Common agents of food spoilage and deterioration include temperature, light, moisture, microbial growth and oxygen. Certain oxidizing enzymes that occur naturally in food can also cause degradation. Food that is spoiled can be detected by changes in how it feels, smells, appears or tastes.
Excessive heat increases the rate of natural enzyme reactions and of other food constituents. Too much heat causes proteins to break down and some vitamins to be destroyed while foods dry out and moisture is lost. Excessively cold temperatures that cause food to freeze can lead to spoilage by breaking emulsions or cracking the food's surface which can lead to microbial contamination.
Bacteria, yeasts and molds also cause food to spoil by breaking down fats and proteins. Meat and fish are especially vulnerable to naturally occurring bacteria.
Fats, pigments, vitamins and proteins in food can be degraded when foods are exposed to light. Liquid foods are particularly sensitive to light deterioration.
Moisture content can affect spoilage by encouraging microorganism growth and allowing chemical reactions between components in the food. Excessive moisture encourages molds to grow, especially in cereal and grain. Moisture may cause the caking and lumping of dry mixes and powders.
Oxidation of fats forms compounds with strong, undesirable odors and flavors. Certain enzymes speed up reactions between oxygen and food, leading to spoilage.