Role of heat in control of microorganisms.

High temperature:

The range of temperature at which microorganisms can grow varies from low temperature to very high depending on the type of microorganisms. Psychrophiles can grow in very low temperature while thermophiles need the high temperature for growth. Each microorganism has three different range of temperature called minimum, optimum, and maximum temperature. The high temperature i.e, temperature have the maximum range to kill microorganisms but temperature below the minimum range results in static effect or preservation. Application of heat is the tools of killing by high temperature. The applied heat may be either dry heat or steam heat.

• Dry heat:

Dry heat oxidizes protein and other chemical components of the cell and kills the microorganism. It takes more time to kill and temperature required to maintain is also high. It is utilized to sterilize the materials which are difficult to sterilize by moist heat due to various reason. for e.g. substance cannot be sterilized by moist heat because moisture cannot penetrate. Dry heat cannot kill some of the bacterial endospores due to high heat pressure resistive nature of complex endospore. Some of the application of dry heat are;

1. Hot air sterilization: This type of sterilization technique is used for those objects which undergo damage with moist heat ,like some glassware oils, powders etc. About 2 hrs of heating at 160^oc is required to make those objects sterile. The instrument used are hot air oven even kitchen oven etc.
2. Incineration: It is the application of dry heat by burning the materials to be sterilized. It can be done simply by flaming or by burning with fires. Most of the contaminated and risky waste products are incinerate.

• Steam/Moist heat:

It is the application of heat under controlled steam pressure. The mechanisms of killing actions of moist heat are the coagulation of proteins present in the microbial cells. Steam heat can kill bacteria rapidly and it is effective to sterilize bacterial endospores. .It is one of the most effective methods of killing microorganisms. it kills the microorganisms by coagulating their protein and is much more effective then dry heat which kills the microorganisms by oxidizing their chemical subistance Some of the application of moist heat are as follows;

1. Autoclaving; It is the process of sterilizing the given object by heating it at 120^oc for 15 min. at 15lb/inch² pressure of steam under the controlled system. The instrument used for this purpose is called autoclave. An autoclave is modified pressure cooker like apparatus having parts of sampling temperature, pressure etc.
2. Fractional sterilization: it is the process of heating microbial suspension at 100^oc in three successive days. This method was first used by John today so called Tyndallization. Most of the vegetative cells die in 1^st day of heating but spores do not die. The bacterial endospore undergoes germination so 1st-day heating and grows as vegetative cell during the intermittent germinated cells also die and remaining spores undergo germination. Finally, all germinated spores get killed on 3rd-day heating. Nongerminating cells cannot be killed by this technique.
3. Boiling; boiling of water for sometime can kill most of the pathogenic vegetative cells but not a bacterial endospores. this method is used in the laboratory for the purpose of disinfection rather than sterilization.
4. Pasteurization: Most of the dairy products and alcoholic beverages are subjected to pasteurization before marketing. This technique was first developed by Luis Pasteur so-called pasteurization. In this technique, the suspension or other products are heated under the controlled system. This treatment can kill some of the pathogens that lower bacterial load up to safe level of consumption. It can kill microorganism Mycobacterium tuberculosis but not Coxiella Burnett passed on variation of times and temperature, the pasteurization technique can be categorized as;

• Thermal death time(TDT): It is the shortest period of time required to kill as suspension of bacteria at a certain temperature and specific condition. In thermal death time, cure temperature is fixed and the time varies. It is the measured time required to kill 100% of microorganisms.
• Decimal reduction time(DRT); It is the quantity of time in minute required to reduce the total bacterial population by 10% It is the time taken by bacterial TDT curve to pass through one log cycle.

Low-temperature control:

temperature below the optimum for growth depresses the rate of microorganisms growth and metabolism. Growth and metabolism will case if temperature lowers down to the minimum value. It doses not bring sterilizing. it depresses the rate of metabolism of microorganisms and if the temperature is sufficiently Low temperature is useful to preserve microorganism culture but is not a sterilizing agent. killing by lower temperature occurs due to the formation of ice crystals inside cooling cause formation of large ice crystal than rapid cooling.

References:

C.H.Collins. (1995). Microbiological Methods. Oxford Univ Press.

D.Greemwood, R. S. (2002). Medical Microbiology. London: Churchill Livingstone.

M.J.Pelezar. (1993). Microbiology. Tata McGraw hill.

M.T.Madigan, J. a. (1997). Biology of microorganisms. London: Prentice hall International Inc.

R.N.Atlas. (1984). Fundamentals and Applications. NewYork: MacMiHiam Publishing Co.