Controlling Microbial Growth

Heating

Control of microbial growth via heating is accomplished with dry heat, moist heat, or pasteurization.

Physical control of microbes relies on killing microbes through the use of methods such as heat. Physical control of microbes is used more often than chemical control when the goal is sterilization rather than disinfection.

Heat is the best method of sterilization for materials not damaged in the heating process. Heat can penetrate surfaces deeply and is relatively safe and inexpensive. Heat can kill bacteria and fungi and inactivate viruses. There are two measurements of sterilization by heating: thermal death point and thermal death time. Thermal death point is the temperature at which all target microbes in a culture are killed in a given time frame. Thermal death time is the time required to completely kill a specific target microbe in a specific culture at a specific temperature. Each measurement has practical applications specific to the desired outcome. For example, in a medical setting the goal may be to sterilize equipment as quickly as possible, so thermal death point is preferred because sterilization can be completed quickly at high temperatures. However, in food preparation, the goal may be to retain the flavor and texture of the food, so thermal death time is preferred because higher temperatures might ruin the food.

The three types of heat sterilization are dry heat, moist heat, and pasteurization.

Dry heat includes ovens and flames and is useful for the sterilization of metal and glass because metal can corrode when exposed to moisture. In ovens, objects are sterilized when subjected to 180°C for 30 minutes, 170°C for one hour, or 160°C for three hours. Flames are used to sterilize glass and metal objects during use, such as when inoculating cultures.

Moist heat has excellent penetrating power. The simplest method of moist heat sterilization is boiling. However, although boiling kills vegetative bacterial cells and many fungi and destroys some viruses, it is ineffective against endospores. Boiling water under pressure increases the temperature at which water boils, which aids in killing the more heat resistant microbes. This is accomplished using an autoclave, a machine that uses steam under very high pressure to sterilize surfaces and fluids. An autoclave can achieve a pressure of 15 pounds per square inch, which raises the boiling point of water to 121°C, high enough to kill endospores and destroy many viruses in 15 minutes. Importantly, the benefit of the pressure of the autoclave is indirect—the increased temperature of the steam is what kills the additional microorganisms, not the pressure itself. Moist heat is often preferred because it takes less time and energy to sterilize objects. Moist heat is used to disinfect culture media since the steam can clean porous material well. Quality control measures assure that appropriate sterilization conditions are reached during each autoclave run. These measures include include methods such as sensors and recorders. They also include simpler methods such as autoclave tape and biological indicator spore tests.

Autoclave

An autoclave uses moist heat to sterilize objects, surfaces, and liquids. The steam is fed into a jacket and from there flows into the steam chamber. Air is then removed from the steam chamber. When steam hits the automatic ejector valve, the valve closes, ensuring the steam chamber is kept full of steam.
Pasteurization is another form of microbial control that uses heat. Pasteurization is the process of heating liquids in the absence of oxygen to kill microbes that often make people sick or spoil liquids such as milk. Importantly, pasteurization disinfects, rather than sterilizes, liquids. There are two pasteurization types, depending on the desired outcome of the process. High-temperature short-time (HTST) pasteurization exposes the liquid to a temperature of 72°C for 15 seconds, while ultra-high temperature (UHT) pasteurization exposes the liquid to a temperature of 138°C for 2 or more seconds. Liquids pasteurized via UHT pasteurization can be stored without refrigeration, but the higher temperatures alter the structures of some proteins, giving the liquid an altered taste and smell. Milk, beer, wine, and juices are commonly pasteurized to eliminate dangerous bacteria such as species of Salmonella, Mycobacterium, Campylobacter, Coxiella, Yersinia, Listeria, and Escherichia (E.coli O157:H7). While pasteurization is a safe, highly effective method for controlling microbes affecting food and beverages, it is not sterilization. Thus, even pasteurized foods eventually spoil.

Process of Pasteurization

Pasteurization is a heating method used to disinfect liquids and kill dangerous microbes that can make people sick. The liquid is sent through a system that first heats the liquid to either 72°C for 15 seconds (high-temperature short-time) or 138°C for 2 or more seconds (ultra-high temperature). This denatures the enzymes in the liquid and renders them unable to aid in microbial growth. The liquid is then cooled and prepared for consumption.