I did a Google Search of Apple Pectin and found that when mixed with water (juice of apple) it forms a collodial solution. It gels. Pectin is used as a gelling agent by the food industy. It is an indegestible fiber, balances the colon and because it's fiberous, absorbs and expels fat. It is recommended that one should flush their system with it when fatty foods are eaten. Pectin is also found in many other citrus fruits.
As they used to say.................An Apple a Day,
I think it might be the gelling of these horrid stones that softens them. Just a guess. But I know I will continue too drink alot of apple juice after reading that.