WO2005047562A1

WO2005047562A1 - Apparatus for preparing or storing food

Info

Publication number: WO2005047562A1
Application number: PCT/GB2004/004753
Authority: WO
Inventors: Dawn Yvonne Lloyd-Davy; Ivan Paul Parkin
Original assignee: Dawn Yvonne Lloyd-Davy; Ivan Paul Parkin
Priority date: 2003-11-11
Filing date: 2004-11-11
Publication date: 2005-05-26

Abstract

The present inventions relate to an apparatus for preparing or storing food, particularly, utensils, a refrigerator or freezer or an oven. In order to provide an oven which has a self-cleaning facility or a cleaning-assisting facility, the oven interior (2) which defines the cooking space is provided with a layer of photocatalytic material (3), which is preferably titania. A source (5) of ultra violet radiation in the band 380-230nm is provided for photoexciting the photocatalytic surface of the oven interior to photo oxidise organic matter deposited on the oven interior. The ultraviolet radiation has the additional effect of making the oven interior hydrophilic so that removal of any remaining organic material by a damp cloth is much easier. There is also a controller for controlling the source of radiation so that the source of radiation irradiates the photocatalytic surface: (a) within a predetermined period for use; (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combinations of (a) (b) (c) above. A fan (9) is provided for creating an airflow over the photoactive surface during at least part of the irradiation process. At least three walls of the oven can be made of glass, because the walls can be kept clean very easily. A refrigerator or freezer (20) is also provided. Internal surfaces (21) are coated with photoactive material. A source of radiation (27) is provided for photoexciting the photoactive surface, as described above in relation to the oven.

Description

APPARATUS FOR PREPARING OR STORING FOOD

The present invention relates to apparatus for preparing or storing food and to methods of cleaning such apparatus. The present invention is particularly concerned with ovens for cooking food, refrigerators, freezers and utensils for preparing food.

Background Of The Invention Self-cleaning ovens or ovens that make cleaning easier are widely known. Normally, provision is made for very high temperature operation which allows organic matter deposited on the inside of the oven to be burnt off. Sometimes, catalytic surfaces are provided in the oven to enhance the thermal oxidation of the organic matter. The oven may be designed to make cleaning easier. For example, the inside of the oven may be suitably treated. The company NEFF provides an electric oven which has a cleaning assisting facility. In this facility, water with a small amount of detergent may be placed in a part of the oven which is then heated to 60°C. The resulting water vapour helps to loosen organic matter deposited on the inside of the oven, facilitating cleaning. High temperature self-cleaning ovens have the disadvantage that they require a large amount of energy during the self-cleaning cycle. They also give off unpleasant smells and gases which can be dangerous to pets due to the high temperature oxidation. Ovens with cleaning-assisting facilities still require substantial effort to clean them once the dirt has been loosened. WO98/41482 discloses a photocatalytically - activated self-cleaning appliance in which a photoactivated self-cleaning substance is coated on the inside of the appliance. This is capable of being activated by natural or artificial radiation to photocatalytically remove organic contaminants on the surface of the oven. This appliance can be cleaned photocatalytically without raising the temperature inside the appliance to excessive temperatures. However, it is still possible that organic matter may build up on the inside of the appliance, which is difficult to remove. The present inventors have set out to provide an oven which has a self-cleaning facility which is highly effective and which does not require high temperature operation, and which is capable of being operated in such a way as to give optimum self cleaning properties.

Brief Description Of The Invention The present inventors have discovered that certain photoactive materials, when irradiated with photoexciting radiation can become hydrophilic. The present inventors have realised that this property can be put to use in an apparatus for preparing and storing food, hi particular, a very great deal of organic matter which becomes deposited on such apparatus is hydrophobic in nature. By rendering a surface hydrophilic, deposition of organic matter on the material can be reduced. The physical properties of a layer of organic material deposited on such a surface can be altered so that the organic material is easier to remove by other means. Accordingly, in a first aspect, the present invention provides apparatus for preparing or storing food, the apparatus comprising at least one photoactive surface, wherein the photoactive surface may be made hydrophilic by exposure to radiation. The first aspect of the present invention further provides a method of cleaning apparatus for preparing or storing food, the apparatus comprising at least one photoactive surface, comprising exposing the photoactive surface to exciting radiation, to make it hydrophilic, and subsequently removing organic matter deposited on the photoactive surface. The present inventors have also discovered that the effectiveness of the self- cleaning in an oven is greatly improved if a photoactive surface is activated before cooking starts, before any organic material is deposited on it. Similarly, the effectiveness of the self-cleaning can be greatly improved if a regular cleaning cycle is used, with the oven being cleaned at least once within a fixed period of time or within a fixed period after use. In this way, the build up of organic matter can be prevented. Further, organic matter deposited during a cooking cycle can be degraded or loosened before it is subsequently re-heated and hardened by further cooking. Accordingly, in a second aspect, the present invention further provides: an oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photoactive surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, and a controller for controlling the source of radiation, so that the source of radiation irradiates the photoactive surface: (a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combination of (a) (b) and (c) above.

The second aspect of the invention further provides: a method of cleaning an oven, comprising irradiating a photoactive surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, wherein the irradiation is carried out. (a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combination of (a) (b) and (c) above.

