You want as little oxygen as possible on top of your juice. Oxygen causes oxidation of the juice and gives unwanted bacteria and fungi a chance to live in and on your juice.
As the two above said you want to limit head space as oxygen helps unwanted bacteria and mold to grow as well as causes oxidation which destroys flavors. You could purchase food safe CO2 or an inert gas like argon to flush the headspace. This technique is sometimes used in scaled brewing. But at that rate you can just order two 1 gallon demijohns on Amazon with airlocks.