TOMATOES are considered an all-year-round vegetable, but it's in midsummer that they get one's undivided attention. By August they hit the top of the list and stay there until we have indulged ourselves with the truly ripe, delicious, juicy, bright-red fruit with flavor that's perfectly balanced with sweetness and acidity.Americans consume an average of nearly 18 pounds of fresh tomatoes a year, proving this is a country of tomato lovers - exceeded perhaps by the Italians. Americans consume an additional 22 pounds in processed tomato products like ketchup and sauces. Sophisticates like fresh tomato slices alternated with thinly sliced mozzarella or provolone cheese, or with fresh basil or oregano and olive oil. Others say they sprinkle sugar on their fresh tomatoes. Honey and sliced tomatoes on English muffins are a favorite of Russ Morash, producer of "The Victory Garden" on public TV. Four out of five American home gardeners say tomatoes are their choice crop. Eighty-five percent of the 30 million Americans who have gardens or tubs of vegetables grow tomatoes. And there are dozens of kinds to choose from. About 30 are listed in the catalog of W. Atlee Burpee, the largest supplier of vegetable and flower seeds by mail. Some seed companies specializing in only tomatoes list as many as 75 varieties Tomatoes are by no means a hobby vegetable. After potatoes, tomatoes are America's most important commercial vegetable. Since tomatoes are so popular, why are there so many complaints about the lack of flavor in tomatoes sold in supermarkets? Clark W. Nicklow, professor and plant breeder, has done advanced research on tomatoes (as well as on squash and peppers) at the University of Massachusetts Suburban Experiment Station in Waltham. Dr. Nicklow has some answers to the questions people ask about ripe tomato flavor. "For one thing, it's a common belief that bright red tomatoes have the best flavor," he says, "but color actually has little to do with taste. The meatier a tomato is - the greater the proportion of seeds and gel - the milder it tastes, regardless of color." "Taste and quality are personal," Nicklow noted last week at the Seventh Annual Taste of Massachusetts Tomato Festival held at Boston's City Hall Plaza. "Some people want the stem on the tomato when they buy it. This doesn't have a thing to do with flavor, nor does it mean the tomato will last longer, as some think. A tomato must have the right balance between acidity and sugar to produce a good taste." But there is no single answer or reason for pale, underripe, tasteless tomatoes. Last year this reporter took an investigating trip through the tomato farms and packing plants of Florida, which produce half of all the tomatoes grown for fresh consumption annually. My conclusion: Growers have gotten the message that consumers want quality flavor. But it's a slow project. Home gardeners, knowing first-hand how wonderful their own tomatoes taste in summer, ask how can the supermarket tomatoes be so different? One answer is that supermarket tomatoes are cold. "The refrigerator can be the tomato's worst enemy," says Beth Mahaffey, director of education for the Florida Tomato Committee. "If at any point during transport anybody along the way - the trucker, wholesaler, or retailers allow the tomatoes to be chilled below 55 degrees F., they're destroying the flavor," Ms. Mahaffey explains. "The ripening process stops and the tomato will develop a mushy texture and loss of flavor." Refrigeration hinders the formation of hundreds of volatile chemicals responsible for a tomato's flavor. The problem is getting tomatoes to the grocer when they are ripe - but not too ripe to handle. "Tomatoes are delicate when they're at the peak of ripeness," says Nicklow. "When overripe they'll be mushy, not easy to slice, and the flavor is not quite as good. So the aim is for a tomato that's vine-ripened but firm. "I started in 1979 to breed tomatoes for chain-store selling," he says, "but only 2 to 3 percent are purchased by these stores. The old-fashioned fruit and vegetable store owner knew how to handle tomatoes, checking ripeness every day, putting the ripest in front and less ripe in back to slowly ripen." Today, most market tomatoes are picked in the green-mature stage, gassed with ethylene to start ripening, then shipped. (Ethylene is normally released by the fruit as part of the ripening process.) In the past, consumers have demanded that tomatoes be attractive and commercial growers have emphasized uniform shape and large yield, neglecting flavor. Some consumers say that although many stores display tomatoes at room temperature, the fruits are held in refrigerators in the back room. Unfortunately, there's no way of telling until the fruit is cut and tasted. "We're dedicating two-thirds of our budget to educating the trade in the handling and correct temperature for tomatoes," says Mahaffey of the Florida Tomato Committee. "We're hoping this will help get better-quality fruit to the consumer." Meanwhile, tomato lovers are taking advantage of summer crops from farmers' markets, roadside stands, and backyard gardens.