Abstract:
A vessel for cooking food under pressure wherein the shape of a bottom surface of the vessel and the position of heating elements which heat cooking fluid contained within the vessel act to form a cool zone in a region near the bottom surface. Within the cool zone, the temperature of the cooking fluid is maintained below the cooking temperature so that food particles dislodged from the food being cooked do not burn and contaminate the cooking fluid. In addition, the temperature in the cool zone is maintained above the boiling point of water so that moisture contained within the food particles is continuously evaporated during the cooking cycle, preventing boil over conditions. A novel pressure release system is provided for intermittently operating a pressure release valve near the end of the cooking cycle. A computer controller controls operation of the release valve based upon the temperature of the cooking fluid to eliminate sudden drops in temperature.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 07/839,612, filed on Feb. 21, 1992 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to cooking appliances and, in particular, to a pressure fryer designed to prevent accumulation of condensed moisture within the cooking fluid. 
     Pressure cookers are well known in the field of cooking appliances. In such pressure cookers, the pressure inside a cooking vessel is increased above atmospheric pressure to cook food quickly while also retaining moisture in the food. Items such as chicken and fish are commonly cooked in this type of appliance. The food is often first coated with a breading and then cooked in a cooking fluid, such as oil, for a predetermined time period. Near the end of the time period, or cooking cycle, a valve is opened releasing the pressure inside the fryer so that the food items can be safely removed therefrom. 
     During the cooking process, small particles of the breading typically used to coat the food items being cooked often become dislodged and fall to the bottom. If these particles, commonly referred to as “cracklings”, remain in the cooking oil during subsequent cooking processes, they can burn and contaminate the cooking oil. 
     In order to prevent such contamination, many prior cooker designs have incorporated what is commonly termed as a “cold zone”. This cold zone typically comprises a deep trough formed in the bottom surface of the cooking pot to collect the cracklings which accumulate during operation. The burners or heating elements are typically positioned well above the cold zone such that the zone is maintained at a temperature substantially lower than the cooking temperature. In this manner, cracklings may accumulate during successive cooking operations without burning and contaminating the cooking oil. 
     An example of such a pressure cooker is shown in U.S. Pat. No. 4,930,408 to King. The pressure cooker comprises a cooking vat divided into an upper cooking zone and a lower cold zone interconnected by a tapered annular wall. The cold zone is substantially smaller than the cooking zone, preferably comprising 20-30% of the volume of the vat. The cooking oil is heated by heating elements which line the walls of the vat in the lower portion of the cooking zone. The cold zone is maintained in essentially quiescent condition and at a substantially lower temperature than the temperature of the oil in the cooking zone to minimize cooking of cracklings and other contaminants. 
     U.S. Pat. No. 4,273,991 to Barnhill discloses a pressure fryer including strip heaters on the front wall, rear wall, and side walls of the fryer. The bottom edge of each lower most strip heater on each of the walls is spaced above the bottom edges of the tank&#39;s side walls to create a relatively cold zone for the cooking liquid within the tank&#39;s trough. Food particles which fall to the tank floor migrate toward the center of the floor since it is sloped downwardly relative to the bottom edges of the side wall, and are less likely to turn to charcoal. 
     U.S. Pat. No. 1,990,351 to Shroyer discloses a grease kettle including heating element which extends substantially through 270° of arc of the circumference of the kettle and at its ends is provided with terminal portions which extend through the bottom of the kettle. A second heating element is arranged into a series of vertical convolutions spaced at intervals about the portion of the periphery of the kettle and is provided with terminal portions extending through the bottom wall of the kettle. The patent discloses that by reason of the arrangement of the heating elements, grease in the lower portion of the kettle over substantially the entire bottom surface will remain at a temperature below that at which food sediment will char or burn. At the same time, grease at the frying level will be heated to the frying temperature. 
     In these types of pressure fryers, the cracklings which become dislodged from the food being cooked naturally contain moisture. During subsequent cooking cycles, this moisture collects and condenses in the cold zone generally without problem. However, if the cooking process is interrupted for a short period of time between cooking cycles, oil stratification may occur. To provide a uniform oil temperature prior to resuming cooking operation, the oil is typically stirred. In the known pressure fryers described above, when stirred entrapped water in the cracklings contacts the cooking oil which is at temperature greater than the boiling point of water, 212° F., and can be as high as 350°. The water thus converts to steam, causing a boil over condition which can injure the operator of the cooking appliance. Boil over also results, due to the same effect, from the naturally occurring convection currents within the oil which cause entrapped water to move up into the hot oil. 
