Ensuring the safety of food through heat sterilisation is crucial. To understand the basics, this article focuses on two main methods: moist heat sterilisation and dry heat sterilisation. We delve into why these methods show different efficiencies against microbes, exploring how humidity affects sterilisation efficiency and how the moisture content in the environment affects microbial survival. Additionally, we examine examples of heat-resistant spore-forming microbes, investigating why they resist common sterilisation methods from a scientific standpoint. From fundamental principles of microbiology, we deepen our understanding of the food sterilisation process.
Why is there such a difference in sterilisation efficiency between moist and dry heat?
First, let's discuss the basic principle when considering heat sterilisation of microbes: the relationship between microbial heat sterilisation efficiency and moisture content. In basic microbiology experiments, we learn about autoclaves and dry heat sterilisation. Autoclaves, which use steam under pressure, are employed for sterilising items like microbial culture mediums that contain moisture, operating at around 120°C for about 20 minutes. On the other hand, dry heat sterilisation, used for sterilising items like glassware, requires higher temperatures, around 160°C for about an hour. Despite being a method of sterilisation, dry heat sterilisation necessitates higher temperatures and longer durations.
Why does dry heat sterilisation require higher temperatures and longer times than moist heat sterilisation?
Why, then, does dry heat sterilisation require higher temperatures and longer times? Let's use an example to explain. Imagine dipping your finger into a bath of hot water at 60°C; you'd feel intense heat. As mentioned before, 60°C is sufficiently high to denature proteins. Now, think about being in a sauna. The air temperature in a sauna can reach 100°C. Despite this, you wouldn't feel as hot. Why? The answer lies in the heat conductivity of air being much lower than that of water. Water has a thermal conductivity of 0.582, while air's is only 0.0241, making water 25 times more efficient at conducting heat. Hence, when you dip your finger into water, it quickly transfers heat to your finger. In contrast, in the air of a sauna, even if the air temperature is 100°C, heat is transferred much more slowly. Therefore, one can endure being in 100°C air. For microbes, an environment rich in moisture is akin to dipping your finger into a hot bath: heat is quickly transferred to the microbes, effectively sterilising them. In the case of dry heat sterilisation, it's akin to being in a sauna; microbes can endure even high temperatures like 160°C, making it harder to sterilise them. This demonstrates the close relationship between sterilisation efficiency and moisture content.
Heat-resistant spores are highly dehydrated, hence their resistance
Let's discuss why heat-resistant spores can survive even at high temperatures. The detailed mechanism is still largely unknown. However, one thing is clear: heat-resistant spores contain very little water. During the process of spore formation, water is squeezed out of the cell, leaving the spores almost completely dehydrated.
