ENSURING THE SAFETY OF MICROWAVEABLE FOODS: HOW TO DETERMINE THE WATTAGE OF YOUR MICROWAVE

Colorado State University Extension
SafeFood Rapid Response Network

ENSURING THE SAFETY OF MICROWAVEABLE FOODS: HOW TO DETERMINE THE WATTAGE OF YOUR MICROWAVE

Edited by: Marissa Bunning, Ph.D., Mary Schroeder, M.S., R.D, Pat Kendall, Ph.D., R.D., Colorado State - Spring 2008

Microwave ovens have become commonplace appliances in homes, work environments, college dormitories, hotel rooms and convenience stores. The variety of microwavable foods has increased in recent years with the latest generation of products including single-serve packages, organic products and reformulations focused on improving healthfulness (Bertrand, 2005). The availability of fresh and frozen vegetables in microwaveable bags has also increased and plastic bags specifically for steaming foods in the microwave are now available. The timesaving benefits and ease of operation of microwave ovens have helped to establish their popularity; however, these factors may also contribute to improper use that could result in insufficiently cooked products.

Between January 1, 2007, and October 29, 2007, at least 272 people in 35 states contracted salmonellosis, likely from consumption of Banquet brand frozen pot pies produced by the ConAgra Foods Company. Isolates of Salmonella I 4,[5],12:i:- (pronounced four five twelve eye minus) with an indistinguishable genetic fingerprint were subsequently collected from all infected persons (CDC 2007). On November 1, 2007, General Mills Operations voluntarily recalled approximately 3.3 million pounds of frozen meat pizza products because of possible contamination with Escherichia coli O157:H7 and the possibility that the pizza may have been linked to a multistate outbreak of E. coli O157:H7 illness, which sickened 21 people in 10 states (FSIS 2007). The possibility exists that in at least some of the cases, the pizzas were cooked in the microwave, even though they were not designed to be microwaveable.

Cooking instructions on packaged products often vary according to the power or wattage of the microwave. Many different types of microwave ovens have been manufactured, and consumers may not know whether the unit they are using is high wattage or low wattage. Wattage information is often displayed near the door or on the back of the oven but some manufacturers only list the wattage in the operation manual. For determining the wattage level of microwave ovens, USDA recommends the following method in the Microwave Ovens and Food Safety Fact Sheet (USDA, 2007).

Time-to-Boil Test
Measure a cup of water in a 2-cup glass measure. Add ice cubes; stir until water is ice cold. Discard ice cubes and pour out any water leaving 1 cup. Set the microwave oven on high for 4 minutes, but watch the water through the window to see when it boils.

If water boils in less than 2 minutes, it is a very high wattage oven 1000 watts or more.
If water boils in 2½ minutes, it is a high wattage oven about 800 watts or more.
If water boils in 3 minutes, it is an average wattage oven 650 to 700 watts or more.
If water boils in more than 3 minutes or not by 4 minutes, it is a slow oven 300 to 500 watts.

Use the minimum cooking time given for high wattage ovens; use the maximum cooking time for slow ovens. The minimum cooking time may need to be reduced for very high wattages.

Unlike most conventionally heated foods, microwavable foods often have "active" packaging that can play an important role in the proper cooking of the product if consumers follow the directions on the package (Bertrand, 2005). Such directions may include a specified "standing time" to allow for further heat penetration, although this may not be made clear to consumers.

The use of microwave ovens can have a positive impact on food safety since food can be thawed and cooked quickly, spending less time in the temperature zone that is conducive for microbial growth. However, consumers need to be informed about safe microwave cooking methods and the proper use of food thermometers to ensure food is cooked or reheated adequately and safely. Any food that has reached an internal temperature of 165º F after the recommended standing time should be safe to eat.

Sources:

Bertrand, K. (2005). Microwave Foods Satisfy Need for Speed and Palatability. Food Technology. 59(1): 30-34.
CDC (2007). Investigation of outbreak of Human Infections Caused by Salmonella I 4,[5],12:i:-. Updated Oct. 29, 2007. Accessed March 12, 2008. Available at: http://www.cdc.gov/salmonella/4512eyeminus.html
USDA Food Safety and Inspection Service (2006). Appliances & Thermometers: Microwave Ovens and Food Safety. Accessed February 12, 2008. Available at: http://www.fsis.usda.gov/Fact_Sheets/Microwave_Ovens_and_Food_Safety/index.asp

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