Dry heat sterilization - Pharmaceutical Microbiology Third Semester PDF Notes

Dry heat sterilization


• Dry heat sterilization methods

• Applications of dry heat sterilization

Intended Learning objectives

At the end of this lecture, student will be able to

• Classify the methods of dry heat sterilization

• List the applications of incineration, flaming and hot air oven

• Explain the construction and working of a hot air oven

Dry heat sterilization

• The lethal effects of dry heat on microorganisms are due largely to oxidative processes

• Less effective than the hydrolytic damage which results from exposure to steam

• Dry heat sterilization usually employs higher temperatures in the range 160–180°C and requires exposure times of up to 2 hours depending upon the temperature employed

• Dry heat application is generally restricted to

– glassware

– metal surgical instruments

– non-aqueous thermostable liquids

– thermostable powders

• The major industrial application is in the sterilization of glass bottles which are to be filled aseptically

• For the purposes of depyrogenation of glass, temperatures of approximately 250°C are used

Dry heat sterilization methods

Depending on the application, different dry heating methods are employed

 Sunlight and drying

 Red heat

 Flaming

 Incineration

 Hot air oven

Sun light and Drying

• Sun light possess UV rays which have germicidal activity

• This is a natural method for sterilization of water in tanks, rivers and lakes


• These rays cannot penetrate through glass

• Drying in air has deleterious effect on many bacteria's


• Spores are unaffected by drying

Red Heat

• It is used to sterilize metallic objects by holding them on a flame till they are red hot


• Metallic objects such as inoculating wires, tips of forceps and needles are held in the flame of a Bunsen burner for instant sterilization


• Burning contaminants to ashes

• Very effective method of sterilization

• Preferred use: sterilization of inoculating loops


• Burning to ashes

• Very effective method of sterilization

Preferred use:

• Disposing of Paper cups, contaminated dressings, Animal carcasses, bags and wipes

Hot air oven

• Dry heat sterilization is usually carried out in a hot air oven

• Comprises an insulated polished stainless steel chamber, with a usual capacity of up to 250 litres

• Surrounded by an outer case containing electric heaters located in positions to prevent cool spots developing inside the chamber


• A fan is fitted to the rear of the oven to provide circulating air, thus ensuring more rapid equilibration of temperature

• Shelves within the chamber are perforated to allow good airflow

• Thermocouples can be used to monitor the temperature of both the oven air and articles contained within

Sterilizer operation

• Articles to be sterilized must be wrapped or enclosed in containers of sufficient strength

• Suitable materials are paper, cardboard tubes or aluminium containers

• Container shape and design must be such that heat penetration is encouraged in order to shorten the heating-up stage

• Articles must be carefully arranged within the chamber to avoid obscuring centrally placed articles from wall radiation or impending air flow

• The temperature variation within the chamber should not exceed ±5⁰C

• Following sterilization, the chamber temperature is usually allowed to fall to around 40°C


• Glasswares like syringes, petridishes, test tubes, flasks, pepettes, spatula, swabs can be sterilized

• Chemicals such as powders which would clump or form into cake in presence of moisture

• Surgical instruments like forceps, scalpels, and scissors

• Oily fluid swhich are impermeable to water such as oils and fats


• It is a continuous process

• It can be used for substances that would be harmed by moisture

• It is suitable for assembled equipments

• Provides sufficient time for penetration

• It is less damaging to glass and metal equipments than moist heat


• It requires long heating up times, high temp and long exposure time

• Thermo labile substances cannot be sterilized by this method

• Not suitable for surgical dressings, rubbers, plastics

• Not suitable for preparation containing water, alcohol or other volatile substances

Differentiation between dry heat and moist heat sterilization

Dry heat sterilization

Moist heat sterilization

• Less effective

• More effective

• Mechanism : destructive oxidation

• Mechanism : denaturation of enzymes

• Requires longer time for action

• Requires  lesser time for action

• Heat resistance of the organisms is found to be more

• Heat resistance of the organisms is found to be less

• Conduction of heat is slow

• Conduction of heat is faster

• Latent heat of vapourisation

• No Latent heat of vapourisation


Dry heat Vs Moist heat

• When water comes to boiling point it needs sufficient heat to convert into vapour, on condensation the vapour are released

• Hence moist heat is more efficient


• Sunlight and drying – uv radiation

• Incineration is burning a substance to ashes

• Flaming is burning the contaminants – used for sterilizing inoculation loops

• Equipment used for dry heat sterilization – hot air oven

• Used mainly for sterilization of dry articles

• Sterilizing conditions - 160⁰C for 2 hours

• Used for moisture sensitive articles

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