As I posted in my October 13th blog entry, I've enrolled in an on-line course about the relationship between science and cooking. Still haven't had the chance to try any of the recipes, still trying to a get a better grasp of the science portion of the lessons. Not enough hours in the week, but no one ever said taking a Harvard on-line course would be easy.
Here's what went down on the second week.
- This week's guest lecturer is Dave Arnold. Week 2's focus is on energy, temperature, and heat, so naturally we begin with a recipe to carbonate a cocktail to show what happens when you mix ethanol and water. I knew I signed up for this class for a reason.
- Because they're scientists, the instructors like to make things quantitative, in the belief that when we start to make ideas quantitative, then we'll start to understand qualitatively better.
- The benefits of sous-vide cooking is then demonstrated in an example using eggs cooked at various temperatures in a very small window between 57 Celsius and 70. Accurately controlling the temperature can make a great deal of difference - something to keep in mind when you're trying to make the perfect Eggs Benedict or eggs on toast.
- Equation of the week: equals mc sub p delta t. (Or heat equals mass x specific heat capacity x change in temperature). You can use that equation to calculate how much energy you have to put into heating a cup of water.
- Start with water that's at room temperature, in this case, about 23 degrees Celsius. One cup of water is 237 grams. We are going to heat it to the boiling point, 100 degrees Celsius. The specific heat (c sub p). for water is 4.18 joules per gram degree Kelvin.
- How much energy do I have to dump in? Using the equation Q is equal to mc sub p delta t, put in that m is equal to 237 grams, that c sub p is equal to 4.18 joules per degree Kelvin, and that delta T is 77 degrees Celsius (100 degrees Celsius - 23 degrees Celsius, the final temperature minus the initial temperature) If we multiply all of these things out, we got 237 times 4.18 times 77. So that's how much energy it takes to boil a cup of water -76 280 joules.
- What does that mean? Think about it like this, how much wood would you have to burn in order to make this much energy? If you take the energy density of wood, 14 megajoules per kilogram, then you can calculate for yourself that the amount of wood that it takes is 76 280 joules divided by 14 megajoules per kilogram, which is 5.4 grams of wood.
- Just for fun!: Go look at the power output of your microwave, calculate how long does it take for you to generate the 76 280 joules needed to boil water. If you put water in a cup in your microwave for that amount of time, does it actually boil? If not, why not?
- NOTE: Most microwaves have a power output of about 400 watts.
The watts to joules calculation is the energy E in joules (J) is equal to the power P in watts (W), times the time period t in seconds (s): E(J) = P(W) × t(s). - I got 190.7 seconds, feel free to let me know if my math is off (scroll over to see).
- You really see how much you don't know about a topic when you answer all the week's pre-questions.
- The liquid nitrogen lecture reminds me of an experiment I've been meaning to try, and of a cool food truck I saw on an episode of Eat St. recently.
- Think of heat as the most used and the most mysterious ingredient in your kitchen --> Harold McGee
- Heat: the total energy that results in the temperature of a system
Temperature: the measure of energy in the motion of molecules in the material - Want a quick way to calculate the number of calories in what you're eating? Use the 449 rule.
- We end with a demonstration of the coffee-infused rum cocktail the cafe Touba being made, and some more fun with equations involving latent heat.
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