And following C s comment - chemists sell citric acid in a powder form - 100g will cost about 75p or so. Citic acid is used in food production as a preservative in commercial production.
Do you actually need to freeze to store them? Apples are a good keeper - its the way nature made them - keep in a dark, frost free, dry, airy place. Info oin varities at the National Fruit Research Centre at Brogdale if you want any specific info -
http://www.brogdale.org. Call them and they are happy to help if any queries.
Make cider with the excess? Make a cider vinegar? make apple pies and freeze these, rather than the whole apple? Make apple jelly? Add to onion relish or make up some chutneys with them - best way to keep rather than risk freezer burn....