Patent Publication Number: US-2020288545-A1

Title: Process and system for microwave cooking food products with humidified air control

Description:
TECHNICAL FIELD 
     The present disclosure relates to processes and systems for cooking food products using microwave radiation, and more particularly, to processes and systems for cooking food by microwave radiation in which the relative humidity in the cooking chamber is controlled during cooking. 
     BACKGROUND 
     Microwave ovens are commonly used to cook food products, both commercially and domestically. Cooking food products using microwaves, for example microwave cooking of bacon slices, particularly when performed commercially on a high-volume basis, often results in uneven tenderness in the cooked food product in which a significant percentage of slices are undesirably chewy and tough. Microwave cooking of bacon slices also frequently produces bacon slices in which some bacon is not fully cooked when it is discharged from the microwave oven. 
     High volume commercial microwave cooking systems typically utilize conveyors, such as belt conveyors, that transport the food product to be cooked through cooking enclosures such as microwave cavities in which the food product to be cooked is exposed to microwave radiation generated by a magnetron and transmitted to the cooking enclosure through a waveguide. In order to cook a food product, for example bacon slices, to a higher temperature, it is often necessary to reduce the conveyor speed to increase residence time of the food product in the cooking enclosure or microwave enclosure. 
     Starting and stopping the food product conveyor through the cooking enclosure in such commercial systems also can result in an undercooked or overcooked food product, such as bacon slices, due to stopping and starting of the magnetrons. Accordingly, it is desirable to minimize starting and stopping the food product conveyor. Cooking cavities typically include manually adjustable exhaust dampers that are adjustable to remove excess moisture from the microwave cavities created by the food being cooked. However, there is no internal measurement of the dry-bulb or wet-bulb temperatures in such conventional cooking enclosures utilizing microwave cooking systems. 
     Reducing the speed of the food product conveyor through a commercial microwave cooking system reduces throughput and cooks the food product higher temperatures, which adversely affects yields and can overcook portions of the food product. Starts and stops of the food product conveyor are inherent in high-volume microwave cooking systems and therefore are unavoidable, resulting in overcooking, undercooking, variable texture, and variable yields for some percentage of the food product. Manual damper controls are not effective in responding quickly to temperature changes in the microwave enclosure and do not automatically compensate for variable incoming product loads. 
     Conventional continuous microwave processes in which the moisture content or relative humidity of the air in a microwave cooking enclosure is not controlled result in significant percentages of food product, such as bacon slices, that are tough and/or undercooked. Maintaining sufficiently moist air in the microwave enclosure increases the lethality of the microwave cooking process at a given temperature because non-desiccated bacteria are more vulnerable to being killed than desiccated bacteria at a given temperature. 
     Existing forms of moisture control in microwave ovens do not measure the wet-bulb temperature directly, and do not inject moisture into the microwave cooking enclosure to control or modulate the wet-bulb temperature, and thus the relative humidity, in the enclosure. Existing microwave cooking systems typically have only a manually operated damper on the cooking enclosure exhaust stack, but do not have any way to automatically measure or control the wet-bulb temperature, dew-point, or relative humidity of the air in the microwave cooking enclosure. 
     Accordingly, there is a need for a microwave cooking system and process having active measurement and control of the wet-bulb temperature in the microwave cooking enclosure. There also is a need for a microwave cooking system and process that allows precise control of the tenderization of the microwaved food item and the requisite lethality of the cooking process. 
     SUMMARY 
     In an embodiment, the disclosed process and system utilize microwave cooking during which the wet-bulb temperature in the microwave cooking enclosure is maintained by a closed-loop control to modulate the wet bulb temperature in a microwave cooking enclosure both to optimally tenderize and fully cook food products and to provide requisite lethality to microorganisms on and in the food products being cooked and the conveyor equipment that transports the food products through the microwave cooking enclosure. For example, the process and system tenderize and fully cook bacon slices when discharged from the system. The disclosed process and system also assure that no microorganisms, such as bacteria, survive on the belt and supporting structure of the microwave conveyor, including in harborage areas that otherwise would not be heated to lethal temperatures using microwaves alone. 
     In an embodiment, the disclosed process measures and actively controls the moisture level in a continuous series of microwave ovens, such as used for microwave cooking bacon slices. Moisture is injected into the microwave oven in the form of steam, in embodiments saturated steam, or a water mist to control the wet-bulb temperatures in the microwave ovens at a level that will assure the destruction of microorganisms such as pathogenic bacteria on the slices and on the belt. Also, the wet-bulb temperature in the microwave cooking enclosures is modulated to maintain a predetermined minimum temperature that tenderizes the bacon slices by degrading the collagen-laden connective tissue that causes toughness in bacon. 
