Place cornstarch in a medium bowl and whisk in pineapple juice, soy sauce, ketchup, garlic, brown sugar, vinegar, sesame oil, curry powder, ginger, pepper flakes (if using), salt and pepper. Set aside. 1 can pineapple tidbits in juice, 1 tablespoon cornstarch, ¼ cup soy sauce, ¼ cup ketchup, 2 cloves garlic, 2 tablespoon packed brown sugar, 2 tablespoon cider vinegar, 1 tablespoon sesame oil, 2 teaspoon curry powder, 1 teaspoon ground ginger, ½ teaspoon red pepper flakes, ½ teaspoon salt, ½ teaspoon black pepper,