Dry shaking refers to shaking without ice. Why, I hear you ask, would I possibly want to do such a thing? Well, the answer lies in a brief exploration of the physical properties of the main ingredient bartenders use for foams, namely egg whites... Now for the science bit:
Creating an Egg Foam
‘The physical stress of beating the egg whites can create a foam. There are two types of physical stress caused by the beating of the egg whites with a whisk, the first being that the whisk drags the liquid through itself, creating a force that unfolds the protein molecules. This process is called denaturation.
The second stress (which occurs through shaking without ice) comes from the mixing of air into the whites, which causes the proteins to come out of their natural state. These denatured proteins gather together where the air and water meet and create multiple bonds with the other unraveled proteins, and thus become a foam that holds the incorporated air in place.
Ways to Stabilize your Foam:
Copper bowls have been used in France since the 18th century to stabilize egg foams. The copper in the bowl assists in creating a tighter bond in items such as egg whites. The bond created is so tight that the sulfurs are prevented from reacting with any other material. A silverplated bowl will have the same result as a copper bowl. A pinch of powdered copper supplement from a health food store used in a glass bowl will yield the same result as well. Drawbacks of the copper bowl include the expense of the bowl itself, as well as the fact that the bowls are difficult to keep clean. Copper contamination from the bowl is minimal as a cup of foam will contain a tenth of one's normal daily intake level.
Adding an Acid
Cream of tartar (potassium bitartrate) is an acidic salt that can be used to change the pH of the egg white to an acidic range by boosting the number of free-floating hydrogen ions in the egg white. This has the effect of stabilizing the foam, and is therefore an alternative to using a copper bowl. 1/8 teaspoon/0.5 grams of cream of tartar should be used for each egg white to create this effect. 1/2 teaspoon/2 milliliters of lemon juice can also be used to achieve the same results.
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