Marinades are used both to flavor and to tenderize meats before cooking. There is no need to tenderize the meat for American- and European-style soups, because the long cooking will break down the meat’s fiber and tenderize it anyway, or for Asian-style soups that call for an already tender (and expensive) cut. But when an Asian-style soup recipe calls for an expensive cut of meat (many recipes just say “steak”), you can replace it with a less expensive, tougher cut and tenderize it using some ingredient in the marinade to help break down the fiber and soften it. Finely chopped tropical fruits such as fresh mango, papaya, and pineapple are the best natural meat tenderizers. Acidic ingredients such as lime juice, wine, or vinegar are also effective tenderizers. Marinate the meat strips in a mixture containing 2 tablespoons finely chopped tropical fruit for about an hour.