Is there a physiological reason why certain foods are commonly served at breakfast and others are regarded as strange choices to eat first thing in the morning? I view breakfast as a time to raid the refrigerator and use up leftovers. With my new microwave oven, it is even easier for me to reheat a bowl of soup or slice of pizza. However, one of my friends looks askance at this habit and says I am endangering my health. Could that be?

Your friend's advice is well-intended but wrong. It is true many people find certain traditional foods more appealing choices with which to begin the day, and are quite content with a rather repetitive morning meal. A lot of people are satisfied with a glass of orange juice and a bowl of the same dry cereal day after day.

Others like yourself prefer variety, and happily eat foods in the morning that the majority would not eat until later in the day. Any food that fits into your diet and contributes to meeting your day's nutrient requirements is just fine first thing in the morning. That can include anything from a leftover chicken leg and a slice of whole-wheat toast to a serving of pasta with grated cheese. We suggest washing these or other leftovers down with a glass of vitamin C-rich citrus juice to ensure that you'll get your day's supply of that nutrient at the outset.

This fall, for the first time, I noticed quinces in the produce section of the market. They look like yellow delicious apples. I wasn't sure whether they were to be eaten raw or needed cooking, so I didn't buy them. Can you tell me about them?

Quinces are native to Central Asia and occupied an important spot in ancient Greek and Roman mythology as the fruit of love, marriage and fertility. It is the original fruit for marmalade, which gets its name from the Portuguese word for quince: marmelo. Because quince contains lots of pectin and acid, it is a good fruit for making preserves. Until recently in this country, quinces were found only in jellies, usually on the specialty-food shelf.

Some varieties of quinces can be eaten raw when they are very ripe, but are more often cooked a little, with or without a sweetener. Quinces can be baked or stewed and are especially good as an accompaniment to fowl. You can also dice a single quince, which has a rather aromatic flavor all its own, and stew it with either apples or pears.

When ripe, quinces are yellow. But at that stage, they bruise easily. It is best to buy them on their way to turning yellow and let them ripen in a brown bag with several air holes.

A whole quince, weighing about 5 ounces, contains 55 calories, along with small amounts of a variety of vitamins and minerals.

I have mastered the art of soup-making without spending a lot of time in the kitchen. Especially during the winter months, I make a large pot of soup every week, usually enough for one main course and a first course on the second night. To prevent monotony, I vary the ingredients, but often it is a mixture of dried beans, some meat or chicken, herbs and spices, and fresh or frozen vegetables. With the exception of onion and celery, I add the vegetables and some precooked pasta just before serving. Here is my question: Since I simmer the meat for a long time, are there any nutrients left in it by the time I serve it?

Yes. It is commonly believed that by the time a piece of chicken or beef has been cooked long enough to flavor a broth, it is devoid of any nutrient. That is simply not so. True, some B vitamins vulnerable to heat are destroyed with prolonged cooking. Thus much of the thiamine originally present would have disappeared by serving time. That cannot be avoided. It is also true that some water-soluble B vitamins may leach out in the broth. But that is of little nutritional consequence, since the broth will be consumed anyway. Beyond that, most of the protein and iron and some of the B vitamins remain in the beef or chicken.

There are a number of strengths to both your methods and general recipe outline. Your procedure of adding vegetables just before serving minimizes loss of both B vitamins and vitamin C. Moreover, the combination of beans, pasta and vegetables with just a small amount of animal protein improves the usefulness of the nonmeat proteins in the body. Finally, the presence of both animal protein and some vitamin C remaining in the briefly cooked vegetables increases the absorption of iron from the dried beans.

Can you tell me why cream of tartar is sometimes used in whipping egg whites?

Cream of tartar makes foam more stable. Ironically, both cream of tartar and salt, often added to egg whites for flavor, delay the formation of foam in the first place. For that reason, directions in many recipes specify that you whip egg whites to a foamy stage before adding either of these ingredients.

Cream of tartar is derived from tartaric acid, which occurs naturally in fruits, particularly in currants and grapes. In fact, in making wine, the tartrates normally present in grape juice must be removed before it is bottled. If not, tartrate crystals will form naturally as it sits on the shelf.

On a trip to Europe last summer I fell in love with the taste of red, black and white currants. They were sour, but when mixed with other fruits and fresh mint, a touch of sugar brought out a delicious flavor. What is their nutritional value and how do they compare to dried currants?

Curiously, the fresh currants you ate are unrelated to the so-called Zante dried currants available in this country. These are actually a type of grape called Black Corinth.

Calorically, currants of different colors are about equal, containing around 30 calories per cup. But for some reason, black currants are loaded with vitamin C, providing nearly twice the Recommended Dietary Allowance in just one-half cup. White or red varieties have considerably less. Regardless of hue, they all contain small amounts of a medley of minerals and vitamins. Unfortunately, fresh currants are pretty difficult to find in the United States.

Zante currants, by contrast, are higher in calories. A half-cup provides about 200 calories, very little vitamin C, and small amounts of several vitamins. If eaten in sufficient quantities, they can be a good source of iron: The half-cup serving would provide 13 percent of the RDA for an adult woman and 23 percent of that for a man.

1987, Washington Post Writers Group