquid egg product to a temperature from about 140°F to about 155°F prior to said heating step.
5. A method according to claim 1, said radio waves having a frequency of from 15 MegaHertz to 150 MegaHertz .
6. A method according to claim 1, said radio waves having a wavelength of from about 2 meters to about 20 meters.
7. A method according to claim 1, said radio waves having a wavelength of about 10 meters.
8. A method according to claim 1, wherein said flowable egg product is a liquid egg product .
9. A method according to claim 1, wherein said flowable egg product comprises a particulate egg product in a liquid medium.
10. A method according to claim 1, wherein said flowable egg product comprises shell eggs in a liquid medium.
11. A method according to claim 1, wherein said flowable egg product comprises prepackaged egg products in a liquid medium.
12. An apparatus for pasteurizing a flowable egg product, comprising: (a) an egg product holding tank; (b) an egg product line connected to said holding tank; (c) pumping means connected to said egg product line for establishing an egg product stream in said egg product line, said egg product stream flowing away from said holding tank; (d) a radiolucent conduit interposed in said egg product line; (e) a generator which generates high frequency radio waves operatively associated with said radiolucent conduit, with said generator positioned so that radio waves generated pass through said radiolucent conduit and heat said egg product stream; and (f) a holding section in said egg product line, said holding section placed after said radiolucent conduit relative to the direction of flow in said egg product line.
13. An apparatus according to claim 12, further comprising a preheater connected to said egg product line before said radiolucent conduit, relative to the direction of flow in said egg product line.
14. An apparatus according to claim 12, further comprising a homogenizer connected to said egg product line after said holding section, relative to the direction of flow in said egg product line.
15. An apparatus according to claim 12, further comprising a cooler connected to said egg product line after said holding section, relative to the direction of flow in said egg product line.
16. An apparatus according to claim 12, further comprising an aseptic filler connected to said egg product line after said holding section, relative to the direction of flow in said egg product line.
17. A method of pasteurizing an egg product, comprising: heating said egg product to a predetermined temperature by subjecting said egg product to high frequency radio waves; and then holding said egg product at said predetermined temperature for a predetermined time; wherein the total thermal treatment of said product is sufficient to pasteurize said egg product without coagulation thereof.
18. A method according to claim 17 wherein said egg product is selected from the group consisting of shell eggs, prepackaged liquid egg products, and liquid egg products.
19. A method according to claim 17, further comprising the step of aseptically packaging said egg product following said holding step.
20. A method according to claim 17 further comprising the step of preheating said egg product to a temperature from about 140°F to about 155°F prior to said heating step.
21. A method according to claim 17, said radio waves having a frequency of from 15 MegaHertz to 150 MegaHertz .
22. A method according to claim 17, said radio waves having a wavelength of from about 2 meters to about 20 meters.
23. A method according to claim 17, said radio waves having a wavelength of about 10 meters.
24. An apparatus for pasteurizing an egg product, comprising: (a) egg product holding means; (b) a generator which generates high frequency radio waves, said generator operatively associated with said egg product holding means and positioned so that radio waves generated heat egg product contained within said egg product holding means .
25. An apparatus according to claim 24, further comprising conveying means, said conveying means configured to move said egg product holding means from a first position to a second position, said first position such that radio waves generated by said generator do not heat egg product contained in said holding means, and said second position such that radio waves generated by said generator heat egg product contained in said holding means.
26. An apparatus according to claim 25, further comprising a cooler operably associated with said conveying means, said conveying means configured to move said holding means to a third position such that egg product contained in said holding means is cooled by said cooler.
27. An apparatus for pasteurizing egg products, comprising: (a) conveying means for transporting an egg product along a predetermined product path; (b) a generator for generating high frequency radio waves, said generator operatively associated with said conveying means and positioned so that an egg product conveyed on said conveying means is exposed to high frequency radio waves generated by said generator; and (c) holding means for holding the egg product for a predetermined time, said holding means operably connected to said conveying means for receiving the egg products and positioned after said generator, relative to the movement of the egg products along the product path.
28. An apparatus according to claim 27, further comprising a cooler operably connected to said holding means.
29. An apparatus according to claim 27, further comprising a preheater connected to said conveying means, said preheater positioned so that an egg product on said conveying means is preheated prior to exposure to high frequency radio waves.
Description:
MEIHM ) AND APPARATUS FOR PASTEURIZING EGG PRODUCTS USING HIGH FRBQUENCY

Field of the Invention

The present invention relates to methods and apparatus for the pasteurization of egg products utilizing high frequency radio waves. The methods may be used in either batch or continuous pasteurization processes.