The present inventors have further realised that, where a photoactive surface is included in a refrigerator, it is beneficial to operate the irradiation in accordance with a predetermined schedule. For example, irradiation could be only performed at night when it is relatively unlikely that a user will open the door of the apparatus, thereby interrupting the procedure. Also, according to some systems, electricity is cheaper at night than in the day. Further, any accompanying release of heat into the refrigerator or freezer is less of a problem at night when ambient temperatures are lower outside the apparatus, thereby making removal of the additional heat straightforward. Accordingly, according to a third aspect, the present invention provides a refrigerator or freezer comprising at least one photoactive surface a source of radiation for photoexciting the photoactive surface to photocatalytically degrade organic matter deposited on the photoactive surface, to make the photoactive surface hydrophilic, or both, and a controller for controlling the source of radiation so that the source of radiation irradiates the photoactive surface in accordance with a predetermined time schedule. The third aspect of the present further invention provides a method of cleaning a refrigerator or freezer having at least one photoactive surface and a source of radiation for photoexciting the photoactive surface to photocatalytically degrade organic matter deposited on the surface, to make the surface hydrophilic, or both, the source of radiation irradiating the photoactive surface in accordance with a predetermined time schedule. The present inventors have further realised that, where a photocatalytic degradation process is used, it is largely driven by oxidation by atmospheric oxygen and atmospheric water vapour, the rate of degradation can be accelerated if a fan is provided for creating an airflow over the surface, whereby the transport of oxygen and water vapour to the surface is improved and the transport of oxidation products away from the surface is improved. Accordingly, in a fourth aspect, the present invention provides an oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photocatalytic surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, and an airflow generator for creating an airflow over the photocatalytic surface during the irradiation thereof. The fourth aspect further provides a method of cleaning an oven, comprising irradiating the photocatalytic surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, wherein an airflow over the photocatalytic surface is created during at least part of the irradiation process. The present inventors have further realised that, with photoactive systems, a very high level of cleanness can be obtained within apparatus such as ovens, refrigerators, and freezers. Accordingly, it will now be possible to provide a clean apparatus which is constructed on at least three sides out of glass. Although glass vision panels have been provided in cooking appliances for many years so that the state of the food being cooked can be observed, it has not been desirable to make a large part of the oven of glass, as it would rapidly become stained and unsightly. However, with the present methods, clean glass surfaces can be obtained so that an aesthetically appealing structure can be maintained over a long period of time. Accordingly, in a fifth aspect, the present invention further provides an apparatus for preparing or storing food, having a photoactive surface, a source of radiation for photoexciting the photoactive surface to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, wherein the walls of the apparatus are made of glass on at least three sides of the apparatus. Preferred or optional features of the invention will now be described.

Apparatus For Preparing or Storing Food The apparatus for preparing or storing food according to the first aspect of the invention may be any suitable utensil, oven, refrigerator or freezer. It is particularly preferred that the first aspect of the invention comprises an oven, refrigerator or freezer. The apparatus of the fourth or fifth aspect of the invention preferably comprises apparatus having an interior space in which the photoactive surface and source of radiation are mounted, for example an oven, refrigerator or freezer. Any aspect of the invention may be combined with features of any other aspect of the invention, where appropriate. Accordingly, the apparatus of the first aspect of the invention may be operated in accordance with the time schedule of the second aspect of the invention if it comprises an oven or in accordance with the time schedule of the third aspect of the invention if it comprises a refrigerator or freezer. The apparatus of the first aspect of the invention may comprise a fan or it may be made of glass, as in the fourth and fifth aspects of the present invention. An apparatus according to the second and or third aspect of the invention may incorporate a fan or it may be made of glass as required by the fifth aspect of the invention. The apparatus of the fourth aspect of the invention may have at least three glass sides as required by the fifth aspect of the invention. It is particularly preferred that the apparatus of the first, second, fourth and fifth aspect of the invention is an oven. This may be any type of oven, including a gas oven, electric oven, microwave oven, or any other apparatus for cooking food. In particular, the first, second, fourth and fifth aspects of the invention could be applied to apparatus which, in normal cooking use is open, such as a grill, but which may be closed for example when not in use, so that it can be safely irradiated with radiation, as will be described further below.

Photoactive Surface and Photocatalytic Degradation reaction Photoactive coatings are already known for providing self-cleaning windows. A very thin layer of photoactive material, typically titanium dioxide, is applied to the window. The semi conductor material acts as a photocatalyst for photodegradation of organic pollutant. The thin film exhibits photo induced hydrophilicity, allowing rain water to form sheets that give effective dirt removal. By 'photocatalytically degrade' it is meant that some change occurs in the organic material in contact with the photocatalytic surface when it is irradiated with radiation of a suitable wavelength. Suitable wavelengths are discussed below. By using a photodegradation system, it has been found that an efficient cleaning of the oven interior can be obtained at low temperature. In particular, cleaning may be carried out at ambient temperature or at a temperature not higher than 100°C. In practice, a source of radiation will generate a certain amount of waste heat when running. Suitably, cleaning is carried out at a temperature produced by release of this waste heat into the cooking space in the oven. As discussed further below, this temperature may be in the range 30-70°, preferably 40-60°C .

