Q. I am single and live alone. I enjoy cooking and often make interesting casseroles. However, I don't like to eat the same thing for several nights in a row. How long can I safely keep a meat casserole in the refrigerator?

A. We cannot really give you a precise answer to this question, because the time a cooked casserole dish may safely be kept in the refrigerator depends, to a great extent, upon how it is refrigerated. Ideally, it should be put into a shallow container, refrigerated promptly at the end of the meal and kept at a temperature of 40 degrees Farenheit or below. Under these conditions, leftovers should be safe for four or five days.

On the other hand, foods that are allowed to cool slowly on a counter over an extended period, particularly if the mixture is in a deep dish, provide a favorable growing medium for bacteria and the production of toxins.

Unfortunately, while thorough reheating does afford considerable protection against harmful bacteria, some bacteria toxins are quite resistant to heat. Reheating cannot be relied upon as a universal antidote against food-borne infection.

We should mention that in discussing food-borne illness, people often observe that they have eaten food stored considerably longer than five days and suffered no ill effects. That may be true. The point is that guidelines for food safety are set with a margin of safety. To our way of thinking, it seems to make little sense to push that margin to its limits.

Incidentally, if you have freezer space available, for the sake of both food safety and variety, you may want to consider packaging single servings in individual boiling bags or aluminum foil containers.

Q. I am taking diuretics, and the doctor has told me I must be sure to eat plenty of foods rich in potassium. He recommended fresh oranges, orange juice and bananas, but I am really sick of these by now, and frankly I don't like too many fruits. Are there some potassium-rich vegetables I could use instead?

A. Orange juice has become the classic "chaser" with which to wash down diuretic pills, and from a practical point of view, there is a good reason for this. Setting up a routine of this type is an excellent way of insuring that when you take your diuretics you will get all the potassium you need.

Orange juice is a very convenient source of potassium, providing 480 milligrams per cup. But you can get just as much from a small baked potato, or a serving of spinach or summer squash.

Certain vegetables contain even larger amounts. Among the richest sources are cooked kidney beans, winter squash, dandelion and mustard greens. These provide between 700 and 900 mg. per cup. A great number of vegetables, such as broccoli, brussel sprouts, sweet potato, cabbage, caulifower, beets and corn, also provide considerable amounts.

Other vegetables may contain lesser amounts, but remember, it's the total that counts. There is no reason you must stick with oranges, bananas and others fruits for potassium as long as you include generous amounts of a variety of vegetables in your diet.

Q. In your column, you often write about polyunsaturated and saturated fatty acids. Could you please explain the differences bewteen the two?

A. Fats are made up of three molecules of fatty acids and one molecule of glycerol, which is an alcohol. When we talk about polyunsaturated and saturated fats, we are really speaking about differences in the chemical composition of these fatty acids - particularly, the differences in the amount of hydrogen they contain.

Fatty acids are made up of chains of carbon atoms with an acid at one end. Along the carbon chain are attached hydrogen atoms. The more hydrogen, the more "saturated" the fatty acid is.

Such saturated foods include meat fats, butter and dairy products, plus coconut oil used in many manufactured foods. All not only, contain predominantly saturated fatty acids but also raise the level of blood cholesterol.

Others, the monounsaturates, still have room for two more hydrogen atoms. These fatty acids, which seem to have little effect on blood cholesterol, are found mainly in peanut and olive oil.

And finally, the polyunsaturated fatty acids have even more vacancies, or double bonds as they are called, along the carbon chain. These fatty acids, which tend to lower blood cholesterol, predominate in fish and in oils such as corn, cotton-seed, safflower and soybean.