If one has followed the Master-Cleanse protocol 'precisely' and NOT cheated,the digestive system has been essentially shut down-one of the main benefits of the MC...
a break for the system.
The flora/bacteria in the lower tract has been largely killed and washed out by the SWF.
OJ and the Cooked veggies get the digestive system jump started naturally.
The body begins once again to produce the 'good' flora/bacteria in the digestive tract. Some folks take probiotics post break...that's cool. POST BREAK...
Just my take from experience and others...I don't have all the answers though