Lecture 13 - Physical Methods - Heat

Heat can be used to control anything that is not heat labile. Heat is ideal because it can easily controlled and contained, there is no residue in heat, as opposed to chemicals, and bacteria do not develop resistance to heat.

==part of Dry Heat=

Incineration
Dry heat is a method of heating that involves little water. For example, the Bactericinetaor uses a dry heat of 700°C to 1500°C, usually around 1000°C. The only mode of action is protein oxidation, which occurs by either adding oxygen (fast) or removing hydrogen (milder oxidation). All organic material is burned off, only leaving behind inorganic ashes. The incinerator desteroys everything, including spores and prions more effectively than an autoclave.

The disadvantage with incineration is that you cannot sterilize things like drugs because everything will be destroyed. Incineration is useful for any medical waste.

Hot Air Oven
The hot air oven uses a temperature of 160°C (320°F) for about 2 hours to sterilize. This is only useful for things that are dry because heat penetration is an issue, and water would lower the temperature of the oven by boiling. The advantage to this method is that it will not cause a metal instrument to rust as opposed to the moist heating methods.

The hot air oven uses three modes of action: dehydration, protein oxidation (mild), and protein coagulation. You can tell that protein coagulates when it changes physical properties. However, proteins are more stable in dry heat.

Moist Heat
Moist heat methods use water to better penetrate a sample, disinfecting the sample with a lower temperature than the dry heat methods. Therefore, the only mode of action for moist heat is protein coagulation.

Boiling
Water boils at 100C, which kills vegetative cells within 15 minutes, which is a method of disinfecting. Killing the spores would take 5-9 hours of boiling, which is not realistic. Scrubbing helps reduce the bacterial load, and salting the water will increase the boiling point.

Free Flowing Steam
The steam is at about 100°C and disinfects the metal instruments. Water is boiled in a room and the steam is transferred to a chamber to disinfect.

Steam Under Pressure
Autoclaves are gold standard for sterilizing, and goes for 121.5°C for 15 minutes to sterilize a sample that is less than 75mL. Works much faster than a hot air oven because it uses moist heat. The pressure helps get the water to a higher boiling point, which raises the temperature of the steam without boiling off the sample's water.

High Vacuum Autoclave
Goes up to 30 PSI and temp goes to 135°C for 3 minutes to sterilize a sample. Prions are still not destroyed.

Pasteurization
The idea is to disinfect pathogens without changing the flavor by heating, and is used for foods. Milk contains Mycobacterium bovis, which causes a tuberculosis-like disease, and Coxiella burnetii, which causes Q Fever. Milk was heated up to 61.7°C to kill Mycobacterium bovis, and then was raised to 62.9°C when the Rickettsia-like bacteria was discovered.

Flash
The flash method is also used for milk, and raises the temperature to 71.6°C for 15 seconds through a tube.

Milk Sterilization
Milk can be sterilized by bringing it up to 140°C for 1 second. It has a longer shelf life, and does not require refrigiration, but tastes weird.