MICROWAVE OVENSCOOKING METHOD

Abstract: A microwave oven has a forced hot air system for delivering a forced flow of hot air through the oven cavity and a magnetron for delivering microwave power to the cavity. The hot air temperature is sensed after a predetermined time, between 8 and 15 minutes, and the further cooking time beyond the predetermined time is determined from a characteristic relating sensed temperature at the predetermined time and the further cooking time. The characteristic is preloaded into a microprocessor of the oven, and different characteristics may be provided appropriate to different sorts of meats, such as poultry (Figure 1), beef and port (Figures 2 and 3 respectively)).

Description: Field of invention This invention relates to a method of controlling the cooking time of food in a microwave oven. Background to the Invention The Applicants' Patent Specification EP-A-0187543 (which is not a full prior publication but is prior art according to Article 54,3) and the Applicants' Patent Specification EP-A-0122710 disclose methods of cooking food in microwave ovens, but do not disclose a method in which the total cooking time is derived by reference to a preset characteristic, as in the present invention. Disclosure of the invention According to the invention there is provided a method of controlling the cooking time of food in a microwave oven having a food receiving cavity, a magnetron for delivering microwave power to the cavity, a forced hot air system comprising an electrical resistance heating element and a fan operative to blow air over the heating element and thence through the cavity simultaneously with the production of microwave power, temperature sensing means for sensing the temperature of the hot air flow and timing means for timing cooking from the commencement of cooking with the cavity in a cold condition and control means for controlling the operation of the magnetron and the hot air system, characterised by the steps of: (i) sensing the temperature at a predetermined time after commencement of cooking, (ii) determining the further cooking time beyond the predetermined time in accordance with a predetermined characteristic relating sensed temperature at the predetermined time to the further cooking time, and (iii) ceasing production of microwave power and hot air power after the further cooking time has elapsed to terminate cooking. The predetermined time is preferably between about 8 and 15 minutes, conveniently about 10 minutes. The further cooking time which is added after the predetermined time may be anything from zero to up to about 65 minutes, the characteristic relating the sensed temperature at the predetermined time to the further cooking time being determined empirically and conveniently being represented as a polynomial equation preloaded in the oven. The temperature sensing means preferably comprise a thermistor located adjacent the fan, and the microwave oven may have a time display which, when the predetermined time is reached, displays the further cooking time and counts down to zero as the further cooking time elapses, reaching zero when cooking is complete. The invention was devised primarily to cook meat items. It will be appreciated that for larger cuts of meat the further cooking time can take the total cooking time to well beyond 30 minutes. For these longer cooking times, it is desirable that the foodstuff should not be exposed to high temperatures for a prolonged period of time because this tends to cause the foodstuff to dry out. To avoid this problem, the temperature sensed at the predetermined time may govern thermostatic control of the hot air system, so that the maximum hot air temperature reached during the further cooking time decreases as the further cooking time increases. Preferably, a second thermistor is used to monitor the hot air temperature immediately downstream of the electrical resistance heating element, and the temperature sensed by the second thermistor is used in a thermostatic control circuit which selectively energises and de-energises the electrical resistance heating element during the further cooking time to prevent the hot air temperature reaching undesirably high levels. The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a composite temperature/time graph showing below 15 minutes on the time axis the variation of hot air temperature with time and after 15 minutes shows the characteristic relating sensed temperature at 15 minutes to the further cooking time, when cooking poultry items, and Figures 2 and 3 are similar graphs for cooking beef and pork items, respectively. The microwave oven forming the preferred embodiment of this invention is similar in construction to the microwave oven disclosed in the applicants UK specification No. 2127658 and European patent specification No. 0099705. In particular, the oven comprises a food-receiving cavity, a magnetron for suplying microwave power to the cavity and a forced hot air system for forcing a supply of hot air through the cavity simultaneously with the delivery of microwave power. The forced hot air system comprises an electrical resistance heating element and a fan, both of which are accommodated in a compartment disposed behind a rear panel of the cavity. Inlet and outlet apertures in the rear panel enable a flow of air to be forced by the fan over the electrical resistance heating element and thence through the cavity. Figure 1 constitutes between 0 and 15 minutes a family of curves showing the variation of hot air temperature with time for poultry items of differing size. Curve 10 shows the temperature/time variation for a small chicken drumstick which is cooked at about 11 minutes when the sensed temperature reaches the maximum of 184°C. Curve 12 shows the temperature/time variation for a small chicken quarter, the sensed temperature reaching 184°C at time 15 minutes. Curves 14, 16, 18, 20 and 22 show temperature/time variations for a two pound chicken, a three pound chicken, and a twelve pound turkey respectively. It will be noted that for these items the sensed temperature at 15 minutes varies between about 153°C and 172°C. After 15 minutes, Figure 1 shows a curve 24 representing the variation of sensed temperature at 15 minutes with the further cooking time required beyond 15 minutes. For example, for the two pound chicken of curve 14, the curve of characteristic 24 shows that the further cooking time required is represented at 26, which is about 7 minutes. Hence, the total cooking time for the two pound chicken is 15 minutes plus 7 minutes, making a total of 22 minutes. For the small chicken quarter of curve 12 no further cooking time is required so that this particular food item is cooked at the predetermined time of 15 minutes. It will be seen that for the largest item, the 12 pound turkey, the further cooking time required is about 65 minutes, represented by time 28 in Figure 1. The hot air temperature sensed up to the predetermined time of 15 minutes is detected by a thermistor placed adjacent to the fan. For larger food items which require a total cooking time of more than about 30 minutes, it is desirable to limit the maximum hot air temperature to prevent drying out. This is conveniently achieved by using the temperature sensed at 15 minutes to control a second thermistor in a thermostatic control circuit. The second thermistor is located immediately downstream of the electrical resistance heating element. For example, if the temperature at 15 minutes is below 155°C, as for curve 22, the thermostatic control is set to limit the hot air temperature in the further cooking time to 180°C. If the temperature at 15 minutes is between 155°C and 163°C, the thermostatic control limits the hot air temperature to 190°C. If the temperature at 15 minutes is above 163° or 165°, no thermostatic control is applied. The thermostatic control acts to energise or de-energise the electrical resistance heating element to keep the hot air temperature at or closely below the pre-programmed level. It will therefore be appreciated that the temperature sensed at the predetermined time of 15 minutes not only determines the length of the further cooking time, but also sets the maximum hot air temperature so as to prevent larger food items drying out. When the further cooking time has elapsed the control means of the oven turn off the magnetron and the hot air system which completes the cooking process. Figure 2 is a composite graph similar to Figure 1, but for beef items. In this case the predetermined time is 10 minutes and it is the magnitude of the temperature sensed at 10 minutes which determines the further cooking time and any thermostatic control for the larger meat items. Curve 30 shows the temperature/time variation for a fourteen ounce piece of beef, and curves 32 and 34 show similar temperature/time variations for pieces of beef of two pounds four ounce weight, and four pounds eight ounce weight, respectively. Curve 36 shows the extent of further cooking time required as a function of the temperature sensed at 10 minutes. Hence, the piece of beef following curve 30 requires 5 minutes of further cooking time, making 15 minutes in all. The piece of beef following curve 32 requires a further cooking time of 26 minutes and the piece of beef following curve 34 requires 45 minutes further cooking time. As before, the temperature sensed at the predetermined time (in this case 10 minutes) is used to limit the maximum temperature of the hot air by thermostatic control. If the user requires a rare or well done result the curve 36 is effectively shifted up or down so as to reduce or increase the further cooking time. Figure 3 shows between 0 and 10 minutes the temperature/time variation for two differing sizes of pork joint. Curve 40 shows the temperature/time variation for a two pound pork joint and curve 42 shows the temperature/time variation for a pork joint weighing three pounds six ounces. As for beef, the hot air temperature is sensed at the predetermined time of 10 minutes, and the magnitude of this temperature governs the magnitude of the further cooking time, in accordance with the characteristic 44 relating sensed temperature to further cooking time. Figure 3 shows that the pork item following curve 40 requires a further cooking time of 28 minutes and that the pork item following curve 42 requires a further cooking time of 42 minutes. In all cases once the predetermined time has been reached the oven displays the required further cooking time on a digital display which counts down to zero as the further cooking time elapses, reaching zero at the end of the further cooking time, so that the user has an indication of when cooking will be completed. Also, in all cases the microwave and hot air power levels are maintained constant throughout at 1100 watts hot air power and 200 watts microwave power into the cavity. The oven described is preferably semi-automatic in operation, in that the user touches one of a plurality of pads, for example marked "Red Meats", "Poultry", "Baked", depending on the food item being cooked. If the user selects "Red Meat", the oven senses the temperature at the predetermined time of ten minutes, and if the user selects "Poultry", the predetermined time is fifteen minutes. However, the predetermined times may be pre-programmed by the oven manufacturer to other values, and may be the same for all meats, depending on power levels.

