Archive for the ‘Kitchen’ Category

Physics in the Kitchen

Thursday, June 4th, 2009


Chocolate SouffleNot all science is done in labs. In fact, most of us are scientists around our house every day without even realizing it. Step into the kitchen for example. You don’t have to channel Rachael Ray if you want to be Mrs. Wizard — even the most simple culinary tasks are chock-full of science! The academics call this Molecular Gastronomy. But I call it Chemistry in the Kitchen.


Let’s say you want to bake a cake. Even if you use mix out of a box, guess what, youre running a number of science experiments! Step 1: As you add the water, egg and oil to the powdered mix you are creating what is called an emulsion — though you call it batter. Other examples of emulsions in the kitchen are mayonnaise and butter. An emulsion is a stable combination of two liquids that normally do not mix. Oil and water are famously difficult to combine, as you probably already know from the phrase “they’re like oil and water” or from endlessly shaking up your salad dressing. But, while salad dressing separates into two layers unless constantly shaken, the oil and water you add to your cake mix form a batter. Why? Well the secret ingredient — the emulsifier — is the egg.


Egg yolk contains molecules called lecithins. These molecules are rod-shaped and each end has a different property — kind of like a magnet. One end of the magnet is hydrophilic — it attracts water — while the other is hydrophobic — it repels water. The lecithins in the egg yolk pull the batter together: hydrophobic ends grab the oil; hydrophilic sides grab the water. The emulsifying egg coaxes these two stubborn liquids together into a stable batter. And there you have it: oil and water together at last, with the help of one little egg.


Ok, Step 2: time to mix. The directions on the box tell you to beat your batter for 2-3 minutes after the ingredients are combined, much longer than you might thinkWhy? Well, mixing is one form of leavening  — the process that changes your cake from a dense batter to the light and fluffy treat that comes out of the oven. Leavening works by creating gas-filled bubbles in the batter — these small gaps cause the cake to expand, and rise up in the pan. Think of a piece of cake or bread: look closely and you’ll notice that much of what you’re eating is empty space. Creating these spaces is what leavening is all about.


Mixing your cake batter is a form of mechanical leavening; other examples include creaming, beating, stirring and kneading. What you are really doing during those 2-3 minutes is physically adding air molecules to the batter. Think about how you stir a bowl of hot soup to cool it down; the motion of swirling the soup with your spoon adds air to the hot liquid. By beating your batter, you are doing the same thing: adding air molecules that will expand in the oven and create the gaps of fluffiness. If you like your cake lighter, beat it a little bit more; if you like a denser cake, beat it a little bit less. You’re the scientist, after all!


Though you aren’t adding them, your cake mix also contains chemical leaveners. You probably have some chemical leaveners in your kitchen, but you call them baking powder and baking soda. Chemical leaveners are used because they release carbon dioxide when they combine with moisture and heat. So your cake rises due to bubbles of air created mechanically and bubbles of carbon dioxide produced chemically. No wonder it is so fluffy and delicious!


Step 3:  time to bake. The instructions on the box suggest different cooking times for different baking dishes. Why? Well, heat moves through solids when atoms vibrate against each other and exchange electrons, in a process called conduction. Metals are good conductors because the electrons in their atoms are easily transferred — loose, in a way — so heat moves faster through metal than through, say, woodBecause metal conducts so well, putting your cake in a metal pan will allow the heat from the oven to move more quickly through the pan to the batter, so you can cook it for less time than in a glass or ceramic dish.


Because heat moves through conduction, each heated-up molecule transfers heat to the one next to it. So, the outside of your cake will be cooked first while the center is the last to receive the vibrations. That’s why you check the center of the cake to see if it’s doneConductive heat transfer creates texture and heat gradients in the food you cook: think of a seared steak with a juicy pink centerDo you like your cake slightly crispy on the outside and softer in the middle? Experiment with the concept of conductive heat transfer until you find the temperature, material and baking time to create your perfect cake.


Congratulations kitchen chemists — you’ve not only baked a delicious cake but dropped some major science on the way. And now for the best experiment of all: bon appétit!