Q: I need a recipe for marshmallow topping. I can't find one in any cookbooks. Is it really hard to make?

A: Seven-Minute Icing is really a form of marshmallow topping. You can find a recipe for it in just about any general-purpose cookbook. It differs from marshmallow in its lack of gelatin. Gelatin tightens the foam -- makes the air bubbles smaller -- and by doing so, also makes it whiter. Gelatin also prevents evaporation of moisture, so a marshmallow topping won't develop the crusty surface of icings.

Here's a recipe. It will tax your home-mixer's motor windings, by the way, and will taste only marginally better than the stuff one buys in a jar. MARSHMALLOW TOPPING (Ices 9-inch cake or tops 8 portions of ice cream)

1/4 cup boiling water

1/4-ounce envelope plain gelatin

6 egg whites (absolutely clean of yolk)

1 cup granulated sugar

1 teaspoon vanilla extract

Pour boiling water into a mixing bowl and sprinkle gelatin over its surface. Whisk vigorously until foamy, then let sit a couple minutes to finish dissolving. Place egg whites and sugar in a saucepan, blend together with a whisk and heat on a low burner, stirring continually with the whisk. When the mixture is hot to the touch (you cannot leave your finger in more than 2 seconds; at that point, it is about 140 degrees), pour into the small mixing bowl of an electric mixer.

Turn mixer on highest speed and beat until the meringue reaches the soft-peak stage. That is, if you dip a spoon into the foam and withdraw it, a peak forms that immediately falls over. At this point, add the dissolved gelatin and continue beating for about 10 minutes -- still on highest speed. The topping is done when it forms stiff peaks. You cannot overbeat it, by the way.

Stir in vanilla extract and transfer to a container. Marshmallow will not spoil, due to its high sugar content, so it needs no refrigeration.

Q. Where does gelatin come from? I heard that it is extracted from bone marrow.

A. Gelatin is produced by the prolonged cooking of tissues rich in connective tissue. The major ingredient of connective tissue is collagen. This fibrous protein is the most common protein of two- and four-legged animals (insects rely on chitin).

Collagen is composed on three strands helically wound about each other in much the same fashion as a rope. Each strand is in turn made up of smaller protein molecules that are interlinked to form a network. This arrangement makes for a very tough, resilient, elastic foundation of skin, bones and ligaments.

Prolonged cooking of collagen strands splits out the individual protein molecules of which each strand is composed. Whereas the very large molecules of collagen are quite insoluble (that is, they will not dissolve in water), the much smaller molecules of gelatin do.

This dissolution produces a slimy mixture of water and gelatin molecules called a sol. Sols are noted for their abilities to form gels when cooled. Gelatin sols are no exception. As they cool, their molecules cross-link to form a giant matrix enclosing pools of water. The macroscopic result is a shimmering semisolid that, on heating, turns back into a solution.

The traditional sources of gelatin are pork rind (skin), bone (a collagen network that is 25 percent of the bone's weight containing deposited salts of calcium and phosphorus) and connective tissue in the areas around the joints. Bone marrow is the bone's fatty interior. It contains little or no collagen. Bone marrow's main function is the production of new red blood cells.

Send you cooking and food-related questions to Tom Neuhaus, The Food Section, The Washington Post, 1150 15th St. NW, Washington D.C. 20071.