     In an exemplary embodiment, a system for microwave cooking food products with humidified air control includes a microwave cooking enclosure, a source of microwave energy connected to transmit microwave radiation into an interior of the microwave cooking enclosure, a microwave-shielded sensor positioned within the interior of the microwave cooking enclosure, a steam nozzle connected to provide steam to the interior of the microwave cooking enclosure to make humidified air therein, and a control. The control is connected to receive a signal from the microwave-shielded sensor indicative of a wet-bulb temperature within the interior of the microwave cooking enclosure and in response modulate a flow of steam into the interior of the microwave cooking enclosure to maintain a wet-bulb temperature of the humidified air therein at or above a predetermined value and to actuate the source of microwave energy to transmit the microwave radiation into the interior of the microwave cooking enclosure, whereby the microwave radiation and the humidified air simultaneously cook the food products in the interior of the microwave cooking enclosure and kill microorganisms in the interior of the microwave cooking enclosure and on a the surface of and within the food products. 
     In embodiments, the system uses microwave-shielded dry- and wet-bulb sensors located directly inside the microwave cooking enclosure. The microwave shields enclose the dry- and wet-bulb sensors and act as Faraday cages having openings sized to allow ingress of the moisture-laden air in the enclosure so that the sensors can measure the wet bulb and dry bulb air temperatures inside the enclosure, but block microwaves from entering the cages to protect the temperature sensors from damage from microwave radiation and to prevent arcing in the microwave cooking enclosure. 
     The wet-bulb sensor measures the temperature that moisture is evaporating inside the enclosure. In embodiments, the wet bulb sensor uses a nozzle that mists water onto a temperature sensor, so that the localized moisture evaporation within a cloud of water vapor around the sensor cools the sensor down to the wet-bulb temperature. In other embodiments, the wet-bulb sensor uses a conventional design in which an end of a sock made of moisture-wicking material is inserted in a pan of water and an opposite end is draped over the wet bulb sensor. 
     In embodiments, the wet-bulb temperature sensor, and optionally the dry-bulb temperature sensor, are used as the input or sensing element as part of a feedback control loop in which a control modulates a valve to regulate injection of steam or atomized water into the microwave cooking enclosure to maintain a predetermined wet-bulb temperature. Optionally, the closed loop system is connected so that the control modulates an exhaust damper that regulates the exhaust of moisture-laden air from the microwave cooking enclosure. In embodiments, this feedback control loop system is configured to automatically control the wet-bulb temperature inside the enclosure using the steam valve to increase or decrease the actual wet-bulb temperature to maintain a pre-programmed set-point and/or temperature range. 
     In another exemplary embodiment, a system for microwave cooking food products with humidified air control includes a microwave cooking unit, the microwave cooking unit including a microwave cooking enclosure, a source of microwave energy connected to transmit microwave radiation into an interior of the microwave cooking enclosure, a microwave-shielded sensor positioned within the interior of the microwave cooking enclosure that detects one or more of a wet-bulb temperature, dew point temperature, and relative humidity within the microwave cooking enclosure, a steam nozzle connected to provide steam to the interior of the microwave cooking enclosure to make humidified air therein, and an exhaust damper connected to the interior of the microwave cooking enclosure to exhaust the humidified air from the interior of the microwave cooking chamber. The system further includes a conveyor that conveys food products into and out of the interior of the microwave cooking enclosure and a control connected to receive a signal from the microwave-shielded sensor indicative of the one or more of the wet-bulb temperature, the dew point temperature, and the relative humidity within the interior of the microwave cooking enclosure and in response modulate a flow of steam from the steam nozzle into the interior of the microwave cooking enclosure and/or modulate the exhaust damper to maintain the wet-bulb temperature within the interior of the microwave cooking chamber at or above a predetermined value and simultaneously actuate the source of microwave energy to transmit the microwave radiation into the interior of the microwave cooking enclosure, whereby the microwave radiation and the humidified air combine to cook the food products on the conveyor in the interior of the microwave cooking enclosure and kill microorganisms on the outer surface of and within the food products, on the conveyor in the interior of the microwave cooking enclosure, and on the interior of the microwave cooking enclosure. 
     Meat collagen degrades within a temperature range of approximately 158-210° F., and preferably 176-194° F. As such, meats that contain high amounts of collagen are much more tender when cooked to temperatures of at least 158° F., preferably 176° F. or higher, to degrade the collagen to a more tender state. By way of example, bacon slices are high in collagen, and therefore if bacon slices are cooked to a temperature of 176° F. or higher, the slices are much more tender than if the slices are cooked to lower temperatures. In a conventional microwave oven in which moisture content of the air in the microwave enclosure is not controlled, the final bacon slice temperatures are highly variable. Any slices that are cooked to temperatures of less than 176° F. tend to be much tougher than slices that are cooked to 176° F. or higher, and hence the variable slice temperatures from a conventional microwave oven will result in slices that have varied degrees of tenderness/toughness. 