The photo degradation process can be carried out until removal of the organic matter is substantially complete. Alternatively, it can be carried out sufficiently long to loosen the organic matter which is contact with the surface, facilitating manual cleaning. It is particularly preferred that the photoactive surface is for photocatalytic oxidation of the organic matter. In all aspects of the invention, it is particularly preferred that both photocatalytic degradation and rendering a surface hydrophilic occur. Preferably, the same photoactive surface is capable both of photocatalytically degrading organic matter and of becoming hydrophilic upon irradiation. It is particularly preferred that the same component of the photoactive surface is photoactive to give both hydrophilicity and photocatalytic degradation. Suitable components will be discussed further below. The science underlying the invention will be described in more detail further below. However, without being bound by theory, it is preferred that the photoactive surface comprises a semiconductor material in which an electron can be promoted to the conduction band of the semi conductor, leaving a hole in the valence band. Photocatalytic oxidation can then occur if the band gap between the conduction band and the valence band is sufficiently great to generate radicals from atmospheric oxygen or water, as described further below Photocatalytic oxidation is particularly advantageous, because it employs atmospheric oxygen which is obviously readily available. It is preferred that the photoactive surface comprises a layer of a photoactive material deposited on an interior surface of the apparatus. In this way, the apparatus interior can be constructed of normal materials, particularly glass or metal, while the photoactive surface can be applied to the interior before or after assembly into the apparatus. Any suitable interior surface of the apparatus may be provided with a photoactive surface. The photoactive surface may be applied to any or all of the apparatus walls, the apparatus door and any interior fittings of the apparatus, for example, racks, grilles or grill pans which may for example be provided in an oven. Conventionally, racks and grilles are treated with a chrome surface. The photoactive surface may be applied to such a chrome surface, or directly onto the material out of which the grilles or racks are made. The photoactive surface suitably comprises a photoactive component. This photoactive component may be mixed with other materials, such as binders or carriers. However, it is particularly preferred that the photoactive material is applied so that the photoactive surface comprises at least 50% by weight and most preferably at least 75% by weight and most probably at least 80% by weight of photoactive material. The photoactive component may be included in an enamel or glass if appropriate. Preferably, the photoactive surface comprises titanium dioxide, tungsten trioxide, Nb₂O₅, SrTiO₃ or mixtures thereof. It has been found that the photoactive surface suitably comprises titanium dioxide, tungsten trioxide or a mixture thereof. These materials are particularly suitable for use in photocatalytic surfaces. Titanium dioxide and tungsten trioxide are each capable both of photocatalytic degradation and of being made hydrophilic upon irradiation with radiation of suitable wavelength. These may be supplied in any suitable form. It is particularly preferred to use crystalline titanium dioxide, for example anatase or rutile. This material can be obtained from manufacturers of fillers with great ease. Titanium dioxide is also refened to as titania. The photoactive material is preferably applied as a thin layer of a finely divided solid material. Suitably, the particle size is greater than 50nm. The thickness of the layer of photoactive material may be any suitable thickness. However, it is preferably in the range 50nm-1000nm. In practice, it is found that increasing the thickness beyond 400nm does not increase the rate of photodegradation of organic matter in contact with the surface. Where a photoactive material such as titanium dioxide is deposited to a thickness in the range 50nm -150nm, it is found it can be substantially transparent, so that the interior material of the apparatus can still be seen. At these thicknesses, titanium dioxide is found to have a high percentage of internal reflection, meaning that the exciting radiation is used efficiently. Dopants can be added to the photoactive material, for example to induce colour changes, to include a phosphor or pigment, or to give other effects. For example, if tungsten trioxide is added to titanium dioxide, a blue colour can be obtained. The photoactive material is suitably applied over substantially the entire internal surface of the apparatus wall, so that there are substantially no places where organic material may accumulate after cleaning. Preferably, there are no gaps in the layer of photoactive material. Titania has many advantages. It is hard, durable, refractory and an excellent photocatalytic material. The hydrophilic nature helps to prevent adhesion of organic matter to the surface and which means that the oven interior can be subsequently cleaned manually with a damp cloth. When ultraviolet light is absorbed by the photocatalytic material, it is believed that electrons and positive holes are generated which migrate to the surface of the photocatalytic material. During photocatalytic oxidation, the organic debris left inside the apparatus can be converted to a greater or lesser extent into carbon dioxide and water. As the photocatalytic degradation occurs at the surface of the apparatus interior, any adhering organic material is effectively loosened making it easier to remove manually. An apparatus according to the present invention may be constructed as a new apparatus or it may be provided by taking an existing apparatus and applying a photoactive surface to the apparatus interior and providing a source of photoexciting radiation. The photocatalytic surface is suitably applied to the oven interior of an existing or new apparatus by a process selected from chemical vapour deposition (CVD), physical vapour deposition (such as sputtering and evaporation methods), sol-gel spray coating, dip coating, spin coating or screen printing. Chemical vapour deposition (CVD) includes all forms of CVD such as atmospheric pressure CVD, low pressure CVD, plasma assisted CVD, molecular organic CVD, vapour phase epitaxy, laser enhanced CVD, aerosol CVD and liquid injection CVD. Chemical vapour deposition (CVD) methods are preferred. They are particularly prefened to deposit titanium dioxide. CVD deposited titanium dioxide is found to have a long life and a great chemical and abrasion resistance. No binder or other material are required in order to allow the titanium dioxide to deposit. CVD techniques are found to deposit titanium dioxide well onto materials such as glass, and enamel If the photoactive material is to be deposited onto a metal surface, it may be preferable to anodize the metal surface first. Titanium dioxide is found to be stable in a pH range from greater than 1 to less than 13. Physical vapour deposition techniques such as sol-gel deposition can be carried out using commercially available precursor materials which are sprayed onto the interior of the apparatus.

It is found that where a photoactive surface such as titanium dioxide is made hydrophilic, the hydrophilic property of the surface may have a relatively long half-life. For example, the half-life may be of the order of twelve hours. It is found that sufficient hydrophilicity can be given upon irradiation for a useable level to remain up to 48 hours later. Further, where a photoactive surface is irradiated, the hydrophilicity may survive to relatively high temperatures, of the order of 500°C which is very suitable for use in an oven.

