Thermal Stabilization

THERMAL STABILIZATION

The use of heat to sterilize is dependent on the magnitude (T), duration (t), and amount of moisture present.

T ∝ 1/T

It is thought that the heat coagulates protein in the living cell. The temperature required for this phenomenon to occur is inversely proportional to moisture present.

Dry Heat

Relatively stable substances that resist degradation at high temperatures ( >313 K) are suitable candidates for dry heat sterilization. Two-hour exposure at 353 K or 45-minute exposure at 433 K kills spores as well as vegetative forms of microorganisms. These exposure periods do not include the lag time from loading of the oven until sterilization temperature is achieved. The lag time depends on the geometry and operating features of the oven and characteristics of the load.

The oven types that can be employed are natural and forced convection, both of which are described in chapter 7, “Drying.” The forced convection oven offers the advantages of uniformity of heat distribution and reduction in lag time in comparison with the natural convection system.

This method is reserved almost exclusively for glass or metal, as other materials char (cellulose), oxidize (rubber), or melt (plastic) at these temperatures.

Moist Heat

Moist heat offers the advantage of greater effectiveness at low temperatures. The thermal capacity of steam is much greater than that of hot air.

Spores and vegetative forms of bacteria may be effectively destroyed in an autoclave employing steam (394 K) under pressure (1.03 x 10⁵ N/m²) for 20 minutes or (1.86 x 10⁵ N/m² at 405 K) for 3 minutes. The lag time to complete exposure of the material to be sterilized is important.