The Half Decent Pharmaceutical Chemistry Blog

Yesterday I wanted to write something about the ongoing rubbish crisis in Naples, focusing mainly on the danger linked to people burning uncollected garbage in the streets and the resulting overproduction of carcinogenic dioxins. This issue is having a huge impact on the Italian public opinion. Moreover, because Italy is surprisingly a member of the EU and Naples is a major tourist spot, the crisis is causing a lot of embarrassment to the government and institutions, with international press pointing out what inefficiencies and lack of organization has resulted in.

There are two reasons why such a juicy post wasn’t written, though. The former is that the person who should have lectured me on dioxins and their nasty, toxic effect, my professor of Toxicology, was an ape and when I look at my notes and the book she wrote for a decent bunch of information on these chemicals, I realised she actually told us nothing about it.

The second reason is that, when I got back home in the evening, I was fighting to keep my eyes open, so, writing was beyond consideration. Don’t think I’m making it up: I agree that no assay could make you feel so tired. What wore me out was cleaning the autoclave with my supervisor. So, instead of dioxins, of which I have no knowledge whatsoever, I am here to introduce you to the mysteries of autoclaves and steam sterilization.

This is it: a Fedegari (I told you this post was sponsored) autoclave. Last afternoon, despite the articles I must read, my task was to clean it; something no one had ever done, although almost a year ago, while sterilizing a bag of Petri’s plates before throwing them away, the content of one leaked out and because of the high temperature solidified at the bottom of the chamber. At least this is what everybody guessed, as they said the leak wasn’t too bad and the machine remained sterile, albeit a bit dirty.

The cleaning procedure (sorry for the picture quality: they were taken with a 2 Mega pixels camera from a mobile phone) took a couple of hours and, towards the end, when the machine was empty, I literally got the upper part of my body entirely inside it to remove the rubbish our cheap detergent had only softened. Meanwhile, my supervisor, not tall enough to do this, had to keep the autoclave down to the ground as the machine is remarkably light when empty and my weight was sufficient to make it eventually fall to the ground on one side and begin to roll.

Fortunately, as proved by the pictures, the autoclave wasn’t that dirty, after all. That’s not a surprise when you think that stuff remained immerged in water which is heated up to 121°C for at least 16 minutes. These parameters aren’t arbitrarily chosen, but result from a series of consideration whose aim is to guarantee the most efficient sterilization in the worst case (a common assumption when it comes to good manufacturing practices).

In fact, there are mainly three parameters a good…autoclavist must always bear in mind when optimizing the settings for a cycle. A degradation reaction follows a first order kinetic and, therefore, you can easily calculate how many of the initially present micro organisms have been destroyed. This, in turn, leads to the simple and very practical assumption that the relationship between the number of contaminants and time has the characteristics of an exponential decrease, which means the more organisms you had at the beginning, the longer it’ll take for having an acceptable sterilization. By the way, the first parameter which regulates a sterilization is the D-value, or decimal decay time, which measures the interval required to achieve a reduction of microbial content of a logarithmic unit, at a given temperature. Generally, at 121°C, this time is a minute.
This might look a pretty useless value, though, given that the temperature is everything but constant throughout the process. In fact, the second value, z, is often called temperature coefficient, as it tells us how many degrees are needed to have a 10-fold variation of D. It’s practical to assume z = 10, which means 1°C varies D of 24%. Mind you, it’s not only very useful, but also consistent with empirical observation.
Finally, both (arbitrary) assumptions (D = 1 minute and z = 10) come together in the last value: F0. This parameter is the equivalent exposure time at 121°C for an ideal organism, whose z is 10. The importance of F0 is massive: it allows you to determine the efficiency and effectiveness of a sterilization carried out in the real world, where temperature isn’t constant. For example, a sterilization cycle seems to last less than what it should because a simple computer can easily include in the theoretical 16 minutes at 121°C, both heating and cooling. Here is the formula with which you can enjoy yourself during cold, rainy evenings: