I'd say insects because the question says through infestation and insects infest and invade
enzymes, enzymes are wildly used in nutrition industry. https://www.creative-enzymes.com/cate/Health-Diet-And-Nutrition_109.html
i think it's bacteria. (in my opinion) im not sure about this answer though!
its yeast it makes it harder which decreses its nutritnal value but it could be bacterie since yeats IS bactiria
The clue to get the correct answer is "infestation".
When we talk about infestation, we are referring to the presence of insects, more than usual, which infest causing damage and diseases. So, we can see infestation as insect invasion. Then, those insects will leave things like waste or eggs, which will affect the zone, causing contamination, and of course, it will reduce drastically nutritional values, because that food won't be for human consumption.