Moist heat sterilization describes sterilization techniques that use hot water vapor as a sterilizing agent. Heating an article is one of the earliest forms of sterilization practiced. The various procedures used to perform moist heat sterilization process cause destruction of micro-organisms by denaturation of macromolecules.

Description

Heating an article is one of the earliest forms of sterilization practiced. Moist heat sterilization processes sterilize using hot air that is heavily laden with water vapor, which plays the most important role in the sterilization.

Boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Th ...

a sample for 30 minutes or more will kill virtually all vegetative cells present, but will not kill

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

s, which can

germinate Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...

shortly thereafter and resume growth. Therefore, boiling is an insufficient method to achieve sterilization.

Action on micro-organisms

Moist heat causes destruction of micro-organisms by denaturation of macromolecules, primarily proteins. Destruction of cells by lysis may also play a role. While "sterility" implies the destruction of free-living organisms which may grow within a sample, sterilization does not necessarily entail destruction of infectious matter.

Prion Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It ...

s are an example of an infectious agent that can survive sterilization by moist heat, depending on conditions.

Validation

To facilitate efficient sterilization by steam and pressure, there are several methods of verification and indication used; these include color-changing indicator tapes and biological indicators. When using biological indicators, samples containing spores of heat-resistant microbes such as Geobacillus stearothermophilis are sterilized alongside a standard load, and are then incubated in sterile media (often contained within the sample in a glass ampule to be broken after sterilization). A color change in the media (indicating acid production by bacteria; requires the medium to be formulated for this purpose), or the appearance of

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can ...

(cloudiness indicating light scattering by bacterial cells) indicates that sterilization was not achieved and the sterilization cycle may need revision or improvement.

Methods used

Tyndallization

A more effective method is

Tyndallization Tyndallization is a process from the nineteenth century for sterilizing substances, usually food, named after its inventor John Tyndall, that can be used to kill heat-resistant endospores. Although now considered dated, it is still occasionally u ...

, which uses three successive steam treatments to achieve sterilization over the course of three days. This works by killing vegetative cells, allowing germination of surviving spores, and killing the resulting vegetative cells before they have time to form further spores. Any surviving spores from the first treatment, or incidentally formed spores during the first incubation period, are killed in a third steaming cycle.

High pressure

A more commonly used method when extended heat is not a concern is to use an

autoclave An autoclave is a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure in relation to ambient pressure and/or temperature. Autoclaves are used before surgical procedures to perform sterilizat ...

pressure cooker Pressure cooking is the process of cooking food under high pressure steam and water or a water-based cooking liquid, in a sealed vessel known as a ''pressure cooker''. High pressure limits boiling, and creates higher cooking temperatures which c ...

. When sterilizing in this way, samples are placed into a steam chamber on a shelf or raised floor, and the chamber is closed and heated so that steam forces air out of the vents or exhausts. Pressure is then applied so that the interior temperature reaches , and this temperature is maintained for between 15 and 30 minutes. This elevated temperature and pressure is sufficient to sterilize samples of any commonly encountered microbes or spores. The chamber is then allowed to cool slowly or by passive heat dissipation; it is rare for forced cooling to be applied, or for pressure to be vented deliberately. Pressure sterilization is the prevailing method used for medical sterilization of heat-resistant tools, and for sterilization of materials for microbiology and other fields calling for

aseptic technique Asepsis is the state of being free from disease-causing micro-organisms (such as pathogenic bacteria, viruses, pathogenic fungi, and parasites). There are two categories of asepsis: medical and surgical. The modern day notion of asepsis is deriv ...

. In cases when items need to be sterilized for immediate use, ''flash sterilization'' may be employed. Flash techniques generally run for the minimum time, temperature, or pressure, and may sacrifice some safeguards, such as the abilities to validate with biological indicators or prevent contamination. Additional protocols are generally taken to mitigate the sacrifices; flash sterilization equipment is often kept in an operating room's

sterile field Sterile or sterility may refer to: *Asepsis, a state of being free from biological contaminants * Sterile (archaeology), a sediment deposit which contains no evidence of human activity *Sterilization (microbiology), any process that eliminates or ...

, steam-penetrative protective packaging may be used to prepackage items, and specially designed rigid sterilization container systems can be reused.

References

{{reflist Medical hygiene Sterilization (microbiology)