Background of the Invention

Large quantities of liquid egg products are pasteurized each year to produce products sold in refrigerated form with limited shelf life. See generally, Egg Pasteurization Manual (USDA Agricultural Research Service 1969) . More recently, the provision of techniques for ultrapasteurizing liquid egg products has led to the marketing of liquid egg products which have extended shelf lives under refrigerated conditions.

Methods of ultrapasteurizing liquid whole egg products using indirect heat, such as contacting the liquid whole egg product to a heated surface, or direct heat, such as contact to or the injection of steam, are known. See U.S. Pat. Nos. 4,808,425 to Swartzel et al . ; 5,105,724 to Swartzel and Ball; and 4,957,760 to Swartzel et al.

U.S. Patent No. 4,091,119 to Bach discloses a method and apparatus for uniform heating of a foodstuff for preservation using high-frequency electromagnetic

fields. The foodstuff (in particular, a jam, marmalade or jelly) is located in a container open at its top, and two fields, one high frequency and another ultra-high frequency, are directed as different portions of the foodstuff container. Use of a high frequency field of between 1 - 300 MHz, used in conjunction with an ultra- high frequency field of between 300 MHz and 5GHz, is disclosed.

U.S. Patent No. 4,853,238 to Huang discloses a method for treating liquid egg white or liquid whole egg in order to extend the shelf-life of the liquid egg material. The liquid egg material is subjected to microwave energy while it flows in a conduit for a time sufficient to inactivate or destroy a sufficient amount of microorganisms in the material without significantly reducing the egg protein functionality of the material.

U.S. Patent No. 4,975,246 to Charm discloses a method and apparatus for pasteurizing and/or sterilizing heat-sensitive material using high-temperature, short- time heating to substantially destroy selected microorganisms without substantially affecting other desirable properties of the material. Heat-sensitive material, such as liquid egg product, is continually passed through a flow path, such as plastic tubing, subjected to microwave radiation to heat the material to a high temperature for a short period, and then rapidly cooled.

Summary of the Invention

Disclosed is a method of pasteurizing a flowable egg product in a continuous flow apparatus. The method comprises the steps of: (a) passing a flowable egg product continuously through a conduit transparent to high frequency radio waves; (b) heating the flowable egg product to a predetermined temperature by subjecting it to high frequency radio waves; and then (c) holding the flowable egg product at said predetermined temperature

for a predetermined time; wherein the total thermal treatment of the product is sufficient to pasteurize the product. Preferably, the total thermal treatment is sufficient to achieve a five, six, seven or even nine log cycle reduction in Listeria monocytogenes without significantly reducing the egg functionality.

An apparatus for pasteurizing a flowable egg product is also disclosed. The apparatus comprises: (a) an egg product holding tank; (b) an egg product line connected to the holding tank; (c) pumping _ means connected to the egg product line for establishing an egg product stream in the egg product line; (d) a radiolucent conduit interposed in the egg product line; (e) a generator capable of generating high frequency radio waves and placed so that radio waves generated pass through the radiolucent conduit to heat the egg product stream; and (f) a holding section in the egg product line after the radiolucent conduit .

Also disclosed is a method of pasteurizing a flowable egg product comprising the steps of: (a) heating the flowable egg product to a predetermined temperature by subjecting it to high frequency radio waves; and then

(b) holding the flowable egg product at said predetermined temperature for a predetermined time; wherein the total thermal treatment of the product is sufficient to pasteurize the product. Preferably, the total thermal treatment is sufficient to achieve a five, six, seven or even nine log cycle reduction in Listeria monocytogenes without significantly reducing the egg functionality.

An apparatus for pasteurizing an egg product is also disclosed. The apparatus comprises: (a) egg product holding means; (b) a generator capable of generating high frequency radio waves and operatively associated with said egg product holding means and positioned so that radio waves generated heat egg product contained within the egg product holding means .

Another apparatus for pasteurizing egg products is disclosed, comprising (a) conveying means for transporting an egg product along a predetermined product path; (b) a generator for generating high frequency radio waves, the generator operatively associated with the conveying means and positioned so that an egg product conveyed on the conveying means is exposed to high frequency radio waves generated by said generator; and (c) holding means for holding the egg product for a predetermined time, the holding means operably connected to the conveying means for receiving the egg products.

