you beat egg whites, does denaturation occur?"
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
youre 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 wouldnt.
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 arent being denatured?
It was a great discussion...and it was great to find
your site. Its definitely been bookmarked! If
you cant help with an explanation, do you know
of any other resources wed check? Youre
really the only good resource our searches have produced.
Thanks very much,
Colleen Fox and the Hazen High School AP Biology Class
Ms. Fox and Hazen High School AP Biology Class,
is well worth exploring because its 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 thats crème caramel.
Denaturation ("changing the nature") happens
when protein molecules unravel from their naturally
coiled state. With eggs, this happens most frequently
when theyre 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.
Its 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.
can learn more about how eggs are affected during cooking
on our science
of eggs page.
Youve asked two terrific questions. Weve
already chosen your second question on butter for another
"Question of the Week." And we think youve
discovered one of the most fascinating ways of learning
Anne & Sue