When cream or eggs are whipped, air gets trapped inside and causes the ingredient to foam, grow in volume, and become stiff. Whipped eggs, especially egg whites, are great leaveners for baked goods. In the oven, the trapped air expands, causing something like a cake or soufflé to rise. But sometimes whipped whites can be unstable and difficult to work with, so a stabilizer such as sugar or an acid (like vinegar or cream of tartar) is often added to the whites. Sugar also increases the coagulation temperature of the egg protein, allowing the cake or soufflé to rise more before it sets. Recipes usually instruct you to whip egg whites or cream to a particular firmness, or peak stage. The photos show you what those stages should look like. We’re showing sugar-stabilized egg whites, but the characteristics of each stage apply to cream as well.
Medium peaks hold their shape pretty well, except that the tip of the peak curls over on itself when the beaters are lifted.
Stiff peaks stand straight up when the beaters are lifted. (Medium-stiff peaks are just stiff enough to stand up firmly but with a slight curl at the tip.)