Microorganisms are of great significance to foods for the following reasons: (1) microorganisms can cause spoilage of foods, (2) microorganisms are used to manufacture a wide variety of food products, and (3) microbial diseases can be transmitted by foods.

Foods can be considered as a medium for microbial growth. Considering the vast array of sources, substances, and methods with which food is produced, practically every kind of microbe is a potential contaminant. Given a chance to grow, microbes will produce changes in appearance, flavour, odour, and other qualities of the food. The changes vary according to the type of food degraded but can be summarized by examining the fates of the major nutrients found in food: proteins, carbohydrates, and fats.

Protein-containing foods, particularly meats, are putrefied by organisms (e.g., Proteus, Pseudomonas, and Clostridium bacteria) that break down the long peptide chains of proteins into amino acids and foul-smelling compounds such as amines, ammonia, and hydrogen sulfide (H2S). Carbohydrates (sugars and starches) are fermented into acids (e.g., the acetic acid in vinegar), alcohols, and gases, especially carbon dioxide. This process is responsible for the bursting of spoiled chocolate cream candies by yeasts. Fat-containing foods such as dairy products are spoiled by microbes that break down lipids into fatty acids and glycerol. Rancid milk, which can be caused by bacteria, yeast, or mold, is an example of this process.

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