Can't help you until you tell me how you conducted your lemon battery experiment. I don't think you were generating hydrogen, BTW. I believe that the acid was only the charge carrier (electrolyte) and didn't actually enter into the redox reactions. You were probably just using a couple of electrodes (zinc and copper?) inserted into a lemon. The lemon just acted as an electrolyte.
Citric acid has three acidic protons.