Thermal sterilization methods
Moist Heat Sterilization
Contents
• Mechanism of sterilization by heat
• TDP, TDT and DRT
• Classification of heat sterilzation methods
• Moist heat sterilization – autoclaving
Learning
objectives
At the end of this lecture, student will be able to:
– Explain the mechanism of heat sterilization methods
– Outline the significance of Thermal death point (TDP),
Thermal death time (TDT) and Decimal reduction time (DRT, or D value)
– List the heat sterilization methods
– Explain the construction and working of an autoclave
Thermal sterilization / Sterilization by heat
Heat
sterilization – Mechanisms
• Heat is the most reliable and widely used means of
sterilization
• Heat appears to kill microorganisms by denaturing their
enzymes and other essential cell constituents
• The changes to the three-dimensional shapes of these
proteins inactivate them
Heat sterilization – mechanisms
• Excessive heat acts by coagulation of cell proteins
• Less heat interferes metabolic reactions
• Sterilization occurs by heating above 100°C which ensure
killing of bacterial spores
• Sterilization by hot air in hot air oven and sterilization
by autoclaving are the two most common method used in the laboratory
Measures of
heat resistance
• Thermal death point (TDP) is the lowest temperature at
which all the microorganisms in a particular liquid suspension will be killed
in
10 minutes.
• Thermal death time (TDT), the minimal length of time for
all bacteria in a particular liquid culture to be killed at a given
temperature.
• Decimal reduction time (DRT, or D value) is the time, in
minutes, in which 90% of a population of bacteria at a given temperature will
be killed
Heat
sterilization methods
Types of heat sterilization methods:
1. Sterilization by moist heat
2. Sterilization by dry heat
• Moisture-sensitive products – dry heat sterilization
(160–180°C) procedures are used
• Moisture-resistant items – moist heat sterilization
(121–134°C) procedures are used
Sterilization
processes
• Heating-up stage –
the articles to be treated must first be raised to sterilization temperature
• Holding stage –
timing for the process
• Cooling stage –
after the process is complete
Thermal
sterilization / Sterilization by heat
Moist heat sterilization / Steam sterilization
a) Sterilization at temperature above 100⁰C (Saturated
stream)
– Autoclaving / steam sterilization
b) Sterilization at temperature of 100⁰C (Boiling water)
– Tyndallisation
– Sterilization by boiling water
c) Sterilization at temperature below 100⁰C
(Hot water below Boiling Point)
– Pasteurization
– Vaccine bath
d) Heating with a bactericide
Moist Heat
Sterilization
Moist heat kills microorganisms primarily by
1. Coagulating proteins (denaturation), which is caused by
breakage of the hydrogen bonds that hold the proteins in their
three-dimensional structure
2. Breakage of DNA strands
3. Loss of functional integrity of cell membrane
Sterilization
at temperature above 100⁰C (Saturated stream)
Autoclaving / Steam
sterilization
• Reliable sterilization with moist heat requires
temperatures above that of boiling water
• These high temperatures are most commonly achieved by
steam under pressure in an autoclave
• Autoclaving is the preferred method of sterilization,
unless the material to be sterilized can be damaged by heat or moisture
• The higher the pressure in the autoclave, the higher the
temperature
• Sterilization in an autoclave is most effective when the
organisms either are contacted by the steam directly or are contained in a
small volume of aqueous (primarily water) liquid
• Under these conditions, steam at a pressure of about 15
psi (121 °C) will kill all organisms and their endospores in about 15 minutes
• Autoclaving is used to sterilize culture media,
instruments, dressings, intravenous equipment, applicators, solutions,
syringes, transfusion equipment and numerous other items that can withstand
high temperatures and pressures
• Large industrial autoclaves are called retorts
Sterilizer
design and operation
• Autoclaves are stainless steel vessels designed to
withstand the steam pressures employed in sterilization
Sterilizers can be
Portable sterilizers
Large scale sterilizers
1. ‘Portable’
sterilizers / Bench autoclave, where they generally have internal electric
heaters to produce steam and are used for small pilot or laboratory scale
sterilization and for the treatment of instruments and utensils
2. Large-scale
sterilizers/ Horizontal Autoclave for routine hospital or industrial use,
operating on ‘dry’ saturated steam from a separate boiler
Autoclaving
– procedure
• Water is boiled to produce steam, which is released
through the jacket and into the autoclave’s chamber
• The air initially present in the chamber is forced out
until the chamber is filled with saturated steam and the outlets are closed
• If the air is not completely exhausted, the container will
not reach the temperature expected for a given pressure
• Hot, saturated steam continues to enter until the chamber
reaches the desired temperature and pressure, usually 121°C and 15 pounds of
pressure
• At this temperature saturated steam destroys all
vegetative cells and endospores in a small volume of liquid within 10 to 12
minutes
• Treatment is continued for about 15 minutes to provide a
margin of safety
• The chamber should not be packed too tightly because the
steam needs to circulate freely and contact everything in the autoclave
• Bacterial endospores will be killed only if they are kept
at 121°C for 10 to 12 minutes
• When a large volume of liquid must be sterilized, an
extended sterilization time will be needed
Source of
steam
‘Wet’ steam
– Steam for sterilization is generated within the sterilizer
– It is constantly in contact with water
‘Dry’ saturated steam
– Steam is supplied under pressure (350–400 kPa) from a separate
boiler
– No entrained water droplets
General
design features of an autoclave
• Steam sterilizers are constructed with either cylindrical
or rectangular chambers
• Preferred capacities ranging from 400 to 800 litres.
• They can be sealed by either a single door or by doors at
both ends (to allow through passage of processed materials)
Autoclave
• During sterilization the doors are held closed by a
locking mechanism which prevents opening when the chamber is under pressure and
until the chamber has cooled to a pre-set temperature, typically 80°C
• In the larger sterilizers the chamber may be surrounded by
a steam-jacket which can be used to heat the autoclave chamber and promote a
more uniform temperature throughout the load
• The same jacket can also be filled with water at the end
of the cycle to facilitate cooling and thus reduce the overall cycle time
Stages of
operations – autoclave
1. Air removal and steam admission
– Downward displacement or evacuation
2. Heating-up and exposure
3. Drying or cooling
• Dressings packs must be dried before removal from the
chamber
• For bottled fluids cooling must achieved as rapidly as
possible to minimize thermal degradation of the product
• Steam exhaust can be achieved by application of a vacuum
Precautions to be
taken during loading in an autoclave
• Articles which require different sterilization time must
not be sterilized together
• The articles should be loosely packed to allow maximum
penetration of stream
• Articles should not touch the chamber wall
• When vacuum is applied, liquid materials should not be
kept for sterilization
• Solid and liquid materials should not be combined because
vacuum will be applied to replace stream with air
• Make sure that the doors are shut tightly and locked
Summary
• Heat acts by denaturing bacterial enzymes other essential
cell constituents
• Thermal death point (TDP), Thermal death time (TDT) and
Decimal reduction time (DRT, or D value) are measures of heat resistance of
organisms
• Stages of heat sterilization are heating, holding and
cooling
• Most important equipment used for moist heat sterilization
is the autoclave
• It sterilizes articles using pressurized steam at 121⁰C,
and 15lbs/sq.inch pressure – duration 20 min
• Autoclaving is the method of choice for sterilization of
heat and moisture resistant articles
