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@Carl_Pham can you please help me?
It depends what you mean. Most obnoxious spoilage -- smelly, slimy results -- come from the action of bacteria, and what they are doing is digesting the food and excreting their own waste products, so you are gradually replacing the food with a bacterial colony and its sludge products. On the other hand, the part of spoilage that consists of going rancid, turning brown, stuff like that, is the result of direct oxidation of the molecules in the food by atmospheric oxygen. (This is one reason various substances are added to packaged foods "to prevent spoilage" which are chemical that absorb oxygen.)
So, roughly speaking, you have two chemical processes:
(1) Direct oxidation of the chemical constituents of the food (proteins, carbohydrates and fats), which usually results in depolymerization (reduction of the big molecules to smaller molecules), which produces changs in color, taste and consistency.
(2) Digestion by bacteria. Digestion is a number of complicated chemical processes, but generally involves depolymerization as well. Then you have the presence of the bacteria themselves, and their excretions.
Hope that helps. If you have more specific questions, ask.