     Desiccated bacteria are much harder to kill than non-desiccated bacteria. In a conventional microwave oven without moisture control, the bacon slices may be dehydrated during cooking so that the bacteria are desiccated before lethal temperatures are achieved, and thus all the desiccated bacteria may not be destroyed during cooking. 
     In yet another exemplary embodiment, a process for microwave cooking food products with humidified air control includes placing food products into an interior of a microwave cooking enclosure of a cooking unit; transmitting microwave radiation from a source of microwave energy into the interior of the microwave cooking enclosure to cook the food item; generating a signal from a microwave-shielded sensor positioned within the interior of the microwave cooking enclosure indicative of a wet bulb temperature within the interior of the microwave cooking enclosure; and providing a flow of steam to the interior of the microwave cooking enclosure to form humidified air therein. The process further includes maintaining the wet-bulb temperature within the interior of the microwave cooking enclosure at or above a predetermined value by a control receiving a signal from the microwave-shielded sensor indicative of the wet-bulb temperature within the interior of the microwave cooking enclosure and in response modulating the flow of steam into the interior of the microwave cooking enclosure. Microwave radiation is transmitted into the interior of the microwave cooking enclosure simultaneously with maintaining the wet-bulb temperature to cook the food products in the interior of the microwave cooking enclosure, and kill microorganisms with a combination of the microwave radiation and the steam in the interior of the microwave cooking enclosure and on the outer surface of and within the food products therein. 
     In still another exemplary embodiment, a process for microwave cooking food products with humidified air control includes placing food products into an interior of a microwave cooking enclosure of a cooking unit; transmitting microwave radiation from a source of microwave energy into the interior of the microwave cooking enclosure to cook the food products; providing a flow of steam and/or water spray to the interior of the microwave cooking enclosure to form humidified air therein; maintaining a wet-bulb temperature of the humidified air within the interior of the microwave cooking enclosure at or above a predetermined value; and simultaneously with maintaining the relative humidity, transmitting the microwave radiation into the interior of the microwave cooking enclosure, the microwave radiation and the humidified air combining to cook the food products in the interior of the microwave cooking enclosure and kill microorganisms in the interior of the microwave cooking enclosure and on an outer surface of and within the food products. 
     With the disclosed process and system, the wet-bulb temperature in the microwave cooking enclosure is modulated by the closed-loop system to be maintained at a predetermined temperature and/or temperature range, for example 176° F. or higher for bacon slices, to ensure that all slices are treated with a tenderizing cooking process that sufficiently degrades the collagen, thus tenderizing all of the bacon slices to a uniform state of tenderized bacon. The moist heat at a lethal temperature assures that all microwaved food, for example bacon slices, is heated to a lethal wet-bulb temperature, for example 160° F., that assures that any pathogenic bacteria are destroyed in a hydrated state before being desiccated. 
     Other objects and advantages of the disclosed process and system for microwave cooking food products with humidified air control will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, perspective view of an exemplary embodiment of the disclosed system for microwave cooking food products with humidified air control; 
         FIG. 2  is a schematic side elevation of the system of  FIG. 1 , showing two representative microwave cooking units of the four units depicted in  FIG. 1 ; 
         FIGS. 3A, 3B, 3C, and 3D  are top plan, perspective, front elevation, and side elevation, respectively, of an exemplary embodiment of a wet bulb temperature sensor unit of the embodiment of  FIG. 1 ; 
         FIGS. 4A, 4B, and 4C  are side elevation, perspective, and end views, respectively, of the microwave shield of the wet-bulb temperature sensor unit of  FIGS. 3A-3D ; 
         FIGS. 5A, 5B, and 5C  are top plan, front elevation, and side elevation, respectively, of the top plate of the wet-bulb temperature sensor unit of  FIGS. 3A-3D ; 
         FIGS. 6A, 6B, 6C, and 6D  are top plan, perspective, front elevation, and side elevation, respectively, of an exemplary embodiment of a dry-bulb temperature sensor unit of the embodiment of  FIG. 1 ; 
         FIGS. 7A, 7B, and 7C  are side elevation, perspective, and end views, respectively, of the microwave shield of the dry-bulb temperature sensor unit of  FIGS. 6A-6D ; and 
         FIGS. 8A, 8B, and 8C  are top plan, front elevation, and side elevation, respectively, of the top plate of the dry-bulb temperature sensor unit of  FIGS. 6A-6D . 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIGS. 1 and 2 , an exemplary embodiment of the disclosed system for microwave cooking food products with humidified air control, generally designated  10 , includes discrete microwave cooking units  12 A,  12 B,  12 C, and  12 D. In other embodiments, the system  10  includes more or fewer cooking units  12 , and in still other embodiments a single cooking unit  12  is used. In embodiments, the cooking units  12 A- 12 D each include a housing  14  that encloses and defines a microwave cooking enclosure  16 , where food products are cooked. In embodiments, one or more of the housings  14  has more than one microwave cooking enclosure  16 . The housings  14  each include a top wall  18  that is attached to front and rear wave guides  20 ,  22 , respectively, that open into and communicate with the microwave cooking enclosures  16 . In other embodiments, a single waveguide  20  is used for each microwave cooking enclosure  16 , or more than two waveguides are used for each microwave cooking enclosure. In an embodiment, the housing  14  includes a side wall  34  with an access door  36 . 
