The addition of Acid

Citric acid (lemon juice), tartaric acid solution, and cream of tartar (which is the solid salt of tartaric acid mixed with cornstarch) have no effect on the volume of the meringue, but they help stabilize the foam by decreasing the pH level in the albumen, making the foam less apt to collapse. Only a small amount of any of these acids should be used, as too much, in addition to adversely changing the taste of the meringue, will impede coagulation during baking.

It has been commonly accepted that a copper mixing bowl produces a superior and more rapidly whipped egg white foam. However, current research indicates that the degree to which a copper bowl is preferable to a stainless steel bowl is questionable. While you should certainly avoid plastic and wood, which are hard to clean of fat, aluminum, which is corrosive and tends to impart a grayish color, and glass, which is not suited for use in the kitchen, copper’s alleged ability to impart an acidity to the whites as it comes in contact with the albumin is now disputed. Although there is certainly no proof of any disadvantage to using a copper bowl, the addition of acids generates the same result.