Mold is a normal part of food as it ages. It grows on almost all foods because it is set in motion when the food is exposed to excessive oxygen. However, there are specific foods that grow mold more quickly than others, and conditions in which they are more likely to mold. Mold in and of itself is not bad. Some molds can even be beneficial, but on the other hand other molds can be extremely dangerous to your health.
Mold is a fungus that lives on plants and animals, and is typically microscopic individually. It is when numerous fungi form colonies that it can be seen with the naked eye. Foods that grow fungi contain spores that give mold its coloring. Moreover, mold is often invisible until the bacteria have been on the food for a period of time, at which point you may have already ingested it.
Certain foods are notorious for getting moldy quickly, and that is largely due to their composition and storage methods. Moist, warm conditions are breeding grounds for mold, and therefore juicy foods or those that welcome moisture are the most likely to become moldy quickly. These foods include breads that are left in the cupboard or on top of the counter as opposed to being refrigerated, cheeses, and fruits and vegetables that are left for long periods of time in the refrigerator or out on the counter.
The timing in which bread molds has a lot to do with the water content. Some breads are dryer than others, such as Indian bread and pitas. Other breads have higher water content, such as breads containing fruits, white and wheat bread, and some brown breads. The breads that are dryer have a much longer shelf life; those that are frequently moister can have a longer shelf life by storing them in an airtight container in the refrigerator.
Cheeses, much like breads, depend upon their water source and bacteria present at the time of creation to determine how quickly they will mold. Cheeses with a lot of moisture, such as ricotta, cottage cheese and other soft cheeses, are most likely to mold very quickly, as bacteria will be able to grow easily. Hard cheeses such as Parmesan will remain healthy and keep their taste much longer than cheddar or mozzarella. In some cases, a healthy mold is grown on the cheese on purpose, as with bleu cheese, since it is what provides the cheese with its strong flavor.
Fruits and vegetables should typically be consumed within a few days of purchase to avoid ingesting mold. Refrigerate produce and check it prior to eating, as sometimes mold can be in one area of a fruit or vegetable and not another. Again, moisture and temperature play a large part in the quickness to mold. As moisture allows bacteria to breed, juicier fruits and vegetables, such as peaches, plums, tomatoes and cucumbers, are more likely to get moldy quicker than are those with harder peels and rinds, such as bananas, apples, peppers and squash. You can cut off moldy parts of harder fruits and vegetables and use the rest of them, but you should consume them right away if you do.