Day 134

Our first assignment in Microbiology Lab was interesting, and the results might be even more interesting. We were sent out into the college in pairs, armed only with three petri dishes and three sterile swabs (i.e. oversized Q-Tips). The petri dishes had each been prepared with a thin layer of agar, an ideal growing medium for most microbes. The idea was to swab a surface, then transfer any microbes we had collected to the agar in one of the petri dishes, then repeat for the other two petri dishes. In five days, we will examine our petri dishes to see what–if anything–has grown in the agar.

The kicker was that we were also to put the lid back on the first petri dish after one minute,  put the lid back on the second dish after ten minutes, and put the  lid back on the third dish after twenty minutes. So there are actually two experiments here: what microbes can we actively collect from various surfaces; and how many airborne microbes can we passively collect?

Before leaving to find samples, there was some chatter about where everyone was going for their samples. Someone cleverly suggested arriving at the college’s wine store in white lab coats, studiously but silently taking samples from various surfaces, then saying to the lab partner just before exiting, “Interesting. Very interesting. We’ll have the results in five days.” Don’t know if anyone followed through with that suggestion.

My partner and I ended up at one of the college’s small lunch counters with the very clever idea of taking samples from one of the lunch tables. (This was a clever idea because after we had swabbed our samples, we could sit at the table and chat over a coffee while our petri dishes gathered airborne microbes. The disadvantage of this idea, as I realized afterwards, was that if our petri dishes reveal the presence of any particularly exotic microbes, I may not be able to bring myself to eat at that lunch counter again.)

A large part of proper bottling and kegging revolves around the use of carbon dioxide, so in Packaging, it was all about gas laws today:

  • Ideal Gas Law. What is an ideal gas? I suggested it was a gas that didn’t mind splitting the restaurant bill, but apparently that is an ideal date. An ideal gas is one that follows the Ideal Gas Law: PV = nRT, where P is pressure, V is volume, n is the number of moles of gas, R is a gas constant and T is temperature. Simply stated, pressure, volume, the amount of gas and temperature are all linked such that you can hold any two of them constant, but changing the third will have an effect on the fourth. For instance,  if you keep the pressure and temperature constant and increase the amount of gas, then the volume will increase. Hold the amount of gas and the volume constant but increase the temperature, then the pressure will increase. Apparently this is what most gases do most of the time, which makes them ideal candidates to be called an ideal gas.
  • Dalton’s Law of Partial Pressure. If you have a container filled with a bunch of different gases, the sum of the pressures of all of the different gases will be the total overall pressure (Ptotal = P1 + P2 + P3  … + Pn) So if I mix together carbon dioxide at 200 kiloPascals (kPa) and nitrogen at 150 kPa, the total pressure in my gas container will be 200 + 150 = 350 kPa.
  • Henry’s Law. This was a tough one to understand: At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Whuh? Parsing it down, I came to understand that this means that different gases will dissolve into a given liquid at different rates–some will dissolve easily, and other not so much. For instance, at a given temperature and pressure, about 200 times more carbon dioxide will dissolve into water than will nitrogen. This brought up the interesting idea of “beer gas”, which is a premixed bottle of nitrogen and carbon dioxide that bars use to push beer from the keg to the draught tap, the idea being that the nitrogen will not readily dissolve into the beer and affect its flavour.

After all that, I should carefully examine a bottle of beer to see if it is obeying these laws. Excuse me.

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3 Comments on “Day 134”

  1. Canageek Says:

    It should be PV = nRT: Volume is always capitalized (At least, I have never seen a lower case v used for volume.)

    Chemists normally work as constant pressure (atmospheric) since it is easiest– You just add liquid to an open beaker, but a lot of physicists for example use constant volume (injecting gases into a closed cylinder).


  2. […] January, we tried to grow petri dish cultures using bacteria and wild yeast you can probably find around the house (or college). Today in […]


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