The presented paper reviews the existing literature dealing with rice puffing and optimum conditions of this process. The rice pre-treatment procedures and puffing conditions (dry heat, microwave, gun-puffing) are considered. The optimum composition of the raw material is also mentioned.

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The microwave power is a parameter greatly influencing the rate of heating. Several authors reported on certain differences between the rated power output as a parameter used by oven manufacturers in the labelling of ovens, and the power actually delivered to the heated product. A review of the respective information is given in this article together with the results of own experiments following the influence of the oven type and the heated substance parameters on the power actually absorbed in the substance volume during its heating. As the heated substance, water and solutions of NaCl and sucrose of different concentrations were used. For the heating, four types of domestic microwave ovens and glass and plastic containers, were used. The decreasing of the efficiency of heating with the decreasing volume of the heated substance and a certain relation between the rate of this decrease and the types of oven and of substance was estimated. With the small cavity ovens, a lower rate of the decrease of the absorbed microwave power with the decreasing volume of the substance was found as compared to the large cavity oven. A certain influence of other technical oven parameters is shown in the comparison of the tests results with the ovens of the same rated power and the cavity volume. In addition to the substance volume, also its dielectric properties probably influence the microwave power absorbed in small samples during the heating. No simple dependence can be seen on the basis of the tests results between the type of container used in the tests and the power absorbed in the heated substance.  

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Susceptors have been used with microwaveable foods in the most widespread types of active packaging systems since the late 1970s. Although different types of susceptors have been proposed, only those consisting of a polyethylene terephthalate (PET) film lightly metallized with an elemental aluminium laminated onto a dimensionally stable substrate (paper or paperboard) are currently used commercially (Figure 1). In spite of more than 20 years experience in the susceptor application in the food industry, relatively little information on the susceptor's performance and their changes during microwave heating has been published in the literature. The influence of the thickness of susceptor metallization on the heating capability was experimentally followed by Z�������� and M���� (1992). Samples of susceptors of three different levels of metallization were tested in this work. The susceptors with optical density (parameter used for the characterization of the metallic layer thickness) of 0.3 were shown in this work to be optimal. The electric and morphologic changes of susceptors during the microwave heating were also followed experimentally by Z�������� and M���� (1994). The formation of holes and cracks in the susceptor surface and the change in the crystallinity of PET film were presented by the authors as the main cause of the decrease of the susceptor heating ability during heating.The aim of the present study was (i) to test the optical as well as the electrical properties (especially optical density, DC square resistance and impedance) of laboratory prepared susceptors and to determine the relationships between them, and (ii) to determine the optimum thickness of metallization from the point of view of the maximum microwave energy absorption in the film. Samples of susceptors of different metallization have been prepared for this study.As mentioned above, the commercially utilized susceptors consist of a metallized active layer (acting as heat source) deposited on PET film. The Thin Al films of varying thickness, i.e. 3 to 30 nm, were deposited onto polyethylene-terephthalate film by evaporation in the vacuum of 3 × 10 -3 Pa. The dependence of DC (direct current) surface resistance on thickness was measured using a four-point method. The surface resistance exhibits the size effect in accordance with the FuchsSondheimer theory. The microwave absorption properties of the prepared films of various metallization thickness were measured in a microwave field at the microwave power of 1.8 mW. The maximum microwave absorption at 2.45 GHz was found to occur in a layer of optical density of about 0.22. Properties of Thin Metallic Films for Microwave Susceptors