The killing action of heat is, as we have seen, a time temperature relationship affected by numerous conditions that must be taken into consideration in selecting the time and temperature required to reduce the microbial population to the desired level. Practical procedures by which heat is employed are divided into two categories: moist heat and dry heat
Moist heat
Steam under pressure: Heat in the form of saturated steam under pressure is the most practical and dependable agent for sterilization. Steam under pressure provides temperatures above those obtainable by boiling, it has the advantages of rapid heating, penetration, and moisture in abundance, which facilitates the coagulation of proteins.
Fractional Sterilization: Some microbiological media, solutions of chemicals and biological materials cannot be heated above 100 degree Celsius without being damaged. If, however, they can withstand the temperature of free-flowing steam(100 degree Celsius), it is possible to sterilize them by fractional sterilization. This method involves heating the material at 100 degree Celsius on three successive days with incubation periods in between. Resistant spores germinate during the incubation periods; on subsequent exposure to heat, the vegetative cells with be destroyed. If spores are present and do not germinate during the incubation periods, the material will not be sterilized. An apparatus known as the Steam Arnold is used for this technique; however, it is also possible to operate an autoclave with free-flowing steam for this purpose.
Boiling Water:
Contaminated materials exposed to boiling water cannot be sterilized with certainty. It is true that all vegetable cells will be destroyed within minutes by exposure to boiling water, but some bacterial spores can withstand this condition for many hours. The practice for exposing instruments for short periods of time in boiling water is more likely to bring about disinfection rather than sterilization. Boiling water cannot be used in the laboratory as a method of sterilization.
Pasteurization: Milk, cream, and certain alcoholic beverages are subjected to a controlled heat treatment which kills microorganisms of certain types but does not destroy all organisms. Pasteurized milk is not sterile milk.
Dry Heat
Hot Air Sterilization: Dry heat, hot air, sterilization is recommended where it is either undesirable or unlikely stem under pressure will make direct and complete contact with the materials to be sterilized. This is true of certain items of laboratory material, such as Petri dishes and pipettes, oils, powders and similar substances.
Incineration: Destruction of microorganisms by burning is practiced routinely in the laboratory when the transfer needle is introduced into the flame of the Bunsen burner. Incineration is used for the destruction of carcasses, infected laboratory animals, and other infected materials to be disposed.