Amaranth, a plant "lost" for 500 years after the Spanish conquistadors saw it used in ceremonies of human sacrifice, may turn out to be the popcorn of the 1990s and a flour of the 21st century.

The high-protein grain was cited at an American Association for the Advancement of Science symposium here today as one of the foods of the future.

Initially, it will be an exotic product to be eaten occasionally, but eventually it may be as commonplace as avocados have become over the past decade, according to Dr. Noel Vietmeyer of the National Academy of Sciences.

Amaranth was once a staple of the Aztec diet, but the Spanish, seeing it mixed with blood in conjunction with human sacrifice, banned its cultivation. "It died as a crop," Vietmeyer said, and continued to be cultivated only in remote hills of Mexico and in parts of Asia.

Containing 16 to 18 percent protein, considerably more than most cereals, amaranth is seen as a new source of protein as well as of lysine and other amino acids in which grains normally are deficient. When roasted, it pops and tastes like fine-kernel popcorn without salt or butter. It can be mixed with flour and made into bread, and can be grown everywhere, Vietmeyer said.

Foods like amaranth are being promoted by scientists for their nutritional value and hardiness. Some of these plants have particular promise in Third World countries where desert and salt conditions complicate the feeding of the population.

Quinoa, a staple of the Incan diet that has flourished in the high Andes for more than 3,000 years, was also mentioned here as a food of the future. Quinoa is another extremely hardy grain, according to David F. Cusack of the Agroclimate Development Program in Boulder, Colo. It grows in arid climates, swamps and at high altitudes, and can be used to make cereals, biscuits, desserts and liquor.

Fifty years ago, scientists might have hesitated to consider the introduction of such exotic new crops in this country. But within the last few decades, American tastes have become cosmopolitan. New waves of immigrants have brought with them tastes and products previously unknown in American supermarkets. Air freight has made it possible for fruits like the kiwi from New Zealand to become widely available.

The increasing costs of transcontinental shipping have also opened up new markets for locally grown products that will in turn add variety to the American diet. Vietmeyer said more kinds of apples from regional orchards may begin to reappear to supplement the standard handful of varieties that has come to dominate the market.

"Ten years ago people went to supermarkets based on meat," Vietmeyer said. "Now they pick their supermarkets for produce. Specialty products have become a very big thing."