|MadSci Network: Biochemistry|
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.
Try the links in the MadSci Library for more information on Biochemistry.