Radio frequency energy has been reported as being used to scramble eggs ( See, e . g. , "Magna-Tube" heat processing equipment sales brochure, Proctor Strayfield, Horsham, Pennsylvania) , but has not heretofore been suggested for use in pasteurizing egg. Advantages of high frequency radio waves for egg pasteurization include, among other things, the uniformity of heating provided thereby, the avoidance of adding water to the product such as by steam injection, and the avoidance of fouling of heated surfaces.

The foregoing and other objects and aspects of the present invention are explained in detail in the specification set forth below.

Brief Description of the Drawings

FIGURE 1 schematically illustrates a continuous flow apparatus for carrying out the present invention.

FIGURE 2 schematically illustrates a continuous movement processing apparatus for carrying out the present invention.

Detailed Description of the Preferred Embodiments

Examples of pasteurized liquid whole egg products which may be produced by the method of the present invention include whole egg, fortified whole egg

(whole egg with added yolk) , salt whole egg (e.g., salt 10%) , sugar whole egg (e.g., sugar 10%) , blends of whole egg with syrup solids, syrups, dextrose and dextrins and/or gums and thickening agents, blends of whole eggs with less than 1% sugar and/or salt, scrambled egg mixes (for example, a mix of about 51% egg solids, 30% skim milk solids, 15% vegetable oil and 1.5% salt) , reduced cholesterol egg products and blends thereof, custard blends, and the like, that is products containing at least about 10% egg solids. Products which are extremely sensitive to thermal processing and which are particularly suitable for pasteurization by the present invention include, for example, liquid whole egg and blends thereof (less than 2% added non-egg ingredients) , fortified whole egg and blends thereof (24-38% egg solids, 2-12% added non-egg ingredients) , liquid salt whole egg, liquid sugar whole egg, and other liquid whole egg blends which are 24-38% egg solids and 12% or less of added non-egg ingredients. Terms used herein have their standard meaning in accordance with industry and regulatory usage. See, e.g., 7 C.F.R. § 59.570(b) (1985) .

Methods and apparatus of the present invention are also suitable for pasteurizing shell eggs (i.e., eggs contained in the natural shell) and for pasteurizing packaged egg products, for example, restructured reduced- cholesterol eggs in individual consumer packages. Shell eggs and packaged liquid eggs or egg products to be treated can be placed in a liquid media so that they are flowable, and therefor suitable for use in continuous flow pasteurizing apparatus. Additionally, egg components (such as de-cholesterolized egg yolk) may be encapsulated in an edible material to form capsules which can then be carried in a liquid medium (preferably another egg component such as egg white) for pasteurization using methods of the present invention and continuous flow apparatus. As used herein "flowable"

means that the product to be pasteurized is capable of flowing through continuous flow pasteurizing apparatus, either without the addition of a carrier (for example, liquid whole egg) or when placed in a liquid carrier medium (for example, whole shell eggs placed in water or saline solution) . As noted above, examples of flowable products include liquid egg products, as well as particulate products carried in a liquid medium, and shell eggs or packaged liquid egg products placed in a liquid medium.

Suitable liquid carrier media for use in practicing the methods of the present invention include, for example, water and saline. Carrier media may include sterilizing or other additives, or may be heat sterilized prior to use in the present invention.

As used herein, the terms pasteurization and pasteurized refer to treatments sufficient to kill sufficient pathogenic microorganisms contained within the e 99 product being treated to render the egg edible without threat of infection by, e.g., Salmonella species. Pasteurization may be thought of as a treatment which, for all practical purposes, renders pathogenic microorganisms into a state in which they are incapable of reproducing or growing. Pasteurization methods preferably cause at least a nine log cycle reduction of Salmonella bacteria in the product, as required by the USDA Egg Pasteurization Manual (1969) . Pasteurization methods preferably cause a 5 log cycle reduction in Listeria in the product, and more preferably cause a seven to nine log or greater reduction in Listeria bacteria in the product .

