DRY HEAT STERILISATION By Charlotte, Michelle and Rebecca

Dry heat sterilisation is most commonly used for non aqueous preparations that are stable at high heats.  This includes powders, some types of containers and certain impregnated dressings. 

Dry heat sterilisation makes use of hot air free from (or with minimal) water vapour, which plays no role in this sterilisation process.  It is usually carried out in a hot air oven making use of radiation, convections and conduction.  Heat is passed from the source to the load, and ‘blown’ about the chamber to achieve uniform temperatures in all areas.  Heat is then conducted from the outer layer to inner layers of the item.  Items must be dry prior to sterilisation to prevent water interfering with the process.

Published evidence shows the following temperature-time exposures needed to kill certain pathogens by dry heat.

Time	Temperature	Microbe type
90 minutes	110°C	Vegetative bacteria and most viruses
3 hours	140°C	Resistant bacterial spores and resistant viruses
90 minutes	115°C	Mould spores

Recommended time – temperature combinations

Different products require different combinations of time and temperature.  Three cycles listed in the BP (1993) include:

• Minimum of 180 °C for at least 30 minutes
• Minimum of 170°C for at least 1 hour
• Minimum of 160°C for at least 2 hours

For glass containers used for large volumes of dosage forms for injection, a cycle of 250°C for 45 minutes is most appropriate as this is considered sufficient for denaturing pyrogens that may be absorbed into the surface of the glassware.

During a cycle it is imperative that the entire contents of each container is maintained at a certain temperature for the specific length of time, allowing for temperature variations in hot-air ovens, to ensure effective sterilisation.

Design and operation of the dry heat steriliser

Ovens for dry heat sterilization are designed and equipped with forced air circulation, these ovens are stainless steel with a high quality argon welding and have curved joints for cleaning and hygiene purposes.  They have structure reinforcement for chambers to prevent thermal shocks and vibration and can sterilise glassware and metal material and reduce the reduction of bacteria by sterilising and de-pyrogenation. This can reach a maximum temperature of up to 250 degrees. The ovens for dry heat also have duly insulated doors with a silicon gasket for positive sealing and have positive door interlocking with pneumatically operated cylinders which prevent simultaneous opening of both doors.

Dry heat ovens are recommended to be maintained at a positive pressure and all air entering the chamber should be via a bacteria proof filter. The efficiency of dry heat process depends on velocity and the designed circulation pattern. Only a small amount of heat is transferred from the heat source to the articles in a hot air oven by conduction because, the limited pathways and small areas of the contact. Radiation is the chief form of heat transfer so the heaters need to be arranged all round the chamber. It is useful to have ovens fitted with the facility for automatic boost heating to give minimum heat up times but it is essential to have accurate temperature control by easily set regulators. A loaded oven temperature variation should not exceed 5 degrees once the sterilising temperature is reached. The temperature variation is normally measured as the difference between the temperature at the centre and any other point.

When loading the dry heat ovens it is best to load it will the same material of the same size. it is also beneficial not to have dull or blackened equipment as it does not reflect heat, If using glassware it must be cleaned thoroughly because heat transfer will be impaired if the surface is coated with a greasy film.

           

                                             

Infrared conveyor oven

This is one type of oven that can be used for dry heat sterilisation. This works by placing the items (generally smaller items such as glass syringes and other glass wear) on the conveyor belt and allowing to pass through the tunnel under the heated infrared radiators at about 180 oC. Infrared is a thermal type of radiations that absorbs the energy and converts it to heat. The oven is used for a continual process as it is highly effective constantly carrying out heat transfer from the source. It is unaffected by the thermal resistances of the static surface air films.

                                                                                  

Heating with bactericide

This is a method used for sterilizing aqueous solutions that are unable to handle normal autoclaving conditions, this sterilizes at a temperature range 98 to 100 for 30 minutes with the presents of a bactericidal substance to help kill or inactive the bacteria contaminating that certain product. The chose of bactericidal all depends on the compatible with the product its container and the closure chosen. This is to prevent any toxins being exposed which may cause potential harm to the patient. This method of sterilisation is no longer recognized by the BP (1993) due to the risks it carries.