Radiation Source and Irradiation Process The apparatus of the invention preferably comprises an apparatus which has an interior which can be closed. In particular, the invention is suitably applied to an oven, refrigerator or freezer. Suitably, such apparatus comprises a door to allow access to the interior. The apparatus may be designed to be light tight, to prevent escape of harmful radiation. In practice, conventional oven, refrigerator or freezer designs provide an adequate level of light tightness without substantial redesign It is particularly preferred that the source of radiation is an ultraviolet light source. Ultraviolet light typically has an energy which is sufficient to promote electrons from the valence band of a semiconductor such as titanium dioxide to the conduction band, which are separated by an energy gap which is sufficient for oxidation of organic matter deposited on the surface. The ultraviolet radiation may comprise UV A, having a wavelength in the range 315-400 nm. This radiation is particularly suitable where photoactive material comprises tungsten trioxide. This type of radiation penetrates certain types of organic material, particularly ones containing benzene rings particularly well, so that photoactive surface can be irradiated. The irradiating radiation may comprise UV B, having a wavelength in the range 315nm-280nm. A mixture of different wavelengths may be used. For example the wavelength may span a range from 400nm to 230nm, preferably 380-230nm. Alternatively, a single wavelength inadiation sources may be used, having a wavelength falling in this range. Preferably the ultra violet light source comprises a UV black light bulb. The power of the light source need not be very high, being preferably in the range 5-20W, suitably around 8-10W. A normal 8W germicidal lamp of the type available from BDH has been found to be suitable. The radiation density may be in the range 2-10 W/m², preferably W/m². The use of ultra violet radiation also has the additional benefit of killing bacteria and viruses located within the apparatus, providing an additional cleaning effect. In the method of the invention, it has been found that a cleaning effect is obtained after an exposure of at least 30 minutes, preferably at least an hour. More resistant stains may require longer exposure to remove. An additional manual or cleaning step may be suitable provided to remove any remaining organic matter. However, for the reasons set out above, this additional manual cleaning step is made particularly easy by the present invention. It is important that substantially the entire interior surface of the apparatus is irradiated. It has been found that, because of the relatively high reflectivity of photocatalytic material, particularly titania, substantially the entire surface area is kradiated due to multiple internal reflection.

By using a photodegradation system, it has been found that an efficient cleaning of the oven interior can be obtained at low temperature. In particular, cleaning may be carried out at ambient temperature or at a temperature not higher than 100°C. In practice, a source of radiation will generate a certain amount of waste heat when running. Suitably, cleaning is carried out at a temperature produced by release of this waste heat into the cooking space in the oven. As discussed further below, this temperature may be in the range 30-70°, preferably 40-60°C.

The photo degradation process can be carried out until removal of the organic matter is substantially complete. Alternatively, it can be carried out sufficiently long to loosen the organic matter which is contact with the surface, facilitating manual cleaning. If necessary, more than one source of radiation may be used. The sources of radiation may be positioned so that substantially all surfaces of the interior of the apparatus are ireadiated. This is particularly important in refrigerators and freezers, where objects will be placed inside the interior and which may block the passage of radiation from one source to certain areas of the refrigerator or freezer. In an oven according to the present invention, to protect the source of radiation from the cooking temperature, the source may be mounted behind a temperature resistant surface which is transparent to the radiation, the source being mounted in a mounting which is thermally insulated from the interior of the oven. For example, the ultraviolet source may only be able to withstand temperatures up to 80°C. A heat resistant closure may be provided which is movable between a first position in which it allows radiation from the radiation source to enter the oven interior and a second position in which the source of radiation is insulated from the oven interior. The closure may be operable so that it is only closed when the oven is operating or only open when the radiation source is switched on, or both of them. For example, a heat shield may be used. The light source may be movably mounted between a first position in which it can radiate surfaces of the interior of the oven and a second position in which it is shielded from the interior of the oven. For example, it may be rotatably mounted. Active or passive cooling means may be provided for selectively cooling the radiations source. A temperature sensor may be provided for determining if the radiation source is becoming too hot and for giving a signal in response to which cooling means are activated. The cooling means may comprise active cooling means. The cooling means may comprise a forced draught cooling means, radiative fins, water cooling etc. The surface of the interior of the apparatus may comprise a flat surface. Alternatively, it may comprise a surface with depressions or projections, to increase the local surface area. Preferably a smoothly curving depression or projection is used, so that corners for trapping organic material are not created. For example, a wavy or crenellated surface may be used. According to the second or third aspect of the invention, the apparatus comprises a controller for controlling a time schedule of irradiation. The controller can provide selectable or programmable control of irradiation. The controller may comprise a timer. The controller may include a timer and means for commencing or terminating kradiation in response to signals received from the controller. The controller may be programmed by the manufacturer of the apparatus or it may be programmed by the user. According to the second aspect of the invention, the source of radiation inadiates a photoactive surface according to at least one following conditions:

(a) Irradiation occurs within a pre-determined period before use. Preferably, inadiation of the photoactive surface occurs within a period of 24 hrs before use, more preferably within a period of 12 hrs before use and suitably within a period of 6 hrs before use. There are number of practical way this may be attained. According to a first embodiment, it is assumed that majority of cooking operations will occur within a fixed period of the day, for example, from 7am to 7pm, or from 11 am to 11pm, depending upon domestic cooking patterns. Given this assumption, the controller may be programmed to inadiate the surface at a fixed time of day before the beginning of the normal time at which cooking occurs, for example at 6am. The controller may be programmable by the user to inadiate the surface at a selected time of day, depending upon the likely use by the user.

(b) Irradiation occurs at least once within a pre-determined period of time

The operation maybe carried out to render the surface of hydrophilic, in preparation for subsequent use or in order to remove organic matter deposited on the surface. Irradiation of the surface may occur at regular intervals, separated by a fixed period. The pre-determined period is suitably equal to the half-life of excited species on the photoactive surface, so that a high level of excited species is always present. For example, the controller may be configured to inadiate the surface every 12 hrs more preferably every 8 hrs, and most preferably every 6 hrs. It is found that, when an oven is new, an initial inadiation period of six hours before use is preferable. After that inadiation periods of 10-40min preferably 15-30 min. at least once a day are prefened.

