As far as I understand it the apple juice is for the malic acid. There's also malic acid in lemon rind, or is it the pith. In any case there is a recipe that calls for blending the whole lemon.. juice, pith, rind and seeds.. and then sieving the results. I'm currently experimenting with this.. but unfortunately also put the oo in with the mixture and ended up after refridgeration with a solid mousse which has to be eaten with a spoon... this means you can't down the lot in one go.. so I'm eating it over several days to see what happens!