The baby-boomer's tastes graduate to the gourmet hamburger

''The baby-boom generation has grown beyond McDonald's and Burger King; they want to reward themselves in this type of place.'' So says Bill Palmer of T. J. Applebee's, the small but growing chain of restaurants of which he is president.

Applebee's is typical of a new wrinkle in the restaurant business - a trend that goes by the somewhat (misleadingly) racy name of ''adult fast food.'' Applebee's and its counterparts represent a gastronomic middle ground between the hamburger chains and a serious pay-attention-to-every-bite dining experience.

The plastic-laminated menu here, somewhat broader than at most ''gourmet hamburger'' establishments, includes variations on the burger theme. But there are also trendy munchies like guacamole and fried mozzarella sticks, plus daily chalkboard specials including quiche and homemade soups. ''Thirty percent of our items are made from scratch,'' says Mr. Palmer in an interview in his Abernathy Square restaurant in this community northwest of Atlanta. Applebee's and other adult fast food chains, unlike the traditional burger chains, also have bars in their restaurants.

The prices here are steeper than in mainstream hamburgerland, but still modest. The New York strip steak, at $6.45, is the most expensive thing on the menu.

''I'll be with you in a minute, ma'am,'' says Karen, the waitress, a blur in the corner of my eye as she flies by as if on roller skates. The wait at Applebee's averages 10 minutes from ordering to receiving your dinner: just long enough to size up the decor, which is heavily butcher block, with exuberant ivy and spider plants. (''One of our waitresses has a greenhouse business on the side, and she takes care of them,'' says Mr. Palmer.)

The popular ''greenhouse look'' with its big, easy-to-peer-through windows is a distinct part of the marketing strategy at Applebee's. ''Women feel comfortable here; it's not like a bar where they can't see inside before they go in,'' says Mr. Palmer.

Also in evidence are brass railings and stained glass; the effect being striven for is a sort of Gay '90s motif, with a light Irish accent - ''natural, '' as opposed to typical fast-food ''plastic''; a friendly neighborhood pub, albeit an un-dark and un-smoky one. The name derives from Mr. Palmer's wife's name - Theresa Joan - ''and Applebee's just sounded like a good, old-time proprietor's name,'' Mr. Palmer says.

It's a bit hard to see much of either the Last Hurrah or Norman Rockwell small-town feeling at Abernathy Square, a one-story suburban shopping center wrapped around three sides of a parking lot along Roswell Road. Yet Palmer maintains that 60 percent of Applebee's business are customers who are here once a week or more.

Of a weeknight the tables are filled with groups of young women, presumably co-workers at one of the nearby office parks, or sartorially mismatched suburban families (Dad in a business suit, Mom in jeans and T-shirt, Junior in Little League outfit) who have piled into the car to go for a quick bite. Off by the window a couple of men are scribbling at legal pads and punching at calculators between sips of coffee.

Applebee's, now a unit of W. R. Grace & Co., the chemical company, began in November 1980 when Palmer, a former Burger King franchisee, discovered how hard it was to find a good place for a moderate-priced business lunch - without eating at his own shop.

The new chain, launched with a second mortgage on his house, has grown to six restaurants plus five more under construction. The goal is 200 stores by 1989, 75 percent to be franchised and each with $1.5 million annual sales.

Applebee's restaurants, typically about 3,500 square feet, are about one-third the size of many of the gourmet burger places; this cuts overhead and means they don't require such a big population to support them.

Meanwhile, over in San Antonio, Texas, Clark Crowdus, director of marketing for Fuddruckers Inc., characterizes business as ''outstanding'' - with 21 company-owned restaurants, mostly in Texas, operating today, and the 22nd to open in Los Angeles tomorrow. There are also eight franchised restaurants, mostly across the Sunbelt, with more scheduled to open soon. ''You'll see them opening across the country.''

The menu at Fuddruckers, which Mr. Crowdus characterizes as a ''gourmet or upscale hamburger'' restaurant, is heavy on the basics: burgers, hot dogs, French fries. But each Fuddruckers has its own butcher shop and bakes its own burger buns.

Another Texas entry into the gourmet-burger business is Chili's Inc. in Dallas, with 28 company stores plus two franchise operations. Executive vice-president Ron McDougall, who prefers to characterize Chili's as ''casual adult dining,'' says business is excellent and tells of plans to open another 11 company owned stores by the end of 1984.

The gourmet-burger chains have generally been launched in the Sunbelt, but many are optimistic about spreading farther north.

Robert Wisely of Restaurant Research Associates of Tustin, Calif., calls gourmet hamburgers ''very much an emerging trend, with substantial potential,'' and, like other observers, ascribes the development to the rise in disposable income among the baby-boomers. ''A lot of (restaurant) chains are hurrying and scurrying to get their entries into the field up and running.''

He sees gourmet-hamburger establishments as not only upscale from traditional fast food, but as an alternative to the traditional family restaurant, which has , in his view, fallen on hard times. ''Those places are so mediocre, with no positioning clarity, no menu clarity.'' The ''adult fast food'' place, with high volume and a limited menu and no obvious comparison with either traditional burger chains or coffee shops, is also under less pressure to control prices.

''At the root of the experience (at an upscale burger emporium) is the entertainment value - they hire college kids in interesting outfits to wait on tables,'' Wisely says.

What about profitability? ''As long as these people can take in $2 million-plus a year per unit, and if they can keep their square footage down to 6,000 or less, they shouldn't have a problem,'' says Mr. Wisely.

And market saturation? ''If you look at the market segment drawn to these places (typically the 21- through 49-year-old group) you'll see there are adequate numbers of bodies to fill these places up. Saturation is no problem. The big problem is the cost of the initial investment.''

Lest the euphoria get to be overwhelming, though, hearken to the words of William Trainer, a restaurant-industry analyst for Merrill Lynch in New York: ''Yes, there's a market for the 'gourmet hamburger,' but like anything else, there will be many entrants into the market, and there will be a shakeout.

''What I think is ironic is the implication in all this talk about the 'post-McDonald's' generation that there is a need that isn't being met. The traditional hamburger chains are doing fantastically well. The big-three hamburger chains - McDonald's, Burger King, and Wendy's - which account for 30 percent of sales in the whole fast-food industry, had sales up 22 percent for the first quarter this year (over the first quarter of 1983), and that was up from a strong base.''