Demand for food is highly elastic. Food is not a homogenous substance that costs the same to produce each kind. When people are feeling more flush, they will consume more expensive food products (lobster, beef, fish, berries, mussels) and when they are feeling more price conscious they will economize on cheaper products (wheat, lentils, rice, tofu, chicken, beans, vegetables), and by wasting less food. We're talking about literally thousands of different ingredients and products, produced in different ways in different regions, that can to one extent or an other be substituted for each other.