Week 6: Heat Transfer, Harvard Style

Molten Chocolate Lava Cake from the Week 6 Lab, a tasty view of how heat moves from the outside to the inside.

The first note I wrote this week: Heat moves by random walk. Then I looked at the Harvard food science online review sheet, which begins differently: "Heat is transferred between molecules through random collisions, resulting in the diffusion of heat via random walks similar to those discussed previously." Er, right. I'm sure I wrote that somewhere.

Why not just say, "When heat moves, worlds collide?" Heat's effect on food is the reason someone invented the toaster oven, the waffle iron, the George Foreman grill, microwave, pressure cooker and sous-vide wand, you name it.

We're now halfway through the Harvard online course and diving into how heat moves through food. Time and temperature have always been an issue in my macarons class, because macarons crave precision. Bake at 325 degrees for exactly 13 minutes and nobody gets hurt. Under or over, and we've got a tray of head for the back door. 

When I worked the ovens at the gourmet-to-go shop Eatzi's, I shoved volumes of dough into ovens that ranged from hot to hotter and could eyeball how many more or less minutes were needed for a happy ending. Some chefs I know set their ovens at 400 and just bake everything at that temperature because it's easy to remember. How heat moves from the outside into the food, and for how long, is the stuff of Thanksgiving nightmares as you think of Chevy Chase carving into a deliciously golden brown bird in the film "Christmas Vacation" and it collapses.

A cool way to learn about heat transfer is with chocolate - what couldn't we learn from chocolate? There's a lab involving a molten chocolate lava cake, it's delicious and I wrote about the method - using a hot water bath to cook the cake just so, instead of blasting it in a hot oven and hoping for the best.

Now here's where I carved a star on my notes page. I'd written: "In order to properly cook something, it's not enough to cook inside to a target temperature (such as 165 for that Christmas turkey). The outside has another target, and it's much higher - higher than the boiling point of water. And the inside target is much below the boiling point of water. You have to try to achieve both heating protocals at the same time."

From here, we delved into browning reactions - and it makes sense. I've had guests ask in the croissants class why the croissant isn't golden brown and crunchy on the inside like it is on the outside. The previous paragraph tells you - the outside of the croissant browns because it exceeds a temp above the boiling point of water, while the inside stays soft because it remains below the boiling point. A-ha!

Remember how Julia Child told us to dry the meat before trying to brown it? That's because it wouldn't brown if it was wet. For browning to occur, water on the edge of the steak must evaporate and the external temperature has to reach the boiling point of water for the browning reaction known as Maillard to occur.

To learn more about browning as a factor of heat transfer, we also get a terrific lesson in french fries. Read on. But first, here comes you know what.

Equation of the Week: L = the square root of 4DT where D is the diffusion constant and T is the time (left over from last week but still good). With it, you can estimate cooking time for a particular food.

A-ha Lesson of the Week: The path to perfect french fries. Throw potatoes into room temp oil, then turn up the heat. This mimics the process of boiling water and allows heat to diffuse through the inside without burning the outside. Traditionally, potatoes are fried twice - once at a lower temp to cook the interior, and then again at a high temp to brown and crisp the outside. As Dan Souza of America's Test Kitchen explains, by going with room temp oil and raising the heat, the potatoes cook in one step, not two. As the oil warms, the interior cooks through and water begins to escape from the potato, just as it would in the traditional first fry. As the oil continues to heat, the fries begin to crisp and brown. This part of the process mimics the second high-heat fry, Souza says. Instead of tossing fries in the hot oil and retrieving them twice, just start with cool oil and remove the fries once when they're done.

Try this at home: Fry those potatoes. Dan likes to use Yukon Golds, less starchy than russets. Cut the potatoes into sticks. Dan chooses peanut oil, which produces light and crispy fried foods, he says. Use about 6 cups of room temp oil. Add potatoes and turn heat to high. Cook for about 5 minutes. The oil will mimic boiling water, so let the fries cook until they begin to firm, about 15 minutes. Don't stir. Don't panic. These beauties are soft at this point and easily broken. After the 15 minutes, go ahead with tongs and gently stir the fries around. Give them about 5 or 10 more minutes until golden crisp. Using a wire skimmer, remove to a wire rack. While still hot, season with kosher salt.

Delicious, this science!

Next week: Viscosity and Polymers