Autoclave: Autoclave is a sealed devise that allows the entrance of steam under pressure. The use of moist heat facilitates killing of all microorganisms including heat resistance endospores. The underlying principle is, with increase in steam pressure, the temperature also rises. The water boils at 100°C upon the vapor pressure of atmosphere. Temperature will increase if vapor pressure is increased. The autoclave is operated at 15lb/inch2 for 30 minutes. This temperature for a period of 30 minutes is sufficient to kill all the vegetative and spore cells of microorganism. Pressure temperature relationship is given as
|Pressure (lb./inch2)||Temperature (°C)|
Autoclave is used to sterilize media, broth, aprons, rubber tubes and rubber gloves. This type of sterilization is also used for commercial canning of fruits and vegetables. While handling the autoclave, precaution should be taken that the level of water should be checked before operating. After completion of operation, air should be completely evacuated from the autoclave before opening the lid.
Hot air oven: in hot air oven, sterilization is accomplished by hot air. Dry heat of given temperature is not nearly as effective as moist heat of same temperature. In other words, sterilization process in an oven is longer than autoclave. Hot air oven is operated at the temperature of 160°C – 180°C. for period of 2 to 4 hours. An oven consists of an incubated cabinet which is held at constant temperature by means of electric heating mechanism and thermostat. It is fitted with fan to keep hot air circulating at constant temperature and thermometer for recording temperature of oven.
Hot air oven are most commonly used for sterilization of glassware like petridish, test tubes, pipettes and other metal instrument that can tolerate prolonged heat exposure. Precaution should be taken that temperature should be checked at which materials are sterilized. Hot air oven should not be used to sterilize culture media as liquid would boil to dryness.
Laminar air flow: It is used for reducing the danger of infection while working with infectious microorganism and for preventing contamination of sterile materials. There is special filter system; High Efficiency Particulate Air Filter (HEPA filter) which can remove particles as small as 0.3 micrometer. In front of blower, there is also a special mechanism from which the air blown from the blower produces uniform air velocity along parallel flow lines. These are horizontal and vertical types.
This HEPA filters can produce dust free air current with uniform velocity thus helping safe handling of microorganism. Within chamber of laminar flow, we can transfer any media for culturing bacteria or fungi or any microbes without contamination.
Incubator: an incubator is an equipment that consist of copper of steel chamber around which warm water of air circulates either by electricity or by means of small gas flames. Incubator is used to culture organism in suitable medium at proper temperature. The temperature of incubator is kept constant by thermostat control. Temperature variation should not be more than one 1°C.
The door of incubator should be opened only when necessary. If the tubes are to be incubated for a long time or at high temperature, the medium become too dry due to excessive evaporation. In such case, cotton plug should be pushed inside the neck of tube and a media rubber cap should be placed to cover the plug.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.