Non Sterilizing Methods to Control Microbial Growth

Many physical and chemical technologies are employed by our civilization to control the growth of (certain) microbes, although sterility may not the desired end-point. Rather, preventing spoilage of food or curing infectious disease might be the desired outcome.

Applications of Heat

The lethal temperature varies in microorganisms. The time required to kill depends on the number of organisms, species, nature of the product being heated, pH, and temperature. Autoclaving, which kills all microorganisms with heat, is commonly employed in canning, bottling, and other sterile packaging procedures. This is an ultimate form of preservation against microbes. But, there are some other uses of heat to control growth of microbes although it may not kill all organisms present.

Boiling: 100^o for 30 minutes (more time at high altitude). Kills everything except some endospores. It also inactivates viruses.  For the purposes of purifying drinking water, 100^o for five minutes is a "standard" in the mountains" though there have been some reports that Giardia cysts can survive this process. Longer boiling might be recommended for Mississippi River water the closer to the Gulf.

Pasteurization is the use of mild heat to reduce the number of microorganisms in a product or food. In the case of pasteurization of milk, the time and temperature depend on killing potential pathogens that are transmitted in milk, i.e., staphylococci, streptococci, Brucella abortus and Mycobacterium tuberculosis. But pasteurization kills many spoilage organisms, as well, and therefore increases the shelf life of milk especially at refrigeration temperatures (2°C).

Milk is usually pasteurized by heating, typically at 63°C for 30 minutes (batch method) or at 71°C for 15 seconds (flash method), to kill bacteria and extend the milk's usable life. The process kills pathogens but leaves relatively benign microorganisms that can sour improperly stored milk.

During the process of ultrapasteurization, also known as ultra high-temperature (UHT) pasteurization, milk is heated to temperatures of 140 °C. In the direct method, the milk is brought into contact with steam at 140°C for one or two seconds. A thin film of milk falls through a chamber of high-pressure steam, heating the milk instantaneously. The milk is flash cooled by application of a slight vacuum, which serves the dual purpose of removing excess water in the milk from condensing steam. In the indirect method of ultrapasteurization, milk is heated in a plate heat exchanger. It takes several seconds for the temperature of the milk to reach 140°C, and it is during this time that the milk is scalded, invariably leading to a burned taste. If ultrapasteurization is coupled with aseptic packaging, the result is a long shelf life and a product that does not need refrigeration.

A review of protocols and recommendations for the use of heat to control microbial growth is given in Table 1.

Low temperature (refrigeration and freezing): Most organisms grow very little or not at all at 0^oC. Perishable foods are stored at low temperatues to slow rate of growth and consequent spoilage (e.g. milk). Low temperatures are not bactericidal. Psychrotrophs, rather than true psychrophiles, are the usual cause of food spoilage in refrigerated foods. Although a few microbes will grow in supercooled solutions as low as minus 20^oC, most foods are preserved against microbial growth in the household freezer.

Drying (removal of H₂O): Most microorganisms cannot grow at reduced water activity (A_w < 0.90). Drying is often used to preserve foods (e.g. fruits, grains, etc.). Methods involve removal of water from product by heat, evaporation, freeze-drying, and addition of salt or sugar.

Irradiation (UV, x-ray, gamma radiation): destroys microorganisms as described under "sterilization". Many spoilage organisms are readily killed by irradiation.

In some parts of Europe, fruits and vegetables are irradiated to increase their shelf life up to 500 percent. The practice has not been accepted in the U.S.  UV light can be used to pasteurize fruit juices by flowing the juice over a high intensity ultraviolet light source. UV systems for water treatment are available for personal, residential and commercial applications and may be used to control bacteria, viruses and protozoan cysts.

The FDA has approved irradiation of poultry and pork to control pathogens, as well as foods such as fruits, vegetables, and grains to control insects, and spices, seasonings, and dry enzymes used in food processing to control microorganisms. Food products are treated by subjecting them to radiation from radioactive sources, which kills significant numbers of insects, pathogenic bacteria and parasites.

According to the FDA, irradiation does not make food radioactive, nor does it noticeably change taste, texture, or appearance.  Irradiation of food products to control food-borne disease in humans has been generally endorsed by the United Nation's World Health Organization and the American Medical Association. Two important Disease-causing bacteria that can be controlled by irradiation include Escherichia coli 0157:H7 and Salmonella species.

Control of microbial growth by chemical agents

Antimicrobial agents are chemicals that kill or inhibit the growth microorganisms. Antimicrobial agents include chemical preservatives and antiseptics, as well as drugs used in the treatment of infectious diseases of plants and animals. Antimicrobial agents may be of natural or synthetic origin, and they may have a static or cidal effect on microorganisms.  

Types of antimicrobial agents

Antiseptics: microbicidal agents harmless enough to be applied to the skin and mucous membrane; should not be taken internally. Examples include alcohols, mercurials, silver nitrate, iodine solution, alcohols, detergents.

Disinfectants: agents that kill microorganisms, but not necessarily their spores, but are not safe for application to living tissues; they are used on inanimate objects such as tables, floors, utensils, etc. Examples include, hypochlorites, chlorine compounds, lye, copper sulfate, quaternary ammonium compounds, formaldehyde and phenolic compounds.

Common antiseptics and disinfectants and their uses are summarized in Table 2. Note: disinfectants and antiseptics are distinguished on the basis of whether they are safe for application to mucous membranes. Often, safety depends on the concentration of the compound. 

Preservatives: static agents used to inhibit the growth of microorganisms, most often in foods. If eaten they should be nontoxic. Examples are calcium propionate, sodium benzoate, formaldehyde, nitrate and sulfur dioxide. Table 3a and 3b are lists of common preservative and their uses.