     Canadian Patent No. 622,425 to Kelton discloses a pressure cooker including a cylindrical portion at the bottom of the vessel which constitutes a sump or sediment chamber into which fall particles of food. An elongated metal equalizer mounted in the center of the vessel is intended to conduct heat from the hot fat downwardly into the sump zone thereby keeping the sump above the boiling point of water, but also maintaining it below the burning point of the crumbs. However, in practice, this design has not proved successful. Also, the addition of such an equalizer to the interior of the cooking vessel limits the volume available for cooking and further restricts the use of baskets or racks typically placed in the vessel to support food items. 
     Another problem associated with the use of a cold zone is that as the water evaporates from the cracklings there is a period of time during which it passes through the cooking oil. When the cook cycle is complete and the pressure relief valve is opened, these small bubbles of water evaporate. Such evaporation results in a sudden undesirable temperature drop in the cooking oil. 
     SUMMARY OF THE INVENTION 
     The present invention provides a unique pressure fryer design in which the shape of the fryer and the placement of the heaters which heat the cooking liquid therein act to provide a cool zone at the bottom of the fryer. Within this cool zone, the temperature is maintained above the boiling point of water such that moisture contained within food particles is evaporated and the above-described boil-over condition is prevented. In addition, the temperature within the cool zone is kept well below the cooking temperature such that the food particles do not burn and contaminate the cooking fluid. 
     In accordance with the present invention, the pressure fryer includes a vat for containing cooking fluid. Preferably, the vat has a gently sloping bottom surface surrounded by a plurality of side walls. The bottom surface includes a central flat portion defining a lowermost point on the bottom surface. A plurality of heating elements are positioned on the side walls of the vat a sufficient vertical distance above the lowermost point on the bottom surface to form a cool zone between the heating elements and the bottom surface. In this manner, the temperature within the cool zone is maintained below the cooking temperature, but above the boiling point of water. 
     Another object of the invention is to prevent and/or control a sudden unwanted temperature drop in a pressure cooker caused by the sudden release of pressure and evaporation of water in the cooking oil. It is also noted that the invention has applicability in any environment where it is desirable to control the sudden decrease in temperature due to a pressure decrease. 
     In accordance with this aspect of the invention, a method and apparatus are provided for releasing the pressure from within the fryer and eliminating or minimizing the associated temperature drop. In accordance with the invention, a pressure release valve is operatively connected to a computer controller, which includes, as one of its inputs, output signals provided by a temperature sensor or sensors which are used to detect the temperature and rate of change of the temperature of the cooking oil. Near the conclusion of the cooking cycle, the pressure release valve is opened by the controller and the pressure within the fryer is released. The temperature and rate of change thereof are monitored by the controller. When the temperature of the oil detected drops to a predetermined temperature level and the rate of change detected by the controller indicates that the temperature is decreasing, the pressure release valve is closed and the cooking medium is allowed to absorb more heat. When the temperature rate of change stabilizes and/or the temperature of the cooking oil increases slightly, the pressure release valve is once again opened. The process is repeated at frequent intervals, with each opening of the pressure release valve allowing a small amount of pressure release and shutting to stop the release of pressure from the fryer when the temperature decreases and falls below a predetermined level. 
     Thus, the resulting arrangement provides for pressure release as quickly as possible while maintaining the temperature of the cooking medium at or near a predetermined set temperature. The intermittent release of pressure prevents undesirable rapid temperature drops. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a pressure fryer in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of a further embodiment of a pressure fryer in accordance with the invention; and 
     FIG. 3 is a schematic diagram of the pressure control system in accordance with another aspect of he invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a device for cooking food under pressure according to the invention. Preferably the device comprises a pressure fryer  10  having a generally rectangular or square vat  12  for holding a supply of cooking fluid  13  (shown in FIG.  2 ), such as cooking oil. The vat includes a front wall  14 , a back wall (not shown), two side walls  18 ,  20 , and preferably a gently sloping bottom surface  22 ; although flat bottoms may be utilized. A cover  24  seals the top of the vat  12  in a known manner so that the pressure within the vessel may be increased above atmospheric pressure to effect pressure cooking of food contained within the vat. The bottom surface  22  of the cooking vat  12  comprises a substantially horizontal or flat base portion  26  defining a lowermost point on the bottom surface  22 . The base portion  26  is surrounded by a plurality of sloping portions  28  which originate at a respective wall of the vat  12  and slope downwardly, terminating at the flat base portion  26  of bottom surface  22 . 
