Yingtai: Why is sterilization specifically set at 121°C? Can't it be 120°C or 122°C?
In fields such as healthcare, pharmaceuticals, and scientific research, sterilization is a crucial step to ensure safety and the accuracy of experiments. Among various sterilization conditions, the 121°C for 15-20 minutes is widely adopted, often referred to as the "golden standard." However, have you ever wondered why it is specifically 121°C and not 120°C or 122°C? The reasoning behind this temperature involves principles from microbiology, thermodynamics, and engineering, and also reflects historical development and industry standards. Let’s explore why 121°C became the benchmark.
1. The Origin of 121°C
121°C became the standard temperature for high-pressure sterilization for a reason rooted in history. The autoclave, or high-pressure sterilizer, was invented in 1879. Back then, there were no advanced temperature sensors, and sterilization was primarily controlled using a pressure gauge, with sterilization conditions expressed in terms of pressure. At one atmosphere of pressure, water boils at 100°C. However, when pressure is increased to 2 atmospheres, the boiling point of water rises to around 121°C, establishing the initial connection between 121°C and pressure.
Moreover, the choice of 121°C is tied to the use of Fahrenheit in the United States. The sterilization temperature in Fahrenheit was 250°F, which, when converted to Celsius (C = (F − 32) ÷ 1.8), is approximately 121.1°C. Over time, as technology advanced, 121°C became widely accepted worldwide and is still in use today.
2. The Microbial Kill Mechanism
(a) The Action of Moist Heat Sterilization
Moist heat sterilization works through the combined action of the heat and humidity of saturated steam to break down the protein structure and cellular functions of microorganisms. The high temperature causes proteins to denature and lose their activity, while water molecules enhance the penetration of the cell wall, destroying enzymes and genetic material, thereby thoroughly inactivating the microorganisms. Compared to dry heat sterilization, moist heat sterilization is more efficient because heat is transferred more effectively. For example, sterilizing glassware requires dry heat at 160-170°C for 1-2 hours, whereas moist heat sterilization at 121°C only requires 15-20 minutes.
(b) Killing Heat-Resistant Microorganisms and Spores
Spores are dormant forms of certain bacteria that can withstand harsh environmental conditions. Their multilayered, dense membrane structure and low moisture content allow them to survive extreme conditions for years. However, at 121°C in a moist heat environment, the spores’ defense mechanisms are overcome, leading to protein denaturation, enzyme inactivation, and damage to genetic material, ultimately killing them. Experiments show that a 15-minute treatment at 121°C kills 99.9999% of heat-resistant spores, eliminating the risk of microbial regrowth.
(c) Sterilization Kinetics
In sterilization kinetics, the D-value and Z-value are key parameters used to measure the effectiveness of sterilization. The D-value represents the time required to reduce microbial populations by 90% at a given temperature. For example, if a spore has a D-value of 1.5 minutes at 121°C, after 15 minutes of sterilization, the spore population would decrease to one-millionth of its original number. The Z-value indicates the change in the D-value for every 1°C increase in temperature, which is typically around 10°C for spores. This means that increasing the temperature from 120°C to 121°C reduces the D-value by a factor of 10, significantly improving sterilization efficiency. This is why 121°C became the critical standard in sterilization operations.
3. The Key Role of Saturated Steam
In high-pressure sterilization, 121°C of saturated steam strikes the best balance between sterilization effectiveness and equipment requirements. Its heat content and penetrating ability are particularly notable.
(a) The Heat Content Advantage of Saturated Steam
At 121°C, saturated steam has a heat content (enthalpy) of 2700 kJ/kg, much higher than air or water at the same temperature. When the steam comes into contact with sterilization items, it rapidly condenses on the surface and releases a large amount of latent heat, quickly raising the temperature of the items to the sterilization standard, which significantly improves sterilization efficiency.
