I've frozen and defrosted the same boilies many many times and still caught on them. Also letting them go a bit mouldy will work too.
Or possibly try this, put one or 2 ml of flavouring in a plastic bag , drop boilies in bag, and make sure the flavour is spread about as wide as possible, try and cover all the boilies. This sometimes acts as a preservative and if you squeeze the air out of the bag will prolong their shelf life.