With the beginning of agriculture and a decreasing dependence on hunting and gathering, the need to preserve surplus foods became essential to survival. Despite a long tradition of efforts to preserve food from spoilage, it was not until the nineteenth century that the microbial spoilage of food was studied systematically. Louis Pasteur established the modern era of food microbiology in 1857, when he showed that microorganisms cause milk spoilage. Pasteur’s work in the 1860 proved that heat could be used to control spoilage organisms in wines and beers.
Removal of Microorganisms
Microorganisms can be removed from water, wine, beer, juices, soft drinks, and other liquids by filtration. This can keep bacterial populations low or eliminate them entirely. Removal of large particulates by prefiltration and centrifugal ion maximized filter life than pasteurized to better preserve the flavor and aroma of the original product.
Refrigeration at 5 C retards microbial growth, although with extended storage, microorganisms eventually grow and produce spoilage. Slow microbial growth at temperature below – 10 C has been described, particularly with fruit juice concentrates, ice cream and some fruits. Some microorganisms those are very sensitive to cold and their numbers are reduced. Thus although refrigeration slows the metabolic activity of most microbes, it does not lead to significant decreases in overall microbial populations.
Controlling microbial populations in foods by means of high temperature can significantly limit disease transmission and spoilage. Canned food is heated in special containers called retorts at about 115 C for intervals ranging from 25 to over 100 minutes. The precise time and temperature depend on the nature of the food. Sometimes canning does not kill all microorganisms, but only those that will spoil the food. After heat treatment the cans are cooled as rapidly as possible, usually with cold water.
Pasteurization involves heating food to a temperature that kills disease-causing microorganisms and substantially reduces the levels of spoilage organisms. In the processing of milk, beers, and fruit juices by conventional low-temperature holding (LTH) pasteurization, the liquid is maintained at 62.8 C for 30 minutes. Products can also be held at 71 C for 15 seconds, a high temperature, short time (HTST) process; milk can be treated at 141 C for 2 seconds for ultra-high-temperature (UHT) processing. Shorter – term processing results in improved flavor and extended product shelf life. Such heat treatment is based on a statistical probability that the number of remaining viable microorganisms will be below a certain level after a particular heating time at a specific temperature.
Dehydration, such as lyophilization to produce freeze-dried foods, is a common means of eliminating microbial growth. Grains, meats, fish, and fruits were dried to improve self life.
Chemical Based Preservation
Various chemical agents can be used to preserve foods, and these substances are closely regulated by the US food and Drug Administration and are “generally recognized as safe”. They include simple organic acids, sulfite, ethylene oxide as a gas sterilant, sodium nitrite, and ethyl formate. These chemical agents may damage the microbial plasma membrane or denature various cell proteins. Other component interfere with the formation of nucleic acids, thus inhibiting cell reproduction.
Radiation, both ionizing and nonionizing, has an interesting history in relation to food preservation. Ultraviolet radiation is used to control population of microorganisms on the surface, but it does not penetrate food. The major method used for radiation sterilization of food is gamma irradiation from a cobalt – 60 source; however, cesium – 137 is used in some facilities. Gamma radiation has excellent penetrating power, but must be used with moist foods because the radiation produces peroxides from water in the microbial cells, resulting in oxidation of sensitive cellular constituents. This process of radappertization.
Electron beams can also be used to irradiate foods. The electrons are generated electrically, so they can be turned on only when needed. Also, this approach does not generate radioactive waste.