Discuss purpose & level of microbial control achieved by heat.

High temperatures combined with high moisture is one of the most effective methods of killing microorganisms. Dry heat is used to sterilize surfaces, and materials which are not likely to break down in high heat and which do not contain any liquids, e.g., glass Petri dishes and culture vessels, and metal surgical instruments. Dry heat penetrates more slowly than moist heat which destroys microorganisms by coagulating their proteins and also destroys microorganisms by oxidizing their chemical constituents. Moist heat penetrates more quickly than dry heat, and is used to sterilize culture solutions and agar preparations, and to sterilize surgical instruments etc. Pressurized steam heat is needed to kill bacterial endospores, which can withstand boiling. Typically a pressure of 15 psi (pounds per square inch) is needed to create steam at a high enough temperature (121°C) to kill endospores. Spores of Clostridium botulinum are killed in within 20 minutes by moist heat at 120°C, whereas a 2-h exposure to dry heat at the same temperature is required.

The thermal death time refers to the shortest period of time to kill a suspension of bacteria or bacterial spores at a prescribed temperature and under specific conditions. The thermal death point is the lowest temperature at which all microorganisms in a particular liquid will be killed in ten minutes. The decimal reduction time is the time in minutes that it takes for 90% of a given population of microorganisms to be killed at a given temperature. It is the time in minutes for the thermal death-time curve to pass through one log cycle. A disinfectant has been applied to a contaminated surface and the result is shown in the graph-1 (figure-1).

Dry heat. Dry heat kills microorganisms by reacting with and oxidizing their proteins. Dry heat can be used in incineration devices, such as the Bunsen burner or the hot-air oven. In the hot-air oven, a temperature of about 170°C for two hours will bring about sterilization.

Moist heat. Moist heat is used to kill microorganisms in such things as boiling water.Most vegetating microorganisms are killed within two or three minutes, but over two or three hours may be required for destruction of bacterial spores. In moist heat, the microbial proteins undergo denaturation, a process in which the three-dimensional form of the protein reverts to a two-dimensional form, and the protein breaks down.

Moist heat is used in the autoclave, a high-pressure device in which steam is superheated. Steam at 100°C is placed under a pressure of 15 pounds per square inch, increasing the temperature to 121°C. At this temperature, the time required to achieve sterilization is about 15 minutes. The autoclave is the standard instrument for preparing microbial media and for sterilizing instruments such as syringes, hospital garb, blankets, intravenous solutions, and myriad other items.

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