Answer to The Fatal Feast

The killer was Madam Bing. The poison was in the ice cubes. Since the food taster drank first, the ice cubes had not melted and released the poison, so the drink had no effect on him. After 10 minutes, the ice cubes had started to melt and release the poison into Mr. Xie's drink (which he later dropped onto his food). Madam Bing knew where the poison was, which was why she focused on eating rather than drinking. She was feigning sickness, assuming that if she was seen as a victim too, it was unlikely anyone would suspect her.