You can give only ripe eggplant to guinea pigs, without green parts. Dangerous green parts, and the unripe fruit contain solanine. Special granulated food guinea pigs should receive daily. Most guinea pigs do not overeat, but the amount may need to be limited if the mumps starts gaining excess weight.
Choose a good quality food, designed specifically for guinea pigs. Since vitamin C in feeds loses its activity over time, look for food that uses a stabilized form of vitamin C, or at least pay attention to its freshness and the date of manufacture. Purchase food in small quantities and store in a cool, dark place.
For growing guinea pigs, alfalfa-based food is suitable. And for adults, it is better to find food based on hay from timothy grass.
Most of the feeds for guinea pigs contain vitamin C in their composition, but unfortunately, this vitamin is rather unstable and, over time, collapses. Keep dry food for guinea pigs in a cool dark place, this will help preserve vitamins in them.