Pressure Points: Risks of Using Sealed Glass ISO Bottles for Sterilizing Liquids

Steam sterilization (Autoclaving) is a widely used method for sterilizing various materials and equipment in laboratory, healthcare, and other industries. Compared to other sterilization methods, autoclave steam sterilization offers several benefits, including its efficiency, effectiveness, and convenience. Unlike other sterilization methods, such as gas or chemical sterilization, autoclaving does not require any hazardous chemicals, making it a safe and cost-effective option.

DURAN? Autoclave Bottles with rubber caps from DWK Life Sciences are a safe and reliable solution for processing sterile aqueous liquids. Made from thermally shock resistant, high-purity, USP/EP Type I borosilicate 3.3 glass, these bottles use an innovative rubber closure system that offers both self-venting and self-sealing capabilities, along with the added convenience of a visual check of continued seal integrity.

However, using non-venting closures (such as a fully tightened screw cap) to sterilize liquids in autoclaves can create a dangerous situation, which has the potential to cause personal injury and damage.[4] During the filling and capping of the bottle, an atmosphere of air gets trapped within the bottle. Upon heating, the air in the sealed container expands and at the same time the air is compressed by steam vapour released from the heated liquid, resulting in a rapid rise in pressure within the bottle. These pressures reach several atmospheres greater than the external pressure exerted on the outer walls of the container by steam in the autoclave chamber.[5]

After sterilization, the liquid exhaust phase of the cycle gradually releases the chamber pressure, which can be tracked and equalized by the internal pressure of the bottle if self-venting type closures are used. Additionally, the liquid cools through vaporization that occurs due to the pressure change. However, with sealed containers, the pressurized state persists until the load cools through the walls of the glass container by conduction. As there is insufficient time during the liquid exhaust phase for the sealed container system to significantly cool, the internal temperatures and pressures remain high. Unlike a vented bottle that relieves the excess pressure, the pressures within sealed glass containers continue long after the cycle is completed. As a result, there is a risk of the bottle bursting due to either thermal shock, such as the opening of the door and contact with room temperature air, or sudden jarring or striking of one bottle against another while unloading. The bursting of one bottle can lead to a dangerous chain reaction of multiple bottles bursting that can cause severe personal injury and damage. It is crucial to use appropriate closures for autoclaving liquids to ensure the safety of laboratory personnel.

The DURAN? autoclave cap perfectly fits over the specially shaped beaded rim of the DURAN? bottle neck. The cap design allows the venting of steam and air during the heating phase of the steam sterilization cycle. Later the cap automatically creates a hermetic seal at the end of the cooling phase as a vacuum is created within the bottle. The higher external atmospheric pressure pushes down the concave center of the cap. This offers a visually conformation of continued container integrity after autoclaving. The cap can be easily removed when required by pulling on the side tab, reducing the risk of contact with the sterile contents.

[1] When using closed top screw caps for autoclaving the recommendation is to loosen the cap by a quarter or half turn.

[2] The DURAN? GL 45 membrane caps (such as 291182809) may be autoclaved at 121 degrees centigrade for 20 minutes. The screw cap can be tightly closed as the membrane permits an automatic sterile gas exchange, allowing safe pressure equalisation during the autoclave cycle, and at the same time ensuring the sterility of the bottle contents. However this product is difficult to validate in a GMP process.

[3] A better option for GMP processing is to use a single-port GL 45 tubing connector cap (292612607) with a commercially available sterile venting filter. The DURAN? single-port GL 45 stainless steel cap can connected to a suitable sterile-air 0.2 micron 50 mm disk or capsule filter. This option is especially recommended for larger DURAN? bottles. Examples of suitable 50 mm filters: MilliporeSigma 50 mm 0.2 micron Millex-FG and Sartorius Midisart? 2000 Venting filter: (these filers have the advantage that they can be used up to 20 times)

[4] Links to examples of autoclave accidents

• https://ehs.weill.cornell.edu/sites/default/files/ll_autoclave_explosion.pdf
• https://www.osha.gov/ords/imis/accidentsearch.accident_detail?
• https://www.labmanager.com/lab-health-and-safety/safe-use-of-autoclave-sterilizers-1835

[5] Allwood, M. C., Hambleton, R. and Beverley, S. (1975), Pressure changes in bottles during sterilization by autoclaving. J. Pharm. Sci., 64: 333-334. doi:10.1002/jps.2600640232