     The cooking units  12 A- 12 D are connected in series by a conveyor, which in embodiments is an endless belt conveyor  24 , that passes through the cooking enclosures  16  of the cooking units and through entrance and exit openings  26 ,  28  in the front and rear side walls  30 ,  32  respectively of each cooking enclosure. In embodiments, the conveyor takes the form of a conveyor belt in a single lane, or conveyor belts in 2 lanes, 3 lanes, 4 lanes, or more, and the lanes are arranged horizontally in parallel and/or are stacked vertically. Conveyor  24  supports food products  100  to be cooked in the microwave cooking enclosures  16 . Optionally, a second belt conveyor  24 A, spaced vertically above the belt conveyor  24 , passes through the cooking enclosures  16  such that the food products  100  are held between the belt conveyors  24 ,  24 A. In embodiments, the conveyors  24 ,  24 A include belts made of polypropylene to avoid interference with the microwave radiation within the microwave cooking enclosures  16 . In embodiments, any support structure within the cooking enclosures  16  also is made of polypropylene, or other material that does not interfere with microwave radiation. 
     The system  10  and process are ideally suited for such food products  100  as bacon strips, and in other embodiments, the food products take the form of various forms of animal protein, such as beef, pork, lamb, poultry such as chicken, duck, and turkey, and fish. For food products  100  in the form of strips, such as bacon strips, the strips are held between the two belt conveyers  24 ,  24 A. 
     As shown in  FIG. 2 , in an embodiment the system  10  includes a source of microwave radiation, which may take the form of front and/or rear magnetrons  38 ,  40 , respectively, that are connected to the waveguides  20 ,  22  of the housing  14  of each cooking unit  12 , respectively, to transmit microwave radiation through the waveguides  20 ,  22  into the microwave cooking enclosures  16 . In embodiments, each of the cooking units  12 A- 12 D includes an exhaust damper  102  that is selectively modulated to control air flow from the microwave cooking enclosure  16  to the ambient. In embodiments, the exhaust damper  102  includes a duct communicating with the microwave cooking enclosure  16 , adjustable louvers, which in embodiments are be gravity controlled or remotely adjustable by control  92 , and/or an exhaust fan, such as a variable speed exhaust fan, which in embodiments is actuated by control  92 , and/or is thermostatically controlled. The exhaust damper  102  controls the flow of humid air exhausted from the microwave cooking enclosure  16 . In other embodiments, each of the cooking units  12 A- 12 D includes two or more exhaust dampers  102  that are independently controlled. 
     In an exemplary embodiment, each cooking unit  12 A- 12 D includes a wet-bulb temperature sensor unit  42  and optionally a dry-bulb temperature sensor unit  44 . As shown in  FIG. 1 , optionally, one or more of the cooking units  12 A- 12 D includes a second wet-bulb temperature sensor unit  46  and a second dry-bulb temperature sensor unit  48 . As also shown in  FIGS. 3A, 3B, 3C, and 3D , and  FIGS. 4A, 4B, and 4C , each wet-bulb temperature sensor unit  42  includes a microwave shield  50 . In embodiments, the microwave shield  50  is a cylindrical tube  52  with open ends  54  and a plurality of openings  56  along its length. In embodiments, the openings  56  are arranged in rectilinear rows along the length of the tube  52 . In other embodiments, the openings  56  are arranged randomly or in other patterns, and there may be more or fewer than illustrated in the figures. In embodiments, the openings  56  are circular to eliminate microwave arcing points but may take different shapes. In still other embodiments, the shield  50  takes the form of a metal mesh screen in which the openings  56  are in the form of screen openings. The openings  56  of these embodiments are sized to block microwave radiation from entering the interior  58  of the shield. In embodiments, the shield is made of stainless steel, but other corrosion-resistant conductive materials may be used. 