(c) Irradiation occurs within a pre-determined period after use

Organic material can be easier to remove when it is still hot. Accordingly, inadiation may occur within a predetermined period after use, for example within one hour of use, more preferably within 30 minutes of cooking, more preferably substantially immediately after cooking. Preferably, the oven comprises means for giving a signal when cooking has been completed. Preferably, the signal is given to the controller and the controller is configured to either initiate inadiation immediately or to initiate inadiation after fixed period of time. Further, the controller may comprise a timer for determining the time for which inadiation has occuned and means for giving the signal after inadiation has been completed for a pre-determined period of time, the controller switching off the inadiation after the pre-determined period of time.

(d) Irradiation may occur in accordance with a combination (a) to (c) above. The controller may be programmed to operate a combination of the procedures described above. For example, it may be programmed to initiate inadiation after a fixed period after cooking has ceased. It may then be programmed to initiate another inadiation operation a fixed period after the first inadiation period has ceased. Alternatively, it may be programmed to cany out inadiation operations at fixed periods of the day provided that inadiation has not taken place within a fixed period of time. Further, optional additional inadiation procedures may be selected by the user. The controller maybe configured to inadiate the interior of the oven at time intervals separated by a predetermined time period, being further configured to inadiate the oven at times different to the predetermined time periods, example immediately after cooking or when an extra inadiation cycle is selected by the user, wherein the inadiation at regular intervals is interrupted or adjusted to start at different times after an extra inadiation cycle. This can help to save energy by avoiding unnecessary extra inadiation cycles. A refrigerator or freezer according to the third aspect of the invention may be configured so that inadiation of the interior of the refrigerator or freezer occurs at regular intervals separated by fixed periods or at predetermined periods of the day for example, during the night. The predetermined periods are suitably equal to the half-life of excited species on the photoactive surface, so that a high level of excited species is always present. For example, the controller may be configured to inadiate the surface every 12 hours or preferably every 8 hours and most preferably every 6 hours. Longer inadiation periods may be programmed to occur during the night when access to the refrigerator or freezer is less likely. In the second and third aspect of the invention, it is preferable that inadiation will be interrupted when the door of the apparatus is opened. Preferably inadiation commences again when the door is closed. In order to ensure that adequate inadiation occurs, the controller maybe configured to measure the total time for which inadiation has occuned and not to terminate radiation until the total inadiation time, allowing for interruptions due to the door being opened, has reached a predetermined value. Apparatus design In a prefened embodiment, a safety interlock is provided whereby the source of photoexiciting radiation is switched off if the apparatus door is opened to prevent risk to a user, particularly to the user's eyes The air flow generator of the fourth aspect of the invention preferably comprises a fan. Any suitable design of fan may be used. For example, a conventional propeller or impeller type of fan may be used.

Where an oven is used which is of a design in which a fan is provided as part of the design of the oven, for example for cooling a microwave source or for enhancing conventional cooking, the same fan may be used to direct air to the photoactive surface during the inadiation thereof. Preferably, a controller is provided which is configured to switch on the fan when inadiation commences. It is preferable that an airflow is created over at least 75% of the photoactive surface and preferably at least 90%. This may be obtained by using a plurality of airflow generators for directing air to different parts of the surface. Alternatively, a single airflow generator may be used. For example, a single fan may be used if it is powerful enough. Alternatively, the interior of the oven may be configured to direct airflow to all parts of the surface. Airflow may be directed by baffles, ducts or guides. The airflow generator may re-circulate air within the oven or refrigerator or it may draw air in from the environment so that a fresh air supply is used. Preferably, when the apparatus comprises an oven, air from outside the oven is used, so that the oxygen level within the oven is always at a high level. Preferably, when the apparatus comprise a refrigerator or freezer, air is simply re-circulated, so that additional heat is not inadvertently drawn into the refrigerator or freezer. The present invention does not require a very high airflow per unit area of photocatalytic surface. For example, the flow rate may be of the order of 0.5L/m²s up to 20 L/m²s, more preferably about 1-5 L/m²s. The airflow generator may operate by blowing air on to the surface or by withdrawing air from the oven interior to thereby create airflow through oven interior. Where the same fan is used for the cooking operation and for the inadiation operation, it may be operated at a different power for the inadiation operation, preferably at a lower power. The interior walls of the apparatus may be configured so that if organic material such as grease is projected onto the wall, for example during cooking or use, this grease runs off the wall due to the hydrophilic nature of the wall. Preferably, interior surfaces of the apparatus are configured so that grease can run down the surfaces to a grease collection zone. For example, grease collection zones may be provided at the bottom of vertical wall sections. The base of the interior of the apparatus may comprise sloping surfaces which lead towards a grease collection zone. In this way, accumulation of the grease on the surfaces is prevented and removal of the grease from the collection zone can be made simpler. In order to assist the hydrophilic nature and the dislodging of hydrophobic material, a water supply may be provided. For example, water may be sprayed on to interior surfaces of the apparatus.

Glass Oven According to the fifth aspect of the invention, the apparatus may be formed of glass on at least three sides. Whilst the apparatus may be any shape, it is conventional to construct apparatus such as ovens, refrigerators and freezers from a polyhedral shape employing flat surfaces which are easy to manufacture. It is most common to manufacture such apparatus as a rectangular prisms or square prisms. It is particularly prefened that the apparatus is constructed as a rectangular prism with at least three and preferably four side walls and preferably the top wall formed of glass and, optionally, the bottom side as well. This will give a high level of visibility. Preferably, the surfaces which are formed of glass are glass cover at least 50% preferably at least 75% of that area.

