Only the XR is provided in capsules. Regular Adderall is a tablet.
Agreed with Bonez: acidity of food should not be a factor. Faster gastric emptying due to no food or only liquid could give faster onset of action. It seems unlikely that that would be an issue anyway.
Obviously its not a factor in this case but I thought you might be interested to know this BR (or maybe you already know this), but pH plays a significant role in the rate of drug absorption (i.e. the pronation of a drug and the relative permeability of the cell wall to charged particles).