     Each wet-bulb temperature sensor unit  42  includes a top plate  59  (see also  FIGS. 5A, 5B , and  5 C) that is attached to and covers an upper open end  54  of the tube  52 . The top plate  59  is attached to, or forms a part of, the top wall  18  of the housing  14  (see  FIGS. 1 and 2 ). In embodiments, the top plate  59  includes openings  60 ,  61  that receive and support a wet-bulb temperature probe or sensor  62  and a misting nozzle  64 , respectively, ( FIG. 2  and  FIGS. 3A-3D ) connected to a source of water  66  ( FIG. 2 ). The wet-bulb temperature sensor unit  42  is mounted on the top wall  18  of the housing  14  such that the microwave shield  50 , and thus the temperature sensor  62  and misting nozzle  64 , extend into the microwave cooking enclosure  16 . In embodiments, the shield  50  has a length that extends beyond the lengths of the wet-bulb temperature sensor  62  and misting nozzle  64  sufficiently to prevent microwaves from entering the lower or bottom open end  54  and contacting the probe and nozzle. In other embodiments, the lower or bottom open end  54  is closed or sealed with a metal plate (not shown) integral with the shield  50 . 
     As shown in  FIGS. 6A, 6B, and 6C ,  FIGS. 7A, 7B, and 7C , and  FIGS. 8A, 8B, and 8C , the dry-bulb temperature sensor unit  44  includes a shield  68 . In embodiments, the shield  68  is a cylindrical tube  72  with open ends  74  and a plurality of openings  76  along its length. In embodiments, the openings  76  are arranged in rectilinear rows along the length of the tube  72 . In other embodiments, the openings  76  are arranged randomly or in other patterns, and there may be more or fewer than illustrated in the figures. In embodiments, the openings  76  are circular to eliminate arcing points, but may take different shapes. The openings  76  are sized to block microwave radiation from entering the interior  78  of the shield. In embodiments, the shield is made of stainless steel, but other corrosion-resistant conductive materials may be used. 
     Alternatively, the wet-bulb temperature sensor unit  42  includes a wet-bulb temperature sensor  62  in the form of a dew point sensor or a relative humidity sensor. The dew point sensor or relative humidity sensor  62  generates a signal indicative of dew point and/or relative humidity of the air within the microwave cooking enclosures  16  of the microwave cooking units  12 A- 12 C. Each dew point and/or relative humidity sensor  62  is located within its respective microwave shield  50 , and the misting nozzle  64  is eliminated. 
     In embodiments in which a dry-bulb temperature sensor unit  44  is used, each dry-bulb temperature sensor unit includes a top plate  79  that is attached to and covers an upper open end of the tube  72 . The top plate  79  is attached to, or forms a part of, the top wall  18  of the housing  14  (see  FIGS. 1 and 2 ). The top plate  79  includes an opening  80  that receives and supports a dry-bulb temperature probe or sensor  82 , which in embodiments takes the form of a dry-bulb thermometer. Accordingly, in the system  10 , the wet-bulb sensor can take the form of a wet-bulb thermometer  62 , a combination wet-bulb thermometer and dry-bulb temperature sensor  82 , a dew point temperature sensor, a relative humidity sensor, and combinations of the foregoing. 
     As shown in  FIG. 2 , in an embodiment, the system  10  includes moisture injection nozzles that take the form of steam nozzles  84 ,  86  in the housing  14  of each cooking unit  12 A- 12 D. In embodiments, each cooking unit  12 A- 12 D have more or fewer than two steam nozzles  84 ,  86  are positioned below the conveyor  24 . In other embodiments, the steam nozzles  84 ,  86  are below and/or above the conveyors  24 ,  24 A, and/or on one or both sides of the conveyors. 
     In an embodiment, the steam nozzles  84 ,  86  are supplied with saturated steam through a steam supply line  88 . The flow of steam through the steam nozzles  84 ,  86  into the microwave cooking enclosures  16  is regulated by steam valves  90 . The steam nozzles  84 ,  86  provide heat and moisture to the air in the microwave cooking enclosures  16  to form humidified air, in embodiments heated humidified air, within the cooking enclosures. In other embodiments, the nozzles  84 ,  86  take the form of mist nozzles that provide a fine mist of water droplets through a water line  88 , the flow of which is regulated by water valves  90 , to make humidified air within the microwave cooking enclosures  16 . 
