Patent Publication Number: US-8981270-B2

Title: Method for recording temperature profiles in food packages during microwave heating using a metallic data logger

Description:
This invention was made with government support under contract no. W911QY-07-C-0080 awarded by the U.S. Army Natick Soldier Center. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention is directed to pasteurizing and/or sterilizing foodstuffs using microwave energy and, more particularly, to using a data logger to provide a temperature profile for the foodstuffs during pasteurizing and/or sterilizing. 
     BACKGROUND 
     Microwave sterilization of pre-packaged foodstuffs has been researched for a number of years. See, for example, U.S. Pat. Nos. 4,962,298 and 5,074,200 which are directed to a microwave system for sterilization of pre-packaged food articles; U.S. Pat. No. 4,974,503 which describes and apparatus for irradiating food products to pasteurize, sterilize or uniformly heat food products; U.S. Pat. No. 4,952,763 which describes a system for sterilizing food packages with microwaves under pressure; U.S. Pat. No. 5,750,966 which describes a plant for pasteurizing or sterilizing food products using microwaves in a pressurized chamber; and U.S. Pat. No. 7,119,313 which describes a processing line for pasteurizing and/or sterilizing foodstuffs with microwaves. Microwave sterilization and pasteurization of foods is being commercially used in Europe by TOP&#39;s Foods (Tang,  Microwave Heating News  Vol. 2(3), 2000) where temperature monitoring of the food is measured indirectly by infrared temperature sensors and displacement sensors which monitor bulging of the tray tops. Despite the commercial success of microwave sterilization in Europe, the process has not been widely adopted in the U.S. 
     It would be advantageous in a large scale microwave based food sterilization or pasteurization system to provide a mechanism for assuring proper sterilization or pasteurization for different batches of product and for allowing identification of areas within the processing line which require repair or adjustment. 
     SUMMARY 
     According to the invention, a data logger is positioned within a food product package (tray or pouch) and includes a member positioned for sensing the temperature at the cold spot of the package. The data logger records temperature, and possibly other parameters of interest, against time, so that, for example, an accurate representation of the temperature reached at the cold spot during processing and the time period that the cold spot of the food product was maintained at a specific temperature can be discerned. Information pertinent to a temperature and time period required for pasteurization or sterilization of the food product can be derived from the data logger. This information can be provided during microwave processing or after processing of the food product, and may be provided by wired or wireless transmission (wireless transmission possibly providing greater flexibility for when and how the information is provided). The invention provides exemplary arrangements for positioning the data logger within a tray or pouch such that they do not interfere with microwave transmissions from a wave guide used for pasteurizing or sterilizing the food. That is, the configurations are selected such that the logger and thermal sensor do not alter the position of the cold spot in the food article or otherwise alter the heating provided by the microwaves to the food article. In addition, the data logger does not alter or otherwise corrupt the information detected within the food article during pasteurization or sterilization. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary microwave heating apparatus; 
         FIG. 2  is a schematic diagram of an exemplary microwave heating apparatus; 
         FIG. 3  is a schematic model showing the orientation of a probe for a data logger/thermal sensor within a food tray or pouch as it passes between opposing wave guides, and a showing control tray or pouch or food product tray or pouch not equipped with a data logger/thermal sensor positioned adjacent to the food tray or pouch with the data logger/thermal sensor; 
         FIG. 4  shows an E-field simulation for a data logger with a holder (e.g., tray or pouch) in a wave guide as shown in  FIG. 3 ; 
         FIG. 4   a  shows a heating pattern simulation for a data logger with a holder (e.g., a tray or pouch) in a wave guide as shown in  FIG. 3 ; 
         FIG. 5  shows plastic shielding positioned on a metal base of a data logger in a food tray, as well as a bracing member to hold the thermal sensor in a desired position, e.g., the cold spot; 
         FIGS. 6   a  and  6   b  show a plastic data logger holder and sensor used in conjunction with a food pouch; 
         FIG. 7  shows the heating pattern as indicated by chemical marker-M 2  in whey protein gels in 10 oz trays with and without a metal data logger where the data logger is oriented according to the invention; and 
         FIG. 8  shows a graph of a temperature profile measured with a wireless data logger with a holder in 10 oz mashed potato trays during a microwave sterilization process with comparative results using a fiber optic sensor. 
