In this post we will continue our discussion about the different sterilization methods available. A quick reminder of our goal - total destruction of microorganisms! Sterilization methods can be divided into two: heat or non-heat sterilization methods. This post will focus on the most common methods that use heat to sterilize. So grab a cool icy drink, it's going to get hot. Very hot.

 

Flaming: Sterilization in an Open Flame

The oldest method around is flaming, holding an instrument in an open flame. This is exactly what is done when you burn a tip of a needle to remove a thorn. This method has been around since our ancestor hunters used flaming to make the meat they hunted edible and tasty. They didn't need any knowledge about the dangers of bacteria to understand that barbecued meat is healthier and tastier than raw meat.

Incineration

If you're into total destruction, this method's for you. By using incineration everything is destroyed, not only microorganisms. Incineration is applicable to materials used only once or when the contamination level is so high, that for safety reasons it must be destroyed. Radioactive materials not included, of course. It turns everything into ashes therefore reducing load volume by 90%. 

Typical applications:

  • Final stage in Medical Waste treatment 
  • Biohazard Laboratory Waste treatment

Advantages: 

  • Total destruction when needed prevents reuse of materials
  • Load volume decreases by 90%
  • Elimination of biological agents
  • Works well with large quantities
  • Energy emitted is used for electricity

Disadvantages: 

  • Very high construction and installation costs
  • Requires fuel which is expensive compared to autoclave power consumption
  • Upkeep costs are very high
  • Expensive infrastructure
  • Requires highly trained personnel to run properly
  • Risk of contamination: If filters clog, which is common, this creates incomplete burning and emission of poisonous gasses. 
  • Incineration requires a double chamber, the first between 300-500 degrees and the second between 800-1200. If one of the chambers does not function, which can happen often, the process is incomplete
  • Usually repairs are complex and thus time consuming; therefore it can cause piling up of loads, which, in turn, can create contamination

Dry Heat

With dry heat the bacteria are burned to death or oxidized. Dry, hot air is much less effective in transferring heat than moist heat. This is why microorganisms are much more able to withstand heat in a dry state. The dry heat sterilization process takes a long long time and is done at a high temperature (2 hours at 160°C). The total cycle time, including heating up and cooling down to 80°C can take about 10-11 hours, not the greatest solution for an active clinic that needs its instruments to be readily available. With forced cooling the time may be reduced to five hours.

Applications:

Dry heat sterilization is especially applicable to materials that are damaged by high pressure or moisture and can withstand high temperatures. Dry heat does not cause corrosion of metal instruments; thus it is well suited for the sterilization of metal instruments but cannot be used for sterilization of fabrics, plastics or rubber-ware. It is also suitable for glassware in the lab. The final category is materials that steam cannot penetrate or must remain dry: This includes pharmaceutical powders that need to remain dry, oils and substances that aren't penetrated by steam.

Advantages: 

  • Suitable for sterilizing materials and instruments that cannot be autoclaved
  • Usually cheaper than an autoclave

Disadvantages:

  • Requires very high temperature in order to be effective, 4 hours for a standard 160 degree cycle and 2 hours at 180 degrees
  • Cycle duration is very high thus very low turnaround
  • Much higher power consumption and cost compared to autoclave, therefore much higher running costs
  • Dry heat can damage some instruments 
  • Heat elements tend to break faster than autoclave's heating elements

Our next post will discuss sterilization by steam and we'll explain why this is one of the most commonly used methods and also one of our areas of expertise. We'd love to hear of other methods, advantages or disadvantages, unusual or interesting applications, so please join the conversation.