" When you beat egg whites, does denaturation occur? "

Dear Anne and Sue,

What a great Web site you have! I teach AP Biology in Renton, Washington. We are currently studying biochemistry and a student asked today about making a soufflé. She wanted to know why egg whites stiffen when you beat them. We got into a discussion about whether the proteins are actually denaturing during the process or whether you’re just adding air. Discussion turned to whether or not the whites will return to their original state after the beating, and we agreed that they wouldn’t. So, can you help us? Has denaturation occurred or have you just changed the way the different components of the solution are emulsified? In your March 17, 2003, response to a question about meringue, you stated that the beating of egg whites is just a way to get the strands to unwind so that air can become trapped. We figure that indicates that proteins aren’t being denatured?

It was a great discussion...and it was great to find your site. It’s definitely been bookmarked! If you can’t help with an explanation, do you know of any other resources we’d check? You’re really the only good resource our searches have produced.

Thanks very much,

Colleen Fox and the Hazen High School AP Biology Class

Hi Ms. Fox and Hazen High School AP Biology Class,

Denaturation is well worth exploring because it’s one of the reasons why eggs are able to perform such diverse roles and undergo huge transformations! Eggs can quickly be transformed into heaps of fluffy meringue, but they can also be cajoled into holding milk in the luscious and fragile gel that’s crème caramel.

Denaturation ("changing the nature") happens when protein molecules unravel from their naturally coiled state. With eggs, this happens most frequently when they’re heated or beaten, but denaturation can also be prompted by salt, acids (such as vinegar), alkalies (such as baking soda), and freezing.

When protein molecules are heated, they unfold and extend and their surface area increases. Exposed parts of the molecules become receptive to bonding with other protein molecules, and a network of cross-bonds forms; this causes clumping. Think of the changes an egg undergoes when boiled or fried: from raw to soft to hard. Eggs contain both water and protein, and protein is made up of amino acids. Some amino acids are hydrophilic (attracted to water) and some are hydrophobic (repelled by water). When you add air through beating egg whites, the protein molecules uncurl so that the water-loving parts immerse themselves in the water and the water-fearing parts can stick out into the air. These rearranged proteins then bond with each other, creating a network that holds the air bubbles in the whipped egg whites in place.

If egg whites are beaten until they are stiff, they are fully denatured and have no elasticity; they lose their original properties and aren't able to return to their former state. If egg whites are beaten only until they form soft peaks, the proteins are only partially denatured and retain some of their elasticity. The partially denatured protein strands surround the air bubbles and when heated, these proteins fully denature and solidify, creating a protective wall so that the air bubbles don't burst. This is why such dishes as meringues and soufflés are light and fluffy.

Other protein-rich foods undergo denaturation as well. It’s part of the process of milk becoming cheese, and you can watch it happen when cooking a fish fillet: The meat changes from translucent to opaque as the proteins uncoil and rebond.

You can learn more about how eggs are affected during cooking on our science of eggs page.

You’ve asked two terrific questions. We’ve already chosen your second question on butter for another "Question of the Week." And we think you’ve discovered one of the most fascinating ways of learning about chemistry!

Good wishes,
Anne & Sue