Physical Control of Microorganisms: Heat Sterilization

Introduction and Importance:

The killing effect of heat on microorganisms has long been known. Heat is fast, reliable, and relatively inexpensive, and it does not introduce chemicals to a substance, as disinfectants sometimes do. A visit to any supermarket will demonstrate that heat preserved canned goods represent one of the most common methods of food preservation. Laboratory media and glassware, and hospital instruments, are also usually sterilized by heat.

Mode of Action:

Above maximum growth temperatures, biochemical changes in the cell's organic molecules result in its death. These changes arise from alterations on enzyme molecules and the resultant changes to the three dimensional shape of proteins inactivate proteins or chemical breakdown of structural molecules, especially in cell membranes. Heat also droves off water, and since all organisms depend on water, this loss may be fatal. Heat appears to kill microorganisms by denaturing their enzymes.

Principles and Applications of Heat Killing:

Heat—Preferred Agent of Sterilization:

Heat is preferred agent of sterilization for all material not damaged by it. It rapidly penetrates thick materials not easily penetrated by chemical agents.

Measurements to Determine Killing Power:

The killing rate of heat may be expressed as a function of time and temperature. For example, tubercle bacilli are destroyed in 30 minutes at 58^oC, but in only 2 minutes at 65^oC, and in a few seconds at 72^oC. Several measurements have been defined to quantify and killing power of heat. The thermal death point is the temperature that kills all the bacteria in a 24-hour-old broth culture at neutral pH in 10 minutes. Factor to be considered in sterilization is the length of time required.The thermal death time is the time required to kill all the bacteria in a particular culture at a specified temperature. Both TDP and TDT are useful guidelines that indicate the severity of treatment required to kill a given population of bacteria.

Decimal reduction time (DRT or D value) is a third concept related to bacterial heat resistance. DRT is the time, in minutes, in which 90% of a population of bacteria at a given temperature will be killed (in table 1, DRT is 1 minute). (The temperature is indicated by a subscript: D80°C, for example.)

Table 1	Microbial Death Rate: An Example
Time (mm)	Death per Minute	Number of Survivors
0	0	1,000,000
1	900,000	100,000
2	90,000	10,000
3	9000	1000
4	900	100
5	90	10
6	9	1

Significance of Measurements:

These measurements have practical significance in industry as well as in the laboratory. For example, a food-processing technician wanting to sterilize a food as quickly as possible would determine the thermal death point of the most resistant organisms that might be present in the food and would employ that temperature. In another situation it might be preferable to make the food safe for human consumption by processing foods containing proteins that would be denatured, thereby altering their flavor or consistency. The processor would then need to know the thermal death time at the desired temperature for the most resistant organism likely to be in the food.

Factors Important for Determination of Time and Temperature:

When determining the time and temperature for microbial destruction with heat, certain factors bear consideration.

1) Type of Organism:

One factor is the type of organism to be killed. For example, if materials are to be sterilized, the physical method must be directed at bacterial spores. Milk, however, need not be sterile for consumption, and heat is therefore aimed at the most resistant vegetative cells.

2) Type of Material:

Second factor is the type of material to be treated. Powder is subjected to dry heat rather than moist heat, because moist heat will leave it soggy. Saline solutions, by contrast, can be sterilized with moist heat but are not easily treated with dry heat.

3) Presence of Organic, Acidic or Basic Material:

Third important factor is the presence of organic matter and the acidic or basic nature of the material. Organic matter may prevent heat from reaching microorganisms, while acidity or alkalinity may encourage the lethal action of heat.