Q: I read your description of the different methods used to decaffeinate coffee, but I am primarily a tea-drinker. Is methyl chloride used to remove caffeine from tea? How does the caffeine content of tea compare with coffee?

A: Methyl chloride is not used to decaffeinate tea. Instead, mature tea leaves are steamed and then treated with either ethyl acetate, a naturally occurring compound, or carbon dioxide.

Tea has less caffeine than coffee, although the caffeine content of both varies. According to figures recently compiled by the Institute of Food Technologists' Expert Panel on Food Safety, coffee brewed by the drip method contains anywhere from 110 to 150 milligrams (mg) of caffeine per cup, and if percolated from 64 to 124 mg. Instant coffee contains from 40 to 108 mg. per cup while the amount in decaffeinated, either instant or regular, does not rise above 5 mg.

With tea, caffeine content depends on length of brewing. For example, if brewed just 1 minute, levels range from 9 to 33 mg., but if brewed from 3 to 5 minutes, the amounts rise to between 20 and 50 mg. Instant tea contains from 12 to 28 mg. of caffeine; in a can, from 22 to 36 mg.

Q: What is the nutritional value of guavas?

A: A medium guava, which weighs about 3 1/2 ounces, contains 45 calories. In addition, it provides 165 milligrams of vitamin C, roughly three times the Recommended Dietary Allowance. It also contains some carotene, which the body can convert to vitamin A, as well as small amounts of B vitamins. Like many fruits, it is a good source of potassium. This is of particular interest to people who take diuretics that cause them to excrete potassium along with sodium.

Native to Mexico and South America, guavas are now cultivated in warm climates in this country. They are usually served raw, either alone or as part of a mixed fruit cup, but may.

Q: What causes a "skin" to form on the top of milk when it is heated?

A: When milk is heated in an uncovered pan, water gradually evaporates from the surface. This allows casein, the main protein in cow's milk, to concentrate. The skin is a mixture of this casein, along with some calcium and fat compounds.

You may have noticed that once a skin forms, milk tends to boil over. That is because steam forms under the skin layer. It is possible to prevent skin formation by whipping the milk occasionally as it heats. Not only will you save yourself a stove-cleaning job, but you will conserve the nutrients that would be discarded with the skin.