NutraSweet Co. will begin offering a new line of low-sugar, low-fat desserts in Chicago and San Francisco at Mrs. Fields Bakery Cafes, spokesmen said.

The Desserve line, featuring low calorie frozen yogurt, cheesecakes, pies and cobblers, will be offered on a test market basis. Spokesmen said specialty deli salads and sandwiches will also be offered in the Mrs. Fields Bakery Cafes.Debbi Fields, president of Mrs. Fields Inc., based in Park City, Utah, , said "NutraSweet is recognized as the industry leader in the art of fat and sugar reduction - without compromising taste. Together, we can offer something for everyone, including those looking for healthy dessert alternatives that don't compromise taste."

The Desserve products, developed by NutraSweet, already are available in the Chicago area.

Company spokesmen said the Desserve products will be offered in Mrs. Fields Bakery Cafes in Chicago by the end of the year. San Francisco will begin offering the Desserve line of products in early 1991.