The inventor has found that the adherence of titanium dioxide to glass is extremely good, leading to a tough and durable surface which is particularly suitable for use in the present invention. In an oven, a fan unit and other oven accessories may be provided occupying one face of the cooking space or part of one face of the cooking space. Such oven accessories include controls operable by the user, a processor for controlling the operation of the oven, temperature sensors and other accessories.

Utensils, Refrigerator or Freezer Structure

The apparatus for food storage suitably comprises a storage container or a refrigerator or freezer. The refrigerator or freezer may be of substantially conventional design except that at least part of the internal surface thereof is treated with a photoactive surface. The utensil for food preparation may comprise any conventional kitchen utensil, for example pots, pan, plates, dishes etc. They are preferably manufactured of a material to which a photoactive surface adheres well. For example, glass may be used as titanium dioxide adheres well to glass. As the interior of a refrigerator has a convoluted design, it may be necessary to provide more than one, for example two separate inadiation sources in order to provide illumination for as many parts of the interior as possible. Inadiation periods may be shorter than used for an oven, for example in the region of 10 minutes to 20 minutes. The quantity of organic material deposited in a refrigerator or freezer is commonly lower than that found in ovens. Further, excessive exposure may be avoided to prevent damage to food stored in the refrigerator or freezer. The exposure to ultra violet radiation has the additional benefit of killing bacteria. The internal structure of the refrigerator or freezer and the photoactive coating may be selected for compatibility. In particular, it is desired to give a coating which has a strong adhesion to the underlying surface. For this reason, it may be preferable not to use titanium dioxide where the internal surface of the refrigerator or freezer is made of plastic. Titamum dioxide may be used as a photocatalytic surface for refrigerators and freezers, particularly commercial ones, of which the internal surfaces are made of metal, for example stainless steel or aluminium. Suitable commercially available ultra violet A lamps of the type cunently used for killing bacteria in food storage apparatus may be used. These may need to be replaced at regular intervals, for example, every 3 months, as will be known to the skilled person. The present invention will be described further below with reference to the accompanying drawings in which.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sketch cross section through an oven according to the first, second third and fifth aspects of the present invention. Figure 2 is a schematic diagram showing processes that occur when a semi conductor undergoes photoexcitation. Figure 3 is a diagram showing comparative redox couples for anatase and tungsten trioxide. Figure 4 is a schematic drawing of the control unit of the oven of Figure 1. Figure 5 shows the process steps in the operation of the inadiation cycle according to a first embodiment. Figure 6 shows the process steps involved in an inadiation process according to second and third embodiments of the invention. Figure 7 shows a schematic cross section through a refrigerator or freezer according to the first, third, fourth and fifth aspect of the invention.

DETAILED DESCRIPTION OF THE DRAWING

Figure 1 is a sketch cross section through an electric oven according to the first, second, third and fifth aspects of the present invention. The oven, generally designated 1, comprises a wall 2 defining an oven interior providing the cooking space. The internal surface of the wall 2 is coated with a layer 3 of photocatalytic material comprising titanium dioxide (anatase). An electrical heating element 4 is provided within the cooking space. A source 5 of ultra violet radiation having a wavelength in the range 380-230 nm is provided. A control module 6 is provided for controlling the electrical element 4 and the ultra violet light source 5. The oven interior may comprise conventional fittings such as racks and grilles, which are not shown for clarity and which may be coated with a layer of photocatalytic material. An airflow generator 9 in the form of a fan is provided, for directing airflow over the interior surfaces of the oven. The oven may also contain other conventional fittings, for example a light, which are not shown, for clarity. According to the fifth aspect of the invention, the wall 2 of the oven is made of glass at least in its top and bottom faces and on three sides. The fourth side is closed by the structure 10 enclosing the control 6. The cross section of figure 1 is taken facing towards the door 7 which allows access to the cooking space. An interlock 8 is provided for automatically switching off the ultra violet source 5 if the door 7 is opened while the light source 5 is switched on. In normal use, cooking takes place in the cooking space. As a result, organic matter is deposited onto the oven interior 2. In order to clean the inside of the oven interior 2, the oven door 7 is closed and a programme is selected or actioned automatically whereby the ultra violet light 5 is switched on. The ultra violet 5 may be switched on for a period of approximately one hour in order to provide degradation or loosening of organic material by photocatalytic reaction which will be explained below. Without wishing to be bound by theory, it is believed that the principles underlying the present invention are based on semi conductor photocatalysis, explained below.

Semiconductor photocatalysis Semiconductor photocatalysis refers to the "acceleration of a photoreaction by the presence of a semiconductor catalyst".¹ A semiconductor can be activated by inadiation with photons possessing energy greater than the bandgap. Thus, an electron, e-, is promoted to the conduction band of the semiconductor and a hole, h⁺, is left in the valence band. These species can then recombine, resulting in deactivation of the semiconductor, or migrate to the surface of the semiconductor where the photogenerated hole can oxidise electron acceptors and the photogenerated electron can reduce electron donors, see Figure 2. For a photocatalyst to be efficient, the surface redox reactions must dominate over the electron - hole recombination reactions. The photogenerated electron and hole can recombine either in the bulk (a) or at the surface (b). Alternatively, at the surface the photogenerated electron can reduce electron acceptors (c) and the photogenerated hole can oxidise electron donors (d). It was realised in 1929 that the pigment titania (TiO₂) caused fading in paints." This is due to the photodegradation of the organic polymer binder, otherwise known as chalking.¹" Furthermore, Fugishima and Honda^lv discovered in 1972 that titania electrodes can photocatalyse the splitting of water to evolve oxygen and hydrogen Eqn. 1.

