Science Buddies: "Ask an Expert"

Our son is currently doing the banana, onion and cow poo experiment with mylar balloons tightly sealed, and checking for inflation every day. He has wonderful results with the banana-only bottles with full inflation (looks like they are going to explode!), onion and cow poo bottles are partially inflated after 7 days, and banana & onion bottles are all low inflation except for 1 bottle which is now nearly inflated after 11 days. He did not do any banana & cow poo bottles in this experiment. Why would the banana-only bottles produce so much gas, as I would have expected the bottles with some cow poo added would produce the most biogas? I'm a bit puzzled, and he needs to have project all finished in 2 days. Many thanks

Hi,
Great project! Congratulations on getting interesting results.

I read through the background and other sections of the project (https://www.sciencebuddies.org/science- ... #procedure) to see if I could answer your question, but I cannot say for sure. The idea of this project is to produce a fuel gas, methane. There are naturally occurring bacteria that produce methane by eating various kinds of biomass, but there are also yeasts that ferment biomass, especially fruit, and produce carbon dioxide. I suspect that is what happened with the bananas.

Cow manure has already been through one fermentation process--inside a cow--so the methane-producing bacteria may not have that much left to work on.

I don't know what I would predict for onions as a source of biogas. They certainly get stinky when rotten! I have read that onions and other alliums like garlic have antibacterial properties so this might make them a poor choice for biogas production.

Do some more reading online about biogas production from various kinds of biomass. The answers to your questions are out there. It just takes some time to find them. If you have more questions, post again and we will try to help you.

Good luck!

Sybee

Thank you Sybee, I have read that bananas contain more sugar than vegetables and this could be causing the fast and strong reaction perhaps?

You're welcome!

Yes, there are wild yeasts on fruit and they can use the sugar in the bananas to make lots of carbon dioxide. Bananas would not be a good choice for methane production unless the yeast could be eliminated and only the methane bacteria present.

Let us know if we can help you in making the poster or presentation.

Sybee