Bottles turning yellow

Hi, my new born is just 2 weeks old, and i have been using my haenim 3rd gen uv steriliser to sterilise her bottles after every use. In about just 2 weeks, most of my bottles (avent, pumpables bottles) are turning yellow. Can anyone advise if the uv steriliser is causing the yellowing or whether i am sterilising too much? Do i have to sterilise after every use? Thanks!