2H₂O - 2H₂ + O₂ Eqn 1

Relating this reaction to Figure 3, protons are electron acceptors and are reduced by photogenerated electrons, and oxygen anions are electron donors and are oxidised by photogenerated holes. For the electron to reduce water the conduction band needs to be more negative than the hydrogen evolution potential (HVH^); for the hole to oxidise water the valence band needs to be more positive than the oxygen evolution potential (O₂ H O). Figure 3 illustrates that this is the case for anatase TiO₂.

The splitting of water has received much attention because it can be applied to the production of electricity from photoelectrochemical cells. However, the most important application resulting from the field of semiconductor photocatalysis concerns the destruction of organic pollutants, both in air and in water, Eqn. 2 semiconductor organic pollutant + O₂ CO₂ + H₂O + mineral acids Eqn 2

It was not until 1983 that the semiconductor catalysed photodegradation of organic pollutants was first realised, when Ollis and co-workers^v'^vl demonstrated that TiO₂ could act as a photocatalyst for the oxidative mineralisation of halogenated hydrocarbons. Since then over 200 organic compounds¹ have been shown to be degraded by the reaction described in Eqn. 2including alkanes, alcohols, alkenes, carboxylic acids, aromatics, phenols, polymers, surfactants, herbicides, pesticides, dyes, bacteria and viruses. The majority of this work has involved semiconductor particles as photocatalysts, but there is also a great interest in the application of photocatalytic thin films, especially on glass, ie. self-cleaning windows. The mechanism by which Eqn.2 occurs is not simple. As demonstrated by Fig. 3 because the valence band of anatase is sufficiently positive in energy, the photo- generated holes are capable of oxidising surface hydroxy groups to afford OH^* radicals. These radicals can then oxidise organic pollutants. The role of the photogenerated electrons is uncertain, but because the conduction band of anatase is sufficiently negative in energy, they are capable of reducing oxygen to yield O₂ ^*-. It has been suggested that the consequent reduction of water by these O₂ ^*- radicals generates species such as HO₂", HO₂-, H₂O and OH^* that can oxidise the pollutant.¹ Most of the research concerning the photocatalytic destruction of organic pollutants has concentrated on titania, because it is photostable, non-toxic, inexpensive and very photoactive. Titania has a large bandgap, ca. 3.2 eV, and hence requires inadiation with UV light of wavelengths less than 400 nm to be activated. There are two processes by which self-cleaning coatings are known to act, one being the photocatalysis of organic pollutants, as described above. The second of these processes is photo-induced hydrophilicity. The hydrophilicity of a film is quantified by the water contact angle, whereby large contact angles represent beads of water on the surface and a contact angle of 0 ° represents complete flattening of the water droplet on the coating. For self-cleaning coatings, a low contact angle ensures that water forms sheets across the surface of the material, resulting in effective dirt removal. — -Jthas-been-shαwri-that the contact angle of titania is reduced from ca. 70 ° before UV inadiation to 0 ° after.^vll,vι" Tungsten oxide films have also been shown to posses the same property, with contact angles falling to ca. 5 ° after exposure to UV radiation. Watanabe, Hashimoto and co-workers have suggested that the mechanism by which photo-induced hydrophilicity occurs for titania arises from structural changes of the oxide material.^v"'^vl" Subjecting titania to UV inadiation causes oxygen defects in the material, and hence some Ti⁴⁺ sites are reduced to Ti³⁺. Watanabe and Hashimoto suggest that these reduced sites are favourable for the absorption of dissociated water,^lx and influence sunounding sites to produce hydrophilic domains. Tungsten oxide also readily forms oxygen defects (ie. WO _-x), and is thought to be hydrophilic by the same mechanism.

References i A Mills, S Le Hunte,/. Photochem PhaώkL A: Chemistry, 1997, 108, 1 ii E Keidel, Furben Zeitung, 1929, 34, 1242 iϋ SP Pappas, KM Fischer,/. Paint Teώnd, 1974, 46, 65 iv A Fujishima, K Honda, Nature, 1972, 37, 238 v AL Pruden, DF Ollis,/. Catal, 1983, 82, 404 vi CY Hsiao, CL Lee, DF Ollis, /. Caai, 1983, 82, 418 vϋ R Wang, K Hashimoto, A Fujishima, M Qiikuni, E Kojima, A Kitamura, M Shimohigoshi, T Watanabe, Nature, 1997, 388, 431 viϋ A Nakajima, S Koizumi, T Watanabe, K Hashimoto, Lan mr, 2000, 16, 7048 ix R Wang, K Hashimoto, A Fujishima, M Chikuni, E Kojima, A Kitamura, M Shimohigoshi, T Watanabe, A dv Mater., 1998, 10, 135

Figure 4 shows a schematic view of the control unit 6.

The control unit 6 comprises a processor 11. The processor 11 is connected to a timer 15, a manual control unit 14, a radiation cycle data store 12 for storing records of when inadiation cycles have occuned and an operation data store 13. The operation data store can store operating cycle instructions for operating the heating element 4, the fan 9 and the ultra violet light 5. The processor is connected to these elements and also to the interlock 8 which determines if the door is open or shut. Figure 5 shows the processes which occur after a cooking operation ends. The processor 11 firstly determines whether the door is open by checking the interlock 8. If the door is open, a timer is set to zero and, after a short time period t, for example 5 minutes, the door is checked again. When the door is closed, the inadiation operation begins. This will involve inadiation using the ultra violet light for a predetermined period of time, for example up to half an hour, together with simultaneous operation of the fan at a predetermined airflow rate.