(b) Pressure and Penetration Ability
In an autoclave, the saturated steam at 121°C corresponds to a pressure of 2.1 atmospheres, providing strong penetration ability. This pressure drives the steam into porous materials or liquid media, ensuring that heat reaches every corner, ensuring thorough sterilization. In contrast, at 120°C, the saturated steam pressure is only 1.99 atmospheres, resulting in insufficient heat content and penetration, which may lead to inadequate sterilization of complex structures. Although 122°C has higher sterilization efficiency, the increased temperature and pressure can cause damage to heat-sensitive materials (such as plastic products or biologically active pharmaceuticals), potentially altering their properties or rendering them ineffective. Additionally, the equipment would require stronger materials and more complex designs, leading to higher costs and energy consumption, making it less practical from both an economic and operational standpoint.
4. Standardization and Regulatory Requirements
The selection of 121°C is not only based on scientific principles and practical experience but also closely linked to the consistency of international regulations and industry standards. It has become the "international language" in the field of sterilization.
(a) International Regulations and Industry Standards
On the global stage, several authoritative organizations have adopted 121°C as the benchmark temperature for moist heat sterilization. Both the United States Pharmacopeia (USP) and the European Pharmacopeia (EP) have made 121°C a critical sterilization standard, providing an important reference for the global pharmaceutical industry. The ISO 17665 standard further promoted 121°C worldwide, providing uniform specifications for sterilization operations in different countries. This global standardization ensures that sterilization results are repeatable and consistent, whether in a laboratory in a bustling city or a medical clinic in a remote area, ensuring similar outcomes in healthcare, pharmaceuticals, and scientific research fields.
(b) Industry Practices and Validation
Long-term practical experience has confirmed that the sterilization effect at 121°C is reliable and efficient. The 121°C, 15-20 minutes sterilization condition has been repeatedly validated to effectively kill various microorganisms and meet production and experimental needs. This temperature provides an optimal sterilization time, ensuring the desired effect while avoiding reduced production efficiency. Therefore, it is widely used in medical equipment disinfection, pharmaceutical production, and scientific research equipment processing.
However, for microorganisms with higher heat resistance, 121°C may not be sufficient for complete sterilization. In such cases, a higher temperature of 134°C can be used for rapid sterilization. Although 134°C is more effective in killing highly heat-resistant microorganisms, it places higher demands on equipment and may damage certain items, so it is used cautiously in cases of extreme microbial heat resistance.
5. Why Not Use 120°C or 122°C?
(a) The Inadequacy of 120°C
Though 120°C and 121°C differ by only 1°C, there is a significant difference in sterilization effectiveness. From a sterilization kinetics perspective, spores typically have a Z-value of 10°C, meaning that a 1°C decrease in temperature results in a 10-fold increase in D-value. For instance, if a spore’s D-value is 1.5 minutes at 121°C, at 120°C, the D-value would increase to around 15 minutes. This means that at 120°C, sterilization efficiency is much lower than at 121°C.
In practical application, the sterilization time at 120°C would need to be extended. If the sterilization time were kept at 15-20 minutes, as in the 121°C standard, there might still be residual heat-resistant spores, making the sterilization effect insufficient. In the pharmaceutical industry, the residual spores could proliferate in subsequent production processes, impacting drug quality and even endangering patient health.
(b) The Drawbacks of 122°C
While 122°C would theoretically sterilize microorganisms more quickly, its practical application has many downsides. First, 122°C increases the heat load on both the equipment and the items being sterilized. Higher temperatures result in higher internal pressures, demanding stronger, more durable materials for equipment manufacturing, which increases both cost and manufacturing complexity. Additionally, heat-sensitive materials (such as biologically active drugs and plastic medical instruments) may suffer structural damage at 122°C, altering their properties or causing them to fail.
In terms of energy consumption, sterilization at 122°C requires more energy to maintain the high temperature and pressure, increasing operational costs and demanding higher energy supply and management. From a sustainability perspective, excessive energy consumption is not aligned with energy-saving and environmental protection goals.
