Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2000-01-07
2002-07-09
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S405000, C219S411000
Reexamination Certificate
active
06417494
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of cooking ovens. More particularly, this invention relates to an improved lightwave oven configuration for cooking with radiant energy in the electromagnetic spectrum including a significant portion in the near-visible and visible ranges.
BACKGROUND OF THE INVENTION
Ovens for cooking and baking food have been known and used for thousands of years. Basically, these well-known oven types can be categorized in four cooking forms; conduction cooking, convection cooking, infrared radiation cooking and microwave radiation cooking.
There are subtle differences between cooking and baking. Cooking just requires the heating of the food. Baking of a product from a dough, such as bread, cake, crust or pastry, requires not only heating of the product throughout, but also chemical reactions coupled with driving the water from the dough in a predetermined fashion to achieve the correct consistency of the final product and finally browning the outside of the product. Following a recipe is very important for proper results during the baking operation. An attempt to decrease the baking time in a conventional oven by increasing the temperature results in a damaged or destroyed product.
In general, there are problems when one wants to cook or bake foodstuffs with high-quality results in the shortest times. Conduction and convection provide the necessary quality, but both are inherently slow energy transfer methods. Long-wave infrared radiation can provide faster heating rates, but it only heats the surface area of most foodstuffs, leaving the internal heat energy to be transferred by much slower conduction. Furthermore, the shallow heating depth limits the rate at which heat energy can be introduced to a product, because high radiant powers at the food surface result in a burned food interface. Microwave radiation heats the foodstuff very quickly in depth, but during baking the loss of water near the surface stops the heating process before any satisfactory browning occurs. Consequently, microwave ovens cannot produce quality baked foodstuffs, such as bread.
Radiant cooking methods can be classified by the manner in which the radiation interacts with the foodstuff molecules. For example, starting with the longest wavelengths for cooking, the microwave region, most of the heating occurs because of the coupling of radiant energy into the bipolar water molecules causing them to rotate and thereby absorb energy to produce heat. Decreasing the wavelength to the long-wave infra-red regime, the molecules and their component atoms resonantly absorb the energy in well-defined excitation bands. This is mainly a vibrational energy absorption process. In the near-visible region, the main part of the absorption is due to higher frequency coupling to the vibrational modes. In the visible region, the principal absorption mechanism is excitation of the electrons that couple the atoms to form the molecules. These interactions are easily discerned in the visible band of the spectrum, where they are identified as “color” absorptions. Finally, in the ultraviolet, the wavelength is short enough, and the energy of the radiation is sufficient to actually remove the electrons from their component atoms, thereby creating ionized states and breaking chemical bonds. This short wavelength, while it finds uses in sterilization techniques, probably has little use in foodstuff heating, because it promotes chemical reactions and destroys food molecules.
Lightwave ovens are capable of cooking and baking food products in times much shorter than conventional ovens. This cooking speed is attributable to the range of wavelengths and power levels that are used.
Typically, wavelengths in the visible range (0.39 to 0.77 &mgr;m) and the near-visible range (0.77 to 1.4 &mgr;m) have a fairly deep penetration in most foodstuffs. This range of penetration is mainly governed by the absorption properties of water which is the principal constituent of most foodstuffs. The characteristic penetration distance for water varies from 30 meters in the visible to about 1 cm at 1.4 &mgr;m. Several other factors modify this basic absorption penetration. In the visible region electronic absorption (color absorption) reduces the penetration substantially, while scattering in the food product can be a strong factor throughout the region of deep penetration. Measurements show that the typical average penetration distance for light in the visible and near-visible region of the spectrum varies from 2-4 mm for meats to as deep as 10 mm in some baked goods and liquids like non-fat milk.
It is this region of deep penetration that produces that fast cooking times seen in lightwave ovens. Because the energy is deposited in a fairly thick region near the surface of the food, the radiant power density that impinges on the food can be increased in lightwave ovens without overheating the surface temperature of the foodstuff. Consequently the radiation in the visible and near-visible regions does not contribute greatly to the exterior surface browning.
In the spectral region above 1.4 &mgr;m (infra-red region), the penetration distance decreases dramatically to fractions of a millimeter, and for certain peaks down to 100 &mgr;m (the thickness of a human hair). The power in this region is absorbed in such a small depth of penetration that the temperature at the surface rises rapidly, driving the water out and forming a water-depleted crust. With no water to evaporate and cool the surface, the temperature can climb very fast to 300° F. This is the approximate temperature where the set of browning reactions (Maillard reactions) are initiated. As the temperature is pushed even higher to above 400° F., the point is reached where the surface begins to burn.
It is the balance between the deep penetration wavelengths (0.39 to 1.4 &mgr;m) and the shallow penetration wavelengths (1.4 &mgr;m and greater) that allows the power density at the surface of the food to be increased in the lightwave oven, to cook the food rapidly with the shorter wavelengths and to brown the food with the longer infra-red so that a high-quality product is produced. Conventional ovens do not have the shorter wavelength components of radiant energy. The resulting shallower penetration means that increasing the radiant power in such an oven only heats the food surface faster, prematurely browning the food before its interior gets hot.
Conventional ovens operate with radiant power densities as high as about 0.3 W/cm
2
(i.e., at 400° F.). The cooking speeds of conventional ovens cannot be appreciably increased simply by increasing the cooking temperature, because increased cooking temperatures drive water off the food surface and cause browning and searing of the food surface before the food's interior has been brought up to the proper temperature. In contrast, lightwave ovens have been operated from approximately 0.8 to 5 W/cm
2
of visible, near-visible and infra-red radiation, which results in greatly enhanced cooking speeds.
For high-quality cooking and baking, the applicant has found that a good balance ratio between the deeply penetrating and the surface heating portions of the impinging radiant energy is about 50:50, i.e., Power(0.39 &mgr;m to 1.4 &mgr;m/Power(1.4 &mgr;m and greater)≈1. Ratios higher than this value can be used, and are useful in cooking especially thick food items, but radiation sources with these high ratios are difficult and expensive to obtain. Fast cooking can be accomplished with a ratio substantially below 1, and the applicant has shown that enhanced cooking and baking can be achieved with ratios down to at least 0.6 for most foods, and lower for thin foods and foods with a large portion of water such as meats. If the power ratio is reduced below about 0.3, the power densities that can be used in cooking are comparable with conventional cooking and no speed advantage results.
If blackbody sources are used to supply the radiant power, the power ratio can be translated into effective color temperatures, peak i
Minnear William P.
Romiti Jay G.
Schestedt William H.
Westerberg Eugene R.
Gray Cary Ware & Freidenrich LLP
Pelham Joseph
Quadlux, Inc.
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