Dry heat sterilization
Contents
• 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
Limitation
• These rays cannot penetrate through glass
• Drying in air has deleterious effect on many bacteria’s
Limitation
• 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
Application
• Metallic objects such as inoculating wires, tips of
forceps and needles are held in the flame of a Bunsen burner for instant
sterilization
Flaming
• Burning contaminants to ashes
• Very effective method of sterilization
• Preferred use: sterilization of inoculating loops
Incineration
• 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
Sterilizer
• 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
Applications
• 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
Advantages
• 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
Disadvantages
• 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 |
• 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
Summary
• 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
