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The science of microwave cooking

Microwave ovens can be used to dehydrate herbs, turn butter into ‘ghee’ and cook ‘pulao’. And they don’t cause cancer

Illustrated by Krish Ashok.
Illustrated by Krish Ashok.

It was when I was a teenager that my mother approved the purchase of this newfangled device that she had heard fried papad without oil. It was this use case and not the more general “it reheats food” feature that was the clinching factor in the purchase of our first microwave oven. With three boys in the house and a strong preference for food that had textural diversity, the presence of deep-fried, crisp things at every meal was leading to alarm bells ringing from a budgeting standpoint. The monthly consumption of oil, worries about cholesterol, and a wariness towards reusing frying oil forced an openness to the introduction of unfamiliar new electronics into what was largely a rather traditional kitchen back then.

But for us, the children, it was a disaster. Microwaved papad is among the saddest things one can eat. It tends to be drier than the surface of the moon and uncooked in a few places no matter what you do. When a papad is fried in oil, the spice flavour molecules dissolve in the hot fat and get released when you chew on it. A microwaved papad loses most of its flavour well before you eat it.

This was over two decades ago. Fast forward to now and a microwave oven is more or less a standard fixture in most middle-class homes. Indians, however, continue to be among the most sceptical users of what I think is a tragically underutilised device that features some of the most stunningly magical engineering you will encounter in a home appliance. The misinformation universe that clouds this device is pretty dense. Many believe that its “harmful radiation” that irradiates your food and the rest simply use it for nothing more than reheating food.

So, let me first start by showing you some of the engineering magic that goes into making a microwave oven. Microwaves, like visible light or radio waves, are part of the electromagnetic spectrum. The WhatsApp-Industrial-Misinformation complex tends to compare it to cancer-causing X-rays and ultraviolet rays. It turns out that microwaves have less energy than visible light, meaning that my eye-rolling stare at people afraid of microwave ovens has more energy than the actual microwave. So, you might wonder how on earth it cooks food, especially since we are pretty certain that staring at food with our eyes does not seem to cook it.

This is where quantum physics comes into play. Water molecules in food, which tend to be electrically polarised (meaning they have positively and negatively charged ends) can be flipped by microwaves that have a specific amount of energy. So, a device called a magnetron inside the oven generates microwaves at precisely the frequency required for it to have just the right amount of energy to flip water molecules. And the oven keeps switching the direction of the waves so that the water molecules keep flipping back and forth. When molecules keep flipping like this, they heat up. And that is precisely how your food gets cooked in a microwave oven. The rays heat up water inside your food, and do absolutely nothing else, and the hot water cooks your food. So, for starters, you can’t cook anything that does not have enough moisture in it.

This is why you can’t bake a cake in a microwave. The temperatures needed to cause the Maillard reaction that produces amazing flavours and brown colouring are well above 100 degrees Celsius, and since a microwave oven works by heating water inside food, the temperature will never go above 100 degrees—for once all the water evaporates, the microwaves just pass through the food and don’t have enough energy to cook it.

One feature many users tend to be ignorant of is the power setting. Most ovens will allow you to set how much power (in watts) is used when cooking your food. This is extremely useful because you can use the lowest setting to melt butter to a perfectly spreadable consistency (the default setting, which is high, will almost always cause all the water in the butter to evaporate and turn it into ghee). The middle power setting is great for warming water to just the right drinkable lukewarm temperature if you have a bad throat.

A few microwave hacks—you can dehydrate fresh herbs that are highly perishable. Curry leaves and coriander can be dried and powdered by microwaving them. You can also cook a simple pulao for one (larger quantities will not work here) by adding washed rice, water, ghee, salt and spices to a microwave safe vessel and first cook it for 10 minutes at high power and then 15 minutes at low power. What happens here is that starches gelatinise (cook) at around 70 degrees Celsius in the first 10 minutes and in the next 15 minutes, the cooked rice starches retrograde to form nicely separated, non-mushy grains. You can then squeeze some lime juice and satisfy a late-night rice craving.

And oh, papads in the microwave? They still continue to be sad, disappointing things. You can, however, avoid the uncooked spots by curling them into a glass cup and then microwaving it. It’s still disappointing, but it’s better than having no papad.

Illustrated by Krish Ashok.
Illustrated by Krish Ashok.

Krish Ashok is the author of Masala Lab: The Science Of Indian Cooking.

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