The invention relates to an oven for the preparation of food.

There are three predominant types of oven available to the consumer. The first type to be discussed has a cavity made of sheet metal and is heated from the outside, e.g. by means of an electric heating element disposed near the metal. The transfer mechanism relied on to transfer heat energy to the food is predominantly radiation. The cavity reaches a high temperature and for this reason it is both resistant to this temperature and heavily insulated, and the material of the cavity has a large heat capacity and will continue to radiate heat even after the power has been switched off. This construction is heavy and bulky. The second type is based on the circulation of heated air, either heated by being in the cavity as above or externally generated and re-circulated. The transfer mechanism is predominantly one of convection. The temperature in the cavity is essentially the same as above, and hence the insulation has to have a comparable thickness. This means that this oven has a similar weight and bulk.

The third type is based on the generation of heat inside the foodstuff itself by irradiating it with microwave energy. This means that the cavity does not reach a high temperature, and hence it can be made of a less heat resistant material, and in principle it does not need to be insulated. The main use of the cavity is to prevent the microwaves from escaping. The weight predominantly comes from the transformer and magnetron used. The range of food that may be simply be prepared in a microwave oven is limited, both because it does not heat evenly, but also because the appearance and texture

of the prepared food is not always identical to the results of more traditional ovens.

It is known to provide several of the above features in one and the same oven, but this is additive both as regards bulk and weight, although certain advantages may be had by combining the heating mechanisms for the preparation of food.

There has hence been recognized a need for an oven which will inherently have a low-temperature cavity, reducing the need for insulation thickness and thereby permitting built-in solutions for kitchens without the risk of temperature build-up and consequent fire risk. It has been recognized that this may be obtained without the use of microwaves. The invention is particular in that the bulk means of heat transfer to the food is solely by a heated gas of a temperature not exceeding 150°C supplied at a controlled volume velocity ensuring the transfer of heat energy in dependence of the instantaneous rate required for the preparation of the food.

The invention is based on the realization that the heat energy uptake of the food is a varying quantity over the time it takes to prepare the food, and furthermore that the efficiency with which traditional ovens function is very variable. This means that certain food items may be prepared with simple control of temperature and volume velocity, while other food items may require more complex control, however at much lower temperatures than has traditionally been the case. An advantageous embodiment is particular in that the gas is air with an admixture of a condensable substance such as water vapour. This creates the possibility of both using the water vapour as a transfer medium by condensation and for controlling the moisture content of the surface of the food.

A further advantageous embodiment is particular in that the volume velocity of gas is controlled by the

progress of heating of food. The control may be performed by measuring the instantaneous degree of preparation and by increasing or decreasing the volume velocity. A further advantageous embodiment is particular in that the temperature of the gas is controlled by the progress of heating of food. The control may be performed by measuring the instantaneous degree of preparation and by increasing or decreasing the temperature. In combination with the control of the volume velocity it is possible to obtain exactly the heat transfer which is needed by the food in any particular instant.

A further embodiment is particular in that a radiating line source of heat is scanned across the top of the food in order to dry the surface and/or to brown it. This means that the high surface temperature is applied in a localized fashion exactly where and when it is need for the preparation of the food. The major advantage of the oven according to the invention is the possibility of using low-temperature materials for all oven components. This advantage is obtained, although the preparation time for the food is not increased over traditional methods. The invention will be described in greater detail with reference to the drawing in which

Fig. 1 shows a schematic diagram of the temperature and volume velocity control of an oven according to the invention, and " Fig. 2 shows an embodiment comprising a browning feature.

In Fig. 1 is seen an oven cavity 1 with a supply of heated air through an inlet opening 2. The heated air is generated in a heater 3 which is placed in a duct 4 where air is caused to move by means of a ventilator 5. Food F is present in the cavity. The outlet of air 6 in the cavity 1 is connected to the duct 4. Near the inlet

opening 2 there is provided a nozzle 7 which provides steam from a steam generator 8 via a control valve 9. Temperature sensing means 10 and moisture sensing means 11 as well as surface reflectance measuring means 12 are present in the cavity 1, and the signals are fed to a control computer 13 which controls the heating of the air fed through the inlet 2 and the possible supply of steam via the valve 9. The capacity of the ventilator 5 is also controlled electronically. The control computer receives its input from the user by means of control buttons on an operating panel 14 on which the type of food may also be entered. The control computer 13 also contains a memory with reference values for the food selected. The outlet 6 from the cavity may be connected to a part of the duct (not shown) which is made to condense moisture driven out of the food through heating. In this way, the inside of the food may give off moisture without making the outside or crust too dry, because steam may be supplied when required by the control.

In Fig. 2 is shown an implement which normally is hidden in a compartment 15 outside the cavity. This is a quartz halogen filament in a bulb 16 which is mounted on a carriage which may scan the food F. Hiding it in a separate compartment ensures that the surface is not contaminated by fumes given off by the food F during the cooking. The surface reflectance measuring means 12 provide an input signal to the control computer 13 in order that the correct degree of browning is obtained. In case it is only a drying of the surface which is desired, the change in humidity measured by the moisture sensing means 11 after the halogen lamp 16 has been activated and scanned is used to control the time of scan and temperature of the filament.