As used herein, ultrapasteurization means pasteurization which results in pasteurized products with shelf lives under refrigerated conditions (e.g., 4° Centigrade or less, but above freezing) greater than that obtainable using previously known pasteurization methods. See, e. g. , K. Swartzel et al . , U.S. Patent No. 4,808,425

(the disclosure of all patents cited herein are intended by applicants to be incorporated herein in their entirety) . Thus ultra-pasteurization methods produce extended shelf life products, e.g., products having shelf lives more than 10 days, and preferably more than 14 days, and typically 4 to 6 weeks and up to 36 weeks or more.

The methods of the present invention may be adapted for either continuous flow pasteurization or batch pasteurization. As used herein, continuous flow pasteurization refers to methods in which a continuous stream of product is maintained in the pasteurizing apparatus being used. Continuous flow thermal processing equipment typically comprises heating, holding and cooling sections, in which a continuous stream of product is maintained. Typically such product is liquid egg product. As noted herein, flowable egg product may also be used with the methods of the present invention in continuous flow processing equipment. As used herein, batch pasteurization refers to methods in which a defined amount of product is pasteurized (e.g., heated, held and cooled) as a unit. A typical batch pasteurization system for liquids comprises a processing vat in which the defined amount of liquid egg product is heated and cooled during pasteurization (see, e.g., Stadelman and Cotterill, Egg Science and Technology, 2d Ed. (1977) , AVI Publishing Co., Westport, Conn., pp. 174-175) . Batch pasteurization apparatus may be used with methods of the present invention to pasteurize liquid egg products, shell eggs and packaged liquid egg products. Suitable batch pasteurization systems may comprise vats, trays, buckets, racks, or other containers in which the egg products are held during pasteurization. The selection of an appropriate container for the product to be pasteurized will be readily apparent to one skilled in the art .

The methods of the present invention may further be adapted for continuous movement pasteurization of non-liquid (i.e., non-flowable) egg products. As used herein, continuous movement pasteurization refers to the use of equipment which continually conveys egg products through pasteurization equipment. It is distinguished from continuous flow pasteurization in that a continuous uninterrupted product stream is not established. The products to be pasteurized may be conveyed through the pasteurization apparatus by any suitable means. Suitable conveying means include but are not limited to moving conveyer belts, rails or guides upon which egg products are moved, or upon which receptacles such as buckets or trays which contain the egg products are moved. Suitable egg products for use in continuous movement equipment include shell eggs and liquid egg products which are packaged or otherwise contained.

It is recommended that the equivalent point method for evaluating thermal treatments be applied in practicing the present invention when continuous flow pasteurization is used. This method describes the total thermal treatment received by a product in continuous flow equipment. Procedures for using the equivalent point method for analyzing the thermal effects on products during continuous flow heating have been previously outlined (Swartzel (1982) , J " . Food Science, 47, 1886, and Swartzel (1986) J " . Agric . Food Chem. 34, 397; U.S. Patent 4,808,425 to Swartzel et al . ) and are known to those skilled in the art . The present invention utilizes that portion of the electromagnetic spectrum associated with radio reception (i.e., radio waves having a frequency of from about 500 Kilohertz (KHz) to about 110 Megahertz (MHz) ; or radio waves with wavelengths from about 1 meter to 10 4 meters) . In particular, the present invention uses high frequency radio waves. As used herein, high frequency radio waves (HFR ) means electromagnetic radiation having

a frequency of from about 15 MHz to about 150 MHz, or wavelengths of from about 2 meters to about 20 meters. Alternating currents generate electromagnetic waves of a desired frequency and wavelength, which travel at a speed characteristic of the media in which they are traveling. The wavelength (λ) of a particular wave in a given egg product is determined from a knowledge of the frequency f, which remains constant (a function of the generator) , and v, which depends on the velocity of the wave in the product .

The present invention utilizes high frequency radio waves to produce heat within the products being treated, causing microbial destruction without loss of product functionality, and yielding reduced or eliminated product deposition on surfaces in direct contact with the egg product. It is not yet established whether microbial inactivation using electromagnetic waves is due to thermal effects only, as in conventional heating processes, or includes athermal effects resulting from unknown interactions between biochemical constituents of microbes and an electromagnetic field. See, e . g. , Adey, Biological effects of radio frequency electromagnetic radiation, In: Electromagnetic Interaction with Biological Systems, Lin (Ed.) Plenum Press, New York, pp. 109-140 (1989) . However, microbial destruction with electromagnetic waves producing heat generally yields results similar to that using conventional heat only. See, e . g. Goldblith, Principles and application of radio- frequency energy to food preservation wi th particular reference to concentration and dehydration, In: Freeze Drying and Advanced Food Technology, Goldblith, Rey & Rothmayr (Eds.) , Academic Press, New York, pp. 691-714 (1975) .

