The Thermo Plastic Urethane film used in reservoirs and soft bottles was chosen because it is very durable, soft, safe, and flexible. It’s also somewhat porous, so sports drinks and even some municipal water can cause a yellowing effect. This is normal, not dangerous, and can be lessened by cleaning with “Bottle Bright” manufactured by “Clean Ethics”. You may also try filling it with Coca-Cola or denture cleaner overnight.