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Learn about the comparison between moist heat sterilization and dry heat sterilization.
Microorganisms are killed by heat as a result of the inactivation of their proteins (including enzymes) and, as stated earlier, the heat is applied either in moist or in dry conditions in processes of sterilization called moist heat sterilization and dry heat sterilization, respectively. The conditions and mechanisms of these two lethal processes of sterilization are not the same.
It is important, therefore, to differentiate between moist heat sterilization and dry heat sterilization in any procedure for microbial control. Moist heat kills microorganisms by coagulating their proteins quite rapidly and effectively. When wet proteins are heated they release free-SH groups and give rise to small peptide chains.
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These chains are motile and have capacity to establish new bonds between themselves and form new complex structure different from the original proteins. This process is called as denaturation of protein.
Dry heat, in compression to moist heat, destroys microorganisms by oxidizing their proteins and other chemical constituents. When dry proteins are heated, the polar groups in their peptide chains are less active due to absence of water and their motility is also much reduced.
Therefore, high energy is required to open the peptide molecules in the process called as oxidation of protein operative during dry heat sterilization, and increase of resistance of protein can be observed in this state. In addition to higher temperature, dry heat also requires longer period of exposure as compared to moist heat.
For example, endospores of Bacillus anthracis are killed in 2-15 minutes by moist heat at 100°C, but they are killed by dry heal in 1-2 hours at 150°C. Endospores of Clostridium botulinum are destroyed in 4 to 20 minutes by moist heat at 120°C, but they are destroyed in 2 hours by dry heat at the same temperature.
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Dry heat sterilization, despite its aforesaid demerits in comparison to moist heat sterilization, is preferably used in microbiological laboratories because the dry heat does not corrode glassware and metal instruments as moist heat does. However, a comparative account of temperature and destructive time required by certain bacteria with respect to moist heat and dry heat sterilization is given in Table 21.7.