Once the inadiation cycle is complete, the time at which the inadiation cycle ended is recorded in the inadiation cycle data store. The operation then ends. Figure 6 shows the cycle of operation involved in inadiation according to a first embodiment of the first aspect of the invention, whereby regular inadiation of the photoactive surface can be obtained throughout the course of a normal day.

The cycle begins at START.

The processor 11 checks the time recorded by timer 15. The processor then checks the time from the timer 15 with a cycle recorded in the operation data store. If the time is equal to the time set for an inadiation operation, the processor then checks the inadiation cycle data store 12. In particular, it is determined whether the time since the last inadiation cycle finished is greater than a fixed amount or not. If it is not greater by the fixed amount, there is no necessity to carry out a further inadiation cycle and the system returns to START to go round the cycle again after a predetermined period of time. If, however, the time since the last inadiation cycle exceeds the pre-determined amount, inadiation is commenced. The processor 11 first of all checks using the interlock 8 whether the door is opened. If it is opened, an alert may be given and the system may then return to the start, to go round the cycle again after a pre-determined period of time. If the door is closed, an inadiation operation is carried out as described in relation to figure 5 above. Finally, the time at which the inadiation cycle ends is recorded in the inadiation cycle data store 12 and the system returns to START. In a second embodiment, instead of having fixed times for inadiation cycles recorded in the operation data store 13, it is only necessary to check whether a given time has elapsed since the last inadiation cycle. In this case, the processor 11 can proceed directly to checking the inadiation cycle data store 12 having obtained the time from the timer 15.

In Figure 7, there is shown a schematic cross section through a refrigerator or freezer according to the first, third, fourth and fifth aspects of the invention. It is shown extending from the front at the left to the rear at the right. The freezer or refrigerator 20 comprises an internal space 21 provided with conventional racks 22. Wall surfaces of the internal space are coated with titania. The refrigerator or freezer has a door 23 of conventional type and is provided with conventional freezing/cooling compressor 24 and heat exchanger 25. Further, there is a controller 26 for operating ultra violet lamps of 10 watts power 27. Two are shown, though more may be used. The controller is further linked to a door interlock 28 which extinguishes the ultra violet lamps 27 when the door 23 is opened.

The controller 27 maybe operated to inadiate the interior of the refrigerator or freezer for specified periods of time at specified intervals, for example for 15 minutes every six hours.

The individual lamps 27 may be operated at different times or at the same time.

The present invention has been described above by way of example only and modifications can be made within the invention. The invention also extends to any individual features implicit herein or shown or implicit in the drawings or any combination of any such features or generalisation of any such features or combination.

Claims

CLAIMS:

1. Apparatus for preparing or storing food, the apparatus comprising at least one photoactive surface, wherein the photoactive surface may be made hydrophilic by exposure to radiation.

2. The apparatus according to claim 1, being an oven for cooking food, having: an oven interior defining a cooking space, the source of radiation being for making the photoactive surface of the oven interior hydrophilic.

3. The apparatus of claim 1 or 2, being an oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photexciting a photocatalytic surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior.

4. An apparatus according to claim 1, being a refrigerator or freezer, and further comprising a source of radiation for photoexciting the photoactive surface.

5. The refrigerator or freezer according claim 4, further comprising at least one photocatalytic surface, whereby organic matter deposited on the photocatalytic surface may be photocatalytically degraded by exposing the photocatalytic surface to exciting radiation.

6. The apparatus according to claim 3 or 5, wherein the photoactive surface is the photocatalytic surface.

7. An oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photoactive surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, and a controller for controlling the source of radiation, so that the source of radiation inadiates the photoactive surface: (a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combination of (a), (b) and (c) above.

8. A refrigerator or freezer comprising at least one photoactive surface, and a source of radiation for photoexciting the photocatalytic surface to photocatalytically degrade organic matter deposited on the photocatalytic surface or to make the photoactive surface hydrophilic, or both, and the controller for controlling the source of radiation so that the source of radiation inadiates a the photoactive surface at least once within a predetermined period of time according to a predetermined schedule.

9. An oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photocatalytic surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, and an airflow generator fan for creating an airflow over the photocatalytic surface during the inadiation thereof.

10. An oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photoactive surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, wherein the oven walls are made of glass on at least three sides of the oven.

11 An apparatus or an oven according to any preceding claim, wherein the photocatalytic surface is for photocatalytic oxidation of the organic material deposited on the surface.

12. An apparatus or oven according to any preceding claim wherein the photoactive surface comprises a layer of photocatalytic material deposited on the interior of the apparatus.

13. An apparatus or oven according to any preceding claim, wherein the photoactive surface comprises titanium dioxide, tungsten trioxide, Nb₂O₅, SrTi0₃ or mixtures thereof.

14. An apparatus or oven according to any preceding claim, wherein the source of radiation is an ultra violet light source.

15. An apparatus or oven according to claim 14, further comprising a safety switch for switching off the source of radiation if a door of the apparatus is opened.

17. A method of cleaning apparatus for preparing or storing food, the apparatus comprising at least one photoactive surface, comprising exposing the photoactive surface to exciting radiation, to make it hydrophilic, and subsequently removing organic matter deposited on the photoactive surface.

18. A method of cleaning an oven, comprising inadiating a photoactive surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both wherein the inadiation is carried out. (a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combination of (a) (b) and (c) above.

19. A method of cleaning a refrigerator or freezer having at least one photoactive surface and a source of radiation for photoexciting the photoactive surface to photocatalytically degrade organic matter deposited on the surface to make the surface hydrophilic, or both, the source of radiation inadiating the photoactive surface in accordance with a predetermined time schedule.

20. A method of cleaning an oven, comprising inadiating the photocatalytic surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, wherein an airflow over the photocatalytic surface is created during at least part of the inadiation process.