HOT AIR OVEN
PHYSICAL METHOD OF STERILIZATION BY DRY HEAT
NIVETHA K.
BSc MLT
Vivek institute of laboratory medicine
HOT AIR OVEN
❖ Hot air sterilizer is the most widely used method of sterilization by
dry heat.
❖ It is used to process materials which can withstand high temperatures
for length of time needed for sterilization by dry heat, but which are
likely to be affected by contact with steam.
❖ The benefit of dry heat includes good penetrability and non-corrosive
nature which makes it applicable for sterilizing metal surgical
instruments.
❖ It is also used for sterilizing non-aqueous thermostable liquids and
thermostable powders.
HOT AIR OVEN
❖ Dry heat destroys bacterial endotoxins (or pyrogens) which are
difficult to eliminate by other means and this property makes it
applicable for sterilizing glass bottles which are to be filled
aseptically.
❖ Dry heat kills microorganisms by Oxidation, protein Denaturation
and toxic effects of elevated levels of electrolytes.
Working principle of hot air oven
Working principle of hot air oven
❖ Sterilization by dry heat is accomplished by conduction. It consists of
electrical heating coil that produces dry heat. In the chamber, the
produced dry heat air displaces the cooled air forming heat gradient.
The use of fan allows to the uniform distribution of heat.
❖ The heat is absorbed by the outside surface of the item, then passes
towards the centre of the item, layer by layer. The entire item will
eventually reach the temperature required for sterilization to take place.
❖ Dry heat does most of the damage by oxidizing molecules. The essential
cell constituents are destroyed and the organism dies. The temperature
is maintained for almost an hour to kill the most difficult for the
resistant spores.
HOT AIR OVEN
Components of Hot air oven
❖ Hot air oven consists of the following parts:
1) CABINET:
a) External cabinet: The External cabinet is made of stainless
steel sheets. It covers the inner chamber.
Stainless steel sheets
Components of Hot air oven
b) Glass wool insulation: The space between the inner
chamber and external cabinet is filled with glass wool. It
provides insulation to the hot air oven.
Glass wool
c) Inner chamber: The inner chamber of hot air
oven is made of stainless steel.
Components of Hot air oven
2) A Fan: It helps in uniformly circulating the air within the chamber.
3) Shelves: The cabinet space is divided by shelves, which provide space
for placing the microbial culture.
Components of Hot air oven
4) Thermostat: It plays important role in maintaining the desired
temperature. Thermostat contains the sensor that senses the temperature
and maintain as per the setting.
5) Control panel: The control panel is placed on the front side of the
cabinet, just above the door. The control panel contains switches for all the
parameters. We can set the parameters as per our need.
Components of Hot air oven
6) Door locking controls : The hot air oven looks like small cupboard,
which has insulated door with handle. The door is tight because of asbestos
gasket that allows keeping heat trapped inside the hot air oven.
7) Load indicator: It indicates the hot air oven is overloaded.
8) Mains on/off switch: It helps to turn on/off the hot air oven.
9) Safety Thermostat: It is also known as an over-temperature protection
device. It keeps your oven and specimen safe in case of controller
malfunction.
PREPARATION OF THE LOAD
❖ No overloading: It must not be overloaded and the individual articles
or packs of the load are positioned to allow free circulation of hot air
between and around the item.
❖ Articles: Should be thoroughly clean and dry.
❖ Glasswares: Should be perfectly dry before being placed in the oven.
❖ Test tubes and Flasks: Should be wrapped in paper.
PREPARATION OF THE LOAD
❖ Cotton plugs: May get charged at 180 degree celsius.
❖ Heat-sensitive materials: Dry heat sterilization is slow and not
suitable for heat sensitive materials like many plastic and rubber
items.
❖ Rubber materials, except silicon rubber, will not withstand the
sterilizing temperature.
STERILIZING CYCLE
1) The sterilization hold time: It is set to 180 degrees Celsius for 20
minutes, 170 degrees Celsius for 30 minutes, 160 degrees Celsius for 1
hour.
2) Cutting instruments: Such as those used in ophthalmic surgery,
should ideally be sterilized at 150 degrees Celsius for 2 hours.
