MUNsolved Mysteries

(June 19, 1997, Gazette)

Have a burning question pertaining to science, engineering, medicine, the arts, music, humanities, social sciences, physical education, or any other field? Let us know, we'll do our best to find the answers for you, drawing upon the expertise of members of the university community.

Michelle Kelly, Centre for Institutional Analysis and Planning, is hot and cold on microwave cooking. She sent us the following question:

"If a portion of food is heated in a microwave over on maximum heat for a short period of time...does it cool more quickly than food heated to the same temperature in a microwave, but at a lower heat setting for a longer period of time?"

The short answer is yes, according to Dr. Sue Ghazala, Biochemistry.

However, "the answer to your mystery question depends on the time-temperature combinations in question, and on the nature of the food," she explained. "It is well understood that microwave heating occurs from the inside out. Cooking food at a high temperature for a short period of time will cause faster heating of the centre, however, it will not allow sufficient time for the heat to transfer to the remainder of the food product. Thus, you will be left with a food that is overcooked at the centre and undercooked at the edges."

If you cook food at a lower temperature for a longer period of time, Dr. Ghazala said, the centre of the food will not get as hot as fast. The longer period of time allows heat transfer to the rest of the product, resulting in more even cooking. Therefore, food cooked at a higher temperature for a shorter period will cool off faster, since the heat has not had time to spread throughout the food. Food cooked at a lower setting for a longer period is heated through, and thus takes longer to cool.

"In terms of the actual temperature at the centre of the food," Dr. Ghazala added, "this will be higher with the higher power setting of the microwave if the product is given enough time to reach this temperature. For example, when microwaving evaporated milk, the milk will reach 97 degrees Celsius after 30 seconds at high power. At medium power for the same period it reaches 74 degrees Celsius, and at low power for 30 seconds, 44 degrees Celsius. After 60 seconds at low power, the milk will reach 48 degrees Celsius, and after 90 seconds on low it will reach 51 degrees Celsius."

Dr. Ghazala also pointed out that liquids cook faster than solids in a microwave, so adjust your cooking time accordingly.

Is there a question revolving in your mind? Send it to MUNsolved Mysteries, Gazette, Arts and Administration Building, Memorial University of Newfoundland, St. John's, Nfld., A1C 5S7; e-mail, or fax 709-737-8699. Please include your name and telephone number.