Archive for the ‘kitchen science’ Category

About Eggs

Tuesday, May 17th, 2011

eggs.jpgWe eat them cooked any one of half a dozen ways in the morning.  We use them in cakes and cookies and as a meringue on lemon pies.  The particularly ambitious cook may use them in a mousse or a soufflé.  (For eggs in cooking, revisit Physics in the Kitchen.)  But have you ever stopped to think about how amazing the egg really is?

We all know that eggs should be handled carefully because their shells are incredibly thin.  Drop an egg a short distance and you have a gooey mess to clean up.  One simple tap on the edge of the counter is enough to crack open the shell.  But try this experiment: hold an egg in the palm of your hand and curl your fingers around it.  Now squeeze with all your might.

Did it break?  If you didn’t believe me and didn’t squeeze with all your strength, go back and try again.

What on Earth…?  The key to an eggshell’s strength is the fact that its whole surface is curved.  The strongest shape is a sphere, and an egg is a close approximation of this.  (The reason it’s not exactly a sphere in just a moment.)  With no corners or flat sides to weaken it, the forces you apply to the egg by curling your fingers around it are distributed equally over the egg rather than concentrating at any one point. 

This works even in the following way. Hold the top and bottom of the egg with your thumb and forefinger and squeeze.  Did it break this time?  In physics terms, we say that the egg has a high “compression strength” — you’re compressing the egg, but it doesn’t break!  In fact, an egg has such high compression strength, that (with the proper setup) it can support the weight of a small personwithout breaking.

So why aren’t eggs spherical?  The oval shape is created as the bird lays it, and this turns out to be an advantage for the hen.  The shape prevents the eggs from rolling away!  Spherical eggs would roll and roll and roll… and never return.  For birds like ostriches that nest on the ground, this isn’t an issue, and their eggs are generally more spherical.  But birds that nest on cliffs often lay very conical eggs, which roll in a tight circle around the narrow end and remain on the ledge.

What else?  How can you tell if your egg is fresh?  Eggs contain an air pocket that forms when its contents shrink as it cools after being laid.  As the egg ages, moisture evaporates and the air cell grows larger, reducing the average density of the egg.  An object floats in a liquid if it is less dense than the liquid and an object denser than the liquid sinks.  We can therefore use the egg’s density as a handy measure of the egg’s freshness!  Here’s how it works.  Place your egg in a container of water.  A fresh egg with a small air pocket will rest horizontally on the bottom.  The air pocket in a 1-week-old egg is slightly larger — its density is less — and the end will hover slightly off the bottom, and an egg that’s 2-3 weeks old — even less dense — will rest vertically on the bottom.  Don’t eat any eggs that float!

Now that you know so much about eggs, here’s a bonus question: what do eggs and Roman arches have in common? 

PS. When squeezing your egg: don’t wear any rings, and make sure your egg doesn’t have any cracks already.  My “research” egg was blessed with a crack and I ended up with a handful of broken shell and raw egg oozing between my fingers and onto the floor!

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!