     In an embodiment, the system  10  includes a control  92 , which may take the form of a programmable logic controller (PLC) or a microcontroller or microcontroller unit (MCU). In embodiments, the control  92  is connected to receive signals from the wet-bulb temperature sensor  62  and optionally the dry-bulb temperature sensor  82  from one or more of the cooking units  12 A- 12 D. In an alternative embodiment, the control receives a signal from the dew point temperature sensor and/or relative humidity sensor  62  and a signal representative of temperature from the dry-bulb temperature sensor, and from that data calculates the wet-bulb temperature of the enclosure  16 . For each cooking unit  12 A- 12 D, the control  92  is also connected to control the valves  90  to regulate the flow of steam and/or heated water to the nozzles  84 ,  86  to regulate the humidity of the air in the microwave cooking enclosure  16 . In an embodiment, the control  92  is programmed to modulate the flow of steam and/or water mist from the nozzles  84 ,  86  into the interior of the microwave cooking enclosure  16  as the microwave radiation is transmitted into the interior of the microwave cooking enclosure to tenderize the food products  100  on the conveyor  24 ,  24 A as the food products are cooked and prevent dehydration of, and thus ensure lethality to microorganisms in the interior of the microwave cooking enclosure and on the outer surface of and within the food products in the interior of the microwave cooking enclosure. 
     In the embodiment of  FIG. 1 , in which the system  10  includes a plurality of the microwave cooking enclosures  16  of the cooking units  12 A- 12 D, the conveyor  24 ,  24 A conveys the food products  100  into and out of the interiors of successive ones of the plurality of the microwave cooking enclosures. The control  92  is programmed to modulate the valves  90  and/or the exhaust dampers  102  maintain the wet-bulb temperatures of the humidified air within the interiors of the plurality of the microwave cooking enclosures  16  at or above value selected to maintain lethality and provide even cooking of the food products  100  and to actuate the sources of microwave energy, which in embodiments are magnetrons  38 ,  40 , to transmit the microwave radiation into the interiors of the microwave cooking enclosures, which, combined with the humidified air, cook the food products in the interiors of the microwave cooking enclosures and kill microorganisms therein and on the outer surfaces of and within the food products in the cooking enclosures. In this embodiment, the food products  100  are completely cooked by passing through the plurality of the microwave cooking enclosures  16  of cooking units  12 A- 12 D. 
     In an exemplary embodiment, the microwave cooking enclosure  16  in each of the cooking units  12 A- 12 D is approximately 133 ft 3  for a conveyor  24  having 3 lanes, and in another embodiment the microwave cooking enclosure in each of the cooking units is approximately 164 ft 3  for a conveyor having 4 lanes. With such embodiments, the control  92  actuates the valves  90  to deliver at least 200-225 lbs./hr. of steam to the microwave cooking enclosure  16 , or about 2×10 5  to 2.25×10 5  btu/hr. (1.4×10 4  to 1.58×10 4  gm-cal./sec.). Thus, with this embodiment, steam, in embodiments saturated steam, is delivered to the microwave cooking enclosure  16  at a rate of between 1.37 and 1.50 lbs./hr.-ft 3 , or between 1370 and 1500 btu./hr.-ft 3  (3440 to 3760 gm-cal./sec. ft 3 ). 
     Optionally, the control  92  controls or actuates a control  94  that actuates the magnetrons  38 ,  40 , actuates the valves for the misting nozzles  64 , and/or the control  96  that starts, stops and modulates the speed of the conveyors  24 ,  24 A. In embodiments, the control  92  may take the form of a standalone unit, which may be handheld or incorporated in a tablet, or may be integrated as a module or program into a larger control system (not shown). The control  92  may communicate with the aforementioned components by hardwire, over a network such as a control area network (CAN), and/or wirelessly. The control  92  in embodiments is physically adjacent the cooking units  12 A- 12 D, and in other embodiments physically remote from the system  10  or offsite from the system. 
     In still other embodiments, the control  92  controls a secondary cooking device  98 , which may take the form of a multiple purpose oven (MPO) and/or a forced convection oven, through which the conveyors  24 ,  24 A pass to convey food products  100  before passing through the cooking units  12 A- 12 D. Although  FIG. 2  shows the secondary cooking device  98  positioned upstream of the system  10 , in other embodiments, the secondary cooking device  98  is positioned downstream of the cooking units  12 A- 12 D. In still other embodiments, secondary cooking device  98  takes the form of cooking devices positioned upstream and downstream of the system  10 , and in other embodiments, one or more secondary cooking devices are placed between two or more of the cooking units  12 A- 12 D, or combinations of one or more of the foregoing. 
     The process of operation of the system  10  is as follows. Food products  100  are placed into the interior of a microwave cooking enclosure  16  of a cooking unit  12 . Microwave radiation from the source  38  of microwave energy is transmitted into the interior of the microwave cooking enclosure  16  to cook the food products  100 . The control  92  actuates magnetrons  38 ,  40  of microwave cooking unit  12 A, either simultaneously or sequentially as the conveyors  24 ,  24 A transport food products  100  through the microwave cooking enclosure  16 , to begin to cook the food products in the microwave cooking enclosure. 
