I have never heard of anyone ferementing with the apples present before. I wonder if oxidization of the apples (turning brown with air exposure) is part of the reason it turned sour.
There is a process called keeving that does allow the juice to stay with the pulp for 24 hours. The effect is that the pulp with its pectin reabsorbs nutrients out of the juice. They then let it ferment at 5 degrees C (about 40 degrees F) for a really long time. The result is a naturally sweet cider because it is so nutrient poor with less than ideal temps. I bring this up because I also wonder if the apple fruit is kind of messing with the nutrients, though it is still there. Maybe I'm way off base here. See French & English Tradition - http://www.cider.org.uk/part4.htm