We're not talking laundry here...
Another important technique the home cook really needs to know is folding. Sometimes recipes both in baking and cooking (but more often in baking) require ingredients to be folded rather than incorporated, added, or otherwise mixed in. There is a huge difference when you fold something into a dish rather than just adding it.
Folding is going to involve somehow getting an ingredient that is very delicate in its texture to mix in with something heavier. You're using a technique to very gently "mix in" this lighter ingredient, all the while preserving the integrity of its texture, into something heavier that will crush it if you don't do it right. Soufflés, some cake batters, some pastry dishes will call for folding. You can also fold ingredients in that you don't want broken up -- like folding in delicate raspberries into a batter for muffins where you want to keep the berries as whole as possible rather than breaking them up into mush throughout the batter. Whatever it is you're making, the technique is the same.
You need one hugely important utensil for this: a good spatula.
You can't use wooden spoons, metal spoons, forks, whisks, or anything else for this. Truly a spatula is best and easiest. It's designed in a way where it picks up the delicate ingredient you're trying to fold in, suspends it gently, then cuts through the heavier ingredients without losing the lighter one or being clumsy. It has maximum control, so it's important you invest in some good ones. I like keep a bunch of all different sizes, from teeny tiny ones to big honkers as pictured below. They're pretty affordable and come in all sorts of fun colors, so get a good, sturdy set of spatulas.
Now, the process of folding.
For this post we'll use egg whites to be folded into a cupcake batter. Folding beaten egg whites is the most common scenario where folding is called for, so it makes sense to use that as the example. So here we have pictured the heavier cupcake batter on the bottom, the super light and fluffy egg whites we have to somehow mix into the batter, and my trusty red spatula.
Step 1: Dump the ingredients to be folded in right on top of where they need to be folded into.
Step 2: Take your spatula, and cut directly through the middle of the egg whites and batter.
Go all the way through the egg whites and down into that batter. Feel the spatula grab the batter underneath. Don't be afraid -- you're not killing the egg whites.
Step 3: Begin to turn the spatula clockwise, lifting the ingredients up and together.
Make sure you get all the way down and under. Notice how the spatula is positioned, ready to lift the batter and egg whites together, back up on the flat part of the spatula.
Step 4: Complete the rotation lifting the spatula with ingredients on it straight up into the air, and ready to cut down the middle again.
Notice how the ingredients stuck to the spatula? Now the next time you come down, you're mixing in the batter into the egg whites!
Step 5: Cut through the middle again, straight through the batter, turn, and come up.
Each time you cut through, turn the bowl slightly to the left to help it go faster. Keep this motion of cut through middle, turn spatula, lift up, and back down the middle, each time moving the bowl with your other hand slightly clockwise. Keep doing this and after a few turns it will look like this:
Notice the big chunks of egg whites still left intact? This is good -- we are preserving the air and delicate texture of the batter doing this.
Keep going slightly more and you'll notice the body of the batter has thickened considerably.
You have now fully folded your ingredients.
This technique will feel awkward at first, but you'll get the hang it of pretty quickly as you practice. Don't be discouraged -- it's something that will take you a few times to get the hang of it. Once you do you'll whip up folding in seconds. At first, go slowly and make sure you practice the motions -- pay attention to your hand grip, your wrist flip, how you hold the spatula, and get used to working together with both hands doing something different at the same time (one hand folding, other hand moving bowl). Don't go too fast -- it's important you internalize the motions and make them robotic and then you can add speed.
Egg whites are an inexpensive and easy way to practice this technique. You can also try delicate berries in freshly whipped cream (the opposite -- heavier, denser berries getting folded into a lighter textured whipped cream; same concept and technique though!). Once you've got this down you can feel quite confident attacking those soufflés and fancier French pastry recipes!