Natalie Brockie, of Wellington, asks :-

I often reheat leftovers in the microwave oven but was recently told that destroyed the protein in the food. Is that so?

Phil Bremer, of the Food Science Department at the University of Otago, responded.

When used correctly a microwave oven is a fast and safe way to reheat food, with the reheating having a negligible effect on nutritional quality.

Microwave ovens use a magnetron to covert electrical power into an alternating electric field of microwaves. Many molecules in foods have a partial positive charge at one end and a partial negative charge at the other. For example consider water which has the chemical formula of H2O. The hydrogen ions (H+) have a slight positive charge and the oxygen ion (O2-), which sits in a slight v formation between the two hydrogen ions has a slight negative charge. In a microwave such molecules rotate backwards and forwards trying to align themselves with the changing electric field. As the electric field changes direction 2.5 million times a second, the molecules rotate at great speed, hitting other molecules and setting them in motion. All this movement generates heat by “molecular friction” and the food warms up.

The heat generated by microwave cooking has no more effect on the proteins, fats and carbohydrates in foods than heat generated by a conventional oven. In fact microwave cooking / reheating is generally considered to result in a greater nutrient retention in the food.

To ensure that food reheated in a microwave oven is safe to eat, it is recommended that it should reach an internal temperature of 74C, in order to kill any micro-organisms that can make you sick. A food thermometer should be used to test the food in several places and as food reheated in a microwave oven can heat unevenly it is important to turn food over or to stir dishes of food midway through the cooking process and then to let the food stand for a least 2 minutes after cooking to ensure even heat distribution.