Understanding How Sterilization Is Quantified

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  • 0:05 Quantifying Sterilization
  • 0:59 What Is the D-Value?
  • 2:11 Calculating the D-Value
  • 4:08 Other Values of Sterility
  • 6:22 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will describe what certain values, such as the D-value, F-value, Z-value, and thermal death time mean and how to calculate the D-value given certain specified criteria.

Quantifying Sterilization

When grilling some of your favorite meats, you're more than likely thinking about the great smell coming from the grill, as opposed to the nasty bacteria that are being killed off by the heat of the grill.

However, the fact that you cook your meat in the first place has more to do with preserving your health and safety by getting rid of deadly pathogens than it does with your palate.

In a very similar fashion, getting rid of deadly and harmful pathogens, including bacteria, on things like medical devices is super important during the process of sterilization, which is when we kill off all living microbes on an inanimate object.

In this lesson, we'll go over just the basic components of how it is that we figure out how long it takes to kill off a certain number of microbes when we try to sterilize something.

What Is the D-Value?

If you've ever grilled meat before, you surely know that a certain temperature must be reached to kill off most, if not all, deadly pathogens located in or on your meat. Similarly, in science, we have a way of figuring out how long it would take to actually kill off a certain amount of microbes at a given cooking temperature, so to speak.

This method involves something known as the D-value, where D-value stands for the decimal reduction time.

It sounds far more difficult than it actually is.

The D-value is used in microbiology to know how long it would take to kill 90% of microorganisms in some kind of sample at a very specific temperature.

Let's use a hypothetical, but real-world, example to further strengthen this point. If, at a given temperature, an autoclave, which is essentially a machine that uses high temperatures to sterilize objects contained within it, has a D-value of 3 minutes, then how long would it take to reduce 1,000,000 microorganisms contained within the autoclave down to 10?

Calculating the D-Value

This seems like a really complex question but it's very simple, requiring you to only know 3 - that's right, only 3 numbers - to find out the 4th.

We know 3 numbers based on the question that was asked:

Our D-value is 3 minutes, the time it takes to kill 90% of our sample at a certain temperature in our autoclave. We started with 1,000,000 microbes, and we are left with 10 at the end.

Now, using the following equation we can plug those numbers into it:

D-value equation
d value equation

The equation reads that:

The D-Value = (Total Heating Time)/(log (Original Population) - log(Remaining Population))

In our case, the equation would be:

3 = (X)/(log(1,000,000) - log(10))

By solving for X, we find out that the total heating time is 15 minutes. Meaning, with a D-value of 3 minutes, we'd reduce our microbial population from 1,000,000 to 10 in 15 minutes flat at the temperature the autoclave specifies.

There's one thing of utmost importance that you need to know. You MUST keep in mind that the total heating time has to have the exact same units as the D-value when performing your calculation. If the D-value is given to you in hours, the total heating time must be in hours. If the D-value is given to you in minutes, the total heating time must also be in minutes!

Other Values of Sterility

In any case, as I'm sure you know, when grilling different types of meats, you need to reach different temperatures for different effects. For example, cooking something rare would mean lowering the amount of time you cook something or decreasing the temperature at which you cook it at. If you want something well-done, then you need to increase the temperature. The more you increase the temperature and the more you increase the time at which you grill your meat, the more thoroughly you'll cook your meat.

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