Answers to frequently asked questions and recipes that hit the sweet spot.

Most commonsensical cooks can easily figure out ways to decrease fat in certain dishes: Use less oil in your pasta. Substitute fat-free or reduced-fat dairy items for their full-fat counterparts. Choose leaner cuts of beef and pork, and remove the skin from poultry.

But baking is different. Many cooks quake at the thought of attempting to change the chemistry of a cake or puttering with pastry. Quite right they are to quake, too, for the less-than-precise baker can end up with a doughy mess and a mass of disappointment.

We offer answers to some of the most common questions we receive on the topic, along with helpful advice from Cooking Light Test Kitchens Director Vanessa Johnson and food scientist Shirley Corriher, author of Cookwise. They explain which changes you can easily make and which need more careful consideration. The recipes that follow demonstrate that low-fat baking is an art indeed.

What does sugar do in baked goods?

Shirley Corriher: Sugar prevents the flour proteins from joining and making gluten; gluten development would make a cake or cookie tough. In this way sugar acts as a tenderizer and can replace some of the fat in the recipe. When sugar is present in amounts above 2 tablespoons per cup of flour, the two proteins in flour that normally join with each other and water to form gluten join with the sugar instead.

Sugar also caramelizes in baking, which enriches flavors. Substituting as little as a tablespoon of corn syrup for sugar can make cookies much browner, because corn syrup browns at a lower temperature than sugar. Some sugars, like honey and brown sugar, absorb moisture from the atmosphere, which means that things baked with them will stay soft and moist longer.

Why does butter make cookies crisp, and how can you lighten cookies?

Corriher: Cookies made with butter spread during baking, which means they're thinner. Trimming the amount of fat just a little will limit their spread. If you want to reduce the amount of butter but preserve the crispness, add a little corn syrup to the cookie dough. If you want a puffy cookie that stays soft, use shortening to limit the spread in baking.

Vanessa Johnson: Of all the desserts, cookies are truly the hardest for us to lighten. Because we use less butter, our cookies generally err on the chewy, fudgy side rather than the crisp side. However, small amounts of yogurt, applesauce, or egg whites can help give lower-fat cookies the texture of high-fat ones, as in our Cinnamon Cookies.

Why are butter and eggs necessary in cakes?

Corriher: Butter has three roles in cakes: to make the cake light and delicate by holding air bubbles produced by leaveners like baking powder or soda; to make the cake tender by coating the flour protein; and to carry rich flavors. Stick margarine and shortening can substitute for butter; in fact, shortening is already aerated before you buy it, so it can produce a fine, tender cake. A margarine spread that is soft at room temperature can't substitute for butter, though.

Eggs have two parts, whites and yolks, which do two different things. Whites are an incredible drying and leavening agent, and yolks are nature's great emulsifiers for creamy texture.

Why can't you replace all the eggs in baking with egg substitute?

Corriher: Egg substitutes are usually composed of egg whites and oil, along with other ingredients like coloring and stabilizer. Because they don't have yolks, they can't serve as emulsifiers; it's the natural lecithin in the yolks that helps make an emulsion.

The lack of yolks is the reason you can't use egg substitute to make custards, either. With no yolks, custards wouldn't be smooth and creamy.

Johnson: We generally use fewer yolks and more whites than an equal amount of egg substitute simply from a practicality standpoint: Everyone usually has eggs in the fridge, but not necessarily egg substitute.

What difference does the way you measure flour make?

Johnson: It can make a terrific difference-as much as an ounce per cup of flour!

When fat is reduced in baking, the exact and precise measurement of flour becomes crucial. When a reader says she's had a problem with one of our recipes, one of the first things we suspect is that she's used the measuring cup to scoop flour out of the canister.

We advise readers to use the spoon-and-measure method: Stir the flour to aerate it, then lightly spoon into a dry measuring cup and level with a knife. By this method, a cup of all-purpose flour will weigh four and a half ounces. For more precise measurements, we've included cup and weight measures in these recipes.

Why doesn't Cooking Light use applesauce or fruit puree to replace fat?

Johnson: Because it generally doesn't work to do so. In our opinion, baked products made with fruit purees have an inferior texture and taste. We get far better results by simply reducing the fat.

Why does Cooking Light use sugar instead of sugar substitutes?

Johnson: Sugar is a key ingredient in baking, providing structure and mass in many desserts. Consider a cake: If you substituted artificial sweetener for sugar, you would lose the volume that sugar contributes, and the cake batter would not have enough substance to become a cake. Also, the tastes and textures of sugar substitutes may change when they're heated.

Because sugar also acts as a tenderizer in baking, it can actually replace some of the fat in reduced-fat baked goods. Sugar substitutes don't tenderize like sugar does.