As far as I know, all commercially sold apple juice is pasteurised and most is filtered. If you have a juicer and the time, it's cheaper and better to juice your own.
I find that ten medium sized apples makes 1 litre. So I usually buy a box of apples at the wholesale market when I'm going to flush.
Easy chopping: slice off one side, then the other side of the apple, leaving a thin slice in the middle that contains the core. Then cut each side of this so that only the core remains. Hard to explain in words but if you can follow this, it's much easier than the usual way of removing the core.
To prevent/reduce the juice going brown/oxidising: Before you start chopping, get a big bowl, dissolve some Sea Salt
(1 tspn or less) in a little bit of hot water and fill this will cold/room temp water (preferably filtered).
As you chop up the apples, put them in the bowl of salt water.
Then juice them.
You will find the juice doesn't go brown as quickly. If you have kefir whey, add some of that to the juice, it also helps it not to go brown and will give you a good boost of healthy bacteria to boot.
Hope this helps someone!