In the present invention, high frequency radio waves (HFRW) are converted to heat as they interact with the egg products. Absorption of electromagnetic energy increases the kinetic energy of the molecules of the

absorbing medium, and increases the temperature of the absorbing medium. Because heat is generated within the product being heated, contact with heated surfaces acting as heat transfer surfaces is not required. Thus fouling or burning of egg products in contact with heated surfaces is reduced or eliminated when using HFRW pasteurization techniques. In continuous flow equipment this allows extended process run-times and yields greater efficiency by achieving higher through-put of product before cleaning of equipment is required, while producing product with good functional characteristics.

Most continuous flow pasteurization processes using indirect heat exchangers are designed to maximize turbulent, high-shear flow in order to achieve efficient heat transfer throughout the egg product . In HFRW heating, particulate matter heats at the same rate as liquids, allowing continuous flow pasteurization apparatus to be designed with less concern about the flow characteristics of the egg product . Shear stress on the proteins can be reduced, and the need to make highly homogeneous liquids from eggs can be eliminated. Thus low shear pumps may be used in practicing the present invention in continuous flow apparatus so that the identity of the yolks and egg whites is not lost . HFRW heating is distinguished from higher energy microwave heating in that it is a process that is generally easier to control as the input of energy, and hence heating, is not as rapid. Microwaves are that portion of the electromagnetic energy spectrum with frequencies of from 10 3 to 10 5 MHz. Additionally, larger capacity heating sections can be constructed for use with HFRW (i.e., the cross-sectional area of a HFRW heater can be larger than that used in microwave heating processes) as the penetration depth of HFRW energy is greater than that of microwave energy. It is a well-known phenomena that as frequency increases, penetration of radio wave energy into matter decreases.

HFRW heating is distinguished from ohmic heating in that the heater design and controls are not dependent on the specific conductivity of the material being heated. For example, whole egg and scrambled egg mixes (whole egg with milk and salt) have sufficiently different electrical conductance that it is extremely difficult to heat them with the same ohmic heater, while a HFRW heater should be able to heat each product equally efficiently. HFRW heating does not create free radicals and the resulting deterioration of flavor as is found when high energy ionizing radiation is used to pasteurize egg products.

Any means for generating electromagnetic waves of the desired frequencies may be used to carry out the present invention. Any commercial or industrial generator capable of producing high frequency radio waves may be used. Generators may be added in parallel or in series to increase production or temperature. Generators may be harmonically suppressed or otherwise structured to meet standards for electromagnetic or radio frequency emissions . In a general embodiment of heating using HFRW, the product to be heated is situated between two electrode plates which are charged alternately positive and negative. The plates are connected to the radio frequency generator.

In apparatus used for practicing the methods of the present invention, structures which are interposed between the product to be pasteurized and the HFRW generator are constructed of radiolucent material, that is, material permeable to radio waves. For example, in a continuous flow apparatus as shown in FIG. 1, the conduit carrying the egg product adjacent to the HFRW generator is manufactured of material that is radiolucent. In apparatus designed for batch pasteurization, the container holding the egg product to be treated may be manufactured of radiolucent material . As used herein, "radiolucent" means that the material is

essentially transparent to radio waves of the frequency used in the method of the present invention; while the material may be permeable to electromagnetic waves of other frequencies, this is not required. Examples of suitable radiolucent materials include polytetrafluoroethylene (e.g., the product marketed as TEFLON™ or HOSTAFLON™) , and polycarbonate resins such as LEXAN™, or glass (e.g., KIMAX™ tempered glass process pipe) . It will be apparent to one skilled in the art that the use of radiolucent materials is required only to the extent necessary to allow sufficient exposure of the egg product to the HFRW.

In continuous flow apparatus used with methods of the present invention, any means for establishing a continuous stream of egg product may be used to carry out the present invention. Pumps used to establish the stream may or may not be positive displacement pumps, though positive displacement pumps (timing pumps) are generally needed to precisely define the holding time of a product stream in a holding section. Positive displacement pumps may be used in combination with other pumping means, such as centrifugal pumps. When a liquid carrier medium is used, for example, to convey shell eggs or packaged egg products, it will be readily apparent to one skilled in the art that an adequate flow of liquid carrier medium through the apparatus must be produced so that the egg products are conveyed through the pasteurization apparatus at an adequate rate. Suitable means for producing a flow of liquid carrier medium containing shell eggs or packaged egg products include but are not limited to gravity flow conduits and pumps such as SINE PUMPS ® (Sine Pumps, N.V. ) , auger type pumps, or combinations thereof. Reversible thermal set carrier medium gels may also be used (e.g., methylcellulose solutions) .