     A signal is generated from a microwave-shielded sensor  62  positioned within the interior of the microwave cooking enclosure indicative of a wet-bulb temperature, and hence the relative humidity, of the humidified air within the interior of the microwave cooking enclosure  16 . Alternatively, the control  92  is programmed to receive signals from the dew point temperature and/or relative humidity sensor  62 , optionally from dry-bulb temperature sensor  82 , and from that calculates the wet-bulb temperatures and relative humidity of the microwave cooking enclosure  16 . A flow of steam to the interior of the microwave cooking enclosure  16  is provided through nozzles  84 ,  86 ; alternately, the nozzles provide a water mist. The control  92  opens valves  90  to actuate the nozzles  84 ,  86  to inject steam and/or water mist heat the air within the microwave cooking enclosures  16 . The conveyors  24 ,  24 A are actuated, in embodiments by the control  92 , to convey food product  100 , such as uncooked sliced bacon, that has been placed on the conveyors, in embodiments manually, through the entrance  26  and into microwave enclosure  16  of the first microwave cooking unit  12 A. 
     The wet-bulb temperature, or in other embodiments the dew point temperature and/or the relative humidity, within the interior of the microwave cooking enclosure is maintained at or above a predetermined value by the control  92 , which is programmed to receive the signal from the microwave-shielded sensor  62  indicative of the wet-bulb temperature, and hence the relative humidity of the humidified air, within the interior of the microwave cooking enclosure  16 . In response, the control  92  is programmed to modulate the flow of steam and/or water mist into the interior of the microwave cooking enclosure  16 . Simultaneously with maintaining the wet-bulb temperature and relative humidity at or above a predetermined value, the magnetrons  38  transmit microwave radiation into the interior of the microwave cooking enclosure  16  to cook the food products  100  in the interior of the microwave cooking enclosure. The combination of the microwave radiation and the heated humidified air from the added steam, microwave radiation, and/or water mist provide uniform cooking of the food products and prevent dehydration of microorganisms, thereby promoting killing microorganisms in the interior of the microwave cooking enclosure  16  and on the outer surface of and within the food products. 
     In embodiments, maintaining the wet-bulb temperature, and in other embodiments the dew point temperature and/or the relative humidity, within the interior of the microwave cooking enclosure  16  includes providing steam, in embodiments saturated steam, to the interior of the microwave cooking enclosure  16  through the steam nozzles  84 ,  86 , which are modulated by the control  92 , which is programmed to vary the flow rate of steam into the interior of the microwave cooking enclosure  16 . In embodiments, maintaining the wet-bulb temperature within the interior of the microwave cooking enclosure  16  includes the control  92  being programmed to modulate the exhaust damper  102  connected to the interior of the microwave cooking enclosure to allow humidified air to exhaust from the interior of the microwave cooking chamber. 
     In embodiments, placing the food products  100  into the interior of the microwave cooking enclosure  16  includes placing the food products on the conveyor  24 , and/or conveyor  24 A to convey the food products into and out of the interior of the microwave cooking enclosure  16 . In embodiments, wherein maintaining the wet-bulb temperature within the interior of the microwave cooking enclosure  16  includes actuating the valve  84  and/or  86  by the control  92  to maintain the wet bulb temperature within the interior of the microwave cooking enclosure between 158° F.-210° F. (70° C.-99° C.). 
     In embodiments, as shown in  FIG. 1 , food products  100  are placed successively into the interiors of the microwave cooking enclosures  16  of a plurality of the microwave cooking units  12 A- 12 D as they are transported by conveyors  24  and optionally  24 A into and out of each successive cooking unit  12 A- 12 D. In each of the cooking units  12 A- 12 D, the wet-bulb temperature within the interior of the microwave cooking enclosure  16  is modulated or maintained at or above the predetermined value by the control  92  in response to the signals received from the microwave-shielded sensor  62 , and optionally  82 , indicative of the wet-bulb temperature within the interior of the microwave cooking enclosure. The control  92  adjusts the valves  90  supplying steam and/or heated water to the nozzles  84 ,  86 , and optionally by adjusting the exhaust dampers  102  to allow egress of steam and/or heated mist from the microwave cooking enclosures  16  in order to maintain the desired wet-bulb temperature in the interior of the microwave cooking chambers  16 . 
