It's not your imagination that the offerings on supermarket shelves are changing faster than you can try new products.

It's possible that 2,000 new items have appeared in your favorite grocery store in the past year.Every week, supermarkets add boxes of rice, pasta, potato, brownie, muffin and cake mixes - many made expressly for microwaving - frozen dinners, vegetable concoctions, chicken products, cereals and snack and dairy case items.

During the first 10 months of 1988, food manufacturers presented 6,928 new products, said Mark Friedman, editor of Gorman's New Product News, an industry newsletter. Items available in new sizes or in "new, improved" formulas are counted separately.

The 1988 year-to-date total is an 8 percent increase over the 1987 figure of 6,412 new products. Friedman said that represents a slowdown from 1986, when the number of new products increased 24 percent.

Friedman said the food industry is at the mercy of trends and changes in Americans' lifestyles. About 70 percent of households now have microwave ovens, he said, and the number of one- and two-person households is growing. That means food manufacturers are producing more microwave and single-serving products.

Health concerns are another factor in the volatile food market. "Now the current rage is cholestrol-reducing products like oat bran. . . . No manufacturer wants to be left behind," Friedman said.

Warren Martin, chairman if the marketing department at the University of Alabama at Birmingham said the spiraling number of new products is a long-term trend that creates a dilemma for supermarket executives.

"They're continually being barraged by companies wanting them to put new products on shelves," he said. "If you don't put new products out there, you're going to lose customers."

Winn-Dixie vice president Gus Bergstrom said new products are a fact of life for supermarkets.

"We don't look negatively at them at all. If we did that, we wouldn't have ever had frozen foods."

He noted that frozen foods were considered a fad when they were first introduced, but today the need for more space for frozen foods is one reason supermarkets are getting bigger.