Steam Sterilization: Importance of Steam Quality
Steam sterilization of pharmaceutical equipment is highly dependent on the purity and quality of the steam. If not addressed, it can severely affect the sterilization of your systems.
Along with ensuring microbiologic safety, steam sterilization enables contamination-free handling of product contact containers.
Here are the steam types that are commonly used in life-science applications.
Steam Type | How it’s made? |
---|---|
Industrial Steam (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 |
In this article, we are interested in pure steam quality. Let’s start with the regulatory aspects first.
Table of Content
A Regulatory View on Pure Steam
Regulatory bodies like United States Pharmacopeia (USP), Japanese Pharmacopeia, European Pharmacopeia, etc., outlined compendial requirements for WFI.
All bodies are ultimately trying to make sure that the right steam quality is used for sterilization.
According to USP 39 and ISPE Volume 4 USP monograph, pure steam and clean steam carries the same meaning and quality. Monograph is a detailed written study.
What Does Compendium and Compendial Mean?
Compendium means collection of concise but detailed information about a particular subject. Whereas, Compendial is related to the “compendium” that serves as a standard.
Condensed pure steam shall meet the same WFI requirements as per the USP. But, pure steam excludes the bio-burden requirements due to the below fact.
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
Steam Quality for Sterilization
To effectively sterilize, it is important to introduce the right steam quality as it is the driving force to kill the micro-organisms.
So to avoid poor steam quality, adequate design measures in steam sterilization are required. Out of those, moisture is one of the prime factors.
Based on this aspect, professionals classify steam quality as below:
- Moist Steam (Moisture > Saturated Steam)
- Saturated Steam (The Ideal one)
- Superheated Steam (Negligible Moisture)
Poor conditions for sterilization are (1) and (3). Before understanding them, let us see the ideal one, Saturated Steam, and also the reasons to consider it as the right fit.
Saturated Steam and Its Importance
Let’s see the boiling process in short.
Consider an enclosed bowl of water filled up to half of its volume under heating.
Vapor formation starts as the temperature increases and the boiling occurs at 100°C.
Three phenomena occur at the same time during vaporization.
- High-temperature molecules get less dense and start to travel towards the water surface.
- The water surface applies pressure to the vacant space above it.
- Vapors in vacant spaces apply pressure on the surface of the water (vapor pressure).
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. This is a saturated condition.
Simply put, saturated steam can’t accommodate more steam into it at a constant temperature.
Right 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 (saturated steam has moisture < 5% by mass). This becomes very helpful in increasing the rate of killing micro-organisms rather than dry steam.
In saturated steam, heat is transferred more quickly to microorganisms than in 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.
Irrespective of the extent of moisture present in the micro-organisms, saturated steam effectively transfers the heat and kills the micro-organisms.
Why Not Wet Steam?
Unlike saturated steam, wet steam has moisture > 5%.
As the amount of moisture in saturated steam helps in proper steam penetration, the opposite is the case for wet steam.
This prevents the proper penetration required for sterilization as excess moisture acts as a barrier and results in longer penetration times.
Higher moisture also causes corrosion due to the oxidation of metals when exposed to steam.
As a result of higher moisture in the steam distribution system, condensate oppose smooth steam flow and reduces pressure.
So, the consequences of using wet steam in sterilization are:
- Reduced heat penetration
- Pressure drops across the pipeline due to resistance created by an excessive amount of condensates
- Corrosion due to oxidation
- Failure of sterilization cycle due to increased temperature drop
What causes wet steam to produce?
Well, when the steam comes from Pure Steam Generator, there are a number of reasons for the wet steam formation inside the piping.
Wet steam may be a result of incorrect calculations of the following:
- Pipe sizing
- Pipe routing (frequent 90-degree bends)
- Thickness of insulation
- Location and type of steam trap
- Identification of low point drains
- Sample point locations
- Amount of non-condensable gases
Why Not Superheated Steam?
As with wet steam, superheated steam does the same thing.
Saturated steam when heated further converts to superheated steam causing the following conditions:
- Elevated temperature greater than required
- Does not contain the required moisture content for microbial killing
- Improper heat dissipation
A pressure drop across a pressure regulating valve can produce superheated steam as well. As the pressure in the steam distribution pipeline drops, more superheated steam forms.
Ideally, the steam pressure requirement is 1.5 to 2.5 bar(g). Here’s how ISPE describes the steam sterilization principles.
Sizing Condensate Return Lines
A constant steam velocity of 30 to 35 m/s is recommended at a reasonable pressure drop for a Schedule 40 size pipe. Factors such as corrosion, erosion, fouling, etc., should be considered in the line sizing calculations.
There is a chart called Flow Rate for Dry Closed Returns. You can use that to select the diameter based on the pressure drop.
The general design considers higher velocity in the header than the branch pipes. These velocity requirements vary according to the system pressure.
Conclusion
You see, the saturated steam is main component of the successful SIP cycles.
Whereas, wet and superheated steam are ineffective for your sterilization due to undesired moisture properties.
Excessive moisture in saturated steam can cause a moisture-laden load, resulting in longer sterilization hold times. On the contrary, steam expanding at the system entrance in the absence of moisture can develop superheated steam, making hopeless sterilization.
So, the important point is the dryness fraction for excellent saturated steam shall be 0.95 (95% dry saturated steam and 5% moisture.)
Also, the pure steam must meet the current monograph for WFI of the respective pharmacopeia/s.
FAQs
How long does it take for steam to sterilize?
20 minutes at least. Steam is considered saturated within a temperature range of 121 to 130°C. According to the principle of the F0 value, when saturated steam exposure is maintained at 121.1°C for 20 to 30 minutes of time, the sterilization requirement is satisfied. Similarly, if we maintain the saturated steam at 130°C for 3 to 4 minutes, then also satisfy the sterilization requirement (provided that the steam must be saturated which is difficult to justify scientifically).
Is Autoclave the same as steam sterilization?
Yes, and no. Life-science industries practice steam sterilization with the help of equipment known as an Autoclave.
Yes: The expected outcome from both the concepts is the same, i.e. sterilization.
No: From a functional perspective, they are poles apart. Steam sterilization is used by integrated systems such as equipment and sub-components. While Autoclave is used for mobile components such as flex hoses, plastic containers, clamps, etc.
In short, for Sterilization-In-Place (SIP): Steam Sterilization and for Sterilization-Out-of-Place (SOP): Autoclave
What are the 3 parameters of steam sterilization?
Steam quality for sterilization according to EN 285:2015 – Steam Dryness, Amount of non-condensable gases and Superheated Steam
Hello,
my name is Nina. Thank you for excellent article. I am working in microbiology lab. We use autoclaves for sterilization of nutrient media and aome plastic (tips for autometed pippets). Autoclaves have been qualified every year (vacuum test, temperature mapping, BI checikg). We had audit two years ago and auditor asked for quality of pure steam. What is your opinion about this? Is this mandatory requirements for laboratoy autoclave? Autoclaves are old about 17 years and we didnt ask in specification quality of pure steam. How can we explain in risk assesment that this tests are not mandatory for this kind of equipment.
Glad to know that Nina! Regarding your question. Why do you Autoclave the components? To assure the sterility, right? If your steam itself isn’t pure, how you are going to prove that? That’s why the auditor might have asked about that. Mandatory is nothing unless you can justify other way round. But you need to establish the steam quality parameters. Anyhow. Don’t even think about explaining via risk assessment that the steam quality doesn’t matter. It matters with the very reason of you doing the autoclave. Hope this clarifies.