Bacteria, molds and yeasts are the main causes of the spoilages of unpreserved foods, with bacteria playing the major role in the spoilage of meats, poultry, dairy, and fish products.
Mold and yeast play the major role in the spoilage of fruits and vegetables. The changes in foods due to microbial action can be classified into two types: undesirable changes and desirable changes.
These microorganisms do not cause disease but they spoil food by growing in the food and producing substances which alter the color, texture and odor of the food, making it unfit for human consumption.
All bacteria associated with foods are small. Most are of the order of one to a few micrometers in length and somewhat smaller than this in diameter. All bacteria can penetrate the smallest openings; many can pass through the natural pores of an egg shell once the natural bloom of the shell is worn.
Yeasts are somewhat larger, of the order of 20 um or so in individual cell length and about a third this size in diameter.
Typically, the effects of temperature, pH, and the acidulants, water activity and the humectants, oxygen availability, redox potential and identity and concentration of inhibitory agents are among the variables that must be considered when assessing whether microorganisms will pose a problem in particular food system.
Effects of microorganisms on food
