Food spoilage

The food microbiologist must balance the use of microbes to make foods with the need to keep food fresh.  Many pathogenic and nonpathogenic microorganisms take advantage of improperly protected foods as carbon and energy sources, rendering the food dangerous to eat or inedible. Preservation of food can take the form of storing the food under conditions that do not favor the growth of most microorganisms. Most approaches aim to reduce the amount of water available for growth (the water activity) by altering the salt concentration, pH, temperature, osmotic potential or hydration of the foodstuff.

Few organisms that can  use  human foodstuffs for growth are  capable of extremophilic growth, so food will be more suitable for long-term storage if it is stored at very low pH (e.g. sauerkraut) or very high  salt  (e.g. cured meats such as bacon, salt  beef,  salt  cod). Many fermentation processes leave the foodstuff in such a state. Low temperature (0–4?C) inhibits the growth of organisms, but food spoilage can still occur by the action of psychrophiles such as Pseudomonas spp.  Bacterial growth is very restricted below  -5^?C, but  fungi,  yeasts, and  molds can  continue slow growth at as low as -12^?C (e.g. Debaryo­ myces  spp.).  Very high sugar concentrations prevent the growth of most Bacteria. Jams, preserves, and chutneys use sucrose and fructose to prevent Bacterial spoilage, although some fungi and molds can still grow under these conditions.

Many foods become unpalatable when these approaches are used. In this case, the food microbiologist will try to eliminate or greatly reduce the numbers of microorganisms in foods.  Sterilization by heat can be used to preserve foods such as milk for long periods as long as the food remains protected from contamination. However, very high temperatures (100–121^?C) must be used to accomplish complete sterilization, which can alter the taste of the food considerably. More recently exposure to radioactive material has also been used to sterilize food.

Pasteurization provides a method for medium-term storage of milk, liquid egg or fruit juices while preserving palatability. The temperature and time  used for pasteurization are dependent on the food, with egg being heated for 2.5 minutes at 64.4^?C, whereas milk is heated at 71.7^?C for 15 seconds. The advent of pasteurization of milk was a major step forward in public health, reducing the incidence of brucellosis to virtually zero in the Western world. However, the process is designed to kill pathogens and has the side-effect of increasing shelf-life. Eventually pasteurized products will spoil.