Sunday, August 26, 2012

Steam Sterilization Basics (aka Why You Suck at SIP)

The principles of steam sterilization (SIP) get lost in the shuffle of running a modern pharma production facility.  I think the reason for this is that the specifications get written down way back when, it gets coded into the automation and all that gets validated you sort of forget about it.

...Until you start getting bacterial contamination of your bioreactors.

What most people think is that you can sterilize a bioreactor by heating the process surfaces up to 121 degrees Celsius and hold that temperature for 30 minutes.

As a newbie engineer, that's what I thought.  I'm looking at my Bioprocess Engineering textbook right now and all I see are equations for death kinetics:
kd = αe-E0d/RT
There are mentions of using steam for sterilization, but you'll never understand until you get one of these biotech old-timers who were clamping tri-clovers since before you knew how to read.

Steam Basics

Question: How much energy does it take to heat a liter of pure water from 21o Celsius to 100o Celsius?

Well, if one calorie is defined as the amount of energy it takes to heat one gram of water one degree Celsius, then to heat 1000 grams of water (a.k.a 1L of water), it takes one kilocalorie (kcal).

And if we have to cover the distance of 79oC, then that is:

79 kcal required to heat 1L of water from 21oC to 100oC.

Question: How much energy does it take to make steam from 100oC water?

There's this thing called the latent heat of vaporization. It refers to the extra energy it takes to turn something from a liquid into a gas. So even as you're adding more energy (heat) to the system, the temperature remains the same.

For water, this is 540 kcal/kg. This means:

540 kcal is required to turn 1L of water into steam.

Key message: It takes seven times the energy to make steam from water (at 100oC) than it does to boil water from room temperature.  The reverse is true.  When steam condenses it releases that same energy.

Steam Sterilization

This latent heat of vaporization of water is key in killing bacteria on your cool process surface. Here's why:

When the steam first hits that process surface, the steam loses energy as heat transfers to the process surface.  When the steam loses enough heat, water begins to condense.  And when water condenses, 540 kcal of energy (per liter of water condensate) pummels the organisms that are there.

Also, condensing steam forms a vacuum that draws more steam to the area.  This constant bombardment of disruptive energy ends only when the process surface temperature equilibrates and condensation stops.

This microbial holocaust is achieved through the two properties of steam:
  1. 540 kcal of energy is released per liter of condensate formed. 
  2. Vacuum drawing more steam to the area

Time & Temperature Myth

Clearly to make steam from water, you need high temperatures.

Clearly, it takes time for the bioreactor to reach these high temperatures.

But these are not the only factors.  You need the pressure for steam formation.  You need water to make steam.  And all this depends on how "infected" your process surface is to begin with.

There are a lot of variables in making steam sterilization (SIP) effective: 121oC for 30 minutes isn't all it takes.

Stop Sucking At SIP

See also:


sairgülen said...

Do you think more than 30 minutes sterilization would be harmful for elastomers (or make faster its expiration)

sairgülen said...
This comment has been removed by the author.