3) Oils, glycerol and dusting powder: The British Pharmacopoeia
recommends a holding time of 1 hour at 150 degrees Celsius for oils,
glycerol and dusting powder.
Note:
Cooling may take
Cooling may upto several
take upto hourshours
several and, and,
therefore
therefore do do not
not attempt
attempt toto open
open the
the chamber
chamber
door
door until
until the
the chamber
chamber and and load
load have
have cooled
cooled
below
below 60
60 degrees
degrees Celsius.
Celsius. Glassware
Glassware isis liable
liable to
to
crack
crack ifif cold
cold air
air isis admitted
admitted suddenly
suddenly while
while itit isis
silvery
silvery hot.
hot.
STERILIZATION INDICATORS
1) Biological indicator: The primary biological indicator employed for
assessing the sterilization efficacy of the hot air oven is the spores of
the nontoxigenic strain of Clostridium tetani.
2) Chemical indicator: Browne’s tube No. 3 serves as the chemical
indicator for the hot air oven sterilization process.
3) Physical indicator: Thermocouple, which are temperature-sensitive
devices are employed to continuously monitor the internal
temperature of the oven.
Advantages Of Hot Air Oven
1) Hot air penetrates certain substances that cannot be steam sterilized
(oil,powder).
2) Dry heat is protective method for sterilizing, non corrosive for metal
or sharp instruments.
3) Easy to instal and has relatively low operating cost.
4) Non-toxic and does not harm the environment.
Disadvantages Of Hot Air Oven
1) It is a time consuming method because the dry heat penetrate slowly
as compared to moist heat.
2) It may not be efficient to degrade prions.
3) The heat sensitive material cannot be sterilized using hot air oven.
4) It cannot be operated without electricity.
QUIZ TIME:
1) In hot air oven what temperature is maintained for sterilization of
parenterals?
A) 160 degrees Celsius for 4 hours
B) 160 degrees Celsius for 30 minutes
C) 160 degrees Celsius for 3 hours
D) 160 degrees Celsius for 1 hour
QUIZ TIME:
1) Answer: 160 degrees Celsius for 1 hour
Explanation: The sterilization hold time: It is set to 180
degrees Celsius for 20 minutes, 170 degrees Celsius
for 30 minutes, 160 degrees Celsius for 1 hour.
QUIZ TIME:
2) Which of the following is NOT suitable for sterilization
in a hat air oven?
A) Glassware
B) Metal instruments
C) Rubber items
D) Zinc powder
QUIZ TIME:
2) Answer: Rubber items
Explanation: Heat-sensitive materials: Dry heat sterilization
not suitable for heat sensitive materials like
many plastic and rubber items.
QUIZ TIME:
3) What is the primary mechanism of sterilization
in a hot air oven?
A) Moist heat
B) Ultraviolet radiation
C) Dry heat
D) Chemical disinfection
QUIZ TIME:
3) Answer: Dry heat
Explanation: Hot air sterilizer is the most widely used
method of sterilization by dry heat.
QUIZ TIME:
4) How does a hot air oven primarily kill microorganisms?
A) By denaturing proteins
B) By disrupting cell membranes
C) By oxidizing cell components
D) By inducing DNA mutations
QUIZ TIME:
4) Answer: By denaturing proteins
Explanation: The elevated temperatures have a profound
impact on microorganisms. The heat causes
the water molecules within these organisms
to evaporate. This dehydration process,
leads to the denaturation of vital proteins.
QUIZ TIME:
5) Which biological indicator is commonly used for quality control
testing of a hot air oven?
A) Escherichia coli
B) Clostridium tetani spores
C) Bacillus subtilis
D) Staphylococcus aureus
QUIZ TIME:
5) Answer: Clostridium tetani spores
Explanation: The primary biological indicator employed
for assessing the sterilization efficacy of the
hot air oven is the spores of the
nontoxigenic strain of Clostridium tetani.
Reference:
❖ Textbook of Microbiology - Surinder Kumar
❖ https://microbiologynote.com/hot-air-oven/
#Sterilization_Control_of_Hot_air_Oven
❖ https://biologyreader.com/hot-air-oven-
sterilization.html#google_vignette
Thank
You
