Moist heat sterilization describes

sterilization Sterilization may refer to: * Sterilization (microbiology), killing or inactivation of micro-organisms * Soil steam sterilization, a farming technique that sterilizes soil with steam in open fields or greenhouses * Sterilization (medicine) render ...

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

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 or ebullition is the rapid phase transition from liquid to gas or vapor, vapour; the reverse of boiling is condensation. Boiling occurs when a liquid is heated to its boiling point, so that the vapour pressure of the liquid is equal to ...

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 reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

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 flowering plant, angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the sp ...

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 A prion () is a Proteinopathy, misfolded protein that induces misfolding in normal variants of the same protein, leading to cellular death. Prions are responsible for prion diseases, known as transmissible spongiform encephalopathy (TSEs), w ...

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 both water clarity and wa ...

(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 us ...

, 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 steriliza ...

pressure cooker A pressure cooker is a sealed vessel for cooking food with the use of high pressure steam and water or a water-based liquid, a process called pressure cooking. The high pressure limits boiling and creates higher temperatures not possible at low ...

. 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 deri ...

. 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, 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)