Bill Petty, of Mosgiel, asks :-

When cooking food in a microwave, will it destroy all the organisms that could give you food poisoning?

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

Microwave ovens are a common feature in NZ homes and when used correctly they are a fast way to safely cook or reheat foods. It is however important to understand how microwaves work and what steps you can take to ensure your food is safe to eat.

Microwaves use a magnetron to covert electrical power into an alternating electric field of microwaves. Many molecules in foods (such as water) have a partial positive charge at one end and a partial negative charge at the other. In a microwave these 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”.

This heat kills micro-organisms in the food in the same way that heat from a conventional oven does. However as the food can heat less evenly in a microwave than that in a conventional oven it is important to use a food thermometer and test the temperature of food in several places to make sure it has reached 74°C, which is the recommended temperature to kill the micro-organisms that can make you sick.

Additional steps you can take to enhance the food’s safety include: arranging food items evenly on the turn-table to promote even heating; de-boning large pieces of meat (as the bone can shield meat from thorough cooking), turning food over or stirring dishes of food midway through the cooking process to compensate for uneven heating; covering the food to retain surface moisture, and finally allowing the food to stand covered for at least 3 minutes after cooking to ensure all parts of the food reach the required temperature.