MadSci Network: Biochemistry


Date: Tue Jan 18 16:28:03 2005
Posted By: Todd Whitcombe, Associate Professor, Chemistry
Area of science: Biochemistry
ID: 1103159738.Bc

What a great experiment!!! Did you try doing some controls as well? That 
is, did you put together some bottles with nothing (no dirt or food), 
with just dirt, and with just food? Having controls is important to 
making good scientific observations as it let's you know whether what you 
are seeing is related to the material that you have in the bottle or 
something else.

For example, where I live, it gets cold enough that a capped, empty pop 
bottle will crumple due to the change in temperature that can occur. Of 
course, if you kept your bottles in a room at constant temperature, that 
shouldn't have happened but it is good to have the test present so that 
you can rule out an temperature changes or such in your results.

As to the bottles with "just dirt" and "just food", I am presuming that 
the point of doing your experiment was to examine the gases given off by 
food stuff when it is put into a landfill. That is, when food scraps get 
mixed with dirt. (The same thing happens in a compost pile which is 
really just a miniature version of a landfill.) If that is the point of 
the experiment, then by running samples with "just dirt" and with "just 
food", you can isolate some of the effects and have a better 
understanding of the processes involved.

But none of that is what you are asking about. What gases are produced by 
decaying food? It depends upon how long the food has been decaying and 
what it has been in contact with. You are right in assuming that carbon 
dioxide is the likely output of some of the decompositions. But there 
will also be traces of amines (from the decomposition of protein), 
methane (from anaerobic decomposition), and hydrogen sulphide (again, 
from protein decomposition).

The initial sucking in of the balloon was the period in which the aerobic 
activity continued and, indeed, expanded. This is likely due to the 
surface bacteria continuing to use oxygen as a source of fuel in the 
conversion of the food material into energy. You didn't say which ones 
were sucked in but it would have to be the ones where there were aerobic 
bacteria present along with simple sugars so that the bacteria were able 
to consume the food stuff. (The fruits and vegetables?)

There were also some plain old chemical reactions that likely would have 
happened that had nothing to do with bacteria that would have used up 
some of the oxygen content and contributing to the decrease in gas volume 
that you observed.

After an initial period, the anaerobic bacteria will have taken over and 
yes, the basic reaction is the conversion of sugars (and fats) to short 
chain energy molecules such as pyruvate. Subsequent metabolism would 
produce some carbon dioxide as an end product which is likely the major 
constituent of the gas that is causing your balloons to inflate.

However, as I said above, there are other gases that will form. There are 
a number of compounds in the food that can be used by the bacteria to 
produce a variety of useful compounds. Protein decomposition, for 
example, provides a rich source of amino acids. Hence, as the meat 
decomposes, the bacteria are getting a rich diet. They will also consume 
some of the protein and end up making amines which smell like rotten 
fish. This is, in fact, why rotten fish smell like rotten fish - their 
amines are released as they rot.

But there will be other compounds as well. Hydrogen sulphide (the smell 
of rotten eggs) is another by product of protein decomposition. Methane - 
if their are any methanogenic (methane making) bacteria in the soil. 
Short chain alcohols (methanol and ethanol) will also contribute to the 
gas mixture. Even some esters which are made from acids and alcohols are 
likely to occur.

So, there are all sorts of compounds that will found in the gases. Carbon 
dioxide will dominate but it won't be the only one. As you take off the 
balloons, your nose will let you know that other stuff is present - as 
the air inside the bottles will likely "stink" and carbon dioxide doesn't 
have an odour (after all, it is naturally present in ALL air and if it 
did stink then ALL air would smell!). If you have access to a Chemistry 
department, you could ask if they would be willing to do a gas analysis 
using a technique called "chromatography". But your nose is also a pretty 
good detector for decomposition.

Hope this helps with such a great project.


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