Autoclaves are also known as steam sterilizers, and are typically used for healthcare or industrial applications. An autoclave is a machine that uses steam under pressure to kill harmful bacteria, viruses, fungi, and spores on items that are placed inside a pressure vessel. The items are heated to an appropriate sterilization temperature for a given amount of time. The moisture in the steam efficiently transfers heat to the items to destroy the protein structure of the bacteria and spores.

In healthcare, the term “autoclave” is typically used as the nomenclature to describe a Steam Sterilizer. ANSI/AAMI, which provide standards and guidelines for the processing of medical devices, refers to autoclaves for healthcare specifically as Steam Sterilizers.

You’ve probably heard of pressure cookers? They were all the rage until microwave ovens became popular in the 1980s. They’re like over-sized saucepans with lids that seal on tightly and, when you fill them with water, they produce lots of high-pressure steam that cooks your food more quickly (if you want to know more, please see the box at the bottom of this page). Autoclaves work in a similar way, but they’re typically used in a more extreme form of cooking: to blast the bugs and germs on things with steam long enough to sterilize them.

The extra pressure in an autoclave means that water boils at a temperature higher than its normal boiling point—roughly 20°C hotter—so it holds and carries more heat and kills microbes more effectively. A lengthy blast of high-pressure steam is much more effective at penetrating and sterilizing things than a quick wash or wipe in ordinary hot water and disinfectant. According to one recent review by scientists from New Zealand: “Steam sterilization (autoclaving) is the most widely used method for sterilization and is considered the most robust and cost-effective method for sterilization of medical devices.”

The standard temperature for an autoclave is 121 degrees Celsius. To get an idea of how hot this is, consider that corresponds to approximately 250 degrees Fahrenheit. In other words, it is hotter than boiling water. The reason for this is that simply bringing something up to the temperature of boiling water, 100 degrees Celsius (212 degrees Fahrenheit), is not sufficient to sterilize it because bacterial spores can survive this temperature. In contrast, 121 degrees Celsius is almost always sufficient for sterilization.

Sometimes liquids need to be sterilized. Most biological liquids and laboratory reagents are primarily water. Since water boils at 100 degrees Celsius at standard pressure, it would not be possible to heat it to any temperature higher than that under standard pressure conditions.

As a result, it is necessary to increase the surrounding pressure to 1 atmosphere, or 15 pounds per square inch, above normal pressure in order to increase the effective boiling point of water to 121 degrees Celsius (boiling point varies directly with pressure).

Autoclaves are commonly used in healthcare settings to sterilize medical devices. The items to be sterilized are placed inside a pressure vessel, commonly referred to as the chamber. Three factors are critical to ensuring successful steam sterilization in an autoclave: time, temperature and steam quality.

To meet these requirements there are three phases to the autoclave process:

1. Conditioning Phase (C): Air inhibits sterilization and must be removed from the chamber during the first phase of the sterilization cycle known as conditioning. In dynamic air removal-type steam sterilizers, the air can be removed from the chamber using a vacuum system. It can also be removed without a vacuum system using a series of steam flushes and pressure pulses. Gravity-type sterilizers use steam to displace the air in the chamber and force the air down the sterilizer drain.
2. Exposure Phase (S): After the air is removed, the sterilizer drain closes and steam is continuously admitted into the chamber, rapidly increasing the pressure and temperature inside to a predetermined level. The cycle enters the exposure phase and items are held at the sterilization temperature for a fixed amount of time required to sterilize them.
3. Exhaust Phase (E): During the final phase of the cycle, exhaust, the sterilizer drain is opened and steam is removed, depressurizing the vessel and allowing the items in the load to dry.

Quality steam is vital to a successful autoclave sterilization process. The steam used for sterilization should be composed of 97% steam (vapor) and 3% moisture (liquid water). This ratio is recommended for the most efficient heat transfer. When the steam moisture content is less than 3%, the steam is described as superheated (or dry). Superheated steam is too dry for efficient heat transfer and is ineffective for steam sterilization.