Science Buddies: "Ask an Expert"

My daughter did an experiment to see what kind of fruit would inflate a balloon best while decomposing. She used strawberries, bananas, tomatoes and oranges and ran two seperate tests. The bananas and strawberrries inflate the balloons which then deflated and got sucked into the mouths of the bottles. The oranges and tomatoes just sucked the balloons into the bottle mouths and the orange bottle have actually been crushed in. What causes this? My guess is that the mold used oxygen in the bottles to grow and thus created a vacuum in the bottle. Am I right?

Hi,

This sounds like a great project. There is a similar project on the science buddies website that uses anaerobic bacteria from ruminant animals to ferment fruit and form biogas.

https://www.sciencebuddies.org/mentorin ... p027.shtml


However, it doesn’t sounds like you added an external source of bacteria, so you had just the fruit and the normal flora or microorganisms found on fruit, which would probably be yeast and molds. You don’t mention what happened, but I assume that the fruit was decomposing and that you could see a physical change in the fruit.

Here’s an explanation of what may have happened:

1. Yeast will ferment sugars in fruits to form carbon dioxide and ethanol. The production of carbon dioxide would inflate the balloons. I would expect fruits with higher sugar content to produce more gas. Did you weigh the amount of fruit that was added to each bottle?
This website includes the grams of sugar per 100 grams in various fruits.
Bananas 15.9
Oranges 8.9
Strawberries 5.7
Tomatoes: 2.8
http://www.nal.usda.gov/fnic/foodcomp/D ... herr48.pdf

The sugar content does not explain your results, unless you were using different amounts of the fruit in each bottle. Tomatoes and oranges contain more acid compared to bananas and strawberries, so maybe there was an effect of pH on the production of carbon dioxide.


2. Other microbes could use the oxygen and decrease the volume of gas in the balloon/bottle. Molds and other aerobic microbes could do this. Since you were working with unknown microorganisms, a decrease in volume of the air without a change in temperature (see #4) would confirm this, so yes you were right.


3. It’s highly unlikely that you had any nitrogen-fixing bacteria because this type of microorganism is usually associated with the roots of specific species, but nitrogen fixing bacteria would result in a decrease in the volume of gas.

4. Air can also change in volume due to temperature changes and gas expands with an increase in temperature and decreases in volume with a lower temperature, so there will be a difference in the balloon volume with any change in temperature. If your daughter has time to set the experiment up again, I recommend including a control sample with no fruit and also a temperature measurement at the beginning and at each time of observation. Here are a couple of websites that includes an explanation of the effect of temperature on gas volume. You can actually introduce your daughter to the ideal gas law.

http://en.wikipedia.org/wiki/Gas

http://jersey.uoregon.edu/vlab/Piston/


Donna Hardy