Hot air oven: Principle, Working mechanism, Sterilization control, Advantage and Disadvantage

Hot air oven principle, procedure

Introduction of Hot air oven

Hot air oven is the  most common method of sterilization in the laboratory working on dry heat. Sterilization is the  process of removing or destroying all microorganisms including viruses, bacteria and their spores from the article or surface without destroying its quality and quantity. It is a physical method of sterilization due to dry heat. Factors influencing sterilization by heat are nature of heat i.e dry or moist, temperature and time, number  of microorganism, nature of microorganisms. type of microorganism and presence of organic material. Mode of action: protein denaturation, oxidative destruction of essential cell constituents and toxic effects of elevated level of electrolytes. It works on the principle of conduction where heat is absorbed by the exterior surface of an item and then passed inward to the next layer.This method was introduced by Louis Pasture.

Principle of hot air oven

Electrical device that work on the principle of dry  and  hot air convection (that is circulation of  heated air),conduction and radiation. Hot air convection process is of two types. a. Gravity convection process: Heated air expands and possess less density than cooled air which rises up and displaces the cooler air (the cooler air descends). It produces inconsistent temperature within the chamber thus has a slow turn over. b. Mechanical convection:  Use of fitted  blower or fan that actively forces heated air throughout all areas of the chamber. It is  more efficient.

Structure of Hot air oven

Metallic cabinet with heating filament and fan fixed  in the walls.      -Thermostat, temperature control, double walled :(inner being poor conductor & outer being metallic and air filled space in between the layers) insulation keeps the heat in and conserve energy. Electrically heated, and provided with a fan or a blower to ensure rapid and  uniform. Heating Mechanism:- Killing effect of dry heat on microorganism is due to i) destructive oxidation of essential cell constituents, ii) protein denaturation and iii) toxic effect of elevated level of electrolytes.

Uses of Hot air oven

sterilization of articles that withstand high temperature and not  get burned e.g. Glass-wares, Powders,  forceps, scissors, scalpels, glass  syringes,    Pharmaceutical products like liquid paraffin, fats, grease and dusting powder etc.

Handling procedure of Hot air oven

Wrap the articles or enclose in container of cardboard, aluminium or paper. Mouths of flasks, test tube sand both ends of pipettes must be plugged with cotton wool. Articles to be sterilized such as petri plates and pipettes may be arranged inside metal canisters and then placed. Place the articles at sufficient distances so as to allow free circulation of air in between them and to ensure uninterrupted air flow.  Shut the door and switch on the hot air oven. When the thermometer  shows that the oven air has reached sterilizing temperature, heating is continued for the required period of time (e.g. 160°C for an hour). Allow the temperature to fall up to 40°C (approximately 2 hours),prior to removal of sterilized materials; which prevents breakage of glassware.

Advantages

Do not require water and there is not much pressure build up within the oven making safer  to work. Smaller than autoclave but can still be as effective. Higher temp. can be reached compared to other means. Effective method of sterilization of heat stable articles only method of sterilizing oils and powders.

Disadvantage

some organisms like prion may not be killed or inactivated.Plastic wares or  heat sensitive materials can’t be sterilized. Glasses may become smoky.            sterilization temperatures:  Temperature holding period is at 160°C for  1  hour, 170°C 30 minutes where as at  180°C 20 minutes.

Sterilization control for hot air oven

A) Biological controls: 106 spores of  Bacillus subtilis subsp. niger  or spores of non toxigenic strains of Clostridium tetani on Paper strips are placed inside envelopes and then placed inside the hot air oven after complete sterilization  inoculated in thioglycollate or cooked meat media and incubated for sterility test under strict anaerobic conditions for 3 to 5 days at 37°C. B) Chemical control: Browne’s tube No. 3  which shows a green color after sterilization at 160°C for 60 minutes ( color changes from red to green). C) Physical control: Thermocouples and  temperature chart recorder used.

Precautions

Sterilize dry substances. It shouldn’t be overloaded. Rubber goods, fabrics, any inflammable or volatile substances should not be put inside the oven. Oven is allowed to cool gradually for about 2 hours or up to 40°C before the door is opened.

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