Gregor has turned into a beetle or cockroach, so his tastes change to prefer insect food such as garbage. This helps symbolize and portray Gregor's dehumanization.

Of course, Gregor has been internally dehumanized before he turns into a giant, grotesque insect. The outward form he now assumes only reflects what has happened to him on the inside. He has been transformed from a human being to a kind of worker insect whose sole meaning in life is to provide money to support his family.

Interestingly, while the insect-Gregor is willing to turn to garbage for food, he clings to the part of his humanity which he still has left, which is his love of the arts. He tries to prevent Grete from removing a painting in his room, and he is drawn to the beautiful music he hears. He is also willing to die so as not to be a burden to his family.

Food in the end represents the pressures of material reality that destroy Gregor. Food represents the "garbage" of having to support the corporeal with a meaningless job rather than nurture his artistic side. It is no accident that a piece of food, an apple, kills him.