These ingredients must appear in food ingredient lists. BHT is used as a preservative ingredient in some foods. Canola oil is a key ingredient in many foods. Many skin whiteners contain a form of toxic mercury as the active ingredient. Blueberry sauce is a sweet sauce prepared using blueberries as a primary ingredient. The principal active ingredient in Paregoric is powdered opium. Tomato soup is a soup made with tomatoes as the primary ingredient. Avocado is a primary ingredient in avocado soup. Johnson was unable to find any evidence that homeopathic preparations contain any active ingredient. Caffeine, the major active ingredient in coffee, is associated with anxiety. This is a list of potato dishes that use potato as a main ingredient. Purple yam is an essential ingredient in Undhiyu. Myrrh is a common ingredient of tooth powders. It is a principal ingredient in curry powders. Potato chunks also commonly appear as a stew ingredient.