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