As I posted in my October 13th blog entry, I've enrolled in an on-line course about the relationship between science and cooking. I'm still plugging away at this, good thing I decided to do this for the knowledge.
- This week's session is about the transfer of heat in cooking food.
- Heat is described as a difference in temperatures. Something feels hot because your hand is colder than the thing you touched. When you touch the hot object, energy in the form of heat is transferred from the object to your hand because of the difference in temperatures
- As any cook can tell you, it's tough to make things by applying heat in order to make it taste good. The reason is that in order to properly cook something, it is not sufficient to cook the inside of the thing you're trying to cook to a target temperature. It turns out you also have to have the outside hit another target temperature. And that target temperature for the outside is higher than the target temperature of the inside, higher even than the boiling point of water. So to perfectly cook a meatloaf, or vegetables, or whatever you want to eat, you need to get the outside of the food a temperature higher than the boiling point of water, while getting the inside to a temperature below the boiling point of water.
- Why is it necessary to cook the outside of a food to a temperature that's higher than the boiling point of water? It's because it's needed to make a very important set of chemical reactions, called the browning reactions. Not surprisingly, these are the reactions that tend to take food and make it have a brown colour. Called the Maillard reactions, after the chemist Louis Maillard who discovered and described them in the 19th century, the way these reactions work is that they are a reaction between carbohydrate molecules, like a sugar, and a protein, which could be a single amino acid. At high temperatures, the combination of a carbohydrate molecule and a protein will lead to hundreds of small molecule by-products. Some of these by-products are colour compounds. They tend to be brown, which is the reason food gets that brownish colour when it cooks.
- This is why when you take a piece of fish, seal it in plastic, and cook it sous-vide style (that is, cook it in a constant temperature heat bath), there won't be any flavour molecules produced because the temperature is well below that of the Maillard reactions. Your fish will have a perfect texture, but it would taste quite bland.
- If we put a piece of meat or a piece of fish into an oven at a temperature much higher than 100 degrees, the centre of it, where there's lots of water, can't get higher than 100 degrees, because it can't go above the boiling point of water. The only way that it can actually go above that temperature is if you actually boil off some of the water in whatever it is it you’re cooking, so there's a thin layer around the outside where it's actually dried out. Once it's dried out, in that dried out layer, in that thin layer which is dry, you could push the temperature above 100 degrees Celsius, and you can start to get close to the temperature that's needed for browning reactions. That's the reason really that there's a thin crust of brown around food when you cook it because these temperatures can only be hit in the part of the food where it's actually dried out.
- Carme Ruscalleda, chef and owner of restaurant Sant Pau, demonstrates her method of cooking a steak.
- Nathan Myhrvold shares his secret to cooking the perfect burger; he uses liquid nitrogen. For those of us who don't have access to such chemicals, Heston Blumenthal has a more accessible way to cook meat.
- Michael Brenner and Daniel Rosenburg ttalk about the physics of how heat is transferred to food.
- Microscopically, the air around the food being cooked is hot, meaning the molecules are whizzing around like mad, causing the molecules in the food to start whizzing around like mad, and eventually, they all get hot.
- Heat diffuses into food like a random walk, which brings us to the Equation of the Week,
L=4Dt−−−−√ or L is equal to the square root of 4 times D times t, where L is the length of the food, D is the diffusion constant that's governing the random walk and t is time. Scientists call it the heat diffusion concept. - Different foods have different heat diffusion constants. Knowing this and the above equation allows you to estimate how long it takes to cook different foods. I would still recommend using a kitchen timer, however.
- The week's lecture ends with a demonstration of how to make a better French fry, a lab assignment consisting of making a molten chocolate cake, and a homework assignment of making fried ice cream balls. I'll post my attempts of making at least one of these recipes in the future.
No comments:
Post a Comment