Effect of Fat Content and Food Type on Heat Transfer during Microwave Heating

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Virginia Tech


Microwaves heat food rapidly and foods are prepared in less time. However, due to non-uniform heating nature of microwave cooking, there exists a serious concern over complete elimination of pathogens in the food. There has been an increase in interest to accurately understand the behavior of different food materials in a microwave field and microbial inactivation during microwave cooking.

Recent research showed that fat content in muscle food plays an important role in microbial inactivation by increasing the inactivation level with an increase in the fat level. It was also demonstrated that muscle food heats up differently than a vegetable food product. Cooking food in a microwave oven either by covering the food container or not results in significantly different temperature profiles. The current research attempts to use modeling techniques to analyze impact of these factors on microwave heating.

Mathematical modeling is faster, easier and economically better than actual experiments in determining heating behavior of a microwave-cooked food. Though modeling cannot completely replace actual experiments, it can be used as a tool to understand the effects of various factors influencing the microwave cooking.

A factor that is highly important during microwave processing is dielectric properties of the material. The interaction of microwave with the food is mainly based on its dielectric properties, which can change with temperature. Therefore, determination of dielectric properties of food with respect to temperature becomes critical.

The current research project has two parts. One to determine the dielectric properties of food being tested and another is to employ mathematical modeling techniques to analyze the effect of fat content, food type and the effect of cooking food by covering the bowl using the lid and not covering bowl.

Dielectric properties of ground beef patties at 4%, 9%, 20% fat levels and frozen broccoli were determined using an open-ended, 3.6 mm diameter, semi-rigid coaxial line with copper conductors, connected to a network analyzer. The properties were determined at various temperatures. Foods were measured in triplicate. Results showed that dielectric constant and dielectric loss factor of low fat ground beef were higher than that of high fat level ground beef. In addition, the dielectric properties of florets were lower than that of stem parts for frozen broccoli.

A 1,200W, household type microwave oven was used in this study to heat the food. Food was placed in a microwave-safe glass bowl and cooked for 120 seconds. One headspace and three internal temperature measurements were recorded for every 0.6 seconds. Five replications were performed. Finite element method was used as modeling technique and temperatures were predicted. Experimental and predicted temperature values were compared. Results showed that the model used in the study was more suitable for modeling the uncovered cooking than covered cooking process. Modeling results also revealed that high fat ground beef patties reached higher temperature than low fat patties. In high fat meat products, fat content also contributed to increase in temperature during microwave heating. In vegetable products and low fat meat food, moisture content is mainly responsible for microwave heating.

A more extensive study on critical fat level above which fat content helps in increasing temperature is needed. In addition, inclusion of steam properties in the headspace for modeling the covered cooking is recommended.



Broccoli, Modeling, Dielectric Properties, Ground Beef, Microwave Cooking