Physical Methods of Sterilisation — Heat

To know in detail about different types of sterilisation such as physical type of sterilisation

Heat

Heat is the most important preferable agent for sterilisation as it is fast, reliable and relatively inexpensive. It does not introduce any chemicals to substrates and drives away the water, a prime source of contamination.

Low temperatures retards the growth of the microbe but do not kill them. However, low temperatures like Refrigeration (4ºC), Freezing (-70ºC) and Lyophilisation (-196ºC) are used to preserve microbes preventing contamination but does not achieve sterilisation.

High temperatures are most effective in killing the microorganisms and are applied in moist heat and dry heat.

Moist Heat

Moist heat penetrates faster than dry heat as water molecules conduct heat better. Moist heat dentures and coagulates the proteins and lipids much faster, and the physical state of cells is changed.

Heat destroys the chemical bonds holding the proteins in 3D form and makes the proteins non-functional. Moist heat is categorised at three different as follows

• Below 100ºC (Pasteurisation)
• At 100ºC (Tyndallisation)
• Above 100ºC (Autoclave)

Below 100ºC (Pasteurisation)

Louis Pasteur devised pasteurisation for improving the keeping quality of the wine. This process is now extended for milk and milk products. The process significantly kills all the organisms without significantly affecting milk’s emulsification’s taste and nutritional value. The process is aimed to kill microorganisms Mycobacterium tuberculosis, Mycobacterium Bovis, Brucella abortis, and some species of staphylococci and Salmonella.\

There are three methods of Pasteurisation

Holding Process

Milk is held at 63.7ºC for 30 minutes and constant agitation. Several organisms were killed except Coxiella burnetti, the causative organism of Q fever.

Flash Process

High temperatures and short time, i.e. milk is processed at high temperatures at 71.6ºC for 15–20 seconds and then rapidly cooled.

Ultra Pasteurisation

Ultra Pasteurisation is a further refined process where milk is boiled at 82ºC for 3 seconds. This method removes Coxiella burnetti.