Vegetarianism is more popular than ever, but without meat, vegetarians must find new sources of essential protein. For vegans--vegetarians who don't eat eggs or cheese--this is even more imperative. Three of the best-known non-animal protein foods available are tofu, tempeh and seitan.

Tofu, the most common, is made by separating soybean milk's curd from its whey with a coagulant, then forming the curd into blocks that are packed in water. It's everywhere--this writer ate a delicious tofu dish in a restaurant on Venice's Grand Canal. In addition to tofu itself, connoisseurs can buy tofu hot dogs, tofu cheese, tofu ice cream, even bagels with tofu cream cheese.

Tempeh is a chunky, fermented food made from soy, with grains often added. Held together by a harmless mold, it is usually fried and sold in cakes. Seitan is wheat gluten marinated in tempura sauce and seaweed. Nasoya tofu contains 9 grams of protein per serving; Lightlife tempeh contains 12 grams of protein per serving; and Up Country seitan contains 15 grams of protein per serving.

Joe Holtz, food coordinator of the Park Slope Food Co-op in Brooklyn, says tofu is most popular because it is a staple of Chinese cooking--"Everyone who's ever gone to a Chinese restaurant knows what tofu is. Tempeh comes from Indonesia, and how many Indonesian restaurants are there? We probably sold tofu first, then tempeh, then seitan."

Tempeh is rapidly gaining popularity because of articles in health food magazines, adds Bill Hardesty, one of the managers of Perelandra, a health food store also located in Brooklyn. Still, he said, tofu is easier to cook because of its soft, pliable texture.

Ease is why Ellen Bloom of Loch Sheldrake, N.Y., a retired teacher, prefers tofu for cooking. "Tofu blends into everything, absorbs the flavor, while tempeh has its own special taste," says Bloom. "I use tempeh crumbled up with vegetables or rice, but I use tofu with those things plus everything else. I even mix tofu with spaghetti or tuna." At restaurants, she adds, she enjoys all three protein foods.

Soy, whether tofu or tempeh, may be "so important that it will someday be considered an essential nutrient," says Manhattan nutritionist Richard Kowal. "A lot of exciting research is going on with soy--it reduces the risk of breast cancer. Since vegetarians usually don't have enough protein, it is important that they eat soy at least twice a day." Tempeh has an added edge, according to Kowal--"It's a completely balanced food because it contains both protein and carbohydrates, while tofu is only protein."

As for seitan, Kowal points out that some people have an allergy to gluten or have trouble digesting it. Still, seitan's selling point is its remarkably meat-like taste and texture. If, at a Chinese restaurant, you order "mock beef from wheat gluten," you're ordering seitan.