Introduction:
Definition: Sterilization is defined as complete removal of microorganisms from an object, surface or a product.
Dry heat sterilization uses application of heat in dry form.
Principle:
The killing of microorganisms by heat is a function of the time-temperature combination used.
If the temperature is increased then the time required for killing all the bacteria will be decreased.
Condition: Cycles recommended as per BP 1988 are:
A minimum of 180°C for not less than 30 minutes.
A minimum of 170°C for not less than 1 hour.
A minimum of 160°C for not less than 2 hour.
Mechanism of killing the bacteria:
The vital constituents of cells such as proteins (enzymes) and nucleic acids are denatured by oxidation.
Application of dry heat sterilization:
Dry heat is used to sterilize
Glassware e.g. test tubes, Pasteur-pipettes, petri dishes, flasks, glass syringes etc. The glassware should be prewashed with apyrogenic water.
Porcelain and metal equipment such as forceps, scalpels, scissors etc.
Dry materials in sealed containers like powders.
Fats, oils and greasy materials (like petroleum jelly) that are impermeable to moisture.
Dry heat sterilization is not used in the following cases:
Aqueous solution cannot be sterilized by dry heat because the water will evaporate if the container is kept open. If the container is kept close then it may burst due to pressure developed by the steam.
Surgical dressings cannot be sterilized by dry heat because cotton or other cellulosic material will get charged at the high temperature.
Sompe powders that cannot withstand the high temperature are not sterilised by dry heat method.
HOT AIR OVEN:
A hot air oven is a double walled chamber made up of mild steel or aluminium.
The door is insulated with asbestos gaskets to minimize the heat loss.
To reduce heat loss by conduction insulating material (glass fibre or puff) is used in between two walls.
The inner surface is made reflecting to reduce heat loss by radiation.
It consists of one or more shelves.
It is thermostatically controlled.
Heaters are fitted to heat the air inside.
The heat is circulated by normal convection.
To spread the heat uniformly forced convection is arranged by fitting fans in strategic places.
For effective utilization of the oven and to obtain correct results it should be uniformly packed or loaded so that all the objects are exposed to sterilization temperature for the required period of time.
Glasswares must be dried thoroughly and wrapped in a filter paper or Kraft paper before loading.
Care should be taken so that once in operation the oven is not opened in the middle of the cycle.
The oven should give sufficient time of two hours cooling before opening.
Red Heat:
Used for sterilizing metallic objects which are held on the flame and made red hot.
e.g. insulating wires, needles, forceps etc.
Flaming:
The article is allowed to pass through a flame to make it red hot.
Sometimes the article is dipped into spirit and then set on fire, however it does not produce sufficient heat, it only kills vegetative organisms.
Incineration:
An excellent method of destroying material waste e.g. pathological materials, beddings, solid dressings etc.