Meringues are an essential part of the baker's craft. Not only are they a decorative topping for desserts, they are folded into cakes, make waffles fluffy and can be baked on their own as cookies or decorative containers for other sweets. Salt and an acidic ingredient such as cream of tartar are usually added to the egg whites to help them form a stable foam that holds its shape. The acidity helps by "denaturing" the egg proteins, helping the molecules change from tightly-wound bundles to long, open chains. When cream of tartar is unavailable, alternatives must be used.
The most widely used acidic ingredient in meringues, aside from cream of tartar, is lemon juice. Lemon juice has a number of advantages over other alternatives. It is inexpensive and widely available, and an ingredient that most kitchens already have on hand. Lemon juice is not as neutral in flavor as cream of tartar, but a faint lemony taste will go unnoticed in most baking, or may even complement the main flavors.
Distilled white vinegar is the other widely used acidic substitute for cream of tartar. Like lemon juice or tartar, it is powerfully acidic and will denature the egg proteins with great efficiency. However, it must be used sparingly, or the meringue will have a discernibly sour and vinegary flavor. The effect can be minimized by using a milder vinegar such as white wine vinegar. Depending on the flavors of the finished dish, champagne vinegar or sherry vinegar might also be appropriate.
There is one additional alternative that is seldom mentioned in conjunction with meringues. That is ascorbic acid, better known as vitamin C. Ascorbic acid is often used in commercial bakeries to prevent browning in sliced fruit, such as apples. The acid is dissolved in cool water, and the fruit is dipped in it to stop the action of browning enzymes. Ascorbic acid will serve equally well in meringue making. It is available at health and bulk food stores in crystal form, or vitamin C capsules and tablets may be crushed for the purpose.
The most time-honored replacement for cream of tartar is not an ingredient at all. Cooks and bakers have long known that eggs whipped in a copper bowl make an airier, more stable foam. In recent years, scientists have determined that the copper ionizes one of the proteins in the egg white, allowing it to form stronger bonds with the other proteins around it. The net effect is very similar to that caused by an acidic ingredient. If you use a copper bowl to whip egg whites, omit the acidic ingredient as it will react with the copper.