by Oliver Yu
...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/RTThere 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.
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.
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 SterilizationThis 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:
- 540 kcal of energy is released per liter of condensate formed.
- Vacuum drawing more steam to the area
Time & Temperature MythClearly 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