Subject: How does processing food affect the food's production of gas?

My 7th grade son is designing a science project.  His project hypothesis 
is "The more you process food, the more gas it produces."  His experiment 
is to use one fresh, one frozen, and one canned vegetable.  He will grind 
them separately in a blender and place each type into a test tube with 
some vinegar.  He will place a balloon over the test tube to measure the 
amount of gas generated.  He intends 4 trials of this project.  Exactly 
what chemistry is going on in this experiment?  Does the vinegar act as 
some kind of accelerator, or just reacting to the chemical breakdown of 
the food and the processes it has already undergone?  

Re: How does processing food affect the food's production of gas?