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Autoclave sterilization is one of the most reliable methods to eliminate microorganisms in the laboratory. This equipment combines steam, high pressure, and controlled time, which allows for complete disinfection in just a few minutes: at 121 °C, a cycle can take as little as 15 minutes.
Although most glass bottles can be processed in an autoclave, it is important to consider certain aspects related to the type of material, preparation, and frequency of use. Below are six essential steps to carry out the procedure effectively and safely.
1. Choose the right type of glass
Not all bottles react the same to heat and pressure.
- Borosilicate glass: the most recommended, as it withstands sudden temperature changes better.
- Soda-lime or flint glass: it can be sterilized, but it has a lower safety margin and a higher risk of breaking.
- Plastic-coated glass: also possible, although the coating wears out with each cycle, reducing its lifespan.
As for filling, it is best not to exceed 75% of the container’s capacity. Bottles containing water or aqueous buffer solutions can be filled up to the maximum line, but if sterilizing media with agar, it is recommended not to exceed 50% of the capacity.
2. Prepare and load the bottles
Before placing them in the autoclave, make sure to:
- Clean them thoroughly to remove any residues.
- Use personal protective equipment: lab coat, heat-resistant gloves, closed-toe shoes, and face protection.
- Avoid overloading the autoclave chamber, since steam needs space to circulate.
For large bottles (e.g., 20 liters), it is advisable to add a small amount of distilled water inside to promote steam generation.
3. Handling screw caps
Never fully tighten the caps when sterilizing bottles. The safest method is to leave the cap loose by up to half a turn. If closed completely, the internal pressure may cause the container to burst.
In conventional autoclaves, it is best to use vented membrane caps, which allow pressure equalization while reducing the risk of post-sterilization contamination. For large bottles (over 2 liters), caps with connectors and sterile filters are recommended.
4. Use of aluminum foil
Covering bottle mouths with aluminum foil is a common practice, but it must be done correctly. The foil should be placed loosely, never airtight, otherwise it would prevent steam from entering and sterilizing properly. Also, keep in mind that foil does not protect against contamination once the cycle is complete.
5. Adjust the cycle duration
The sterilization time will depend on the temperature:
- 121 °C → minimum 15 minutes
- 126 °C → minimum 10 minutes
- 134 °C → minimum 3 minutes
It is always advisable to use chemical or biological indicators to confirm that the cycle was effective. Autoclave tape can be used as a visual reference, but it does not replace biological controls.
At the end, open the door carefully, avoiding direct contact with steam, and allow the containers to cool before handling or transporting them.
6. Know how many times bottles can be reused
Durability depends on the material:
- Borosilicate 3.3 glass can withstand multiple cycles, though over time it may deteriorate if scratches or defects appear.
- Soda-lime glass and plastic-coated bottles endure fewer repetitions.
In all cases, it is essential to visually inspect each bottle before use. If you detect any damage —cracks, chipped edges, or scratches— the bottle should be discarded immediately to avoid accidents.
Conclusion
Sterilizing glass bottles in an autoclave is a routine procedure, but it requires close attention to detail. Choosing the right glass, preparing bottles properly, and applying the correct cycle times ensures optimal results and, most importantly, a safe working environment.