     
    
    
     DESCRIPTION 
     The invention is directed to the use of a data logger, such as a metallic wireless data logger, in a processing line for microwave pasteurizing and/or sterilizing foodstuffs. The invention can be practiced in a variety of different microwave based processing lines, including without limitation the processing system described in U.S. Pat. No. 7,119,313 to Tang et al., which is herein incorporated by reference. Pasteurization involves heating foodstuffs to a temperature, typically between 80 ° C. to 100° C., sufficient to kill certain pathogenic bacteria and microorganisms. In sterilization, foodstuffs are heated to a higher temperature, typically between 100° C. to 140° C., to ensure elimination of more resistant microorganisms. Sterilization allows normally perishable foodstuffs to be stored at room temperature for extended periods of time, referred to as “shelf-stable” foods. 
     In the practice of the present invention, microwaves within the 915 MHz ISM band or lower are preferred. Advantageously, microwaves within this frequency band have a longer wavelength and therefore can penetrate deeper into the foodstuff to be heated than can higher frequency microwaves (e.g., microwaves in the 2450 MHz ISM band). Microwave systems that uses 2450 MHz magnetrons and lower power levels typically take about thirty minutes to heat a food product whereas using longer wavelengths and higher power permits heating of the food product for 3-5 minutes. 
     Suitable microwave pasteurization or sterilization lines may include a pre-heating section, a microwave-heating section, a holding section, a cooling section, and an unloading section. In particular embodiments, the pre-heating section, the microwave-heating section, the holding section, and the cooling section comprise respective chambers for heating or cooling foodstuffs therein. The pre-heating section may be used to pre-heat the foodstuff using conventional heating. The microwave-heating section heats the foodstuff for a predetermined time period using microwave heating. The microwave-heating section includes at least one microwave cavity that is operable as a single-mode cavity when microwaves are directed into the cavity for heating the foodstuff. Preferably, the microwave-heating section includes a plurality of cavities spaced along a processing line and through which the food articles are conveyed by a conveyor belt. In the holding section, the foodstuff may be heated to substantially maintain the pasteurization or sterilization temperature of the foodstuff until the foodstuff is pasteurized or sterilized. In the cooling section, the foodstuff may be cooled to a reduced temperature (e.g., about room temperature) for further handling or processing. 
       FIG. 1  is an exemplary microwave-heating apparatus  10  which may be used in the practice of the invention, and is illustrated according to one embodiment of U.S. Pat. No. 7,119,313. The microwave-heating apparatus  10  includes a microwave cavity  12 , a first waveguide  14  for directing microwaves from a microwave source (not shown) to the cavity  12 , and a second waveguide  16  for directing microwaves from a microwave source (not shown) to the cavity  12 . In other embodiments, the microwave apparatus can have only one of the waveguides  14 ,  16  so that microwaves are directed into the cavity from only one direction. In addition, the waveguides  14 ,  16  can be in an alignment or can be offset from one another along a processing line or conveyor belt. Positioned in the cavity  12  may be a support stand  18  for supporting a foodstuff  20  to be irradiated by microwaves introduced into the cavity by the waveguides  14 ,  16 . The support stand  18 , if present in the processing system, desirably is open at the bottom to allow irradiation of the top and bottom of the foodstuff  20  and to allow a fluid medium in the cavity  12  to contact substantially the entire surface of the foodstuff  20 . 