     Simultaneously with maintaining the wet-bulb temperature, microwave radiation is transmitted into the interior of the microwave cooking enclosure  16  to cook the food products  100  in the interior of the microwave cooking enclosure. The microwave radiation and the steam heat kill microorganisms in the interior of the microwave cooking enclosure; and wherein the food products are completely cooked by at least one of the plurality of the cooking units. In one embodiment, the food products are completely cooked by a last one of the plurality of the cooking units. As used herein, the term ‘fully cooked’ and ‘completely cooked’ mean, for example, cooked to a degree to meet applicable food standards and/or kill microorganisms present on and/or in the food product. 
     For bacon slices, the valves  90  are adjusted to modulate the wet-bulb temperature within the enclosures  16  to at least 158° F., and preferably 176° F. or higher. In embodiments, the wet-bulb temperatures may vary from one microwave cooking enclosure  16  to another. The control  92  optionally controls exhaust dampers  102  in the top walls  18  of the housings  14  to modulate the flow of humid air, which in embodiments is steam-laden or moisture-laden air, from the enclosure  16 , which lowers the wet-bulb temperatures in the microwave cooking enclosures  16 . In an exemplary embodiment, this process is repeated as the food products  100  on the conveyor  24  pass through successive cooking units  12 B,  12 C, and  12 D, so that the food products  100  are incrementally cooked, and a fully cooked food product  100  leaves exit  28  of the cooking unit  12 D on conveyor  24 . Alternatively, the food product is completely cooked in a single cooking unit  12 . The resulting food product  100  is thoroughly cooked and active pathogen free yet is moist and provides pleasing organoleptic qualities. In other embodiments, additional cooking, such as browning, is effected by the secondary cooking device  98 , which may occur before, during, or after microwave cooking by system  10 . 
     Thus, the disclosed system  10  and process provide microwave cooking of food, such as bacon slices, in which the cooking is assisted by injection of steam, such as saturated steam, and/or a mist of heated water into the microwave cooking enclosure  16 . Injection of steam and/or mist of heated water is modulated by a control  92  that continuously reads, or in embodiments calculates or derives, the wet-bulb temperature in the interior of the cooking enclosure  16  during the cooking process and continuously adjusts the valve  90 , and optionally the exhaust damper  102  as part of a closed loop system, to control the flow of steam and/or moisture into the enclosure to modulate the wet-bulb temperature to maintain the wet-bulb temperature at or above a predetermined value. This value is selected to provide optimum cooking of the food product and maintain a humid atmosphere in the enclosure  16  that both provides hydrated surface lethality (HSL) of the food and sterilizes conveyors  24 ,  24 A and components thereof, the walls of the microwave cooking enclosure  16 , and other interior components of the cooking unit  12 . 
     When the system  10  and process are used to cook bacon slices as the food products  100 , the result is improved tenderization of bacon slices, consistent tenderization of bacon slices, more consistent product yields, assured destruction of pathogenic bacteria and other undesirable microorganisms on the food products and on the conveyer components, and assured destruction of spoilage bacteria on the food products and the conveyor components, in comparison to systems and processes that do not use steam to maintain a desired wet-bulb temperature. 
     In another embodiment, a closed loop system is not used to maintain the wet-bulb temperature within the microwave cooking enclosure  16  of one or more of the cooking units  12 A- 12 D. Instead, the exhaust damper or exhaust dampers  102  of one or more of the cooking units  12 A- 12 D are adjusted to control the humidified air exhausted from the interiors of the microwave cooking enclosures  16 . In embodiments, this adjustment of wet-bulb temperature of the humidified air within the microwave cooking enclosure  16  takes the form of adjusting louvers of the dampers and/or the speed of the integral exhaust fans of the exhaust dampers to modulate the flow of humidified air from the interiors of the microwave cooking enclosures  16 . 
     Propagation or distribution of the microwaves in the cooking chamber  16  without the presence of steam, and in embodiments a water mist, such as heated water mist, can cause the food product  100  to have a substantial temperature variation during microwave cooking. By microwave cooking the food product  100  in the presence of steam, the temperature of the food products and microwave propagation are more evenly distributed. In addition to maintaining uniform temperature and humidity levels within the microwave cooking enclosure  16 , the steam heats the conveyors  24 ,  24 A, which aids in the overall cooking process by providing direct contact cooking of the food products  100  in addition to cooking by microwave radiation and steam. By combining the benefits of cooking food products  100  in a humid atmosphere with simultaneous microwave cooking, increased food product throughput can be achieved, arcing from microwaves is minimized, increased lethality is achieved, and more even cooking effected. 
     While the forms of apparatus and processes described herein represent preferred embodiments of the disclosed process and system for microwave cooking with humidified air control, it is to be understood that the invention is not limited to these precise processes and systems, and that changes may be made therein without departing from the scope of the invention.