In light of a cookware article I posted recently, some questions have come up regarding heat sources. In that article I repeatedly referenced induction cooktops and it seems more people than I thought are unfamiliar with the system.
What I hope to achieve with this article is to explain the various heating/cooking methods available to us and thereby help us match our cooking utensils to the correct heat source. We will discuss several heating types in this article – conduction, convection, induction, radiation and microwave energy.
Everyone understands the most basic heat source – fire. Humans have been using it for thousands of years. And of course, the first methods most likely involved roasting food directly over or very near to an open fire. Today we would describe this as direct and indirect cooking.
The direct method of cooking would have required the cook to hold the food on stick directly over the fire, either by hand or on a rudimentary spit rod.
Indirect cooking involved putting the food to be cooked on a plank or other support then placing the food very close to the fire for a slower indirect roasting.
The next logical evolution of cooking methods involved placing some form of material between the food to be cooked and the fire or heat source. Most likely this came in the form of stone slabs, which brings us to our first term to be discussed…conduction.
Conduction heat transfers the energy from the heat source directly to the cooking utensil. In the days of old, this might have been the aforementioned stone slab. Today, this involves a cooking pan of some kind, usually metal (see my cookware post). The heat spreads across the bottom and is conducted up the sides of the pan. It is transferred directly to the food as the pan or cooking device heats. For conduction to take place, there must be some direct contact between the heat source and the cookware.
Conduction heating occurs on top of the stove, range, grill or cooktop. These heat sources are direct fire, natural gas and electric.
Convection heat methods require movement of air or liquids around the food using convection currents. Typically, one thinks of convection cooking as being limited to the oven, but this is not true. For example, in a saucepan the fluids or food is first heated by conduction, then the liquids begin to move around the food. There is also air convection occurring in the empty space above the food. This is why it is important to never fill your saucepans too full when cooking.
In the oven, heated air circulates over and around food and cooks it. The most basic method involves either natural gas flame or electric coils which cause heat to rise from bottom to top. In this most basic of oven heating methods, the air cools as it rises. Most modern ovens or ranges are now equipped with some form of convection fan placed in the back or sides which circulates the heated air around the oven space. Some ovens now come what are called aerobake, pure or true convection (this term will vary from manufacturer to manufacturer), which simply means the air from the oven is heated by a secondary heating element before being blown back into the oven.
A word of caution: these mechanisms can cause the oven to actually be hotter than is indicated on the display setting. This increased heat can reduce cooking times and is not always the best setting to use – follow the manufacturers suggestions until you are comfortable with the equipment. To offset the “over heating” issue, use an oven thermometer to get a correct reading and adjust the setting accordingly to achieve the intended temperature.
You may also have heard of a “newer” cooking method in culinary circles called Sous Vide. Heated water circulates around a sealed food for a very long time, sometimes over 24 hours, cooking slowly. For all the fuss surrounding it, Sous Vide is simply a form of convection cooking.
Induction cooking is a form of heat whereby the energy is induced into the cookware, not transferred to it by the cooktop. An electromagnetic coil beneath the ceramic cooking surface creates a magnetic field. This magnetic field passes through the cooking surface to ferrous (iron or steel) cookware, heating the pan and cooking the contents. As mentioned in my cookware article and as may be evident to you, the cookware metal must have magnetic properties.
The best thing about this method is its safety, energy and overall cooking efficiency. The only heat generated is in the pot or pan while the surface and surrounding area remain relatively cool. I have unplugged a portable induction cooktop, picked it up immediately and wasn’t burned. The induction style is able to be efficient because almost no heat or energy is wasted beyond the edge of the pan and it stops when the pan is removed. The efficient exchange of energy can actually speed the boiling of water over more traditional methods.
Radiation heat is a method of transfer using electromagnetic waves. Radiation does not require direct contact with a heat source, liquids or air. Like sound and light, radiation is emitted in waves.
Radiation heat is most commonly found in a broiler or an oven. These heat waves are radiated to the food mass and penetrate the food, cooking it. In a heated oven, over half the heat is radiated energy. The food placed in a heated oven is baked by a combination of convection, radiation and conduction.
Microwave heat is transmitted electromagnetic waves. Microwaves are very short in length and are produced by a special generator in the “oven” called a magnetron. These waves are distributed by a stirrer or, in some ovens, the food rotates on a revolving plate or shelf.
Microwaves are sources of energy, not heat. When the waves are absorbed by the water molecules within the food, the energy is transformed into heat thereby baking or cooking the food. This is why breads or crispy foods don’t cook well in the microwave. The heated water molecules initially soften the dough while warm, but then quickly evaporate causing them to be really tough (some manufacturers have devised utensils to crisp certain dough based foods).
The microwaves are reflected from all angles inside the metal walls of the oven, penetrating the food and passing through the cooking dish. Metallic pans or elements, reflect the waves away from the food, therefore one should use only glass, ceramic or plastic for baking dishes.
Note: not all plastic or glass is suited for use in a microwave; always look on the bottom of the dish for a microwave symbol indicating it has been tested for proper heat tolerances. When selecting a cooking dish, remember that even if a dish is marked as microwave safe, the manufacturer may have intended it for simple reheating not extended cooking. For example, melting or cooking foods such as butter, oils, or sugar have been known to push a “questionable” dish over the top and crack or break it. Always ask the manufacturer if their products are microwave safe or usable for cooking.
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