Why is high pressure steam sterilization usually at 121°C?

What are the methods for controlling microorganisms?

Generally speaking, they can be divided into three categories, namely killing, inhibition and removal. Killing is divided into sterilization and disinfection, inhibition can be divided into preservation and chemotherapy, and removal generally refers to filtration.

Why is high-pressure steam sterilization useful?

High-temperature sterilization is a typical physical sterilization method and the most well-known sterilization method. For example, burning is a typical dry heat sterilization. The pasteurized milk you drink every day refers to milk prepared by pasteurization. Boiling water is a typical boiling disinfection, and high-pressure steam sterilization is widely used in biological laboratories, health care institutions and factories around the world.

The efficiency of hot steam sterilization lies in: hot steam has stronger penetration than hot air; hot steam has a stronger destructive effect on cell components such as proteins; steam has latent heat, and a large amount of heat can be released when the gas is converted into liquid; hot steam can quickly increase the temperature of the sterilized object.

Why is it 121℃?

I believe everyone knows the most commonly used conditions for high-temperature and high-pressure sterilization, namely: 121℃, 15~20 min. Have you ever wondered why it is 121 instead of 120℃?

In fact, the autoclave was introduced in 1879. At that time, there was no temperature sensor, but the sterilization process was controlled by a pressure gauge. The sterilization conditions are expressed in pressure units, which are generally expressed in 1 kg/cm2 in China, equivalent to 1 standard atmosphere = 1 bar = 1 kg/cm2 = 0.1 MPa = 14.5 psi.

In other words, the commonly used autoclave conditions refer to the boiling point of water under 2 atmospheres (pressure difference of 1 atmosphere), which is close to 121℃ but slightly lower than 121. So why did they choose 121 instead of 120 or 119.6℃? In fact, the United States does not use Celsius but Fahrenheit. The temperature used for sterilization in the United States is 250℉. Maybe they think 250 is a very good integer. According to the conversion between Fahrenheit and Celsius, 250℉=121.1℃.

Yes, it is such a simple origin.

Can 121℃ kill all microorganisms?

The answer is no.

In 2002, Kashefi and Lovley published an article in Science. They screened an archaeon from the black chimney on the seabed of the eastern Pacific Ocean. The bacterium can grow at 85-121℃ and was named strain 121. The strain has a generation time of 24 hours at 121℃, sterilized at 130℃ for 2 hours, and then transferred to 103℃, and the strain can continue to grow.

In 2008, Takai et al. published an article in PNAS. They developed a high hydrostatic pressure culture technology that was successfully applied to the cultivation of deep-sea organisms. They isolated the strain Methanopyrus kandleri strain 116 from deep-sea hydrothermal fluids. The strain can grow at 90-122℃ at 40 MPa hydrostatic pressure, and the optimal growth temperature is 105℃. It grows and reproduces under high pressure conditions at 122℃, which refreshes the highest growth temperature of organisms.