Q: What's so special about a key lime? Where did it come from? Are the limes sold in supermarkets really limes or are they some form of green lemon? And, would key limes have any effect on the thickening of key lime pie?

A: The key lime originated in southern Asia and was marketed by the Arabs in Spain and Portugal for hundreds of years before Christopher Columbus bought his boat. During the 16th century, the conquistadors brought these limes to the New World. Their seeds sprouted and established plants, which bear the fruit now called key or mexican limes. At one time, the plant was cultivated in South Florida and the keys. Hence the name.

Today, the only limes cultivated commercially in the United States are called tahiti or persian limes. At the beginning of this century, cuttings of this variety were introduced to California from Polynesia. By the early 1930s it had made serious inroads on key-lime production as the fruit was bigger and the trees more resistant to disease.

The two fruits are quite different in appearance. The key lime has a pale greenish cast that lightens to yellow toward maturity. The persian lime retains its shiny, green color. The key lime is spherical and about 1 inch in diameter. The persian lime looks like a green lemon and is of comparable size to lemons. And the key lime is full of seeds, the persian variety comparatively seedless.

Both varieties of lime are similar in sweetness and acidity. That is, depending on climate, soil and location of the trees, each produces fruits of differing sweetness and differing tartness. Therefore, neither variety is more effective at thickening the filling of a key lime pie, which happens when the condensed milk proteins thicken in the presence of acid. In Florida, as elsewhere in the United States, the great majority of commercially produced key lime pies are made from persian lime concentrate.

Both varieties of lime are similar in flavor and odor. Both contain the characteristic lime essential oils in their peels. In fact, persian limes have more and larger oil glands in their peels.

When juiced, however, key limes release more of their peel oil, since one must press more fruit to obtain the same volume of juice. In addition, key limes have a touch of brightness that many find desirable -- probably because it offsets excessive sweetness. In Texas and much of the Southwest, key limes imported from Mexico are used to prepare and garnish margueritas.

Q: Why, in the preparation of so many bread recipes, must the milk be scalded? I've made my favorite sandwich bread several times without scalding the milk with no apparent harmful effects.

A: The milk is scalded for three reasons: One, to melt any fat in the recipe. Two, to provide a lukewarm fluid for activating the yeast. And three, to denature a group of proteins, which causes dough slackness. It's easy, however, to use a liquid or already melted fat and to start the yeast in lukewarm water. The third reason -- prevention of slackness -- explains how the practice got started.

A slack dough is one lacking in the resilient elasticity normally associated with wheat flour doughs. The symptoms are stickiness and poor oven spring (initial expansion). When baked, the bread is crumbly, low in volume and, if baked free-form, flat.