It will be apparent to those skilled in the art that ingredients may be added to or removed from liquid

egg products treated using the present methods to provide the various egg products given above. Minor ingredients, such as salt, citric acid or phosphate to stabilize color, gums, oils, colors or flavors, may be added to the product stream, and may be added at any suitable time or location in the pasteurizing process. A cholesterol removal step may optionally be included in the process of the present invention. The particular cholesterol removal procedure employed is not critical . Exemplary processes include those disclosed in A. Bayol et al . , European Patent Application No. 89400175.9; U.S. Pat. No. 4,714,556; U.S. Patent No. 4,361,484; U.S. Pat. No. 4,234,619; U.S. Patent No. 3,882,034, and U.S. Patent No. 3,607,304 (applicants specifically intend the disclosures of all U.S. patents cited herein to be incorporated herein in their entirety) .

Using the method and apparatus of the present invention, it is possible to treat egg products from temperatures below 40°F (but above freezing) up to temperatures above 160°F, but below cooking temperatures. The product is then held at the final temperature for a period of time adequate to destroy harmful and spoilage bacteria, as discussed below.

An optional preheating step may be employed prior to HFRW pasteurization to preheat the egg product to a temperature between 140°F and 155°F. Preheating systems may be composed of, but are not limited to, conventional heating systems such as plate, swept, tube heat exchangers, microwave heating systems, ohmic systems, steam injection, hot water injection, hot fluid food injection, etc.

The total thermal treatment received by the egg product during the process must be sufficient to pasteurize the product, as defined above. In continuous flow processing of liquid egg product, the total thermal treatment must be sufficient to cause at least a nine log reduction in Salmonella . In batch systems or continuous

movement systems, a 4.5 log reduction in Salmonella may be equal to the 9-log reduction process used in continuous flow systems. This equivalence is based on the knowledge that the average particle in a continuous flowing fluid may have only half the resident time in a holding tube. For public health safety purposes, the safety factor of a 9 log reduction in continuous flow systems may be equalled by a 4.5 log reduction in batch or continuous movement systems. Proper thermal treatment is typically insured by presetting the holding times. The term "holding time", as used herein, has its ordinary meaning as used in the industry. Preferably, the thermal treatment is sufficient to produce a product having a shelf life of about four weeks to about thirty-six weeks under refrigerated conditions, and more preferably a product having a shelf life of about eight weeks to about thirty-six weeks under refrigerated conditions. The term "refrigerated, " as used herein, means stored at or below a temperature of 4° Centigrade but above freezing. Most preferably the holding time for the egg product is sufficient to cause a five, six, seven or even nine log cycle reduction in Listeria monocytogenes

(Listeria) in the mass mean product element. Listeria is found in a wide range of animals (including man) , on plants, and in soil. See Microbiology, 799-800 (B. Davis et al., Eds., 3d Ed. 1980) (Harper & Row) . A treatment capable of causing a nine log cycle reduction in Listeria is preferred because the wide-spread nature of Listeria makes it difficult to exclude from a pasteurizing plant, because of the pathogenic nature of this microorganism

(see Id.) and because Listeria appears capable of growing to large quantities in egg under refrigerated conditions

(the conditions contemplated for the products of the present process before preparation and consumption) . To produce uniformly treated egg product, each unit of an egg product must receive substantially the same thermal treatment as comparable units (as used

herein, a unit of egg product refers to a defined amount of an egg product, such as a shell egg, a packaged egg product of a defined size, or a defined amount of liquid egg product) . This can be accomplished by exposing each unit of an egg product to the same HFRW energy and holding each unit for the same length of time prior to cooling, with other conditions being substantially uniform. For example, in continuous movement processing apparatus, units of an egg product may be spaced at regular intervals and passed through the HFRW field at a uniform rate, then each unit held for the same time before being cooled. In methods using batch pasteurization apparatus, multiple units of an egg product may be exposed to HFRW simultaneously, then held for a given time and cooled as a batch.

