Microwave ovens are often used for food preparation, raising questions about their effectiveness in eliminating harmful microorganisms. Some bacteria can cause foodborne illnesses. Understanding how microwaves interact with food and the biological processes involved in bacterial destruction is important for food safety.
How Microwaves Generate Heat
Microwave ovens operate by producing electromagnetic radiation within the microwave frequency range. A magnetron generates these waves, which then bounce around inside the oven. These waves are absorbed by specific molecules within food, primarily water, but also fats and sugars.
This absorption causes polar molecules, like water, to rapidly rotate and vibrate as they attempt to align with the oscillating electric field. This rapid molecular movement and subsequent friction generate thermal energy, which heats the food. This process, known as dielectric heating, efficiently transfers energy into the food, raising its temperature.
The Science of Heat and Bacterial Death
Heat eliminates bacteria by disrupting their fundamental cellular structures and processes. When bacteria are exposed to sufficiently high temperatures, their proteins and enzymes undergo denaturation. This means the complex three-dimensional shapes of these molecules, essential for their function, unravel and break down.
Heat also damages bacterial cell membranes, causing them to leak and lose integrity. This disruption leads to bacterial cell death. The temperature reached within the food, and the duration for which that temperature is maintained, are the primary factors determining bacterial destruction, not the microwave radiation itself.
Microwave Effectiveness Against Bacteria
Microwave ovens can kill bacteria, but only if the food reaches a sufficiently high and uniform temperature. Studies show microwaving can reduce bacteria levels, with some like E. coli inactivated in as little as 30 seconds at full power. However, a significant challenge with microwave cooking is the potential for uneven heating. Factors like food density, shape, and water content can create “cold spots” where temperatures remain too low. This uneven heating means some parts may be cooked and safe, while others could harbor surviving bacteria.
Practical Considerations for Microwave Use
To maximize food safety with a microwave, several practices are important. Stirring or rotating food midway through heating helps distribute heat more evenly. Covering food with a microwave-safe lid or plastic wrap promotes more uniform heating by trapping steam.
Allowing food a “standing time” after microwaving is beneficial, as heat continues to conduct throughout, raising its internal temperature. Using a food thermometer to verify the food reaches a safe internal temperature, such as 165°F (74°C) for most leftovers and poultry, is the most reliable way to ensure bacterial destruction.
It is important to note that while microwaving can kill bacteria, it does not destroy bacterial toxins or spores, which may require higher temperatures or longer durations to inactivate. Therefore, microwaves are best suited for reheating cooked foods, not cooking raw foods from scratch, especially large or dense items that may not cook evenly.