     The cooking oil  13  is heated in the vat  12  by appropriate heating elements mounted on the exterior surface of the walls of the vat. The heating elements are controlled by a temperature controller (not shown) as is well known in the art. During the cooking cycle, the heating elements are activated by the temperature controller to heat the cooking fluid contained in the vat to a cooking temperature of typically approximately 260° or higher to effect cooking of the food placed within the vessel. In one embodiment, upper and lower electric heating elements  30 ,  32  are positioned on the front wall  14 , back wall, and two side walls  18 ,  20  of the vat  12 . Alternatively, as illustrated in FIG. 2, gas burners  40  may be used to heat the cooking fluid in the vat. In this configuration, preferably one or two gas burners  40  are attached to each side wall  18 ,  20  of the vat  12 . 
     In accordance with the invention, the lower electric heating elements  32  or gas burners  40  are positioned on the walls of the vat  12  at a sufficient vertical distance (D) above the lowermost point on the base portion  26  to maintain the cooking fluid therebetween at a temperature below the cooking temperature of the rest of the fluid and above the boiling point of water. This arrangement provides a cool zone in the bottom of the vat between the bottom edge and the base portion of the bottom surface, as will be set forth in more detail below. 
     Referring to FIG. 2 as previously described, during the cooking cycle, pieces of the breading or other coating commonly referred to in the industry as cracklings  42  become dislodged from the food products  44  being cooked within the vat and tend to drift downward towards the bottom surface  22  of the vat  12 . In the present invention, the slope of the sloping portions  28  of the bottom surface  22  is preferably sufficiently steep to direct such cracklings onto the flat base portion  26 . In particular, for a vat designed to hold 100 lb. of cooking fluid having a side wall height (H) of approximately 35 inches, a base portion length (L) of approximately 29 inches, and a total volume of approximately 3500 cubic inches, a preferred upward slope angle (θ) of the sloping portions with respect to the base has been found to be about 9° to 21° more preferably approximately 13° to 17°. In this example, the lower electric heating elements  32  or gas burners  40  are positioned at distance (D) equal to approximately 2 to 3 inches above base portion  26 , preferably about 2 inches, so that the temperature of the cooking oil in the cool zone is kept substantially below the cooking temperature. A preferred temperature for the cool zone region within the vat  12  is thus approximately 245° F. In this manner, the cracklings settling on the base portion  26  of the bottom surface  22  are not located near enough to the heating elements to burn and contaminate the cooking oil. 
     The heating elements  32  or  40  are also positioned near enough to the bottom surface  26  of the cooking vat  12  where the cracklings  42  settle so that the temperature of the cool zone is kept higher than the boiling point of water. Because the temperature of the cooking oil is maintained above the boiling point of water, the moisture naturally contained in the cracklings  42  will evaporate and will not suddenly mix with the cooking oil  13  if later stirred. Instead, the moisture contained in the food particles  42  evaporates and passes through the oil  13  continuously during the cooking cycle as does any moisture remaining at the end of a cycle. Thus, dangerous boil over conditions associated with prior art designs are avoided. 
     Thus, the arrangement of heating elements  30 ,  32 , or  40 , in combination with the sloping shape of bottom surface  22  and horizontal base portion  26  create a cool zone at the bottom of vat  12  wherein the temperature of the cooking oil contained therein is maintained at a temperature above the boiling point of water at the highest pressure encountered in the fryer during the cook cycle, yet well below the cooking temperature. A preferred range of temperature of the cool zone is between about 212° F. and 250° F. and, as stated above, preferably about 245°. Those skilled in the art will appreciate that other means for maintaining this temperature range at the bottom of the vat (in particular, within about 2-3 inches of the bottom) could also be used. For example, a heating element  34  could be positioned below the bottom surface  22  of the vat  12  as shown in phantom in FIG.  1  and used in connection with other heating elements to maintain the temperature of the cooking oil between about 212° F. and 250° F. Maintaining the temperature of the cooking oil in this desired range is achieved according to the invention with a completely unobstructed cooking vat. 
     In accordance with another aspect of the invention, pressure release is controlled to avoid significant temperature drops which may occur when the fryer is opened near the conclusion of a cooking cycle. FIG. 3 schematically illustrates a pressure release system in accordance with this aspect of the invention wherein like parts have been numbered accordingly. Although not specifically illustrated, this aspect of the invention can be utilized alone or in combination with the cool zone design according to the invention as described above. In fact, the pressure control system according to the invention may be particularly useful in combination with the cool zone design according to the invention due to the possibility of an increased amount of dissolved water in the hot cooking fluid. 
