Effective sterilization techniques are an essential part of many industries. They make it possible to produce aseptic pharmaceutical and nutraceutical products, and ensure safety in work environments where micro-organisms are manipulated.
Additionally, they allow for monoseptic processes to take place (where aseptic conditions are required as a baseline, and one type of micro-organism is then added). Over the years, many different types of sterilization methods have been developed. Here we provide an overview of seven of the most common – heat, filtration, chemical, UV irradiation, gamma irradiation, X-ray irradiation, and sonication, with particular emphasis on the first two techniques.
Heat is a highly effective way of destroying harmful micro-organisms. This long-practised sterilization technique is divided into two types – wet and dry. The former is direct, utilising steam injection followed by flash cooling, while the latter tends to be indirect, heating the surrounding air or equipment. Wet heat sees faster results, but may compromise the integrity of products because of the added moisture. Both types of heat sterilization can also have the negative effect of degrading vitamins and proteins, rendering it a non-viable option in many situations.
Filtration is used for both liquids and air. All too often, the air in a workplace is improperly sterilized, leading to it being a contamination risk. Filtering systems can help prevent this. Micro-organisms are not destroyed by filters, but can be removed. There are two types of filters – depth and surface, or membrane. Both systems require sterilization prior to use.
Depth filters, which High Efficiency Particulate Air (HEPA) filters fall under, engage porous packing material – typically glass wool, glass fibre or mineral slag wool – to intercept micro-organisms. They are relatively inexpensive, but not absolute – and are often used as pre-filters before exclusion filters.
Surface filters, on the other hand, typically use cellulose nitrate, cellulose acetate, or vinyl polymers and polyamides, and are expensive. However, they are absolute; removing micro-organisms of a particular size entirely.
This may involve either gas or liquid treatment of substances. Chemical disinfectants are used in the daily wash-downs of surfaces, as well as in decontamination scenarios, such as decontamination of bio-safety cabinets. Commonly used chemicals include ethylene oxide, formaldehyde, glutaraldehyde and propylene oxide, as well as alcohols, halogens, dyes and detergents. With some chemicals, exposure may be necessary for an extended period of time to ensure destruction of harmful organisms.
Also known as ultraviolet germicidal irradiation, this technique is commonly used in hospital settings. It uses short wavelength UV light to destroy the nucleic acids and damage the DNA of micro-organisms, thereby killing them. It can be used in combination with filtration systems for effective purification of air and water.
This process utilises ionising radiation to break the covalent bonds of bacterial DNA, thereby destroying them. It has the advantages of generating no heat or moisture, and not leaving residual radioactive materials. However, gamma ray irradiation may require large spaces.
The other major type of irradiation, X-ray sterilization also does not leave any toxic residues and is safe in terms of heat and moisture. It does not require as much space as gamma ray sterilization because, unlike in the case of that more powerful technique, the rays are concentrated in a single direction.
This method involves the use of sound energy to agitate particles, with destructive effects. It uses ultrasonic frequencies. While not as potent or common as the other sterilization techniques, sonication can be used to enhance the efficacy of disinfectants.
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