Autoclave

An autoclave is a device used to heat materials above the boiling point of water, inside a sealed container strong enough to withstand the steam and other pressures generated inside. Small units may have heat applied from a direct flame (e.g., a home pressure canner) or electrical resistance elements surrounding the pressure vessel. Larger units are most commonly heated by pressurized steam from a boiler or other steam generator.

Note: Especially for the food applications, this article begins by using English conventional units for volume and pressure, since the containers and autoclaves are matched for certain specific sizes, and recipe guides in the U.S. state pressures in pounds per square inch. Temperatures are given in Celsius and Fahrenheit. Metric equivalents would be very welcome as well, but both the recipes and equipment have to be considered.

For control and safety, an autoclave must be equipped with a means of monitoring the internal chamber pressure. It must also have one or more pressure release valves, which will open and divert overpressure into a safe direction, rather than allow pressure to increase until the chamber explodes.

Home and small-scale use

Small autoclaves called Pressure cookers or Pressure canners are used for home-scale food preparation and preservation. Larger units, usually horizontal and electrically heated, may be used for small-scale sterilization of medical equipment or biological waste; the need for home or office sterilization has much decreased with the widespread availability both of single-use disposable equipment, and systematic safe disposal of medical waste.

Pressure cooker

A pressure cooker superficially resembles a covered saucepot, with typical capacities from 2-12 quarts/liters. Rather than having a cover that fits loosely by gravity, the cover is much heavier metal, and normally has L-shaped fasteners that drop into matching slots in the metal bottom. The cover is then partially turned, so the horizontal part of the L is firmly under solid metal. Some manufacturers require a flexible gasket between the top and bottom, while others rely on metal-to-metal sealing. Gaskets age and need periodic replacement, and, as do the pressure regulating elements, have a remarkable ability to become lost in the kitchen.

Pressure cookers rarely have a true pressure gauge, but either a set of fixed weights, or a weight with graduated lines, which mate to a metal vent in the cover. The weight is made so that when pressure builds above the desired level, it will tilt on the valve seat and let steam escape until pressure lowers to the desired level. It is generally easier to use the fixed weights, which come in versions for 5, 10, and sometimes 15 pounds per square inch. The variable weights tend to need constant observation and adjustment of the stove heat. There is also a fixed burst diaphragm which will rupture and release dangerous pressures.

Once it is working, it can both prepare normally long-cooking foods such as beans in a small fraction of the usual time. It can produce very good braised meats.

Pressure canner

While pressure canners can be used to cook large amounts of food, they are larger devices intended for food preservation by canning. "Canning" is most often done in heat-safe glass jars, but there are home systems for actual metal cans, usually not justified unless one is preparing farm-sized quantities.

It is best to think of the capacity of a canner not so much in volume, but the number and type of standard canning jars it will hold. The smallest will hold one level of pint jars, of a number depending on the diameter of the narrowest part of the cylinder, which often taper slightly toward the bottom. A serious home canning unit can hold two levels of quart jars and three of pint or half-pint jars or cans. The canners usually come with a flimsy metal basket or rack for the jars, although a few will not drop hot jars onto the floor; wise canners make their own.

Large-scale biomedical use

Considerably larger units will be found in hospitals and laboratories, both for preparing sterile equipment and chemical mixtures, and for making infectious waste safe. The customary sterilization recommendations is 121 degrees Celsius, which is produced by 15psi. For such things as sterilizing microbiological culture media, the temperature of the small containers must be held for a minimum of 15 minutes; more and more sources recommend 20 minutes.

When sterilizing large quamtities of equipment, the heat must reach all parts of the container, so extended periods may be needed. In the past, sometimes sealed bacteria cultures were placed inside the materials to be heated, and it was only certain that the load was sterile after they grew no organisms for 24-48 hours. There are now commercial strips, inexpensive enough to put on individual packages, that change color and display "sterile" when they have received adequate heat.

Industrial autoclaves

Much larger, and potentially higher pressure and temperature, autoclaves are used in industry. They are especially attractive when the presence of flame is undesirable. Specialized autoclaves can have internal stirring vanes, and may have provisions to inject additional chemicals during processing. Their role here is primarily for chemical prduction, not sterilization.