Originally posted February 25, 2011
and Inorganic Chemistry
February 25, 2011
Do you like angel food cake? I do. I love it. I also love chocolate mousse. My friend Lionel makes a chocolate mousse (his grandfather’s recipe) that is to die for. I try not to think about the fact that, while I am fortunate to be partaking of this masterpiece, I have neared the onset of my first coronary by approximately two years. What both of these dishes have in common is that they require whipped egg whites for their structure.
There is this lore about whipping egg whites. The right way to do it is to whip your eggs in a copper bowl. And for the longest time, there was never any scientific reason for this. It was just an observation. Egg whites whipped in copper bowls produce a foam, which holds its shape better than foams produced by whipping in plastic or stainless steel bowls. Of course, there were plenty of theories for why this might be the case. One involves the fact that copper is a better conductor of heat. And, while you are whipping the eggs, this heat helps to set and stabilize the foam. Another theory indicted ovotransferrin, a protein found in egg whites, as a potential culprit. Ovotransferrin binds ions of iron to help give eggs certain antibiotic properties. The thought was that ovotransferrin started taking on copper ions when whipped in a copper bowl and that this somehow transferred stability to an egg white foam. Still, others just thought that copper seemed to be the best choice because humans have been using copper utensils for such a large portion of our history. (The Copper Age, the period in which humans began to use copper tools, began around 7,000 years ago.)
Egg white foams
Foams are really quite amazing materials. A foam is made by trapping gas bubbles among a solid or a liquid. Foams are incredibly strong – some are so strong that they can support an object weighing much more than the foam itself weighs. In the case of the egg white foam, constant whipping of the liquid egg white traps air bubbles within the liquid. Of course, it is not just as simple as putting air inside of the egg white. There are lots of changes on a molecular level that occur to help give the foam structure.
Image Source. A beautiful picture of an foam made with egg whites. Notice how different it appears than an unwhipped egg white. The foam is satiny and will hold its shape without running.
These molecular changes are very similar to the changes I discussed last week: protein unfolding. When you whip an egg white (~90% water and ~10% protein), the energy that you expend by whipping goes to unfolding the proteins in the egg white. As you continue to unfold the proteins, they start to aggregate. At the start of the process, the proteins are relatively small and noticeably clear. However, the masses of aggregated proteins are larger in size and can scatter light (this is why the egg whites turn white when you whip them – the same thing happens when you cook an egg). The final foam that you make is a network of unfolded proteins and water surrounding minuscule air bubbles. The hydrophobic (water-fearing) parts of the protein coat the bubble while the hydrophilic (water-loving) parts interact with the water molecules in the egg white.
Unfortunately, there is a limit to the amount of whipping an egg white can take. Further whipping causes the protein aggregates to pack together more tightly. This will squeeze out the water and air bubbles from the aggregated proteins and you will be left with a whipped version of scrambled eggs – a gritty, watery mess. Obviously this is detrimental to a good chocolate mousse. Fortunately, an understanding of the chemical forces that cause this to happen can help us to make better/more stable foams.
The proteins in egg whites contain an amino acid called cysteine. Cysteine is special because it has a reactive sulfur atom attached to it. Under the right conditions, the sulfur atom can shed a H+ ion (in the same way that an acid like hydrochloric acid does). After this happens, the protein becomes sticky and will search out another protein with a reactive sulfur atom upon which the two proteins stick together. This is what a good pastry chef will keep from happening if he/she wants to make a good egg white foam.
The reaction of two proteins containing the amino acid cysteine. The sulfur atoms that are part of cysteine become reactive when they lose the hydrogen that the are initially attached to. These reactive sulfur molecules can link two proteins together. (A similar reaction occurs during the vulcanization of rubber)
On Food and Cooking
As is the case with all questions about food science, my first move is to open up On Food and Cooking by Harold McGee. For those of you who are unfamiliar with this book, it is THE source for understanding the physical and chemical changes that happen to your food as you cook it. It is a reference for both the home and serious professional cook. And, it is unequaled in the respect that it deservedly receives for teaching us why we cook the way we do.
In the section on egg foams, Harold McGee suggests that bowl of choice for whipping eggs is copper because copper ions themselves will bind very tightly to reactive sulfur atoms and, thus, keep the egg white proteins from linking together through the sulfur bonds. As an inorganic chemist, this makes intuitive sense. Copper is a squishy atom. By that I mean the the electrons that surround a copper nucleus can slosh back and forth very easily. (This property also makes copper a good heat and electrical conductor.) Sulfur, like copper, also is surrounded by an electron cloud that is easily pushed around. In the lingo of inorganic chemistry, copper is called a soft metal and sulfur is called a soft ligand. Now, it is a very dearly held tenet of inorganic chemistry that soft metals like to make bonds with soft ligands. So, naturally, copper and sulfur form a very ideal pair. Another soft metal is silver. And, McGee found that silver, like copper is able to make a stable foam of egg whites. He describes his research briefly in On Food and Cooking. If you are interested, there is a more detailed description in a paper he published in the journal, Nature. So, aside from being THE food scientist, Harold McGee is also a first rate inorganic chemist! In terms of other “soft” metals that might work for this, gold would be a good choice, and mercury would work too … if you could make utensils out of it.
As a side note, adding a little bit of acid (in the form of lemon juice) to your egg whites will also help to keep the proteins from coagulating. Having an acid present will keep the sulfur from initially losing H+, rendering the cysteine unreactive.
Whipped egg whites and my health
I wish that I had Lionel’s recipe for chocolate mousse. But, it’s probably a good thing that I don’t. My mouth is watering and my heart is slowing just thinking about it.
Of course … whipping eggs by hand is a strenuous activity that will more than counteract the effects of eating the mousse, right???
Cheers
-mrh