Following pasteurization the product can then be cooled using conventional cooling systems such as, but not limited to, plate heat exchangers, swept surface heat exchangers, liquid nitrogen injection, C0 2 gas injection, or injection of other inert gases.

Elements' of continuous flow apparatus are interconnected by a product line formed of any conventional sanitary material, such as stainless steel tubing. Raw liquid egg product may be provided by conventional shell egg breaking apparatus, such as a Seymore breaker (not shown) . The breaking apparatus may be in the same plant as the apparatus of the present invention, may be operatively connected to the apparatus of the present invention, or may be located in a separate plant .

To obtain a product with reduced quantities of microorganisms, the pasteurizing apparatus should be sterilized before the egg product is passed therethrough. Sterilizing is preferably accomplished by passing hot water under pressure through the pasteurizing apparatus, as is known in the art, so that hot water is contacted to

those surfaces which contact the product at a temperature and pressure and for a time sufficient to sterilize these surfaces.

Unpackaged liquid egg product, after pasteurization, should be aseptically packaged. Aseptically packaged means packaged to the exclusion of microorganisms other than those carried by the egg product itself. Equipment suitable for aseptically packaging liquid egg products, such as the TETRA PAK™ TBA/9, the TETRA PAK™ TR7-ESL, the TETRA PAK™ Model AB-3- 250, and the Evergreen EQ-4, is commercially available. Also useful in carrying out this step is equipment which packages the product to the substantial exclusion of microorganisms, known in the industry as "clean fillers," but the greater exclusion of microorganisms provided by aseptic fillers makes aseptic fillers preferable, particularly in view of the ability of Listeria to grow under refrigerated conditions, as discussed above.

A homogenization step for unpackaged liquid egg product may optionally be included after the holding step. The term "homogenization" as used herein, means to subject a product to physical forces to reduce particle size. Such procedures are known in the art, and may be carried out on different types of equipment. It is preferable to carry out this homogenizing step with homogenizing equipment at total pressures of from about 500 p.s.i. to about 3,000 p.s.i.

A continuous flow apparatus for carrying out the method of the present invention is shown schematically in FIGURE 1. Arrowheads indicate the direction of flow of the product. This apparatus is readily assembled from commercially available parts. The apparatus of FIGURE 1 comprises a raw product holding tank (10) and a first pump (11) to pump the raw liquid product out of the holding tank and into the radiolucent pasteurization conduit (13) . A preheater (12) may optionally be located intermediate the first pump (11)

and the radiolucent conduit (13) . A generator capable of generating HFRW (14) is located adjacent to the radiolucent conduit (13) , such that high frequency radio waves generated pass through the radiolucent pasteurization conduit. A positive displacement pump

(15) is positioned in the product line after the radiolucent pasteurization conduit, and a check valve

(16) may optionally be positioned in the product line after the positive displacement pump and before the holding section (20) . A flow diversion valve (21) in the product line following the holding section is provided for shunting improperly treated product to a holding tank

(not shown) via a diversion line (22) . The length of the holding section (20) is chosen to provide the desired holding time to the egg product carried therein. Optionally, a homogenizer (23) such as the Cherry Burrell Model HD-6 aseptic homogenizer follows the flow diversion valve. A cooler (24) such as the Cherry-Burrell Model 558X8 No-Bac SPIRATHERM™ tube-in-shell cooler follows the flow diversion valve and the (optional) homogenizer. An aseptic filler (25) such as the TETRA PAK™ Model AB-3-250 receives pasteurized egg product and aseptically packages the whole egg product stream carried therein in individual cartons . It will be readily apparent to those of ordinary skill in the art that modifications of the pumping system described above will be required for pasteurizing non-liquid products, such as eggs in the shell, or pre-packaged egg products. Such products also may not require "traditional" aseptic packaging following pasteurization, such as that provided by the TETRA PAK™ Model AB-3-250, but the products should be packaged with a similar level of barrier protection and to the exclusion of microorganisms. In use for carrying out a method of the present invention for pasteurizing a liquid whole egg product, the apparatus of FIGURE 1 may, for example, be set so

that HFRW having a wavelength of about 10 meters are generated and heat the liquid whole egg product in the radiolucent pasteurization conduit to about 64-70° Centigrade. The holding section (20) is a length suitable to provide a holding time for the liquid egg product of about 2 to 5 minutes to thereby produce a pasteurized liquid egg product. To produce a product with greater kills of Listeria, the holding section is lengthened to give a holding time of up to 9 minutes for some products.