     The system according to the invention comprises a conventional pressure release valve  50 , such as a solenoid valve, mounted by piping connection to an exhaust port in a side wall  18  of the vat  12 . The system further includes a controller  52  mounted adjacent the vat and connected to the release valve  50  by signal line  54 . The controller  52  preferably comprises a microprocessor-based controller, programmed in accordance with techniques well known to those skilled in the art, to monitor the change of temperature and effect the release of pressure accordingly, and to calculate other functions as will be described in detail below. Electric heating elements  30 ,  32  heat the fluid contained within the vat  12  in the manner described above. Gas burners  40  as illustrated in FIG. 2 or other appropriate heating means can also be used. 
     At a predetermined time near the end of the cooking cycle, indicated in a well-known manner by a timer (not shown), the controller  52  sends a release signal to the pressure release valve  50  over the signal line  54 . Alternatively, the predetermined time for the release signal can be calculated by the controller independently for each cooking cycle during the cycle based on factors sensed and input to the controller, such as time, temperature and food product. Preferably, the release signal is sent approximately one minute prior to the end of the cooking cycle although, as stated, the predetermined time may be determined on a case-by-case basis. In response to the signal received from the controller  52 , the pressure release valve  50  opens and begins releasing pressure which has built-up within the vat  12 . The heating elements  30 ,  32  continue to operate to complete the cooking cycle and help maintain temperature. As the pressure begins to release through the valve  50 , the water contained in the cooking oil  13  evaporates and the temperature of the cooking oil begins to drop. 
     The controller  52  is further connected to a temperature sensor(s)  56 , such as a conventional thermistor probes, RTD or thermocouples, mounted within the vat  12  so as to monitor the temperature of the oil  13  during the cooking process in accordance with well-known techniques. In addition, the temperature monitor  56  provides signals to the controller  52  over signal line  58  which enables the controller to determine the rate of change of temperature of the cooking oil  13 . The temperature of the cooking fluid  13  is sampled at predetermined time intervals to compute the rate of change from successive temperature measurements. A preferred temperature sampling rate has been found to be three samples per second, although other rates could be used as desired. As those skilled in the art will further appreciate, this rate of change of the temperature may be determined by any well-known digital or analog technique. The rate of temperature change is then utilized by the controller  52  to control operation of the pressure release valve  50  as described below. 
     In particular, if the controller  52  determines that the rate of change of the temperature of the cooking oil  13  is negative, i.e., the temperature of the cooking oil  13  is decreasing, and if the temperature itself is at or below a predetermined set temperature stored in a memory unit (not shown) associated with the controller, the controller  52  immediately sends a signal to the pressure release valve  50  via the signal line  54  to shut the valve and prevent further release of pressure from the vat  12 . Preferably, the predetermined set temperature is between about 270°-325°, more preferably approximately 280° F. Those skilled in the art will appreciate, however, that other temperatures may also be used in accordance with the operating parameters of the fryer, such as the type of food being cooked and the desired cooking result. 
     After the pressure release valve has been shut, the continuing action of the heating elements  30 ,  32  will cause the temperature of the cooking fluid  13  contained in the vat  12  to increase. The controller  52  continuously monitors the temperature signal received from the temperature sensor  56  over the signal line  58  to determine if the temperature of the cooking fluid  13  has risen above the predetermined temperature or if the rate of temperature change is positive. When the controller  52  has detected either condition, the controller sends a signal to the release valve  50  over the signal line  54  to again open the valve  50  and release pressure. Once the valve is opened, the temperature and rate of change thereof are again monitored as described above to detect whether the temperature of the cooking oil  13  is below the set temperature and falling. If such a situation occurs, the release valve  50  will again be shut and the process repeated. Thus, it can be appreciated that rather than simply releasing the pressure all at once and suffering a rapid and undesirable temperature drop in the cooking oil  13 , the release valve  50  is periodically pulsed open and closed and the pressure is intermittently released. Substantially all of the pressure within the vat  12  is released when the cooking cycle concludes so that the vat can be substantially immediately opened and cooked food items removed safely therefrom. The present invention ensures that pressure is released at the maximum rate possible without experiencing undesirable temperature drops. 
     While the above describes the preferred embodiment of the invention, it can be appreciated that other variations will be apparent to those of ordinary skill in the art. For example, other types of controllable valves may be utilized, other cooking medium may be utilized, etc. Furthermore, rather than the valve being opened and closed fully, it may be opened partially with the amount of opening depending upon the temperature condition. 
     As will be apparent to those skilled in the art, various modifications and adaptations of the embodiments described above will become readily apparent without departure from the spirit and scope of the invention, the scope of which is defined in the appended claims.