The invention also provides an improved method of distributing and heating foodstuffs by packing them in a disposable container having a shield and absorber for converting microwave energy to thermal energy then transporting and heating them in the container to provide surface scorching and reduced direct microwave transmission to the food as will be described more fully below.
Packages in accordance with the present invention can be used for shipping and vending foods both through retail grocery outlets and vending machines. They can be used for a single serving or for several foods in a single container in the manner of a T.V. dinner.
The container body can comprise any microwave permeable nonlossy material and is usually a dielectric such as paperboard or other cellulosic material or plastic resin such as a polyamide or polyester resin having the requisite heat resistance. The container body, e.g., a paperboard box usually includes side, top and bottom walls to enclose and protect the food product.
The lossy microwave energy absorber preferably has the form of a thin sheet or layer that serves as a heating body and is usually part of a composite sheet of heating body composed of a structural supporting sheet that can be either microwabve transparent or microwave opaque such as a ceramic or metal sheet to which the active microwave absorber is applied as a relatively thin paint like layer. The expression paint like layer herein means a coating applied as a layer having a small finite thickness up to on the order of about 1/32 of an inch bonded directly to the structural support layer and having a sufficient flexibility to remain adhered to the layer when the lartter is bent or deformed. When this laminate is used to support the food product, the energy absorbing layer is normally placed on the opposite side of the structural support sheet from the food thus the food is adjacent to and usually contacts the structural support sheet or foil. The geometry and especially the thickness of the microwave absorber is preferably maintained within a specified range to control the saturation i.e., equilibrium temperature reached by the heater after a specified period of heating or indefinite heating. It was discovered that the thickness should be maintained substantially within the range wherein the temperature is positively correlated with the changes in thickness i.e., the temperature response rises with an increase in thickness. The shield which reduces by a controlled amount the quantity of direct microwave transmission to the food product is conveniently applied as a layer or lamination to the inner surface of the container body. It is preferably, but not invariably, formed from an electrically conductive material such as metal foil, e.g., aluminum foil.