Most of us have probably observed the phenomenon of an egg white, once whipped, taking a fluffier, whiter, and more structured form, which is taken advantage of in many types of baking, from souffles to angel cakes, to waffles.
(Also see this post coming soon on my blog: Michael's Test Kitchen !)
Confusing as to the precise language of the term “meringue,” Encyclopedia Britannica describes Meringue as a “mixture of stiffly beaten egg whites and sugar that is used in...desserts” [emphasis added], while many seem to use it to refer to the specific Meringue Cookie, the recipe we will be studying in this post, which is the meringue mixture basically piped into a cookie shape and baked to harden.
This is the most straight-forward dessert, regarding its simple and obvious use of egg whites, in my opinion, so that is why I will be using this form of meringue to explain the science in a basic manner. You will have probably used meringue or seen it used to top Lemon Meringue Pie. Whipped fluffy, then spread over the top, the meringue mix solidifies in the baking process in some sense, but keeps its soft and airy characteristics.
I'll include little basic science notes about each step of the process.
Step 1: Ingredients
5 egg whites
1 cup sugar
½ teaspoon cream of tartar
½ teaspoon vanilla
Pinch of salt
Step 2: Preparation
Preheat oven to 225 degrees. Line a cookie sheet pan with wax or parchment paper. Set aside
You’ll want a low oven temperature so that you give plenty of time for all of the water to evaporate from the egg white before the tops brown too much.
Step 3: Start the Whipping (Denaturation)
In a medium bowl, whip egg whites until they are frothy, 1 minute.
By beating the eggs, you are breaking apart the “building blocks” of protein, unraveling the amino acid chains. This is called denaturation. Unraveling the chains will give the egg white more surface area and structure, because, once separated from each other in long strands, the aminos will connect to form “networks,” which in turn trap the air bubbles.
You’ll need to make sure that there is no moisture like water or grease in your bowl of egg whites, and of course no small bits of egg yolk. All of these things will interfere with the denaturation and prohibit peaks from ultimately forming in the way you need them to. Another thing that is helping the egg white to foam up is obviously lots of air. Air is getting trapped in the egg whites, expanding them, and the proteins are holding everything together.
(See picture graphic [image from wikipedia])
Step 4: Cream of Tartar & Soft Peaks
Add cream of tartar, and continue to whip until soft peaks form.
Even though the proteins are connecting together and structuring themselves, they have a tendency to fall apart after a lot of beating and whipping. In order to keep the proteins from falling apart from their networks of bubbles as easily, recipes such as this one call for you to add cream of tartar as a stabilizer which increases the acidity of the mixture, which aids the proteins by making it harder for them to collapse back to their original state.
Step 5: Sugar & Stiff Peaks
Now add a pinch of salt and gradually add sugar by tablespoons, as you continue to whip egg whites. Stop whipping once you have stiff peaks. Stir in vanilla.
Egg whites can’t expand a great deal without added strength. By adding sugar you are adding strength. Why? Because the egg white proteins can “only expand so far” and so the way to give them more structure is by adding sugar, which bonds with the proteins and “lends them water.” This makes the whole thing a lot less likely to simply collapse, and gives it more potential for expanding and holding shape.
Step 6: Piping the Meringue
Add meringue mixture to a piping bag with a larger star tip. Pipe mixture into cookie shapes on your lined cookie sheets. You’ll need to bake everything at once so make sure you have room to fit all of your mixture.
Bake in preheated oven for 45 minutes - 1 hour. Turn off oven, but leave cookies in oven without opening the door for 1 hour before taking out and finishing cooling.
When you apply the correct amount of heat for the correct amount of time, the water will evaporate and the air bubbles present in the egg whites will expand. At the same time, the proteins will all permanently bond together, or coagulate. For meringue cookies, all of the water should be cooked out, and the result should be much drier than meringue that tops pies, which is generally baked for a much shorter period of time than these cookies.
Step 8: Consume and Enjoy!
Consume. While not consuming, store in an airtight container.
If you leave them out of airtight containers, then the moisture in the air will eventually seep in and ruin the texture and structure.
Not sure what the science is for this part, but they sure do taste good!