The nightmare for any bottler is when complaints start coming in about a bad taste in the water. The immediate question of "what can be the cause?" unfortunately has many possible answers.
Initially consider whether anything has changed: a new bottle wash chemical: a change of machine configuration; a new supplier of bottles or caps; a new customer; a new bottle wash operative. The list is endless. Of most concern is whether the taint is caused by bacterial contamination, however, if all your checks have been carried out correctly, this is unlikely.
There are always unknowns which are linked to the customer and the fate of the bottled water at the customers' premises, such as storage conditions. If you are not responsible for servicing the cooler, this is another area of concern.
Polycarbonate is a porous material and small molecules can enter into the water through the bottle walls. When you consider that coffee has over 1000 flavour components, storage of bottles near to coffee machines is not a good idea.
Polycarbonate is also susceptible to stress cracking and this appears initially in the form of microcracks, often inside the bottle. These are wonderful havens for bacteria, algal spores and chemicals, none of which can be rinsed out easily. The problem is that these residues are not normally detectable by your usual quality checks and the offending material will seep out only after a period of time, usually when the bottle is with the customer.
How can you guard against this? Well, use a bottle wash chemical that does not cause stress cracking and one that contains a detergent ingredient that can penetrate into microcracks and flush out anything lurking in there. Try not to use chlorinated bottle wash products if you can help it, unless you have a very efficient rinse process. Discard older bottles that are beginning to form microcracks.
Keep a stock of selected diagnostic test materials, so that if taint arises, you can undertake rapid assay of the possible contamination. Watch out for bottles that are beginning to look stressed when they come in for washing. Remember, taint issues usually occur only after the water has left your plant.