Look here, engineers and physicists, this is really simplified for home cooks, so don’t get too wrapped around the axle about the lack of precision here. And away we go –
As cooks we are concerned with heat. Cooking is by definition the preparation of food by the application of heat. Heat usually changes the food chemically and/or physically, so that it is safe and flavorful. Yes, we do speak of cooking in regard to some kitchen activity that does not require heat, but then we are just being sloppy with the language.
So, just what is this heat stuff? How does it work? How do we harness it for our taste buds?
We may regard heat as molecular action. The warmer the substance, the more excited and active the molecules are. Conversely, the cooler the substance, the less active the molecules are. Heat always flows downhill – from a warmer object to a cooler object in direct proportion to the temperature difference between the two, as modified by the efficiency of any insulation between the two. Second law of thermodynamics, if you care.
You do know that we can neither create nor destroy energy? We can, however, change form. That’s what we do when we change fuel into heat. We change the energy present in the fuel source to heat, which is just another form of energy. Back to the subject at hand –
Heat may be transferred by three methods:
1. Conduction, or direct touch. When you touch a hot stove and burn your finger it is by conduction.
2. Convection, or hot air (actually currents through a gas or liquid medium). When you feel the warm air coming from the supply registers of your furnace, this is convection.
3. Radiation. The sun’s energy travels to the earth through radiation. When you bask in the warmth of a fire, most of the heat transferred to you is radiant heat.
We cook with all three methods.
The skillet primarily uses conduction to transfer the heat to the food. This is especially enhanced by the use of fats in the skillet as the oil, grease, or whatever. Fat is a very efficient conductor of heat. Although I suppose one could also argue that the use of fat makes it into the convection category. (Sometimes it blurs a bit).
In baking convection is the primary heat transfer method. The air in the oven is heated and transfers to the food. There is some conduction as well as when a loaf is placed on a hot oven stone and the hot stone helps promote maximum oven spring before the top crust forms as well as crust formation on the bottom.
The main place that you see radiant heat in cooking is in broiling, where the heat source is above the food.
So, how come the top of the oven is hotter than the bottom? Heat rises. Lesser heat falls. Doesn’t cold fall? Not really, there is no such animal as cold unless all molecular activity has stopped – absolute 0. Somewhere around -460°F if memory still works. Cold is a nice human word to describe how we feel about it, but there is still heat there. Even when we’re shivering. Anyway, as a given material is heated the molecules become more excited and the material expands, which reduces the density when compared to the same material at a lesser temperature. Gravity works. The denser material flows below the less dense, and voila – heat rises. For the engineering/physics/curious types: Check out Boyle’s law, the ideal gas law, and all the other uglies that describe the heat/volume/pressure relationships on the net. This is why/how air conditioning, cooking, fire, heating, and most industrial stuff works and a significant chunk of our world depends on these little relationships.
Not to confuse things too much more. But I will anyway. There is a significant difference between heat amount and heat intensity. Intensity is easy – how hot is it? We measure in degrees, whether F (Fahrenheit), or C (Celsius [precise] or centigrade [sloppy]) or K (Kelvin), or whatever. We can feel intensity. Amount is measured another way. Here we use BTU, which is British Thermal Unit. A BTU is the amount of heat energy required to raise one pound of water one degree Fahrenheit. (The metric johnnies use the joule). Now this may be a little hard to grasp at first. A simple illustration with two questions should clear it up for you.
I have a one pound piece of steel. The temperature is 100°F.
I have a 10 pound piece of steel. The temperature is 80°F.
Which one is hotter?
Which one contains more heat?
The one pound at 100°F is hotter than the 10 pound at 80°F.
The 10 pound at 80°F contains more heat energy than the 1 pound at 100°F.
There you have the difference between amount and intensity.
It takes more energy to change state than it does to heat the same substance. A pound of water (liquid state) at 32°F. To boil that pound of water at sea level takes 1150 BTU of heat energy. About 84% of the heat energy is spent going from 212°F water (liquid state) to 212°F steam (gas state). There is also a large expansion in the volume that steam occupies vs. the same stuff in the water (liquid) state. That is why boilers can explode and what makes your teapot whistle.
There is a use for all this nonsense in the kitchen! We can never get higher than 212°F water at sea level. So 212°F is the hottest we can get food by boiling. Notice I said at sea level? We can get hotter than 212°F if we change the rules. If we get a pressure cooker, it changes the internal pressure as the water starts to boil! By raising the pressure we raise the boiling point, therefore the food cooks hotter, therefore faster. Conversely, if we try to boil water at high altitude, the boiling point is lower than 212°F and the cooking time is greatly extended. For instance, assuming that barometric pressure is 29.92 inches Hg, the boiling point of water at 1500 meters is 203°F. It gets worse as we go up the hill. I won’t even get into the whole barometric pressure routine, which does affect boiling point. We would also lose pressure as we went up said hill, but let’s not overcomplicate the mess. Main thing to note is that the guys at the top of the hill will probably need to extend boiling time of a given recipe (they already knew that). However – us lower down types need to know that we may need to reduce time a bit for a mountain recipe. Swiss, Carpathian, West Virginia or whatever recipes may need a bit of adjustment and careful watching the first time or two through them.
For those masochists that want more, there is a ton of good physics stuff out on the net. Search and ye shall find (with a good search engine).