Is this the Scibuddies project your daughter did? “From Trash to Gas-Biomass Energy” http://www.sciencebuddies.org/science-f ... p027.shtml
You did not say that equal amounts of fresh cow manure was added to equal amounts of pureed onions and blueberries. Was it? The manure supplies the anaerobic methane-producing bacteria for the biogas production.
The process of bacterial fermentation occurs best in the absence of oxygen but the amount of gas produced also depends on the type of biomass being ‘fermented’. Yeasts use sugar for fermentation but they produce carbon dioxide, not methane.
Assuming that you added methane-generating bacteria to your purees, it would seem that these bacteria prefer onions to blueberries! Onions have sulfur-containing compounds. That’s what causes the bad smell when they rot. Maybe this also causes the bacteria to produce more gas. Cabbage also contains organic sulfur compounds and in my experience at least, cabbage produces more gas than blueberries.
If you haven’t done so, look up what the predominant bacteria are in cow manure and see if you can find out what types of organic biomass they are best at digesting to produce methane. I know there are big landfills in some states that are producing and burning methane to generate electricity so there must be many studies on these bacteria. If you need help finding or understanding scientific journal articles, let us know.
Hope this helps.