     In the embodiment shown in  FIG. 2 , a microwave-heating apparatus  22  as illustrated and discussed in U.S. Pat. No. 7,119,313 includes a first microwave unit  24  and a second microwave unit  26 , each of which include a pair of opposed waveguide applicators  28 ,  30 . Each microwave unit  24 ,  26  has a microwave “cavity” defined as the respective space between each pair of microwave applicators  28 ,  30 . A pressure vessel  32  forms an enclosure around the waveguide applicators  28 ,  30 . The waveguide applicators  28 ,  30  are coupled to a microwave source (not shown) via the waveguides  34 ,  36 , respectively, that extend through the walls of the pressure vessel  32 . A container  38  extends between the waveguide applicators  28 ,  30  of the first and second microwave units  24 ,  26 . A conveyor  40  can be positioned in the container  38  to move a foodstuff  42  between the microwave cavities defined between the microwave applicators  28 ,  30  during microwave heating. A fluid medium (e.g., water) can be introduced into the container  38  through an inlet-fluid conduit  44  to improve heating uniformity during microwave heating. The fluid medium can be discharged through an outlet-fluid conduit  46 . The foodstuff  42  can be heated while immersed in a flow of the fluid medium or in a non-flowing pool of the fluid medium. The illustrated pressure vessel  32  has a gas inlet  48  fluidly connectable to a source of a pressurized gas (e.g., compressed air) (not shown) for establishing a pressurized atmosphere (pressure indicated by a gauge  50 ) inside the pressure vessel  32 . The container  38  may be open to the atmosphere inside the pressure vessel  32  to prevent bursting of the packaging containing the foodstuff  42 . After the foodstuff  42  is heated in the microwave apparatus  22 , the pressurized gas can be released from the pressure vessel  32  through a gas outlet  52 . 
     This invention is directed to properly using a data logger in a package of food that will be heated by microwave energy provided by a waveguide in a microwave based sterilization or pasteurization system. The package may be in the form of a tray or pouch. In the practice of the invention, the data logger can be inserted in an actual food product or a model food (e.g., a whey gel with dielectric properties adjusted to be similar to the actual food product) which has the same attributes of the actual food product which is to be pasteurized or sterilized by exposure to microwaves (for example, according to a process as described above similar to that described in U.S. Pat. No. 7,119,313 or by other procedures). 
     The data logger will be used to measure time/temperature profiles during microwave heating. For example, the data logger will preferably be connected to a thermal sensor which is positioned at the cold spot of a food article (i.e., the coldest spot within the tray or pouch containing the food article), and will detect the temperature at the cold spot for a period of time. Depending on the food article (e.g., peas, carrots, potatoes, fish, meat, soup, gravy, or any other food item to be pasteurized or sterilized), the temperature will need to be maintained at a threshold value for a specified period of time to achieve pasteurization or sterilization. A food provider can use the logged time/temperature profile at the cold spot to assure that the food products have been properly pasteurized or sterilized. If the cold spot does not increase under the influence of the microwave heating to the threshold temperature and is not maintained at the threshold temperature for the specified of time, the food product can be determined to not be correctly pasteurized or sterilized. 
     A variety of data loggers and thermal sensor probes can be used in the practice of this invention. For example, suitable loggers and temperature sensors can be obtained from Elektrolaboratoriet A/S of Denmark and Arvada, Colo. under the trade name “ELLAB” (miniloggers being preferred); MESA Laboratories of Lakewood, Colo. (the “DataTrace” product line are self-contained, wireless, high precision loggers used in critical manufacturing, quality control, and transportation applications); OsPens of Quebec, Canada manufactures fiber optic temperature sensors; and FISO Technologies of Quebec, Canada which manufacturers fiber optic sensors. In the practice of this invention, information pertinent to a temperature and time period required for pasteurization or sterilization of the food product can be derived from the data logger. This information can be provided during microwave processing or after processing of the food product, and may be provided by wired or wireless transmission (wireless transmission possibly providing greater flexibility for when and how the information is provided, e.g., a requirement of connecting a data logger to an external signal conditioning and data acquisition system makes such loggers less desirable for continuous industrial processes such as sterilization of large batches of food products). It is preferable to use a data logger which can be used in high microwave power fields to permit heating of food products in shorter periods of time. Further, it is advantageous to use temperature sensors which are less expensive and fragile. 
