JAMES BEARD stood in the upstairs dining room of The Big Cheese one day last week posing for a photographer. In his hand was a plate of food playing the part of a prop. The photographer snapped and stopped. Beard nibbled. More snaps, more nibbles. Then it was done and Beard was free to sit down to eat some more (virtually the entire menu appeared as the luncheon progressed) and to talk.

Beard surely is America's leading food authority, for want of a better term to encompass his books, his marvelous curiostiy, encyclopedic knowledge and -- not least -- his dauntless appetite. To eat with him is fun. It's not just haute cuisine that intrigues him. Last week he was on a panel tasting fast food fried chicken for the New York Daily News. Now somewhere in the latter half of his 70s, the victim of several medical problems, he controls his intake, but his plate remains razor sharp. He shares his observations willingly and rarely offers pronouncements. sLike Matt Dillon of "gunsmoke," Beard's not pushy. But my money would be on him if someone foolishly instigated a quick-draw tasteout.

Talking with him is even more fun than eating, if only because Beard's patience and generosity is such that he finds himself confronted with more than a fair share of mundane fare. What does he talk about? He is intrigued by politics and world affairs, an avid reader of contemporary literature, a collector of Oriental antiques and, as a former actor, a devotee of opera and theater. But he talks mostly about food because that's what people want him to talk about.

On this particular day the focus soon turned to diets. Beard, in his turn, told of a week-long water diet offered at a spa in England and joined in speculation framing a make-believe "menu" of waters of the world to temp potential customers. This somehow led to talk of a book of recipies put together by prisoners during the last war which, logically enough, brought the conversation to food shops in New York. Beard announced he felt Macy's food emporium has surpassed Bloomingdale's. Everyong took note.

The appearance of a crepe stuffed with pasta inspired Beard to observe that the electric pasta machine is "god's greated gift to man." He chortled as he recalled Marcella Hazan, the leading teacher of Italian cooking, acknowledging the comparability, if not superiority, of his machine pasta to handmade. Then a few food names and food feuds were served up, invigorating the senses in the manner of a taste of brandy between courses of a banquet. Beard went on to recall Michael Field, a skilled teacher with a notorious lack of interest in dining well, and his fondness for two remarkable but radically different personalities, Elizabeth David and M.F.K. Fisher.

Barbara Witt, hostess for the luncheon, reminded Beard that they had met nearly 20 years ago when she approached him to write some material on American cooking for a U.S. exhibit at a European food fair. In time, of course, Beard did his successful "american Cookery." That came about, he explained, after the Perkins family, who held the rights to "Fanny Farmer," had turned down a proposal that Beard revise that classic. Years later, Beard "pushed through" the Fanny Farmer revision with his friend and protegee Marian Cunningham as principal editor. It was published last year by Knopf.

The food kept coming, and so did food tips: talk about a new "fruit dome" that encourages fruit to ripen, about the Devon cream that is being imported from England in bottles, about how the taste and digestive advantages of peeling sweet peppers before cooking with them, about how lucky we are to be "past the time when chefs sent in fluted mushrooms with every course."

Someone mentioned eating flowers. Beard, it turned out, had eaten two meals composed entirely of flowers, one in Japan and one in Thailand. He volunteered a favorite salad recipe: bibb littuce, raw. mushrooms and violets -- the flower and some of the stem "for crunch."

Beard had high praise for several items he was served and offered the cheese-oriented restaurant's young chefs encouragement and advice. Only after the double-diget tasting had ended did he offer the gentle criticism that "cheese is not a gentle food."

Enroute to another restaurant to meet a long-time fan there was time to ponder the rapidly growing popularity of pasta in this country and predict a fall-off in the cooking school boom. "people are beginning to judge in qualitative rather than quantitative dimensions," he said, "to demand value for money."

The sight of a stalk of celery on the bar at Nora's, the second restaurant, led him into a reverie about celery in the public market in Portland, Oregon, during his boyhood and a family that raised and sold "all sizes and types" of blanched celery, and the daughter of that family, who bore the unforgettable name of Xanthia Foltz.

"people use too much celery in salads," he announed, then spoke approvingly about the English custom of stuffing celery with cheese and sadly about the disapperance of celery and olives from restaurant tables. "but you can still get it at Jack's (in San Francisco)," he said brightening.

For an even less apparent reason than earlier twists in conversation, Beard was led back an early book, "paris Cuisine," and to another restaurant, a Russian place called Le Caneton. It was the Paris of the 1920s, but the memory is still clear. "we had blinis with caviar and shashlichs and lots of wine," Beard said. "it was the first time I'd eaten a meal that cost 100 francs (about $20 today) for two people."

Appropriately enough, the penultimate topic was taste. Beard, along with Barbara Kafka, has been conducting-tasting classes for his students. Pepper, oils, herbs are among the subjects."they love it, they discover things," he said. He himself is enthusiastic about a rice new to him, wild pecan rice from Louisiana, and about a Japanese steamer for cooking rice.Vinegar, however, "is being terribly misused nowadays. Some of those raspberry vinegars are very good. Others are just awful. You should do a tasting."

But there wasn't time. An interviewer awaited. There was need for a rest and then it would be time for another meal.

At an age when most people would have slowed down, James Beard steams on like a giant oceanliner. He is giving classes this spring in New York City, is scheduled to do a lecture cruise for the Norwegian-American line in July, classes in Oregon in August, go to San Francisco in October, to France for a pair of food shows in November. "Then I rather hope to rent my house (in New York) and settle down on the West Coast for six or eight months," he said.

If he does, it may be that the successor volume to "delights and Prejudices," the fine autobiography of his early years will finally take form. It's time.