They won't believe that you can inflate a balloon without blowing into it, but here's how it's done. Pour a little vinegar into the bottle. Put two tablespoonfuls of the bicarbonate of soda into the balloon. Carefully slip the neck of the balloon over the neck of the bottle, without spilling the bicarbonate. Lift the balloon so the bicarbonate runs down into the vinegar.
Food entering the stomach is broken down by enzymes that thrive in the acidic conditions.
You can expel the gas by burping.