     Detecting the temperature of the cold spot in food packages and the ability to determine whether a product has received sufficient heating for a desired purpose (e.g., sterilization of pasteurization) is well understood in the art. Determination of the cold spot can be performed by automated or other means well known in the trade. As background, the following articles, which are herein incorporated by reference, discuss identification and use of a cold spot in food product packaging: (1) Wang et al., “Using whey protein gel as a model food to study dielectric heating properties of salmon ( Oncorhynchus gorbuscha ) fillets”,  LWT - Food Science and Technology  42:1174-1178 (2009); (2) Tang et al., “Microwave Sterilization of Sliced Beef and Gravy in 7 oz Trays”,  J. Food Engineering  Apr. 25, 2008; (3) Pandit et al. “Development of a Novel Approach to Determine Heating Pattern Using Computer Vision and Chemical Marker (M-2) Yield”,  J. Food Engineering  78 (2):522-528 (2007); (4) Pandit et al. “A Computer Vision Method to Locate Cold Spots in Foods in Microwave Sterilization Processes”  Pattern Recognition  40(12):3667-3676 (2007); (5) Pandit et al. “Kinetics of Chemical Marker M-2 Formation in Mashed Potato-A Tool to Locate Cold Spots Under Microwave Sterilization”  J. Food Engineering  76(3):353-361 (2006); and (6) Lau et al., “Kinetics of Chemical Marker Formation in Whey Protein Gels for Studying High Temperature Short Time Microwave Sterilization”  J. Food Engineering  60:387-405 (2003). 
     One problem with using a data logger in a food product (e.g., pouch or tray) in connection with microwave heating is that the presence of the data logger itself can impact the heating imparted by the microwave sources and the measurements which are made. Specifically, metal surfaces on the data logger assembly need to be shielded. Further, the data logger needs to be oriented within the tray or pouch in a manner which does not adversely impact the microwave heating imparted to the food product or affect the location&#39;of the cold spot. In addition, the data logger needs to be securely positioned within the food product so that changes in position do not adversely impact the microwave heating imparted by the microwave system or change the location of temperature measurement. The invention provides exemplary arrangements for positioning a data logger within a tray or pouch such that they do not interfere with microwave transmissions used for pasteurizing or sterilizing the food (e.g., they do not alter the position of the cold spot in the food article or otherwise alter the heating provided by the microwaves to the food article), and so that the data logger does not alter or otherwise corrupt the information detected within the food article during pasteurization or sterilization (e.g., the thermal sensor remains positioned in the cold spot during microwave heating and the orientation of the sensor and data logger does not change within the tray or pouch). 
       FIG. 3  shows an isometric representation of an enclosed cavity or continuous processing tunnel used in microwave heating, pasteurization or sterilization processes, such as those described for example in U.S. Pat. No. 7,119,313. In particular, a pair of opposing wave guides  100  and  102  will have trays or pouches  104  and  106  pass between them during operation of a microwave sterilization system. As discussed above, the space between the wave guides  100  and  102  can be pressurized and/or adapted to allow the ingress and egress of fluids for heating and/or maintaining the temperature of the heated food products. Tray or pouch  104  is shown with a probe  108  positioned therein so as to project from a point of approximately zero depth near a side wall of wave guides  100  and  102  towards a depth in the center region of the wave guides  100  and  102 . The end of the probe  110  is positioned in the cold spot of the food article in the tray or pouch  104 . Tray or pouch  106  lacks the probe  110 . 
