Question

Efficiency of Heating Containers Used in Microwave ovens

general objective:

To determine what properties of microwave containers result in the minimum amount of time for heating to a given temperature .

1. sperate the extraneous factors(electricity,size of the container,material of the container) from the factors to be investigated (temperature,time) and decide how you might CONTROL for them (eg. blocking , keeping conditions the same ,etc)

2 . specify the study units

Answer 1

Microwave (MW) heating is rapid but non-uniform. The issue of non-uniform heating is exasperated in frozen foods due to dramatic differences in dielectric properties of ice and liquid water. Many frozen food products are not-ready-to-eat, meaning that they may contain pathogens. Expectation is that the final cooking/heating step in the microwave oven assures food safety. Due to non-uniform heating, part of the frozen foods may not be heated adequately (165°F to kill Salmonella) in microwave ovens. The foodborne pathogens, if present in the cold spots, would survive and cause foodborne illnesses. Improvement in microwave heating uniformity has been a real challenge to both microwave cavity designers and food product development scientist. Vadivambal et al (2010) reviewed various studies on microwave heating and suggested a research need to improve heating uniformity. Improving heating uniformity of a microwave food product can be achieved by modifying food composition and geometry (Ryynänen et al. 1996). In past two decades, many serious efforts have been made by researchers to understand the phenomenon and come with solution to overcome non-uniform heating (Geedipalli et al. 2007; Rakesh et al. 2009; Bradshaw et al. 1997). Many studies have been conducted to understand and improve non-uniform heating experimentally and through computer simulation (Wäppling-Raaholt et al. 2006; Knoerzer et al. 2007). The efforts vary from pure empirical methods to complicated computer modeling software. These approaches need to be validated using experiments.

For measuring performance of domestic oven, IEC 60705 standard suggests to use square container (228mm×228×30 mm) made of material transparent to microwaves (IEC, 2006). Wang et al. (2008) designed a test rig consists of an array of 24 plexiglass cups filled with water and array of 24 thermocouples to asses heating uniformity in a radio frequency heating system. They also used a foam sheet to evaluate heating uniformity of the radio frequency system using an infrared camera. Thermal imaging is an industry standard tool for assessing heating uniformity of product at the end of the heating process.
Our long-term goal is to develop a comprehensive risk assessment model to assess the food safety risk of consuming microwaveable foods. The risk assessment model takes into account of the variation in microwave parameters (power, location of food inside the oven), food composition, layout and its properties, microbial parameters (death kinetics parameters), and consumer behavior (cooking time, standing time, dose-response curve). This study is conducted to assess the variability of microwaves distribution within a cavity and how placement (location) of a food in the turntable affects the heating rate and uniformity. Therefore, the objective of this study is to assess the variation of microwave distribution within a cavity in a range of domestic microwave ovens. Specific objectives of this study are to:
1. Assess the microwave energy distribution within the cavity along the radial direction.
2. Assess the overall heating uniformity within the cavity for ovens of various power
wattage.

A quick and reliable heating uniformity assessment method was developed. For two minutes of heating 1-liter of water load in microwave oven, the temperature rise ranged from
10 oC at the center of the cavity to 28 oC at the edge of the turntable. Just for 2 minutes of heating, the temperature difference can be as high as 18 oC. It was found that water will get more uniform heating in edge of the turntable rather than center of the turntable while allowed to rotate the compartment container. Thus, it is better to place the food at the edge of the turntable rather than at the center of the turntable for rapid and uniform heating. The developed test method can be used for performance testing of range of microwave ovens.
This study was conducted with the water load that covered most of the turntable surface and came up with the recommendation to place the food at the edge of the turntable. However, when the food (load) is placed on a small location within a cavity, the distribution of electromagnetic field changes. Therefore, further studies must be conducted with small water load placed at various radial distance and asses the heating rate and heating uniformity.