In another embodiment of the present invention, a preheater (12) such as a Cherry-Burrell Model E SUPERPLATE™ preheater, preheats the egg product to a temperature between 135°F and 155°F prior to the egg product being pumped into the radiolucent conduit, where the final portion of heating is achieved by HFRW.

A continuous movement pasteurization apparatus for carrying out the method of the present invention is shown schematically in FIGURE 2. Arrowheads indicate the direction of movement of the product. This apparatus is readily assembled from commercially available parts. The apparatus includes a loading area where egg products are placed on the conveying means. It is readily apparent that the loading area must be designed to complement the particular conveying means being used. Exemplary loading areas include accessible sections of mechanical conveying means (e.g., accessible to humans or automated egg transfer devices) , and loading containers such as tanks, trays, or other containers into which the egg products to be pasteurized are placed.

As shown in FIGURE 2, an exemplary continuous movement apparatus of the present invention for use in pasteurizing egg products comprises a loading area, a conveyor belt moving along a predetermined path, an HFRW generator positioned so that radio waves generated pass through the egg products on the conveyor belt at a predetermined point, and a holding section. Egg products

are loaded onto the conveyor belt at the loading area, carried along the predetermined path by the conveyor belt, pass the HFRW generator and are exposed to HFRW, and are then held for the time required to pasteurize the product . Portions of the apparatus may be made of radiolucent material to ensure adequate and uniform exposure of egg products to HFRW. Any suitable means for holding the heated egg products at an elevated temperature for a predetermined period of time may be used. Suitable holding means include, but are not limited to, continuations of the product line or conveyor belt, holding tanks, trays or other containers into which egg products are placed.

The above described apparatus may be readily assembled from commercially available parts, or commercially available systems may be modified for use in the present invention. For example, the THERMOFLO ® RF Heating System (Radio Frequency Co., Inc., Millis, Massachusetts) may be modified for use with the present methods.

An exemplary continuous movement apparatus for carrying out the method of the present invention is shown schematically in FIGURE 2. This apparatus is designed for use with egg products carried on a conveyor belt, such as shell eggs or prepackaged egg products. Arrowheads indicate the direction of movement of the egg products. The apparatus of FIGURE 2 comprises a loading area (30) through which a conveyor belt (31) passes. Shell eggs or prepackaged egg products are loaded onto the conveyor belt at the loading area. The conveyor belt moves the egg products from the loading area along a predetermined product path and past a HFRW generator

(34) . A preheater (32) may optionally be located intermediate the loading area and the HFRW generator. The HFRW generator (34) is positioned adjacent to the conveyor belt such that high frequency radio waves generated pass through the egg products. A holding

section (33) consisting of an extended section of the conveyor belt is placed in the product path following the HFRW generator; the length of the holding section is chosen to provide the desired holding time to the egg product. A cooler (35) such as a chill tank or spray cooler optionally follows the holding section.

In use for carrying out a method of the present invention for pasteurizing shell eggs, the apparatus of FIG.2 may, for example, be configured so that HFRW having a wavelength of about 10 meters are generated and heat the shell eggs to an internal temperature of from 64- 70°C, with all other elements equal to or greater than 64-70°C. The holding section (33) is a length suitable to provide a holding time for the egg product of about 2 to 5 minutes to thereby produce a pasteurized egg product. To produce a product with greater kills of Listeria, the holding section is lengthened to give a holding time of up to 9 minutes for some products. The temperature to which the egg product is heated and the holding time combine to produce a pasteurized product. Those skilled in the art will recognize that different combinations of time and temperature may be used to reach the same result for a given product, and will further recognize that the desired combinations of temperature and time will vary with the product being treated.

Those skilled in the art will appreciate that the present invention can be combined with the features of other egg pasteurization techniques, such as described in U.S. Patents Nos. 4,808,425, 5,105,724, and 4,957,760 (the disclosures of which are intended by applicants to be incorporated herein in their entirety) , and that these techniques can be resorted to for further guidance in practicing the present invention.

Those skilled in the art will appreciate that minor variations can be made in the apparatus and procedures described herein without departing from the spirit of the present invention. Thus the invention is

defined by the following claims, with equivalents of the claims to be included therein.