Q. I have heard that the solvent used to decaffeinate coffee is questionable.

A. The safety of one, methylene chloride, has been questioned. A study conducted several years ago by the National Toxicology Program suggested that giving rats enormous doses in a corn-oil solution caused cancer. But the enormity of the doses (equal to 12 million cups of decaffeinated coffee a day) and the appropriateness of the feeding method raised questions about the relevance of the findings for humans. Because of questions about the data, the report was withdrawn.

The FDA sets allowable limits for methylene chloride at not more than 10 parts per million. Consumer's Union reported that in tests conducted in 1979, they were unable to detect the solvent. This is not surprising in view of the fact that it volatilizes at slightly over 100 degrees Fahrenheit, and the temperature used to roast beans is 400 degrees Fahrenheit.

Nevertheless, according to Consumer Reports, several major coffee companies have switched to the alternative chemical, ethyl acetate. It's a natural ingredient in pineapple, apples and even coffee beans. Allowable levels of this organic compound are set by good manufacturing practices.

Q. Recently a friend served a salad containing a green with a delicious sour flavor. She said it was called sorrel. Can you tell me more about it?

A. Sorrel, sometimes called dock, is aptly described in Jane Grigson's Vegetable Book (Atheneum, 1979) as "halfway between vegetables and flavoring."

In addition to being put raw in salads, it is also used as a filling in omelets or as a major seasoning in soups and sauces for fish, veal and eggs. It can be served as a vegetable by itself, either braised or pure'ed, but because of its strong flavor it is more commonly mixed with other greens, such as spinach or chard.

Sorrel is used widely in France and a number of other countries. But in the United States it tends to be available only in stores that carry specialty produce items. It does grow wild in some places, if you know where to look. But if you really enjoy it and have a garden, your best bet may be to grow it yourself. It is easy to cultivate and comes back each year.

As you might expect from its dark green color, sorrel is a rich source of vitamin A. Even a quarter-cup, a reasonable amount to get in a mixed salad, would provide more than half the day's allowance. It also contains some vitamin C. And it is low in calories.Q Could you explain what EDTA is and why it is added to some processed food? A Ethylene-diamenetetraacetic acid, or EDTA, is one of a number of so-called chelating agents: substances that tie up metal ions to stabilize processed foods.

Many metals in foods, such as the magnesium in chlorophyll and the iron in hemoglobin, exist in a chelated state. When these metals are released, as they might be in food processing, they are free to participate in reactions that can lead to a variety of changes, including discoloration, oxidative rancidity, turbidity and alterations of flavor.

Chelating agents are sometimes added to tie up these metals and prevent these undesirable reactions. EDTA is particularly effective in preventing oxidative changes in foods with an emulsion, such as salad dressing, mayonnaise and margarine.

Because EDTA is such an effective chelator, there have been questions about whether excessive use of it in foods could lead to the depletion of calcium and other minerals from the body. While it appears that this should not be a problem, measures have been taken to regulate the amounts and the ways it can be used.

In order to further reduce any possibility of excess chelation of minerals in food, thereby making them unavailable for absorption, the EDTA sometimes is chemically modified by adding sodium and calcium.