     For experimentation leading to the discovery that the shielding and orientation of the probe matters and must be stable to provide relevant time/temperature profiles for microwave sterilization or pasteurization of a food product, the tray or pouch  106  served as a control. During actual sterilization or pasteurization processes, only one or a few food product trays or pouches containing the food to be heated in a batch of food (or containing model food with properties adjusted to be similar to the batch of food to be heated) will need to be equipped with a data logger and temperature sensor. The data logger equipped food product trays or pouches  104  will be used for calibration periodically for the batch and will function essentially as a witness for the trays or pouches  106  of food that will be sterilized or pasteurized for storage, sale or consumption. 
       FIG. 4  shows an electronic field (E-field) simulation for a metal thermocouple probe  110  oriented as shown in  FIG. 3 . As can be seen from the simulation, the metal thermal couple probe  110  projects to the cold spot  112  in pouch  104  and does not affect the position of the cold spot  112  (compare cold spot  114  in pouch  106  which lacks the probe  110 ), or alter the distribution of energy near the zero depth region adjacent a side wall of the wave guides. From  FIG. 4 , it can be seen that locating the wireless data logger nearest the zero depth region minimizes the effect of the metallic body on the electrical field pattern heating in microwave applicators. If the thermocouple probe  110  was oriented differently, e.g., at an angle, sideways at the depth of the cold spot, etc., the wireless data logger would adversely impact the electric field pattern and would thus make the use of the data logger less reliable for assuring proper heating, pasteurization or sterilization. 
     Similar to  FIG. 4  for the electrical field simulation,  FIG. 4   a  shows the heating pattern simulation for a metal thermocouple probe  110  oriented in  FIG. 3 . As can be seen from the simulation, the metal thermal couple probe  110  projects to the cold spot  112  in pouch  104  and does not affect the position of cold spot  112  (compare cold spot  114  in pouch  116  which lacks the probe  110 ). 
       FIG. 5  shows a 10 oz. plastic tray  200  in which a metallic data logger  202 , such as those sold by Ellab, is positioned. The base  204  of the metallic data logger  202  is shrouded with a plastic material to prevent an adverse impact on the microwave energy which will be imparted to the food article. The probe  206  may have a length of 10-80 mm. To assure that the tip  208  of the probe  206  remains in a desired location, such as the cold spot of the food product (the tip  208  being equipped with a temperature sensor for measuring the temperature of the food product), a brace member  210  is provided to brace the data logger on one side of the tray  200 , with the base  204  bracing the data logger on the other side of the tray  200 . As shown in  FIGS. 3 ,  4  and  4   a , the tray  200  will pass through the wave guides with the base  204  close to zero depth; therefore, the positioning of the base  204  and the shrouding of the base  204  in plastic (e.g., Ultem or other suitable material) will prevent this portion of the data logger  202  from adversely impacting the microwave energy distribution within the tray  200 . In addition, bracing the data logger  202  into position will prevent it from moving around within the tray  200  during processing. In these ways, the temperature profile logged by the data logger  202  provides an accurate representation of the temperature reached in the cold spot for a period of time. Thus, the food processor can accurately determine that the food is being properly exposed at appropriate microwave powers and for appropriate durations to achieve a selected heating result, e.g., sterilization or pasteurization. 
       FIGS. 6   a - b  show a plastic frame  300  (made, for example, of Ultem, or other suitable material) which, for exemplary purposes, encircles a food pouch  302 . The base  304  of a data logger  306  is affixed to the frame  300  in order to hold the data logger  306  in position within food pouch  302 . The purpose of the frame  300  is to stability hold the data logger  306  in position while in the pouch and during use. While the frame  300  may encircle the pouch, the frame  300  may only partially encircle the pouch or may only be positioned at one end, provided that the data logger is held in its configuration of projecting straight into the pouch during handling and use. For further stability, a wing member  308  preferably made from a silicone rubber can be positioned on the probe  307 . The wing member  308  serves to hold the probe  307  generally centrally between the top and bottom of the pouch  302  (i.e., the wing member  308  holds the probe above and below a top and bottom surface inside of said pouch), and to prevent rolling of the probe from its desired location (e.g., at the cold spot of the food product). A variety of mechanisms can be employed to secure the data logger  306  to the plastic frame  300 .  FIG. 6   b  shows the base of the data logger  306  positioned on the frame  300  with a portion that includes a probe with a thermal sensor projecting into the pouch  302  to the cold spot. Once the food product (or a model food) is positioned within the food pouch  302 , the food can be heated by microwave energy, and a time/temperature profile can be obtained from the data logger  306  to assure a selected heating result such as, for example, sterilization or pasteurization of the food product. 
