Application of Moist Heat Sterilisation

            An autoclave is the apparatus used in order to utilise moist heat sterilisation, a method of sterilisation which destroys microbes through the action of steam at high pressures. Moist heat sterilisation allows accurate control and monitoring of the sterilisation process, which means it is energy efficient. Coupled with the fact that moist heat sterilisation exhibits rapid heat transfer from the environment to the items being sterilised, mo ist heat sterilisation is highly cost effective.

            At around 121℃, moist heat sterilisation can be used to sterilise aqueous solutions and suspensions such as injections, ophthalmic solutions, irrigations, dialysis and fluids that dry heat sterilisation cannot. It is even more appropriateIt to use when there are a lot of solutions to be sterilised as the energy carried by the steam flows quickly around every article in the load. This is helped by the fact that as the water condenses it creates a pocket of low pressure into which the much higher pressure steam will readily move. 

            Surgical dressings can only be sterilised by moist heat sterilisation because the porous nature of the dressing means the moisture penetrates it quickly, bringing the dressings up to the required temperature faster than dry heat and preventing it from burning. For this purpose, a typical autoclave would be set at around 134℃ for approximately 3 minutes. Metal instruments e.g. scalpels, can be sterilised in moist heat because the lethal action of water plus heat, destroys micro-organisms more efficiently therefore needing a shorter exposure time at a lower temperature. Immediate drying following sterilisation is needed with these metal instruments to avoid corrosion as the oxidation process is sped up because of the heat.

             Rubber or plastic items (if sterilised separately from the containers) are best sterilised under moist heat because of their thermolability  (destruction or degradation under high temperature).  Moist heat kills micro-organisms by coagulating and denaturing the enzymes and structural protein, but cannot kill pyrogens like dry heat.

            As a method of testing that the required sterilising conditions have been met, it is common for a sealed sample of a thermophilic bacteria e.g. Geobacillus stearothermophillis, to be routinely placed in the autoclave along with the actual items to be sterilised. Following sterilisation, the sample is incubated and tested for any growth. If any growth is noted, it can be said that suitable sterilisation was not achieved and the items should be sterilised again

  

Application of Dry Heat Sterilisation

             Dry heat sterilisation is used to sterilise heat safe items that cannot be disassembled or may be affected by rusting if moisture was introduced. It is used to ensure that items are free from harmful pathogens and deemed sterile to be used under sterile conditions. Equipment to be sterilised by the dry heat method must be exposed to specific temperatures for specific time durations, depending on the item, the possibility of damage to it and the type of microbe that it is could be contaminated with.

180°C	For no less than 30 minutes
170°C	For no less than 1 hour
160°C	For no less than 2 hours

There are two different types of dry heat sterilisers available:

·       Oven type steriliser - this involves coils at the bottom of the oven unit heating to the desired temperature, this then causes the hot air to rise inside the oven via gravity convection. This method is slower as a longer period of time is required to reach sterilising temperature.

·       Mechanical convection – a motor-driven blower circulates the heated air throughout the chamber at a high velocity and allows for a more rapid transfer of heat from the air to the instruments.

            

A Typical Dry Heat Steriliser

Dry heat is used to sterilise numerous types of equipment and materials. Before sterilisation, all equipment must be free from of all signs of visible dirt and be dry.

Glassware – items such as beakers, syringe s, petri dishes and test tubes can be sterilised using dry heat, the items must be cleaned first though with apyrogenic water. This water is free from pyrogens and is essentially water for injections.

Porcelain and metal – items such as pestle and mortars, scalpels and needles

Oils and fats – anhydrous oils (oils which contain no water) and fats

Powders – items such as talcum powder and sulphonamides

  

Summary of Moist/Dry Heat Sterilisation

Moist heat		Dry heat
Advantages	Disadvantages	Advantages	Disadvantages
A high temperature ensures rapid heat transfer	Unsuitable for products which are anhydrous: such as powder and oils	Suitable for products that would be harmed if exposed to moisture	Low heat transfer between items
Destroys microorganisms more effectively than dry heat	It does not destroy pyrogens	Less damage caused to glass and metal than moist heat (except for sharp instruments)	Unsuitable for use on most rubbers and plastics – they are too thermolabile for this method of sterilisation
Suitable for use for a wide variety of injections, solutions, irritants, dialysis fluid.		Suitable for sterilising glass containers and able to destroy pyrogens	Unsuitable for use on aqueous products
Suitable for sterilising surgical dressings and materials due to the moistures ability to rapidly penetrate porous material.		Materials not contaminated with toxic substances	Unsuitable for use on surgical dressings
Suitable for sharp instruments			Accurate control of the process parameters is more difficult than moist sterilisation.
Accurate control of the process parameters is possible
No toxic contaminants are left in the materials that have been sterilised