It is almost impossible today not to use plastic bottles for water and other beverages as packaging (wine in plastic- real classy). Plastic is cheap, light material and gives a good protection from oxygen and other harmful impacts.
What we all sometimes do is reuse the same plastic bottle; when we drink all of what’s originally been there, we just pour in something else, usually water. And that’s maybe ok for the first 2 times. After that time, water starts to taste and smell usually like that one from the swimming pool and it’s just undrinkable. Why is this happening and can it be prevented?
Although it’s a common opinion that the bad smell is because of plastic being deteriorated, that’s actually not the case.
The answer is- bacteria. Bacteria from our mouth, to be precise. Every time we take a sip out of the bottle, we take in bacteria that reproduce inside. One study even showed that after one week of using the same bottle, it contains more bacteria than the average toilet seat.
Considering water has some of the oxygen dissolved in it, first bacteria that will reproduce are the ones that require oxygen to live; they will live in it until the oxygen in the water is used up. After that, anaerobic organisms (those who don’t need oxygen) take over. This happens usually after the third use of the bottle. Anaerobic bacteria are the ones that release that bad smelling compounds, usually containing sulfur (people are actually very sensitive to the smell of sulfur compounds, which even in very small amounts have strong odor; that’s the reason why certain sulfur compounds are mixed with gas used as a fuel for heating- because that gas doesn’t have a smell, so you can notice the eventual leaking). Bacteria from the inside of the bottle are fed with food residues and dead epithelial cells from our oral cavity. Depending on what you had for lunch that day, the smell can range from unpleasant to “something died”.
Soap mold can also be the cause of the bad smell. They are usually found in the residues of detergent if the bottle’s been washed.
Still, if you would like to reuse the bottle, it’s advised to wash it with mild detergent and rinse it well. Most companies do not recommend reusing, but if you really want that, at least do not use it for a long time.