Let’s Talk Heat Sources! – Come on Please!

In light of the cookware article I posted recently, some questions have come up regarding heat sources. Repeatedly, I referenced induction cooktops and many are unfamiliar with the system.

Everyone understands the most basic heat source….FIRE!. Humans have been using it for thousands of years. And of course, the first methods were most likely roasting directly over or very near to an open fire. We know them today as direct and indirect cooking. The direct method would have been holding the food to be cooked on stick over the food, either by hand or on a rudimentary spit rod. Indirect cooking would have involved putting the food to be cooked on a plank or other support and placing the food very close to the fire for a slower roasting.

The next logical cooking methods involved putting some form of material between the food to be cooked and the fire or heat source. Most likely, this came in the form of various stone slabs and this brings us to our first term to be discussed…conduction. We will discuss several in this article – conduction, convection, induction, radiation and microwave energy.

Conduction heat transfers the heat from the heat source directly to the cooking utensil. In the days of old, this would have been our 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. The heat 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 or cooktop. These heat sources are natural gas and electric.

Convection heat method requires 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 (I will cover this subject in an upcoming post).

In the oven, air heated by some means circulates over food and cooks it. The most basic method involves either natural gas flame or electric coils cause heat to rise from bottom to top. In this simplest of methods, the air cools as it rises. Most modern ovens or ranges have some form of convection fan placed in the back or sides and it circulates the heated air around the oven space. Some now have what are called 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 oven setting. They can reduce cooking times and are not always the best setting to use – follow the manufacturers suggestions until you are comfortable with the machine. 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. Sous vide is simply a form of convection. Warmed water circulates around a sealed food for a very long time, sometimes well over 24 hours, slowly cooking it.

Induction heat is a form of heat whereby the heat 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 used must have magnetic properties. The best thing about this method is its energy efficiency. The only heat generated is in the pot or pan and the surface and surrounding area remain relatively cool. The induction style is so efficient since almost no heat or energy is wasted beyond the edge of the pan and it stops when the pan is removed. I have unplugged a portable induction cooktop and picked it up immediately and wasn’t burned. The efficient exchange of energy can 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. This method produces heat waves. These heat waves are radiated to the food mass and penetrate the food, heating it. In a heated oven, over half the heat is radiated energy. The food 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 baking or cooking the food. This is why breads or crispy foods don’t cook well in the microwave (however 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 maker may have intended it for simple reheating not extended cooking. Melting or cooking foods like butter, oils, or sugar have been known to push a questionable dish over the top. Always ask the manufacturer if it is microwave safe or usable for cooking.