dry heat method of sterilisation

1. DRY HEAT STERILIZATION
BY
SENI .M.B
1st yr Msc.MLT
Microbiology

2. DRY HEAT STERILIZATION
PRINCIPLE
• Kills organism by charring, denaturation of bacterial protein,
oxidative damage, and the toxic effect of elevated levels of
elecrolytes
• Materials containing organic materials require more time
for sterilization.

3. 3 methods are available
1.FLAMING
2.INCINERATION
3.HOT AIR OVEN

4. 1. FLAMING
• FLAME OF BUNSEN BURNER
• EITHER LONG TIME OR SHORT TIME
• LONG TIME EXPOSURE IN FLAME TILL THEY BECOME
RED HOT-
INOCULATING WIRE LOOPS , TIPS OF FORCEPS
• FOR SHORTER PERIOD WITHOUT ALLOWING RED HOT
FOR FRAGILE ITEMS
eg: mouth of test tubes
• Hotest part temp: 1500oc

5. Disav: proper sterilization cannot take place.

6. INCINERATION
• Disposal of biomedical waste material
• Burns anatomical waste and biomedical waste
by providing high tem.’
• 870- 1200oc range
• Converting waste into Ash, flue gas , heat.

7. • The ash is mostly formed by the inorganic
constituents of the waste, and may take
the form of solid lumps or particulates
carried by the flue gas.
• The flue gases must be cleaned of
gaseous and particulate pollutants before
they are dispersed into the atmosphere.

8. • heat generated by incineration can be
used to generate electric power
1.Types of incinerators
 Multiple chamber incinerators
 Municipal incinerators
 Micro incinerators

9. 1.  Multiple chamber incinerators:
“ it is an incinerator with two or more
refractory lined combustion
chambers in series separated
physically by refractory walls ,
interconnected by gas passage .

10. WORKING OF MUNICIPAL INCINERATOR
• It involves the combustion of solid waste at 1000ᵒc Waste
converted into ash, gas, and heat.

11. 3. MICROINCINERATORS
• Metal inoculation loops and
needles without using open
flame.
• By using infrared rays inside a
ceramic tube.
• Optimum tem: 900oc.
• Sterilise within 5-7 sec.

12. 3. HOT AIR OVEN( dry heat sterilizer)
• Elecrically heated
• Fan- ensure adequate and even
distribution of hot air.
• Themostat- maintains a chosen
temp.

13. Principle
• Sterilizing by dry heat is accomplished by conduction. 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 of the resistant spore

16. Working
• The most common time-temperature
relationships for sterilization with hot air
sterilizers are
• 170ᵒC (340ᵒF) for 30 minutes,
• 160ᵒC (320ᵒF) for 2 hr, and
• 150ᵒC (300ᵒF) for 150 minutes or longer
depending up the volume.

17. • The contents must not be removed from the oven
immediately as a slow cooling period is necessary –
ideally when the temperature has reduced down to
50°c, but no less.
• The reason for the gradual cooling period is to avoid
the cracking of glassware as well as preventing air
(that could potentially contain contaminating
organisms) entering the oven.

18. Used for glassware, forceps, swabs, water
impermeable oils, waxes & powders
• Before placing in hot air oven
– Dry glassware completely
– Plug test tubes with cotton wool
– Wrap glassware in Kraft papers
• Don’t over load the oven
• Allow free circulation of air between the
material

19. Quality control
To check whether the equipment is working
Properly.
1. Biological controls: paper strips containing
10⁶spores of Clostridium tetani(now Bacillus atrophaeus)
• Place strips in oven along with other material for
the sterilization
• Later culture the strips in thioglycollate broth at
37 ᵒC for 5 days
• Growth in medium indicates failure of
Sterilization.

20. PHYSICAL CONTROL
• Digital display on equipment showing temp. and time.
Chemical controls:
Browne’s tubes
Color change from red to green

21. Advantages
• A dry heat cabinet is easy to install and has
relatively low operating costs
• It penetrates materials
• It is nontoxic and does not harm the environment
• it is noncorrosive for metal and sharp
instruments.

22. Disadvantages
• Time consuming method because of slow rate
of heat penetration and microbial killing.
• High temperatures are not suitable for most
materials.