Food spoilage is caused by bacteria, yeasts, fungi, the food's own enzymes, insects, temperature fluctuation and oxidation. When eaten, food that is spoiled by bacteria or other microbes presents a disease hazard to humans.
Controlling heat, moisture and pH helps to inhibit spoilage by preventing microbial growth and slowing down the food's natural breakdown process. Cooking food deactivates enzymes and kills bacteria and fungi, but this process is only permanent for enzymes. Bacteria and fungi re-infect the cooked food unless measures to prevent their return, such as temperature and humidity control, are taken.
Changes caused by enzymes, oxidation, low temperatures or light are generally cosmetic. They induce undesirable changes in the food's flavor, texture or appearance, but do not render the food unsafe to eat. With the exception of low temperature injury, this damage occurs in conditions that allow for microbial growth; the food may also be contaminated with bacteria or mold and unsafe to eat for that reason. Low temperature injury is an exception because it occurs only in conditions that inhibit microbial growth.
Different foods are more vulnerable to different types of spoilage. Bacterial growth is most prevalent in low-acid, high-protein foods such as meats and dairy products, while yeast growth occurs in food with a high sugar content. Both mold and yeast are capable of spoiling high-acid foods, making them more common in fruit and vegetable spoilage.