MICROWAVE HEATING UNIFORMITY OF MULTICOMPONENT PREPARED FOODS (2002)

During microwave frequency heating many variables in the food, package, and the microwave oven itself affect how the food is heated. Multicomponent foods in particular heat unevenly, causing problems with both sensory and microbiological quality. It is difficult to quantify the influence of factors affecting microwave heating uniformity of foods. This is due, at least partly, to the problems in measuring temperature distribution and the interaction between oven and the food. The objective of this study was to improve the quality of microwave heated multicomponent foods by better understanding of the factors controlling heating uniformity and the consumer perception thereof. Dielectric properties of some food ingredients and components were measured by a cavity perturbation technique at temperatures up to 95oC. Food material used in the heating experiments were chilled ready meals and hamburgers. Temperature was measured by a fibre optic system during microwave heating and by thermocouples after heating. Infrared imaging was used to determine surface temperatures of hamburgers. Water diffusion in a hamburger bun was examined by an NIR water measurement system. The effect of salt content (two types of salt), geometry and placement of food components, as well as the type of tray on heating uniformity of food were investigated. A consumer panel was used to examine how temperature affected the pleasantness of meal components and how the more or less uneven temperature distribution of microwave heated meals was perceived. The measurement of the dielectric properties of starches showed that mainly water content determined the dielectric properties and that, in practice, the variations between the different starches were too small to be significant for recipe modifications. The temperature measurements showed that chemical modifications had either no effect on temperature development or they had only some effects combined with other factors although the dielectric properties varied considerably. The most important factors in modifying the temperature distribution of a ready meal were placement and geometry of components, and the type of tray. Heating uniformity was also studied in a vertically layered system with components of widely different composition and dielectric properties, a hamburger bun. The results showed that uneven heating is difficult to improve by recipe modifications alone. Modelling of the hamburger confirmed that the food itself causes the uneven heating due to focusing and diffraction phenomena. The NIR measurement instrument, which had been found to be suitable for measuring local water content during conventional heating, was found to be very sensitive to both the structure and the temperature of the sample. Structural changes during microwave heating differed from those during conventional heating and, thus, the calibration procedure should be developed and tested separately for measuring water diffusion in a microwave oven. Serving temperature clearly affected the sensory attributes and pleasantness of foods. However, while instrumental measurements indicated large differences in temperatures between two microwave heated ready meals with different component placements, the consumer panel perceived only small differences in some sensory attributes. Regardless of temperature differences, the overall pleasantness of the two microwave heated meals with different placements was judged to be similar and satisfactory. Serving temperature is crucial to the pleasantness of food but, to the extent that it varied in microwave heating, it did not have a major impact on the acceptability of the meal. However, the more uniform alternative is recommended. The results showed that most promising ways of modifying microwave heating uniformity are physical factors, especially the dimensions or geometry of the food. Compared to hamburger, it was easier to achieve more desirable temperature distribution in a ready meal as the hamburger components have widely different properties and the geometry of the product has a very pronounced effect on the heating. Simulation of microwave heating of multicomponent products offers a powerful tool for product development. Close co-operation with package development is essential in solving some problems associated with microwave heating.