Microorganisms、too embarrassed to ask others about！（Part 11）

4.1 Extermination with Heat

We have two major approaches to exterminate microorganisms with heat: heating under the condition where water exists (moist heat), and heating in a dry condition (dry heat). The conditions required for exterminating microorganisms differ largely between those two approaches. The basic principle should be understandable -- you can have a sauna at 100 degrees Celsius but cannot take a bath at 50 degrees Celsius.

In moist heat, the microbial proteins undergo denaturation which generally occurs at around 60 degrees Celsius. In other words, many microorganisms will be exterminated under that condition. For example, it is widely known that Louis Pasteur's invention in 1860s of the technique to treat wine with this condition and stop bacterial contamination saved the wine industry in France back then. This technique has been called "pasteurization" and is used for other products including milk even today.

You can find some microorganisms inhabiting at the deep-sea bed where hot water is discharged from vents. In other words, they live in a environment with high-pressure and high-temperature over 100 degrees Celsius.

In old times when we didn't have autoclaves yet, culture media we prepared went wrong on the following day because bacterial spores survive the temperature at 100 degrees Celsius. Therefore, the standard procedure for culture sterilization was to heat the media to 100 degree Celsius for 30 minutes and repeat that for 3 days. The procedure was invented by a biologist named Heinrich Hermann Robert Koch. The apparatus he used in this procedure is now called "Koch's steam sterilizer". 

In the 19th century, an incident where canned food products were contaminated with Clostridium botulinum due to insufficient heat treatment. C. botulinum is an anaerobic bacterium which forms heat-resistant spores thus those spores survived the insufficient heat treatment, grew in cans, and caused the food poisoning.

The Food and Drug Act was enacted in 1906 because of the incident. Provisions to regulate counterfeit/falsified products and deteriorated products were included in this Act.
Between 1915 and 1921, National Canners Association laboratories conducted research on extermination of microorganisms using spores of C. botulinum. In the research, it was shown that the spores (1011 spores/ml) suspended in phosphate buffer would be exterminated in 2.45 minutes at 250 degrees Fahrenheit (121.1 degrees Celsius).

Since the result was shown, this temperature has been employed even today as the standard temperature for moist heat sterilization (autoclaving). However, you should note that complete sterilization (i.e. extermination) including spores will never be possible when the temperature is below 121 degrees Celsius. I will explain further details in a later section but the lower the temperature is, the longer the time required will be. In other words, the time required for sterilization will be shorter when the temperature is higher than 121 degrees Celsius.

Currently, we use autoclaves for laboratory use for microbiological tests. At plants for medical devices and drugs, large-sized autoclaves for industrial use are often used.

Endotoxins, which often cause problems to drug products and medical devices, are heat resistant thus cannot be inactivated in the autoclave. Therefore, we use dry heat processing at 250 degrees Celsius, a higher temperature than that for the autoclave, for some containers and others such as vials when there is a possible risk of contamination with endotoxins.

By the way, we often use liquid paraffin, highly-refined hydrocarbon which is used in some products such as baby oils, in microbiological tests. In this case, we also use dry heat when we sterilize the paraffin as it doesn't contain moisture. 

We also have some other methods to exterminate microorganisms. 

When it comes to moist heat sterilization, this is a very important method for industrial sterilization. Therefore, I will explain this in a later section