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 tbsp cornstarch, ¼ cup soy sauce, ¼ cup ketchup, 2 cloves garlic, 2 tbsp packed brown sugar, 2 tbsp cider vinegar, 1 tbsp sesame oil, 2 tsp curry powder, 1 tsp ground ginger, ½ tsp red pepper flakes, ½ tsp salt, ½ tsp black pepper,