Meat can spoil or go bad due to a large number of factors, but the most common is microorganisms, such as mold and other bacteria. Microorganisms present in meat cause proteins and fats to break down, spoiling the meat, or breed and reach levels that are unsafe or unsavory for humans to eat.
Once an animal is killed and butchered, the cells in its meat start to break down and cannot be replaced by new cells. As a result, meat can spoil if it is not consumed quickly or stored properly. The chemicals in the meat slowly deteriorate to a point where the meat is unfit for consumption. Some conditions, such as exposure to light and heat, can speed this process.
Bacteria that were either living in the flesh of an animal while it was alive or that found a new home after an animal was butchered can spoil meat for consumption. Any sour, sulfuric or rotten odor indicates that a colony of bacteria has reached unsafe levels, as does a rancid taste or a chalky texture.
Mold likes moist, warm places with lots of food sources — meat makes a great home for a mold colony. Mold can spoil meat by spreading over the surface in fuzzy or colorful patches that change the taste and texture of the meat in a way that most people find unsavory and consider spoiled.
Improper packing techniques can cause a chemical reaction in the meat called oxidation. The fats in the meat react with oxygen molecules and cause the meat to go rancid, producing discoloration and a rotten, sour smell and taste.
References and ResourcesPurdue University: Meat Quality and Safety
Food Service; Why Does Food Spoil?; Lacie Thrall