Energy, Temperature and Heat: Science and Cooking
What is cooking?
Oxford English Dictionary, states that cooking is to make food fit for eating by due application of heat, by boiling, baking, roasting, et cetera. This is a rather narrow definition because you can cook something without applying heat to heat. Eggs can be cooked without changing the pH of its environment, like century eggs. Dishes like Ceviche, where you mix raw seafood and let it marinade in vinegar (acid) overnight is a method of curing. I guess when I make kimchi, I ferment my nappa cabbage for a week or so and that is also a method of cooking without heat.
Making the perfect egg
I have never really understood the importance of eggs in my life till I did this module. I’ve never been a fan of eating eggs but this has been something else all together. Making eggs is incredibly scientific! Just a change of 1 degree Celsius can change the texture of the egg by crazy. Eggs literally can be a thermometer but in a small window of between 57 to 70 degrees. A 13 degree window. And you can figure out the changes by every degree in an egg through an immersion circulator that keeps water accurately heated, low temperature cooking through such instruments is perhaps the greatest cooking invention in the past 200 years. Okay, so with a boiled egg:
57 degrees: An egg is raw
60 degrees: It’s barely set, egg white is made of many proteins so this heat only set some but not all.
62 degrees: We hit the first real egg, basically the perfect egg for Eggs Benedict. It’s a little custard and the egg whites are thick and thin and when you cut it the yolk is running.
63 degrees: This egg is the in between, the yolk is not exactly set and is liquid but will run out.
64 degrees: This is full set yolk, still soft and custardy
65 degrees: The egg becomes more like play dough
66 degrees: The egg becomes like marzipan, yet a little creamy.
67 degrees: Begins to loose creaminess and becomes crystalline in nature.
70 degrees: Hard-Boiled egg
Chemical Energy and Combustion
Heat is an important factor when it comes to chemical reactions. The place where you see energy in food is on nutritional labels (calories) and this energy can be converted into heat. A calorie is a unit of energy and it refers to chemical energy released by food after consumption. When we eat food, we use oxygen from our lungs to burn the food into water and carbon dioxide much like how we burn wood in a fire to release energy in the form of heat. Our body keeps us warm when energy is release by the food we eat. When we eat too much we store the energy we don’t need later as fat.
Cooking with Heat: Wood and Ovens
When we burn fuel and use it to create heat, the heat transforms food from being uncooked to cook. Wood or any full has energy density, that is energy per unit mass- of how much energy can be liberated by burning the fuel.
The energy density of wood is 14 megajoules per kg. That’s 14 million joules per kilogram. Joules is an unit of energy.
Charcoal has a higher density than wood. It is 30 megajoules per kilogram. This explains why if you need less of it for the same amount of food since it is a much higher energy density.
Another way to cook food is to use an oven. Oven are of two types:
electrical ovens that have hot coils which heat the air
microwave ovens which uses radiation and heats the food
The energy reported in ovens are called watts and is the unit of power, and units of watts are joules per second. If I take me leftover Karela sabzi and put it in the oven for 10 minutes. Every minute has 60 seconds, thats 600 seconds. If my oven has 1000 watts that means that the total energy I can dump on the food is 1000 Watts * 600 seconds, which is 600,000J which less than what we would have in a kilogram of wood.
But when I’m heating my sabzi what am I doing to it. What is this heat? When I heat my food, the heat is the total energy that results in the temperature of the system. Higher temperature means more heat. So at a molecular level, temperature is a measure of the motion of the molecules within a material. Molecules therefore jiggle around when it is heated. When I hear the sizzle it is jiggling faster. That is what temperature is. This jiggling of the molecules inside my sabzi causes the material to change and become softer and edible.
Specific Heat Equation
Temperature is the measure of motion the molecules within a matter. Heat is the total energy that results in the temperature.
But different molecules can have different energies based on their chemical nature. How much energy has to be given to food to heat it up. Heat can be measured by:
Q: amount of heat, measured in Joules
m: mass of the food (Kilograms)
∆T: final temperature of the food minus the initial temperature, measured in Celsius
cp: is the number we call the specific heat of a material. It is the number that characterises how much something heats up when you apply a specific amount of heat.
The specific heat of water is 4.18J/gram per degree Kelvin.
The specific heat of eggs is 3.18J/gram per degree Kelvin.
For milk it is 3.85.
For beef it is between 2 and a 1/2 and 3 and a 1/2 joules per gram degree Kelvin.
For air, the specific heat of air is 1.
The specific heat of olive oil is about 1.97 joules per gram per degree Kelvin.
If we apply the equation, it takes 76,280 J to boil a cup of water (237g). So I want to calculate the energy that it takes to boil one cup of water. If we were to use wood to boil water it would take us 5.4grams of wood to create this much energy. (Energy density of wood is 14 mega joules.)
Cooking the Perfect Egg:
This is the best thing I have ever seen being done. My small brain got a little confused but I’ll try to simplify what I understood.
How much wood does it take to cook an egg?
We can do it with Q= mcp∆T equation.
But we need the specific heat of an egg and the mass. The perfect temperature of an egg is 64 degrees. So we use the initial temperature 23 degrees and heat it till 64.
∆T= 64-23=41 degree Celcius
Specific heat of an egg is 3.18J/gram degree kelvin
the mass of our egg is going to be 35g
Q= mcp∆T =35*3.18*41= 4,563 Joules this is the energy to cook a perfect egg.
In contrast, the energy that’s stored in a cup of boiling water is 76,280 Joules. That’s way sufficient to cook a perfect egg. However, its not highly accurate because if we put an egg in boiling water there is some energy that is lost during the process. The heat that you would take from the boiling water is that heat that we are adding to the egg. This is a principle called the principle of conservation of heat.
If we were to take an uncooked egg and pit it in some volume of boiling water, so according to your equation Qegg, the heat thats uptake by the egg, is the mass of the egg, times the specific heat of the egg, times the final temperature minus 23 degrees. Thats the heat that is uptake by the egg. That heat must belong from somewhere that is the boiling water, that starts at 100 degrees and it cools down to some temperature. its going to cool down to whatever the final temeperature is because its only when the the water and egg are the same temperature so there wont be any energy transfer between them. If we were to calculate this then is what we would have to go for. I think this is as far as I can to understanding this…
What is a Calorie?
The “Calories per Serving” is actually a representation of the energy density of a food product based on the combustion reactions of the fats, proteins, and carbohydrates that make it up. This is perhaps more apparent if you look at a nutrition label from a European country, which uses Joules instead of Calories on their packaging.
The definition of a calorie is that it’s the energy that it takes to heat up one liter of water by one degree Celsius.
The 4:4:9 Rule:
When we compare the nutritional labels of food, pasta can have many grams of carbs and proteins but very little fat. Nuts can have more fat and protein however the serving of 28 grams of peanuts have the same amount of calories. The main reason for is the the food molecules store different amounts of energy per gram.The approximate Calorie count per gram is known as the 4-4-9 rule. This describes how much energy is stored in carbs, proteins, and fats.
Total Calories= 4* Carbohydrates + 4*Proteins + 9*fat
We can also cook by freezing, this is what we do when we make ice-cream. A popular cooling agent is Liquid Nitrogen. It is really cold around 196 degrees. Its not such a powerful cooler though, but it can get things much colder than ice and you can stick your hands into it and have particles roll off your skin that insulates you preventing burns. This is called the Leidenfrost effect.