Folding is the process of incorporating a batter or dry ingredients into whipped egg foams - most often beaten egg whites. The point of folding is to allow you to mix the two parts without deflating all the bubbles that you just worked so hard to whip into the eggs. The air bubbles are vital to keeping the dish you are making light and airy.
We’ve seen cookbook writers take pages to tell you how to fold, but the written word is frankly not the best way to teach it (and we can't believe we're saying this!). Watching someone do it in a class, on a TV cooking program, or even in a sequence of photos in a good book is better than reading about it. But the basic process is to use a large rubber spatula to reach down through the center of the foam to the bottom of the bowl and lift the some of the batter up and on top of the foam. As you turn your wrist to deposit the batter on top of the eggs, you turn the bowl a few degrees. Then reach down to the bottom again and lift more of the batter up and over the foam. As the bowl turns, the batter and foam blend, and when there are no traces of egg white left, you’re done.
There are a few tricks that will help you fold. The first is to take a quarter or so of the beaten egg and mix it into the heavier batter You lose some of the bubbles, but it brings the consistency of the batter closer to that of the foam so they mix better.
Another vital point is that the heaviest ingredient must be on the bottom - so don’t pour the batter onto the eggs, scoop the eggs onto the batter. Then begin to fold.
In our experience, neophyte folders are too timid rather than too rough. There are things you can do that will deflate bubbles quickly, such as trying to fold from one edge of the bowl to the other (instead of from the center to a side), which is little more than stirring. Also, you should try to gently deposit the batter on the eggs, rather than dumping it.
But folding should also be a fairly speedy process - perhaps no more than 90 seconds long - and if you go on and on and on, the bubbles that you are theoretically saving by being slow and gentle are going to pop from over-exercise and sheer boredom. Get the job done and get your mixed batter into the pan and into the oven.
Madeleine Kamman, author of The New Making of a Cook, also suggests using a glass bowl when you’re learning to fold, so that you can more easily see what you are doing.
And yes, the eggs should be at room temperature, but that is important for the beating rather than the folding. The proteins in the egg will expand more rapidly if they are at room temperature, so you’ll get more volume from room-temperature eggs.