       FIG. 7  shows a heating pattern as indicated by chemical marker-M 2  for whey protein gels in 10 oz trays with (top gels) and without (bottom gels) a metal data logger. The heating pattern was obtained using well known procedures such as those described in the articles referenced above. What the results show is that when the data logger is shielded, and positioned with its base towards the zero depth position and its tip projecting outward towards the cold spot, the presence of the data logger does not have any adverse impact on the microwave heating of the food product. Thus, the data logger, when shielded, and securely positioned appropriately within the food product, can provide accurate measurements of the time/temperature history for the food product under microwave heating. 
     If the data logger is metal and is unshielded, its presence reflects microwaves and changes the heating which can be achieved within a food tray or pouch, and accurate time/temperature profiles at the cold spot cannot be determined. Similarly, if the data logger is not oriented within the food tray or pouch as described above, the heating pattern will be altered by the data logger and the location of the cold spot may be changed. Furthermore, if the data logger is not stabilized in position in the food tray or pouch, the data logger may move around inside the tray or pouch through simple handling of the tray or pouch, and thus not be able to provide accurate time/temperature profiles which will be required to assure adequate heating for, e.g., sterilization or pasteurization. 
       FIG. 8  shows a graph of a temperature profile obtained with an Ellab wireless data logger in 10 oz mashed potato trays during a microwave sterilization process. Temperature and microwave power are plotted against time.  FIG. 8  shows that the temperature profile matches temperature measurements made with fiber optic sensors. While the temperature profile in  FIG. 8  is a graph, it will be understood that the same information can be provided in a table, or in other forms (e.g., a simple data stream with temperature and time information). The time and temperature profile basically is a representation of data points logged by said data logger which indicate the temperature of the food (or model food) at a point in time after exposure to microwave energy. The time and temperature profile can be used by a food processor to control, e.g., heating, sterilization or pasteurization, of food products in trays or pouches that are similar to those which contain a data logger. For example, if a microwave system such as that described in U.S. Pat. No. 7,119,313 were used for processing vegetables, meat, fish, or other food products of interest (or any other microwave system were employed), and a data logger, used as described herein provides a time and temperature profile showing that under selected conditions (e.g., microwave power, wavelength, time of exposure), sterilization or pasteurization can be achieved (e.g., the cold spot reaches a requisite temperature and is held at that temperature for a requisite time), then other food products subjected to those same processing conditions will achieve the same heating results (e.g., sterilization or pasteurization). Calibration of the system with a datalogger containing tray or pouch of food can be performed at preset intervals (or ad hoc) to assure that the system is continuing to provide the desired microwave heating to the food products. 
     In short,  FIGS. 4-8  demonstrate that heating patterns in a food package are not affected by a holder and/or a metallic data logger under specified microwave power levels , e.g., 7.5 kW per cavity. Using the same design for a holder, virtually any kind of data logger can be used in the microwave field to measure temperature profiles in a food package during microwave pasteurization or sterilization processes. The wireless data logger is located in a position and in an orientation within the food article that minimizes the effect of the metallic body on the electric field pattern heating in a microwave applicator. In addition, the holder with a wireless data logger can be inserted into pouches or trays containing homogenous (e.g., a single food product such as mash potatoes) or heterogenous food products (e.g., a mixed food product such as beef and gravy (pouches or trays) to record temperature profiles during microwave heating processes. 
     The invention has been described in terms of its preferred embodiments. However, those of skill in the art will recognize that the invention can be practiced with considerable variation within the scope of the appended claims.