When an acid solution such as citric acid and an alkaline solution such as Caffenol developer are combined, chemical neutralization results. A salt solution is the product and carbon dioxide gas is liberated (abundantly). Similar reactions occur in baking. Such reactions cause bread and biscuit to rise (leavening). Let me add, generation of gasses during film developing should be controlled. That's why we normally use a dilute solution of acetic acid (vinegar) as the stop bath. What can happen is, blisters often form within the film emulsion if the stop is too strong. This action will spoil the film. Let me add, development takes place in an alkaline environment (some exceptions). You can use plain water rinse to purge the developer or a mild acid rinse. Acetic acid is preferred if developing is to be halted quickly. Acetic acid is one of the ingredients of the fixer; thus we use it to not only stop developing but to reduce fixer contamination with carryover developer.