In a large mixing bowl, mix cornstarch, cayenne, garlic, soy sauce and 1 tablespoon of the oil. Add shrimp, tossing to coat. Let stand for 15 minutes. In large fry pan or wok, heat remaining oil. Add shrimp mixture and vegetables. Stir fry for 3 minutes over medium-high heat. Add grapefruit juice; cover and cook over low heat for 4 minutes. Add grapefruit sections and heat, covered, until grapefruit is heated, about 2 minutes. Makes 4 servings.