Cooking and Thermodynamics

The zeroth law of Themodynamics states that When two systems are put in contact with each other, there will be a net exchange of energy between them unless or until they are in thermal equilibrium, that is they contain the same amount of thermal energy for a given volume (say, 1 cubic centimetre, or 1 cubic inch.) - wikipedia.

Well the above law was cited to appear a little nerdy and later half of this blog will prove that when it comes to practicals my mother knows more about thermodynamics than me... and of course she cooks better.

Cooking is a great art but art is more subjective than objective. I say this because the complexities and factors involved in making tea or boiling an egg are far more than any thermodynamics experiment i have conducted. Don't ask me the complexity of making Mutton Rogan Josh. In this post i would like to give cooking a little objective look, so nerds like me can obtain the pleasure of live demostration of laws of thermodynamics and if they have any fears of cooking, they can extirpate them.

Cooking involves not just heating raw food, but also mixing of n number of substances like salt, water, meat, vegetables, spices, juices etc. Lets first take up heating and learn some basic thermodynamics. The outcome of zeroth law of thermodynamics can be that heat flows from a hotter to a cooler body, if they are brought in contact. This heat transfer will stop when temperatures of both bodies are in equilibrium. Heat Transfer can occur in 3 different ways - Conduction, Convection and Radiation. Out of these the most common way is Conduction, which involves two bodies (generally solid) coming in direct contact to transfer heat. Convection, which is made common by some Microwave ovens, involves transfer of heat through and with the help of fluids (air, liquid). Radiation, doesn't require any medium, this is the way used by Sun to heat up the earth.

Now let us look at the Fourier's law (nerdy for Law of Heat Conduction) which says that the rate of heat transfer through a body in contact with a heat source will depend on Heat Resistivity (good/bad conductor of heat), Surface Area of the body in contact with the heat source, temperature difference between the bodies and the thickness of the body.
So what does it mean in the cooking language -

• Heat Resistivity - Why does it take more time to cook mutton than to cook chicken, because the resistivity of mutton is less than that of chicken. A more direct example will be that of wood and metal. Metal has greater resistivity than wood. So wood will take much more time to heat up than metals. Next time you are cooking keep in mind the resistivity of your raw material.

• Surface Area in contact with Heat Source - The greater the area in contact the more the time it will take to transfer heat. So if you use a flat bottom utensil to heat, you know its going to take more time, but remember your food will be more evenly cooked. Thats why it is said that if you cut the meat/vegetable in proper shape, they will cook well. As the transfer of heat is even through them.

• Temperature Difference - Well this one is quite intuitive isn't it. But don't forget Newtons Law of Cooling (Newton seems to be everywhere from gravity to thermodynamics...i wonder if he was a good chef as well) - it says that the greater the temperature difference between two bodies the faster heat transfer will take place, so if the temperature difference was 30 degrees initially - Newton says that it is going to take lesser time to bring the temperature difference to 20 degrees than from 20 degrees to 10 degrees. It means if you took something out the refrigerator then it will not exactly take twice as much time to heat up...but it will take around 1.5 as much time or so.

• Thickness - Now when heat will exactly reach a point on your vegetable depends on how far is it from the heat source. The farther the distance the more time it is going to take.

Okay much of thermodynamics - now lets come to serious cooking. When you are cooking something you want it to be evenly cooked, you wont like your vegetables to be left raw from inside. It is important to understand that if your cooking something at a very high temperature, then its resistivity is going to decrease with time (this is what is called burning) and this will further slow down the flow of heat inside. Your outer surface will burn and will not transfer any heat inside and hence your food wont get cooked from inside. In general if you want to cook, then initially cook it up on slow heat (low temperature) so that resistivity of the substance doesn't breaks down and heat will get time to reach deep inside. And if you want to make it crispier (i.e. burn it from outside) then just turn up the heat (increase the temperature) for a short period of time.

Well as i said cooking is a great art and it involves not just heating but also mixing up so many other things, and doing them at the right time. The best way to train your mind to such a complex art it go out and explore and experiment. Science is good but taste is heavenly :)
Here is some Dilbert for you...trying to cook like an engineer :)