Dry Heat Oven Sterilization

Dry heat sterilization is any type of heat sterilization that does not include both steam and pressure. Typically this looks like heating something in an oven until the heat kills the bacteria by causing “oxidation of cellular constituents and consequently cell death.”¹

Dry Heat Sterilization Temperatures¹

Depyrogenation Temperatures

Sterilize vs Depyrogenate

Sterilization is a process that removes or kills all forms of microscopic life³.

Depyrogenation is the removal or inactivation of pyrogens⁴.

Sterilization and depyrogenation are distinctly different processes that have different parameters and goals. Conveniently, when using dry heat to inactivate pyrogens, sterilization is also achieved. I therefore recommend always depyrogenating all materials in your brew that can withstand the heat. Ultimately this looks like running all glassware through a depyrogenation cycle in order to achieve the dual goals of sterilization and depyrogenation.

Why Depyrogenate Glass When the CSP Can’t be Depyrogenated?

Pyrogens will naturally be part of your brew. Because we cannot heat the CSP up to extreme temperatures, when all is said and done there will be some pyrogens in the final product. Filtration may remove some of the pyrogens, but likely not all. In this scenario, why bother removing some pyrogens when we can’t remove them all?

Pyrogens are “any substance that can cause a fever”⁴. It is not a single, stray, pyrogen that causes this fever. Instead, it is a critical mass of pyrogens that begin to cause problems for the host⁴. If we have the option to remove, theoretically, 50% of the pyrogens in a CSP by depyrogenating the glassware, then we should do that.

Clean Everything Before Dry Heat Cycle

Before you run the dry heat cycle everything should be properly washed, rinsed, and capped with foil.

Wash: Use a phosphate based detergent such as “Alconox” to achieve lab quality clean. Phosphates are harsh. For small and medium size brews you may choose to use regular dish soap. When reusing glassware, regular dish soap does not remove old oils as well as Alconox. This may result in old oils getting baked onto the glass and essentially ruining the glass as it imparts debris into the brew.

Rinse: Rinse in distilled water. You should use a three stage rinse to ensure all soap is removed. Distilled water is essential to this process as we can’t have tap water or similar leaving minerals behind as the water evaporates out of the glass.

Cap: When glass comes out of the final rinse it should get a foil cap. You want to do this quickly enough that you minimize how long dust has to fall into the glass. Caps should be double layered and have a flute to allow steam to escape, but should be constructed in a way to not allow dust or anything else to fall in. It can be helpful if the caps on vials can be easily removed for a more graceful filling process. Use your head: steam has to escape, dust can’t be allowed in, and they need to be easily removed.

About Temperature Control

This is not baking cookies. When you put the items in the oven, first you have to get the items up to temperature. Once the items are at temperature, only then do you start a timer. You need a laser thermometer to verify the true temperature of your items. If you do not have a laser thermometer you need to get one. If you cannot get one I recommend you leave the items in the oven for at least 30 minutes to get them up to temperature, then after this theoretical 30 minutes of getting up to temp you can start your true timer.

You should use the laser thermometer multiple times during the sterilization/depyrogenation cycle to verify temperatures are holding correctly.

References

View the library page for access to some PDFs.