Tag Archives: Cooking

Why is it not safe to put metal vessel in microwave oven?

We all have heard that one should not put metal inside a microwave oven if one needs to use the appliance for a long time. Some may have had the firsthand experience of the behavior of the oven when they accidentally put a fork or spoon in it.
Contrary to popular belief, it is not unsafe to put all metals in microwave ovens. People often put metal inside the device not thinking about it. The hot pockets is a perfect example of that. It does have a thin aluminum foil around it, which absorbs the microwaves and helps to cook it properly, though leaving it for too long would result in it catching fire. And the inside walls of the oven are made of metal itself. So why can’t we put a metal plate or something in the oven?
To get an answer, we first need to understand the working principle of the oven. They work by electromagnetic radiation. A device called Magnetron is used to produce high frequency microwaves from AC current. These waves vibrate inside the metal walls and get absorbed by the water content in the food. This makes the organic molecules in the food oscillate as well, creating the heat necessary to cook the food.
When a metal object is placed inside the oven, instead of absorbing, it would reflect the waves and they would bounce off the metal walls of the oven, which would result in an erratic oscillation of the waves and a lot of heat, ultimately damaging the device. Other way the metal can damage oven is when it has sharp ends, like fork. Since metals are conductive, electrons will accumulate at the edges and would create an electric arc between the metal and the magnetron, damaging the device. Now you know where those mini lightning bolts came from the last time you forgot to take the fork out.
However, now there are metal containers and plates specifically designed for using in microwave oven. Recently, scientists have deemed it safe to put metal in microwave oven as long as its mass doesn’t exceed 30% of the total mass and there is only one piece of metal, and of course with no pointy bits. Even then, it is generally unsafe to put metals in the oven for its longevity.

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How do induction cooktops work? Are they cheaper than gas?

In an induction stove there is a copper coil under the ceramic cooking plate, whereas a particular type of inductive vessel is placed on the plate. When alternating current (AC) is passed through the coil, oscillating magnetic field is formed. As per physical law of induction this “induces” electric current. The current that passes into the metallic vessel heats up the vessel due to resistance in the metal, and the vessel in turn cooks up the food in it. (See, diagram below.)

Comparing this to a gas stove, the flame of a stove wastes certain amount of thermal energy in heating up the air along with the vessel. During tests the most efficient induction cooker took 2.58 minutes to boil a specific quantity of water, whereas the same quantity of water took 5.36 minutes to reach the boiling point on a gas stove. Thus, induction cooker may very well turn out to be a cost saver.

Why does a popcorn kernel “pop” when it is roasted?

There are six main kinds of corn such as flour corn and sweet corn, but only pop corn would pop when subjected to proper heat.

A pop corn kernel is sort of a sealed container the outer coating of which encloses a layer of hard starch as well as core of soft moisture-laden starch. The amount of moisture is 13.5% to 14%, considered optimum for popping. When the kernel is heated within the range of 200◦ to 230◦ Celsius, the moisture turns into so much steam that not only the hard starch gets cooked but the kernel explodes. It virtually turns inside out, so what you see on the outer surface of pop corn is the hard starch. Popping is essentially the action of turning inside out and that is what creates a puffed up flake out of the hard starch.

Additional reading:
Popcorn (Wikipedia)

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What technique is used in microwave ovens to cook food in minutes?

Microwave ovens have provided the mankind with the whole new way of cooking, quite unlike the conventional method followed since thousands of years. When cooking is done in a customary manner, the heat that comes from a conventional gas stove is transferred to food by conduction. The heat is passed first to the food’s outer layers and then progressively to its inner ones. But microwave ovens produce heat directly inside the food, heating it more evenly and more quickly than gas stove.

Let’s see how this works. Inside the microwave oven, a device called magnetron generates the microwaves which are fields of electrical and magnetic energy. These are directed by an antenna to a hollow tube called waveguide. The tube in turn channels the waves to a fanlike stirrer that disperses them around the oven’s interior. Finally, the waves reflect off the oven’s walls and are absorbed by water molecules in the food.

It is the water that plays key role in cooking. Each molecule of water has a slight positive charge at one end and a slight negative charge at the other. The molecules lie randomly in the food. But as they absorb the microwaves, they align themselves with the waves’ electric field. The electric field oscillates billions of times a second, constantly agitating the water molecules and forcing them to turn round 180° as many times.

In other words, as the microwaves oscillate, so do the molecules. This rapid molecular agitation generates heat and cooks the food. If the food is very thick, the microwaves will not penetrate completely and the food’s inner layers will cook by conduction. Since microwaves don’t produce heat unless they are absorbed by water molecules, many materials such as glass, paper and most plastics can be used safely as cookware.

More reading:
Microwave oven (Wikipedia)

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Why does food cook quicker in a pressure cooker?

Apart from pressure, water and steam also have their roles to play. Ultimately, though, it is the pressure that matters. Here’s what results in quicker cooking. Ordinarily, water boils at 100° Celsius. The temperature rarely rises higher because escaping steam carries excess heat away. If a container of boiling water is sealed, however, the steam is trapped and pressure starts to rise. This force pushes down on the surface of the liquid, preventing water molecules from breaking apart from one another to create more steam. Thus, far less heat is dispersed from the container and, eventually, from the food.
The result of all this is the temperature of the water greatly exceeds 100° Celsius, cooking the food immersed in it much faster and helping to get dinner on the table sooner.

Additional Reading:
Pressure cooking (Wikipedia)

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Why sugar dissolves faster in hot water than in cold water?

Each molecule of water is made of 2 atoms of hydrogen and 1 atom of oxygen. Sugar can dissolve in water only when its molecules can get some space between two molecules (H2O) of water. This is difficult when water is cold and the molecules of H2O are tightly packed with very little space left between the nearest molecules. Vigorous stirring is required in order to dissolve sugar in cold water. Stirring water with a spoon forces molecules of H2O to scatter. This creates some room between H2O molecules, where sugar molecules fit.
When water is heated, its molecules move farther apart. The space between two molecules becomes larger, allowing molecules of sugar to fill the gaps easily and quickly.

More reading:
Sugar (Wikipedia)

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