Steam Sterilization: Importance of Steam Quality

Steam sterilization is practiced in healthcare-related industries to make the product contact containers microbiologically safe. This ensures contamination-free handling and ultimately safe products. Following steam types are commonly considered in almost every healthcare industry.

1. Industrial Steam (Black Steam)
2. Process Steam
3. Pure or Clean Steam

• Industrial or Black Steam: A conventional boiler when fed with soft water produces Black Steam.
• Process Steam: A SS boiler when fed with demineralized water, produces process steam
• Pure or Clean Steam: A Pure Steam Generator (PSG) when fed with Water For Injection (WFI) produces pure steam.

However, this article focuses on the quality aspect of pure steam. Before getting into different saturated steams, let us see the regulatory aspects of pure steam.

Pure Steam Through Regulatory Point of View

Regulatory bodies outline compendial requirements for WFI such as United States Pharmacopeia (USP), Japanese Pharmacopeia, European Pharmacopeia, etc. The ultimate aim of each body is to ensure the right quality of steam being used for steam sterilization. As per USP 39, ISPE Volume 4 USP monograph (monograph is a detailed written study), pure steam is also referred to as clean steam.

What Does Compendium and Compendial Mean?

Compendium: A collection of concise but detailed information about a particular subject. Compendial relates to the “compendium” that serves as a standard.

Pure steam when condensed shall meet the same WFI requirements per the USP. However, pure steam excepted from the bio-burden (microbiological) requirement because of the fact mentioned as:

Finally, because Pure Steam is lethal to microbes, monitoring of microbial control within a steam system is unnecessary, as is a microbial analysis of the steam condensate.

USP-39, 1231

USP defines pure steam as;

Pure Steam is water that has been heated above 100°C and vaporized in a manner that prevents source water entrainment. It is prepared from water complying with the EPA National Primary Drinking Water Regulations, or with drinking water regulations of the European Union or of Japan, or with WHO drinking water guidelines. It contains no added substance. The level of steam saturation or dryness and the amount of non-condensable gases are to be determined by the Pure Steam application.

USP Monograph

Impact of Different Steam Qualities on Steam Sterilization

Ensuring the steam with the right parameters is the key factor for effective steam sterilization. Steam is the driving force in killing the microorganisms. Therefore, adequate design measures required in place to avoid the application of poor quality of the steam in steam sterilization.

Classification of steam to understand sterilization technique:
1. Moist Steam (Moisture > Saturated Steam)
2. Saturated Steam (The Ideal one)
3. Super-heated Steam (Worst for steam sterilization)

Poor conditions for steam sterilization are (1) and (3). Before understanding them, let us detail on ideal one i.e. Saturated Steam, and reasons to consider it as the right fit for steam sterilization.

What Is Saturated Steam and Why It Has Importance?

See boiling process below and let’s understand the importance of saturated steam.

Consider an enclosed bowl of water filled up to half of its volume. Start heating the bowl. As the temperature increases, vapours start to form commonly known at 100°C and during vaporization (steam formation) three phenomenon occur at the same time.

1. High-temperature molecules get less dense and starts travel towards the water surface
2. The water surface applies pressure to the vacant space above it
3. The vapours in vacant space apply the pressure on the surface of the water (vapour pressure)

What Is Saturated Condition?

This process continues to escalate and water starts to evaporate rapidly, resulting in more vapor pressure. The time comes when pressure applied by vapors generated at the water surface comes in equilibrium with the pressure applied by the vapors in the space above it (i.e. space cannot occupy more water vapors in it). This is the Saturated Steam condition.

Simply put, saturated steam can’t accommodate more steam into it at the constant temperature.

Right Amount of Moisture Is The Key

The molecules present in the vapor phase collide with each other. This causes the temperature to drop. Temperature drop results in moisture formation (condensation) in the vapor phase. This moisture in the steam (vapor) is the key factor responsible for the effective killing of the micro-organisms (moisture in saturated steam is less than 5% by mass). This becomes very helpful in increasing the rate of killing micro-organisms than dry steam.

The heat transfer rate from Steam to micro-organisms is higher in Saturated steam than dry steam. The reason being, both (steam and microbe) contain moisture, resulting in effective heat transfer between the same phase (moisture to moisture). Microbes with dry nature take longer to get killed.

Saturated steam (with proper moisture content) therefore can kill both the kinds of micro-organisms, i.e. those with less and high water content.

Why Not Wet Steam?

Like saturated steam, wet steam also posses moisture. The difference is that the moisture content is higher than saturated steam, i.e. > 5%. As the amount of moisture in saturated steam helps in proper steam penetration, the opposite is the case for wet steam. It prevents the proper penetration required for the sterilization because of excess moisture acting as a barrier, causing longer penetration times.

Higher moisture also causes corrosion because of the oxidation of metals under steam exposure. Reduced pressure because of condensate resistance to smooth steam flow is the outcome of higher moisture in the steam distribution system. Following conditions avoid wet steam application for steam sterilization:

1. Reduced heat penetration
2. Pressure Drop across the pipeline because of resistance created by an outrageous amount of condensates
3. Corrosion because of oxidation
4. Failure in the sterilization cycle because of increased temperature drop

What causes wet steam to produce? There are number of reasons for the wet steam formation inside the piping when the steam comes from the Pure Steam Generator. Wet steam may be a result of improper calculations of the following:

1. Pipe sizing
2. Pipe routing (frequent 90 degree bends)
3. Thickness of insulation
4. Locations and type for steam trap
5. Identification of low point drains
6. Sample Point locations
7. Amount of Non-condensable gases

Why Not Superheated Steam?

Just like wet steam’s inefficiency drives unsuccessful sterilization, super-heated steam also has disadvantages. Saturated steam converts to super-heated steam on further heating and result in the following conditions:

1. Elevated temperature than required
2. Does not contain required moisture content for microbial killing
3. Improper heat dissipation

The pressure drop across the pressure regulating valve also causes to form super-heated steam. The rate of formation of superheated steam is directly proportional to the rate of pressure drop across the steam distribution piping. Ideally, the steam pressure requirement is 2 to 2.5 bar (g) at the source. ISPE also described the Steam sterilization principles in a crisp way.

Conclusion

With all said about the saturated steam and perfect dryness (not too moist and not too dry) fulfills the desired duty for steam sterilization. Whereas wet steam and super-heated steam complicates the steam sterilization cycle because of undesirable moisture play.

Excessive moisture in saturated steam can cause a moisture-laden load, resulting in longer sterilization hold times. Similarly, the absence of moisture in steam causes to form super-heated steam when expanded in the system entrance, resulting in ineffective steam sterilization.

The dryness fraction for ideal saturated steam shall be 0.95. This means steam with 95% dry saturated steam and 5% moisture.

Don’t forget to read the related articles

1. What is F0 Value? How to Calculate it?
2. F0 Value and Sterilization Cycle Development
3. Online Calculator for F0 value

Steam Quality for Steam Sterilization: FAQ