Through history, humans have used fire to purify items. Heat generated through application of high temperatures acts by disrupting membranes and denaturing proteins and nucleic acids. Burning, however, is a bit excessive for everyday usage.
Transmissible agents (such as spores, bacteria and viruses) can be eliminated through sterilization. This is different from disinfection, where only organisms that can cause disease are removed.
Some of the methods used to achieve sterilization are:
- Autoclaves: Highly effective and inexpensive. Unsuitable for heat sensitive objects.
- Hot air ovens: Inefficient compared to autoclaves.
- Ethylene oxide: Suitable for heat sensitive items but leaves toxic residue on sterilized items.
- Low-temperature steam and formaldehyde: Effective for instruments with cavities or tubular openings.
- Sporicidal chemicals: Often used as disinfectants but can also sterilize instruments if used for prolonged periods.
- Irradiation: Gamma rays and accelerated electrons are excellent at sterilization.
- Gas plasma.
The preferred principle for sterilization is through heat, the autoclave being the most widely used method of achieving it.
In a dry air oven, it takes two hours at 160°C to kill spores of the bacterium Clostridium botulinium (associated with canned food). Using saturated steam, the same spores are killed in just five minutes at 121°C, proving that moist heat is more effective than dry heat.
Autoclave Design and Control
To be effective against spore forming bacteria and viruses, autoclaves need to:
- Have steam in direct contact with the material being sterilized (i.e. loading of items is very important).
- Create vacuum in order to displace all the air initially present in the autoclave and replacing it with steam.
- Implement a well designed control scheme for steam evacuation and cooling so that the load does not perish.
The efficiency of the sterilization process depends on two major factors. One of them is the thermal death time, i.e. the time microbes must be exposed to at a particular temperature before they are all dead. The second factor is the thermal death point or temperature at which all microbes in a sample are killed.
The steam and pressure ensure sufficient heat is transferred into the organism to kill them. A series of negative pressure pulses are used to vacuum all possible air pockets, while steam penetration is maximized by application of a succession of positive pulses
Typical pressure cycles used in autoclaves are:
- 1. Cycle for fabrics, assembled filter units and discard loads.
- 2. Cycle for laboratory plastic and glassware.
- 3. Cycle mainly used for discard loads.
Process performance can be confirmed by monitoring colour changes on indicator tape often taped onto packages or products to be autoclaved. Biological indicators such as the Attests can also be used. These contain Bacillus sterothermophilus spores, which are amongst the toughest organisms an autoclave will have to destroy. After a run in an autoclave, the internal glass in the Attest vial is shattered, allowing the spores into a differential liquid medium. If the autoclave has destroyed the spores, the medium remains a blue colour. Otherwise, the spores will metabolize, causing a yellow colour change after two days of incubation at 56°C
A control system must therefore provide flexibility in the way in which accurate and repeatable control of the sterilization is achieved and will include the following features:
- Precise loop control with setpoint profile programming
- Recipe Management System for easy parameterization
- Sequential control for complex control strategies
- Secure collection of on-line data from the sterilization system for analysis and evidence
- Local operator display with clear graphics and controlled access to parameters