Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
C428S698000, C428S699000, C428S908800
FIELD OF THE INVENTION
This invention relates to foodware with ceramic food-contacting surfaces.
BACKGROUND OF THE INVENTION
The oldest known type of non-stick cookware is oil-seasoned steel or cast iron cookware. Such cookware is very effective, and has a tough, abrasion resistant surface. Unfortunately, seasoned steel or cast iron cookware is vulnerable to damage by rusting of the substrate metal, and must be handled and particularly cleaned with care to avoid damaging the cookware surface. The critical carbonized surface layer can be lost or damaged if the cookware is allowed to sit in water containing dissolved oxygen for even a few hours.
Another problem with seasoned steel or cast iron cookware is that iron can readily be leached from the cookware's surface by food acids, as are found in tomato sauce for example. Thus, depending on one's food preferences, it is possible to get a dose of iron from food cooked in seasoned iron-based cookware which is well above the daily recommended dose, which can have negative health effects for some people.
Another problem with seasoned steel or cast iron cookware is that reaction products of edible fats and oils (the source of the carbonaceous cooking surface), which are chemically modified by the process of seasoning the cookware, can sometimes escape back into food before the carbonization process is complete. Some intermediate compounds which are produced as fats and oils are chemically converted to the carbonized non-stick cooking surface are known carcinogens (such as trans-fatty acids and peroxidized fatty acids).
Another traditional material for foodware is copper. Copper foodware has excellent heat transfer properties, but is much softer than seasoned cast iron or stainless steel, for example. This makes it delicate and prone to scratching. It is also prone to surface oxidation and various reactions with sulfur compounds in food that lead to tarnishing. Similar to cast iron and steel foodware, copper foodware can introduce copper ions into food at fairly high levels compared to dietary requirements for copper. Copper can be polished to an attractive reflective surface finish, but requires constant work to maintain this surface finish. As a result of the difficulty of keeping copper foodware looking good, a large fraction of copper foodware in modern kitchens today is more for show than for cooking.
The difficulty of keeping copper foodware looking good implies that another approach involving a tougher metal which is better able to hold a shine than copper is desirable. One prior art solution for this is stainless steel. Stainless steel foodware is not normally seasoned with oil in the same way as cast iron or steel cookware. Food tends to stick to stainless steel much more than to seasoned cast iron. However, stainless steel foodware is strong and tough, and holds a finish very well (much better than copper). Furthermore, stainless steel foodware can be soaked in strong cleaning solutions, which makes it more clean-able and more convenient to maintain than seasoned cast iron.
Stainless steel foodware holds its shine much better than copper, but can still be scratched by stainless steel utensils, and it can be pitted by salt water (especially hot brine). Stainless steel foodware can also leach ions into food, especially iron, chromium, manganese, and nickel. Although the actual dose of ions from stainless steel is normally quite low, it is still a concern for some people. (Nickel in particular can leach from some grades of stainless steel at levels that are well above the recommended dietary allowance for nickel.)
Aluminum foodware has the best heat transfer properties per unit weight of any commonly available type of prior art foodware. Unfortunately, aluminum foodware is also prone to leaching of aluminum ions into food, in the form of food acid aluminum salts. Such food acid aluminum salts are much more able to enter the bloodstream than common inorganic forms of alumina (as in bauxite) or aluminosilicates (found in clays for example). Some researchers have expressed concern that blood-borne aluminum can accumulate in the brain, and may increase the risk of Alzheimer's disease.
One way to reduce the amount of aluminum getting into food from aluminum foodware is to coat the aluminum. Anodized aluminum is coated with aluminum oxide to a greater depth than is the case for untreated aluminum objects. (Aluminum that has been exposed to air is always coated with a thin layer of oxide; the oxide is much harder and more stable than the aluminum, and passivates the surface to some extent.) Anodized aluminum is much harder on the surface than untreated, air-oxidized aluminum surfaces because the oxide layer is thicker. Anodized aluminum is also far more resistant to leaching of aluminum ions by food acids than untreated, air-oxidized aluminum foodware. The anodized aluminum surface is not good for release of food during cooking, though; oil must generally be used to prevent foods from sticking.
Anodized aluminum foodware surfaces always contain microcracks, due in part to the difference of thermal expansion rate of aluminum versus aluminum oxide, which leads to thermomechanical stress as the foodware (especially cookware) temperature is changed. These cracks are sites for corrosion, and leach aluminum into food. These cracks can also grow, and yet remain undetectable to the naked eye, so that anodized aluminum foodware can leach more aluminum as it ages.
Anodized aluminum is not dishwasher safe. Typical automatic dishwasher detergents, dispersants, and wetting agents discolor and/or corrode anodized surfaces. Anodized aluminum foodware also can be discolored if oily drips get cooked onto the surface and then get hot enough to be carbonized. Such stains are difficult to remove without changing the appearance of the foodware in the region where the oil drip has been cooked onto the foodware, because the carbonized oil stain must be abraded off.
Another way that aluminum foodware can be modified to prevent leaching of aluminum ions into food is via thermal spraying (flame spray, plasma spray, or high velocity oxidizing flame, known commonly as HVOF spray) the inside of the foodware. In thermal spray methods, a hot gas or plasma is used to melt (or partially melt) a stream of solid particles, which are impinged against a relatively cold metal surface (in this case the aluminum substrate foodware) at a sufficient velocity to adhere to the surface. (The impinging molten particles from the plasma spray also cause localized melting of the aluminum surface, which can improve adhesion.)
Thermal spray methods cannot produce a smooth, specular (mirror-finish) surface. The rough surfaces produced are ideal for anchoring a perfluorocarbon layer, but if untreated, are generally prone to food sticking. Look Manufacturing Company's aluminum-core cookware, which are plasma sprayed with stainless steel, are examples of this approach. The resultant surface is very tough, but is not good for release of foods during cooking. (This type of surface does make an excellent substrate for application of perfluorocarbon-based release coatings, though.) Scan-Pan Inc. sells plasma sprayed aluminum cookware that is sprayed with titanium nitride (TiN). This cookware is much better for release of foods during cooking than otherwise similar cookware (in terms of porosity and surface roughness) which has been sprayed with stainless steel, for example. Scan-Pan's TiN surface coating is quite uneven, though the surface roughness is essentially devoid of sharp edges. The TiN is partially decomposed by the plasma's extreme temperature, so that the surface also contains metallic titanium in addition to titanium nitride.
Perfluorocarbon polymers, such as PTFE (polytetrafluoroethylene, “Teflon™” for example), have been used as non-stick coatings for cookware since the 1960's. The earliest technologies for applying such coatings over relatively smooth aluminum cookware produced very fragile cooking surfaces. Since then, these coatings have bee
Mirick O'Connell DeMallie & Lougee LLP
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