Claims: A method of controlling the cooking time of food in a microwave oven having a food receiving cavity, a magnetron for delivering microwave power to the cavity, a forced hot air system comprising an electrical resistance heating element and a fan operative to blow air over the heating element and thence through the cavity simultaneously with the production of microwave power, temperature sensing means for sensing the temperature of the hot air flow and timing means for timing cooking from the commencement of cooking with the cavity in a cold condition and control means for controlling the operation of the magnetron and the hot air system characterised by the steps of: (i) sensing the temperature at a predetermined time after commencement of cooking, (ii) determining the further cooking time beyond the predetermined time in accordance with a predetermined characteristic relating sensed temperature at the predetermined time to the further cooking time, and (iii) ceasing production of microwave power and hot air power after the further cooking time has elapsed to terminate cooking. A method according to claim 1, characterised in that the predetermined time is between 8 and 15 minutes. A method according to claim 2, characterised in that the predetermined time is substantially 10 minutes. A method according to any of the preceding claims, characterised in that the characteristic relating the sensed temperature at the predetermined time to the further cooking time is determined empirically and is represented as a polynomial equation preloaded in the oven. A method according to any of the preceding claims, characterised in that the temperature sensing means comprise a thermistor located adjacent the fan. A method according to any of the preceding claims, characterised in that the oven has a time display which, when the predetermined time is reached, displays the further cooking time and counts down to zero as the further cooking time elapses, reaching zero when cooking is complete. A method according to any of the preceding claims, characterised in that the temperature sensed at the predetermined time governs thermostatic control of the hot air system, so that the maximum hot air temperature reached during the further cooking time decreases as the further cooking time increases. A method according to claim 7, characterised in that a second thermistor is used to monitor the hot air temperature immediately downstream of the electrical resistance heating element, and the temperature sensed by the second thermistor is used in a thermostatic control circuit which selectively energises and de-energises the electrical resistance heating element during the further cooking time to prevent the hot air temperature reaching undesirably high levels.

Inventors: EKE, KENNETH IAN
Applicants: MICROWAVE OVENS LIMITED
Application year: 1987
Main classification: F24C7
IPC codes: F24C1, H05B6