Heat is considered as most reliable method of sterilization of objects that can withstand heat. It is carried out in two ways viz. Moist and Dry heat. They are the most common sterilizing methods used in hospitals and are indicated for most materials. This method of sterilization is applied only to thermostable products, but it can be used for moisture-sensitive materials for which dry heat (160-180°C) sterilization, and for moisture-resistant materials for which moist heat (121-134°C) sterilization is used.
Advantages:
• Most common method for controlling microbial growth.
• Very effective in destroying unwanted microbes.
• Low cost.
Disadvantages:
• It is probabilistic i.e. not absolute sterilization.
• It can degrade thermally unstable media.
Dry Heat
Dry heat sterilization requires higher temperatures and
longer exposure times than moist heat sterilization. It uses high temperatures
to kill microorganisms and bacterial spores and is used on items that cannot
get wet and for glassware, oils, powders, metal instruments, and items wrapped
in paper. There are three types of temperatures are recommended viz.
|
Temperature |
Exposure time |
|
150°C |
150 minutes |
|
160°C |
60 minutes |
|
170°C |
30 minutes |
Thermal death time (TDT): It is the minimum time
required to kill a suspension organism at a predetermined temperature in a
specified environment. Thermal death time is inversely proportional to
temperature. It is increased in the presence of organic substances, proteins,
nucleic acid, starch gelatin, etc.
Principle: It causes denaturation of proteins and
oxidative damage. There are several different types of dry heat sterilization
such as:
• Red Heat: It is used for straight wires, bacterial
loops, and spatulas.
• Flaming: It is the process of heating over the fire
till it becomes red hot. It is used for bacterial loops, wires, and spatulas.
• Incineration: It is the process that involves the
combustion of organic substances contained in waste materials. It is used for
soil dressing and pathological bedding.
• Hot Air oven: It is an electrical device. The oven
uses dry heat to sterilize articles at 50 to 300°C. The thermostat is used to
control the temperature. It is commonly used in the dairy industry. Glasswares,
forceps, scissors, scalpels, etc. are sterilized by this method but surgical
dressings, rubber items, or plastic materials are not sterilized by this
method.
The temperature used in a hot air oven with the holding time
of articles is described as 160°C for 45 minutes, 170°C for 18 minutes, 180°C
for 7.5 minutes, etc.
Operation:
(i) Articles to be sterilized are first wrapped or enclosed
in containers of cardboard, paper, or aluminum.
(ii) Then, the materials are arranged to ensure
uninterrupted airflow.
(iii) Oven may be pre-heated for materials with poor heat
conductivity.
(iv) The temperature is allowed to fall to 40°C, prior to the
removal of sterilized material (Fig).
Types of Hot Air Oven:
There are two types of hot air ovens. (i) Forced air hot air
oven and (ii) Static air hot air oven. The forced air hot air oven is more
effective than the static air hot air oven. The forced hot air oven works by
heating the oven and using a fan to move the hot air around whereas the static
air hot air oven works by using a heating coil at the bottom of the oven. The
heat rises throughout the oven and takes a longer time to reach the desired
temperature.
Fig: Working of Hot air oven
Advantages:
• It does not cause metals to corrode or rust.
• It is relatively inexpensive.
• It does not release any harmful or hazardous fumes or
pollutants.
Disadvantages:
• It is relatively slow (can take a couple of hours).
• Many objects cannot withstand the very high temperatures
required for dry heat sterilization (for example: some plastics would melt).
Applications of Dry Heat Sterilization:
• The method is applicable for thermo-stable,
moisture-sensitive pharmaceutical and medical devices and materials such as dry
powdered drugs, suspension of drugs in non-aqueous solvents, oils, fats, waxes,
oily injections, implants, ophthalmic ointments, etc.
Moist Heat
Principle: Moist heat is more efficient for
sterilization in contrast to dry heat; it destroys microorganisms by the
irreversible denaturation of enzymes and structural proteins. The temperature
at which denaturation occurs varies inversely with the amount of water present.
Moist heat sterilization involves the use of steam in the
range of 121-134°C. Steam under pressure is used to generate the high
temperature needed for sterilization. Saturated steam acts as an effective
sterilizing agent. There are three types of temperatures are used in this
technique.
At temperatures below 100°C:
• Pasteurization: Food industry.
• Vaccine bath: Vaccine sterilization.
• Serum bath: Serum contaminants, do not kill spores
survive.
• Inspissation: Egg and serum-containing media, can
kill spores.
At temperature 100°C:
• Boiling: Boiling water (100°C).
• Tyndallization (100°C).
At temperatures above 100°C:
• Autoclave.
Pasteurization: This process was first discovered by
Louis Pasteur. It is the process of heating liquids for the purpose of
destroying viruses and harmful organisms such as bacteria, protozoa, molds,
etc. This method is not intended to kill all microorganisms in the food.
Instead, it aims to achieve a logarithmic reduction in the number of viable
organisms, reducing their number so they are unlikely to cause disease.
Process:
• It involves milk.
• In the Holder method, the sample is heated for 30 minutes
at 60°C.
• In the Flash method, the sample is heated for 15-20 seconds
at 72°C.
• Milk is not heated more than this temperature otherwise it
forms aggregate resulting in spoiling of milk.
• By this method all non-sporing bacteria like mycobacteria,
Brucellae, and salmonellae are destroyed.
