Beginning bakers are often taught that cakes are done when the edges pull slightly away from the baking-pan rim. This normal shrinkage, while not the perfect indicator of doneness, is the result of fairly complicated chemical interactions among cake ingredients and heat. A good balance of ingredients and baking conditions yields cake that is both lofty and moist. Several failures in balance can produce cake that continues to shrink after baking.
What Happens in the Oven
Ingredients in cake batter interact with each other and with oven heat in a complex set of reactions throughout the baking process. The air bubbles formed by creaming butter and sugar expand, lifting the sticky elastic result of mixing flour and eggs or other liquid.Additional bubbles, created if a leavening agent like baking powder or soda is used, lift dough even higher. Heat dries batter around bubbles, so that a baked cake is crusty on the outside and moist inside, with a soft crumb. Successful cake baking depends on keeping ingredients and conditions in balance, allowing enough time for heat to raise dough but not enough to overcook it.
For delicate baking projects, such as cakes, accurate oven temperatures are essential, or cake can over-dry and shrink. Place a small oven thermometer in the center of your oven and check it against frequently-used oven settings. Remember that oven heat comes in cycles, so you will need to make several observations over a 15- to 30-minute trial period. At a 350-degree Fahrenheit setting, your oven may actually heat to 375 F, enough of a difference to overcook cake batter. Your cake may begin to shrink during baking and continue as it cools.
Many recipes give a time-range for baking, like 25 to 30 minutes. Set a timer for the shortest time in the range or even a couple of minutes less, like 23 minutes. Testing doneness slightly early can prevent the drying-out that can cause shrinkage. Use a wood toothpick or skewer to test doneness; crumbs will adhere better to wood than metal skewers.
Recipes usually specify a pan size, 8 inches by 8 inches, for example. Dough bubbles let dough expand, and contact with the sides of the pan let dough climb, increasing rise. Like the bottom of the pan, heat on the sides turns the batter outer surface into crust, which helps a cake hold its shape. Using a pan too large for the recipe causes dough to spread and cook too quickly, resulting in shrinkage.
Watch the Beat
At various points in cake recipes, ingredients are combined energetically, not gently stirred. Beating increases the air-bubble content of batter, especially of the eggs it contains. Beating egg yolks with sugar and/or sugar-creamed butter fills viscous egg with bubbles. In a sponge or angel food cake, stiffly-beaten egg whites play the part of baking powder or soda, spreading additional bubbles throughout dough. Thorough beating is a virtue; overbeating creates problems, because the agitation of incorporating air dries out egg white and eventually reduces volume. Overly dry eggs cannot maintain the viscosity dough needs to stay lofty and therefore shrinks. Stop beating peaked egg whites the moment a small clump clings to the inverted bowl of a spoon. Fold beaten whites gently into heavier batter until just barely combined.
A shrinkage particular to angel-food and sponge cakes occurs when cakes are cooled baked-side up. Since neither type of cake typically contains fat, dough is less viscous and more dependent on eggs for retaining loft. Inverting the cake pan for cooling reduces downward pressure on crust and lets the moisture from cooking steam reintegrate into dough. Keeping angel-food or sponge cake moist is a challenge, and letting steam escape can lead to dryness and shrinkage as the cake cools (see Reference 2, Resource 2).
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