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Folding is a gentle mixing method; the aim is to incorporate delicate ingredients or components without causing deflation. Folding also traps extra air in the batter and breaks up existing air bubbles into smaller ones. You use the folding technique to gently combine light and airy mixtures, such as beaten egg whites or whipped cream with a heavier mixture, such as a batter, flour or fruit purees, or when folding in melted chocolate or butter into a batter. For example, when beaten egg whites are folded into a batter, it is done so they will retain as much of their volume as possible.
Before folding, sometimes you will need to temper an ingredient or mixture, such as when adding eggs to a hotter mixture. A technique called "lighten the batter" is done if folding stiffly beaten egg whites into a very thick or heavy mixture, first stir in a few large spoonfuls of the whites. This will loosen the mixture and enable the remainder of the beaten whites to be folded in with ease.
To fold, the lighter mixture, such as beaten egg whites, is placed on top of the heavier one, such as batter, in a large bowl. Starting at the back of the bowl, a large rubber spatula is used to cut down vertically through the two mixtures, across the bottom of the bowl and up the nearest side. The bowl is rotated a quarter turn with each series of strokes. This down-across-up-and-over motion gently turns the mixtures over on top of each other, combining them in the process.
When you fold batter and beaten egg whites together, for example, they must be thoroughly blended and smooth; you need to make sure all of the air bubbles are encased in batter, otherwise the cake won't rise properly. The batter will set around the air bubbles when the cake bakes in the oven, allowing it to puff, and then, set properly.
With UNSHORTENED (FOAM) CAKES, it's normal to see some uneven air holes in the cake's crumb(inside), but not large pockets of air holes, egg whites and/or batter. That means that the folding action was not done enough or was done too much where you managed to pop a lot of air bubbles. Folding takes a lot of practice to get right.
Often a characteristic of cakes that have beaten egg whites over-folded with a batter, is that they don't rise as high when baked; the delicate air bubbles have been popped during the process. On the other hand, when the batter has not been folded sufficiently, there is a sticky wet layer found on the bottom of the pan after baking.
HOW TO FOLD BEATEN EGG WHITES INTO BATTER
When folding, I prefer to use a clean and dry large rubber spatula. Always fold gently but thoroughly, but don't over- or under- do it.
1. To start, use the batter mixture that was previously set aside. Stir it a few times to mix it thoroughly before using.
2. Some recipes call for lightening the batter. Skip this step if not using.
Lighten the batter:
Add a small amount of the beaten whites to a thick egg yolk and flour batter and stir with your balloon whisk to lighten the batter or prepare it for the easier incorporation of rest of the egg whites. Afterwards, fold in the remaining beaten egg whtes in equal portions.
3. Fold the mixtures together:
SARAH SAYS: I recommend folding in the ingredients in 2 to 3 equal portions.
A. Directly add a portion of the whipped whites on top of the heavier mixture, such as batter in this case, and fold, using a large rubber spatula.
B. With the rubber spatula, starting on the top, cut down from the center of the mixture to the bottom of the bowl, then draw the scraper quickly toward you against the edge of it, and up to the left and out. You are thus bringing a bit of the mixture at the bottom of the pan up over the egg whites or "folding it over". Do this a few times.
C. Turn the bowl a quarter turn and fold a few times. Repeat turning the bowl and folding until you only see slight streaks of white. You will need to scrape the sides of the bowl a few times to keep the mixture from climbing up the sides.
D. Repeat with the last two additions or beaten egg whites. As you fold, you will see a few and fewer streaks of eggs whites. The batter and egg mixture will become smoother.
E. Continue the folding movement while slowly rotating the bowl, and cutting down, towards you, and out to the left, until the egg whites have been folded into the body of the batter. You shouldn't see any streaks of batter and eggs whites in the mixture. The mixture should be smooth. Sometimes you'll see with a few small lumps visible; it just means that you slightly overwhipped the egg whites. Break them up with the rubber spatula by cutting straight through the egg white lumps, and continue to fold until the batter is smooth.
4. Immediately divide the batter evenly into the prepared pans and bake in a well-preheated oven. Sometimes the air pockets that occur need to be eliminated by running a small metal spatula or knife in an s-shape through the batter before baking.
Note how light, fluffy and smooth(no lumps from beaten egg whites) the batter is. You should not see any white streaks from the egg whites, either.
HOW TO FOLD FLOUR INTO BATTER
1. Sift 1/2 of the measured flour over the egg mixture.
SARAH SAYS: Make sure it is ONLY HALF of the flour!
2. Fold it in gently but rapidly with a large rubber spatula until the flour has disappeared.
Make sure you scrape the side and bottom of the bowl as you do.
3. Sift the remaining half of the flour over the egg mixture.
4. Fold it in until the flour has disappeared.
HOW TO FOLD FRUIT INTO CAKE BATTER
This technique is used with the Frosted Fresh Cherry Layer Cake Recipe
1. With the last addition of flour, mix until almost incorporated. Stop the mixer and take off the mixer bowl.
SARAH SAYS: If the batter has been already mixed without flour showing, and then you add in the fruit and nuts, you will more than likely incorporate more air bubbles into the batter. If you change the timing to until the flour is ALMOST incorporated, you will lessen the chances of incorporating small air bubbles into the batter.
2. Add the chopped cherries and FOLD with a large rubber spatula until JUST combined. DO NOT OVERMIX.
SARAH SAYS: The reason for folding in the cherries (versus stirring or mixing) is that you help prevent overmixing the moistened batter containing wheat flour, producing too much gluten. Too much gluten toughens and dries a cake when baked. It also causes the cake to develop a peak in the middle and deep cracks on the top as well. If this happens, cut the cake into small, bite-size pieces and frost each one. Place on a serving platter and serve anyway; –no one will know!