I noticed the older bottles (3 mths old) are yellow in colour compared to the newer ones. I have no doubt it's the heat from the steam steriliser that's causing the discolouration. Do you think it's necessary to replace these bottles ? I am concerned that the discoloured bottles might be disintegrating and therefore leaching chemicals into the milk. What are your thoughts on this? Should I be concerned? I might direct my concerns to the company if I can find a way to contact them.