Microwave cooking

Introduced in 1947, the first microwave oven, the Radarange, was 1.8 metres (5.9 feet) tall and weighed 340 kg (750 pounds)! Today, more reasonably sized microwave ovens are found in more than 90% of Canadian homes. Can we learn to use them better?

How microwave ovens work

Microwave ovens are equipped with a small device, a magnetron, that transforms electricity into electromagnetic radiation—very short waves called microwaves. The radiation is similar to that emitted by cell phones, though microwave ovens produce at least 1,000 times more. The magnetron creates an electromagnetic field inside the oven that changes direction five billion times a second. When food is placed in the field, the microwaves penetrate it, causing polar molecules, such as water, to oscillate at the same frequency as the microwaves, a bit like a boat rocking on ocean waves. As the water molecules move back and forth, they bang into each other and become highly agitated. This agitation—a form of energy—generates heat, which is transferred to the rest of the food by conduction. Water molecules are not the only ones that oscillate: sugar and fat molecules also generate heat when exposed to microwaves.

On a microwave oven, except one equipped with Invert® technology, you don’t select a cooking temperature like on a conventional oven but rather the percentage of time the magnetron operates. For example, when the oven is run at full power (setting 10), the magnetron produces microwaves constantly. At setting 7, the magnetron cycles on and off, operating only 70% of the time. These on/off cycles are what cause the different humming sounds you hear when the oven is running. Using the proper setting is important to give the heat time to distribute evenly throughout the food, avoiding hot spots and overcooking.

Why do microwaves cook food so quickly?

Cooking is fast because the microwaves penetrate the food up to three centimetres (a little over an inch) deep. Conventional ovens, on the other hand, heat only the food’s outer surface. What’s more, in a microwave oven, the food is heated directly—not by hot air, which is a poor conductor of heat.

Why doesn't food brown?

For browning and caramelization to occur, the food’s surface has to become very hot, about 150°C (300°F). Since the air around the food stays cool in a microwave oven, the surface doesn’t brown. There are a few exceptions, however. Sugar can be caramelized and nuts and coconut toasted in a microwave oven. Why? These foods contain little water but relatively high amounts of sugar or fat, two molecules that can easily reach and even exceed the temperature at which browning occurs when heated in either a conventional or microwave oven.


Do we end up eating the microwaves?

In a word, no! Turning off a microwave oven is like turning off a light: no more microwaves are emitted, and none remain inside the oven or the food. The reason a resting period is recommended after cooking is not to allow any remaining microwaves to dissipate (since none remain) but to give the heat time to be conducted evenly throughout the food.

Is it dangerous to look through the oven window?  

No, as long as the oven is in good condition, meaning the window is not damaged or cracked and the door is properly sealed. It is true that microwaves, like light, can pass through glass. However, microwave ovens are required to meet very strict standards, and the glass on the window is covered with a thin, perforated metal sheet that reflects the microwaves back into the oven and prevents them from escaping.

Is it true that water heated in a microwave oven can explode?

Well, it’s not an urban legend! Under some conditions, water that is heated in a microwave oven may appear harmless but suddenly erupt and boil over the edge of the cup when jostled or removed from the oven. Fortunately, this is a very rare phenom¬enon and one that tends to occur only when the cup’s inside surface is perfectly smooth. Here’s the reason why. Because microwaves penetrate up to three centimetres deep, a cut of water heats up so quickly that the water superheats, meaning it heats up faster than bubbles of vapour can be produced. When the cup is moved or the water is stimed, all the heat that wasn't able to leave the water in the form of vapour bubbles is released in a flash: the water begins to boil explosively and may even overflow the cup, causing burns. How can protect yourself? Stir the water with a spoon before taking the cup of the oven.

Ten ways to get the most from your microwave oven

1. Soften butter or cold cream cheese

Heat at power level 2 (20%) or 3 (30%), pausing every 30 seconds or so to check the progress.

2. Dissolve gelatin

Microwave softened gelatin for 10 to 15 seconds on maximum power. Stir until the gelatin dissolves. Handy!

3. Rise bread dough

You can use your oven as a warm and moist incubator. Here’s how. Fill a Pyrex measuring cup with water (use a cup that can hold at least 500 ml or 2 cups). Place it at the very back of the oven and microwave it on maximum power until the water begins to boil. Place the bowl containing the bread dough in the oven with the measuring cup. Close the door and, without running the oven, let the dough rise until it has doubled in volume.

4. Melt chocolate

Chop the chocolate into same-size pieces. Microwave on medium power (50%) for 1 minute. Stir and microwave again, pausing every 30 seconds or so to stir, until the chocolate is melted. Don’t go by appearances: chocolate doesn’t lose its shape when melted in a microwave oven and can easily become overheated with the potential to cause burns if you don’t stop at regular intervals and check its progress by stirring.

5. Toast coconut

Spread shredded coconut on the bottom of a Pyrex baking dish. Microwave on maximum power, pausing every 30 seconds to stir, until the coconut is toasted, about 3 minutes in all. Fast and effective.

6. Make floating islands

One of our favourite microwave recipes!

7. Soften brown sugar that has hardened

Spread the hardened sugar on the bottom of a Pyrex baking dish. Lay a slice of bread or apple on the sugar and cover the dish with plastic wrap or parchment paper. Microwave on maximum power until the sugar softens, about 30 seconds.

8. Caramelize sugar

Put the sugar in a Pyrex bowl or cup and add just enough water to moisten the crystals (make sure the container is large enough to contain the syrup when it boils). Microwave on maximum power while watching through the window. Stop as soon as the sugar begins to change colour. Caramelization occurs very quickly, in only 1 or 2 minutes depending on the amount of sugar used.

9. Heat frothed milk for cappuccino

Start by frothing skim milk in a large Pyrex cup with an electric frother. Put the frothed milk in the oven and microwave on maximum power for 30 to 60 seconds (depending on the volume of milk) while looking through the window. When the foam begins to rise quickly like a soufflé, remove the cup from the oven and pour the milk into the coffee. The foam is surprisingly stable and will hold for several minutes.

10. Toast nuts.


Microwave-Toasted Nuts

  • Whole blanched almonds: 4 minutes
  • Slivered almonds: 4 minutes
  • Sliced almonds: 3 minutes
  • Walnuts: 2 1/2 minutes
  • Pecans: 3 minutes
  • Pine nuts: 3 minutes
  • Hazelnuts (filberts): 2 minutes
  • Spread the shelled nuts in a single layer on a microwave-safe dish. Microwave at high power, pausing and stirring every minute. Cooking time will be from 2 to 4 minutes depending on the type of nut and the power of the oven.

Christina Blais

For Christina Blais, explaining food chemistry to the masses is as simple as making a good omelet. Holding a Bachelor and Master degree in Nutrition, she has been a part-time lecturer for over 30 years in the Department of Nutrition at the Université de Montréal, where she teaches food science courses. She has been sharing the fruits of her experience with Ricardo since 2001, during his daily show broadcast on ICI Radio-Canada Télé. And diehards can also read her Food Chemistry on our website. You can follow her on Facebook at @Encuisineavecchristinablais.