Why are acid and cream of tartar used in beating egg white?

Cream of tartar and acid, such as lemon juice or vinegar, are often used in beating egg whites because they help to stabilize the egg white foam and produce a higher, more stable meringue.

Here are the primary reasons why acid and cream of tartar are used:

1. Acidification: Adding a small amount of acid to egg whites lowers the pH of the egg white solution. This acidic environment helps to denature the egg white proteins more effectively, leading to the formation of stronger bonds and a more stable foam.

2. Cream of Tartar as an Acid Stabilizer: Cream of tartar is a mild acid that is commonly used in baking. It consists of potassium hydrogen tartrate, a byproduct of winemaking. Cream of tartar helps to stabilize the acidity of the egg white mixture, ensuring a consistent and controlled acidification process. It also prevents the egg whites from becoming overly acidic, which could result in a grainy or curdled texture.

3. Protein Unfolding and Cross-Linking: The acidic environment created by the acid and cream of tartar causes the egg white proteins to unfold and expose more reactive sites. These exposed sites can then form new bonds with each other through a process called cross-linking. This cross-linking strengthens the protein network and contributes to the stability of the beaten egg whites.

4. Increased Foam Volume and Stability: The combination of acidification and protein cross-linking results in increased foam volume and stability. The trapped air bubbles in the egg white foam become more resilient to collapsing, leading to a meringue that holds its shape well and does not deflate easily. This is particularly important in recipes that require stiff, well-beaten egg whites, such as meringues, soufflés, and angel food cakes.

It's worth noting that the amount of acid and cream of tartar used should be carefully controlled. Too much acid can make the egg whites too thin and runny, while too little may not provide enough stabilization. Typically, a small amount of acid, such as 1/2 teaspoon of lemon juice or white vinegar, and a pinch of cream of tartar (usually about 1/8 teaspoon) are sufficient for every two to three egg whites.

By using acid and cream of tartar, you can achieve better volume, texture, and stability in your egg white-based recipes, leading to more successful baking results.