To beat or not to beat is not the question; how much to beat is the question. Sometimes even the most competent cook is baffled by a recipe that indicates beating an ingredient without telling how much or how long, or by a recipe that instructs kneading or folding without telling how long to knead or how much to fold.
One reason for this confusion is that there simply isn't enough room in newspapers, magazines and cookbooks to explain basic concepts such as beating each time they occur in recipe after recipe. But, whatever the reason, here and now let's clarify the issue.
Whole Eggs: "Lightly beaten" is a phrase that sometimes follows the number of eggs in an ingredients list. It means that the eggs should be beaten with a fork until the yolks and whites are well combined, as you would for scrambled eggs. There is no way to overbeat whole eggs with a fork.
Egg Yolks: "Beat until thick" or "beat until light and pale in color" are two of the traditional ways to describe egg yolks that have been beaten with an electric mixer set at high speed for 2 to 3 minutes until they are almost tripled in volume and are as thick as yogurt. Egg yolks beaten with sugar will be slightly thicker than egg yolks beaten without sugar, and the latter will achieve a greater volume. Even with 4 or 5 minutes of extra beating, the final dish will not be affected.
Egg Whites: "Beat to soft peaks" or "beat until stiff" are common ways of describing the final texture of beaten egg whites. You can overbeat plain egg whites, so be careful.
Egg whites should be beaten at medium to medium-high speed with an electric mixer. When they have tripled in volume, begin to watch them carefully. Stop the beater periodically and lift it out of the egg whites. The beaten egg white adhering to the bottom of the beater will form a soft peak first, that is, it will form a point not quite stiff enough to stand upright, a point that bends gently downward. This is the texture indictated when a recipe calls for the egg white to be beaten to "soft peaks." (It is ideal for beaten egg whites that are to be folded into a savory mixture.)
Continue beating the egg white for about another 30 seconds and the peaks will stand stiff and upright when the beater is lifted out. This is the texture described as "beaten stiff."
With as little as 15 to 30 seconds of beating past the stiff-peak stage, the egg whites will be hopelessly overbeaten and the texture will change from smooth to dry and cracked. Overbeaten egg whites cannot be folded successfully into another mixture without deflating them so much that the final mixture will not rise, or in the case of mousses and souffle's, will not be light.
Overbeaten egg whites are best discarded.
Meringues: Meringues (egg whites and sugar beaten together) are impossible to overbeat. If sugar is added at the beginning, the meringue will beat more slowly than if the sugar is added after the whites have already been beaten to doubled volume. Meringues go through the same stages as beaten plain egg whites (see above), both soft and stiff peaks, but there is an important difference between beaten egg white and meringues. In meringues, the sugar acts as a stabilizer, thus making it nearly impossible to overbeat a stiff meringue. Even 5 or 6 minutes of extra beating will not harm a meringue.
Whipping Cream: Heavy or whipping cream acts in exactly the same way as plain egg whites (see above), but when overbeaten it turns into butter bathed in skim milk. When the beater moves around without the whipped cream itself moving, that is as stiff as it can be beaten safely.
At high altitudes, as little as 3 or 4 seconds of overbeating will turn the cream to butter, at sea level at least 10 or 15 seconds is needed. The addition of sugar will not stabilize cream as it does egg whites.
Folding: Folding is the gentle incorporation of beaten egg whites, meringues or whipped cream into a thicker mixture or batter in which the goal is to combine the two with the least amount of deflation. You can overfold.
For most of my life I have been told, have read and have even taught others to fold by hand with a rubber spatula. Now I realize that there is a much better way to fold, one that is almost foolproof: use an electric beater set on its slowest speed. Fold until the two mixtures are just combined, until the moment when there are no more streaks. That's all there is to it, and I assure you, you will deflate the mixture less with the mixer than you would have by hand.
Creaming: Creaming butter and sugar together simply means softening the butter so that it can easily absorb the sugar, which is then beaten into it. There is no way to overbeat the butter before or after the sugar is added.
When creaming in a food processor, the cold butter and sugar can be placed in the bowl at the same time. Process, scraping down the inside of the bowl with a rubber spatula as needed, until the butter and sugar are well combined and appear soft and smooth. You cannot overprocess.
Cake Batters: Cake batters (unless they are egg white or meringue leavened) should be beaten until they are well combined. A little extra beating won't hurt the cake, but 5 or more extra minutes of beating can activate the gluten in the batter and cause the final texture of the cake to be stiff and dry.
Cookie Dough: Like cake batters, cookie doughs should be beaten until thoroughly combined. At worst, a few minutes of overbeating will only soften the dough more than the recipe writer expected, and so the chilling time may have to be extended before the dough can be rolled out. With drop cookies, a few extra minutes of beating won't hurt the cookies, but it may cause them to spread more than expected when they are baked.
Muffins: If muffin batters are overmixed, even by just a little, the muffins will be flat, dry and full of tunnels (hollow spirals inside). The final beating of a muffin batter should be treated in the same way as plain egg whites or meringues being folded into another mixture (see above).
Kneading: You can't overknead a bread, but you can underknead it -- so err on the side of excess.