• Coxiella burnetii being heat resistant survives Holder’s
method.
Examples:
• Vaccines of non-sporing bacteria are pasteurized in the special
bath at 60°C for 1 hour.
• A temperature of 80°C for 5-10 minutes destroys all
vegetative forms of Bacteria, yeast, and molds.
• Spores of Clostridium botulinum are destroyed at 120°C for
4 minutes.
Inspissator:
It is a system designed to produce large batches of uniform
culture medium (Tuberculum bacteria culture) four to six times per day.
Inspissation takes 50 minutes at 85°C. It is used for sterilization of media.
Example: Lowenstein-Jenson and Loeffler rendered
sterile at 80-85°C for 30 minutes on three successive days inspector.
Boiling:
It is the process of heating the sample to be sterilized
material in a liquid at its boiling point to kill bacteria and other
microorganisms and also spores.
Example: Vegetative bacteria are killed immediately
at 90-100°C.
Steam at 100°C:
It is an inexpensive method of utilizing free steam to
sterilize the culture medium. In this method, the container and the medium are
simultaneously sterilized.
Tyndallization: It is also known as intermittent
sterilization. It helps first exposure to kill all bacteria and spores. It is
used for media with sugar or gelatin which require 100°C for 20 minutes on
three successive days.
Autoclave: It is also known as steam under pressure.
Principle:
High-pressure steam sterilization is used to kill all
microorganisms, including spores. It is the best and most widely used method of
sterilization. Example: Pressure cooker.
Vapor pressure within the autoclave increases which enables
steam to reach higher temperatures. At 103.4 kPa vapor pressure, the
temperature reaches 121.3°C, for 15 ~ 20 minutes. This method is used for
physiological saline, surgical instruments, containers, glass syringes,
dressings, and other articles (Fig).
Autoclaves have lower-row pressure pre-vacuum steam
sterilizers which are classified into two types viz. portable and horizontal.
(i) Under the exhaust pressure steam sterilizer: With
the upper part of the vessel vapor pressure forces the cold air from the bottom
of the exhaust vent. When the pressure is 102.97 ~ 137.30 kPa, the temperature
can reach 121 ~ 126°C, 15 ~ 30 minutes to achieve sterilization purposes.
(ii) Pre-vacuum steam sterilizer: With a vacuum
before injecting steam into the first internal evacuate, a negative pressure is
generated to facilitate steam penetration. When the pressure reaches 105.95
kPa, a temperature of 132°C, 4 ~ 5 minutes are enough for sterilization.
Fig: Principle of Autoclave
Advantages of Autoclave:
• It is rapid and effective.
• It destroys microorganisms more efficiently than dry heat
and therefore a shorter exposure at a lower temperature is possible.
• It can be used for a large proportion of the official
injections.
• It is supplied with dry saturated steam porous materials
and can be sterilized without damage.
• Equipment or components of rubber and certain plastics
such as nylon and P.V.C will withstand the conditions.
Disadvantages of Autoclave:
• Items sensitive to heat cannot be sterilized.
• It is unsuitable for anhydrous materials such as powders
and oils.
• It cannot be used for injections and articles such as
plastics that deteriorate at 115°C.
Advantages of Moist Heat:
• It has good penetration.
• It maintains the integrity of liquids due to the 100%
humidity within the chamber.
Disadvantages of Moist Heat:
• Non-stainless steel metal items corrode in moist heat.
• It may damage rubber and plastic items.
• This method is not suitable for oils, fats, ointments,
oily injections, etc.
Applications of Moist Heat Sterilization:
• This method is a most essential biocidal agent. It is used
for surgical dressings, sheets, surgical and diagnostic equipment, containers,
closures, aqueous injections, ophthalmic preparations, etc. There are some
differences between dry heat and moist heat sterilization (Table).
Table: Differences between dry heat and moist heat
sterilization
|
Moist heat |
Dry heat |
|
1. It has
water and steam. |
1. It does
not require water and steam. |
|
2. It is
based on protein denaturation and coagulation. |
2. It is
based on protein denaturation. |
|
3. Latent
heat is liberated when steam condenses on a cooler surface. |
3. It is
based on oxidative damage. |
|
4. Spores are
killed by exposure to heat at 121°C for 10-30 minutes. |
4. Spores are
killed by exposure to temperature at 160°C for 1 hour. |
|
5. This
process is under pressure. |
5. This
process is on direct flame. |
|
6. This
process takes less time. |
6. This
process takes more time. |
|
7. It is
mainly dependent on boiling and autoclaving. |
7. It is
mainly dependent on flame and incineration. |
Sunlight: It possesses bacterial activity. The action
is due to its content of UV rays. Direct sunlight has a powerful germicidal
action. Exposure to direct sunlight for a sufficient time will kill spores as
well as vegetative cells. Tuberculosis germs are killed in a few hours. UV rays
destroy bacteria in a few seconds or minutes; molds and yeasts are somewhat
more resistant. Ultraviolet light is also used for the purification of drinking
water and swimming pool water.
Ultrasonic Vibration: It is the sound waves above the
frequencies. Vibrations have the property to disrupt the cells. Gram-negative
rods are more sensitive to ultrasonic vibration whereas Gram-positive cocci,
spores of fungi, and resistant to the vibration. The current trend is to use
ultrasonic as a cleaning agent to follow the process of sterilization in an
autoclave.