Monday, August 27, 2012

Steam Sterilization Part 2 (aka Use The Right Steam)

So in the Steam Sterilization Basics post, we established that sterility is achieved when we blast microbes on the process surface with steam.

When the steam touches the cool process surface, it condenses thereby releasing the energy of the latent heat of vaporization, 540 kcalories per liter of condensate.

On top of that, condensate occupies less space than steam such that it draws a vacuum pulling more steam to the cool process surface.

Well, there are three types of steam and two of them are the wrong types for sterilization. A good illustration is as follows:

Figure 1: Water as it is heated

Wet Steam (aka supersaturated steam)
Water vapor at boiling point - liquid water present

Dry Steam (aka saturated steam)
Water vapor at boiling point - no liquid water present

Superheated Steam
Water vapor at a temperature higher than the boiling point.

Wet and Superheated Steam Don't Sterilize

Wet steam exists at boiling point, but does not effectively sterilize because it already contains condensate, it cannot condense and therefore does not possess the latent heat of vaporization required for effective sterilization.

Superheated Steam is too hot to condense and when it comes in contact with the process surface, it just cools to a lower temperature. The heat transferred to the process surface may achieve kill, but not reliably.

Dry Steam Is The One You Want

Dry steam is the steam that collapses easily into condensate when in contact with cooler process surfaces because it is at the boiling point, thereby releasing the latent heat of vaporization.

The effective sterilization of bioreactors depends on dry steam condensing on the process surface until the temperature equilibrates, and this means operating the bioreactor at the boiling point of water, which if steaming at 15 psig is121oC.


Using wet steam? Nope, not enough energy.

Run the SIP hotter than boiling point? Nope - no latent heat of vaporization.

You achieve sterility by delivering the latent heat of vaporization with dry steam, which can only exist slightly above the boiling point.

After that, time it takes to kill the bugs depends on how many bugs are there.

Further reading:

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