Have you ever drunk water that stayed in a glass or an open bottle overnight? In case you have, you might have noticed that it can taste a bit off sometimes. But is that odd taste an indicator that the water has gone bad?
Unlike food, plain water does not contain carbohydrates or proteins and this makes it more susceptible to contamination by germs. There is an actual explanation for the strange taste of water that stayed overnight.
First of all, microbes and dust accumulate in water left in a glass. Interestingly, the microbes aren’t actually behind the stale taste. Namely, when water is exposed to air, it absorbs carbon dioxide and a small part of it is converted into carbonic acid, which means that water exposed to air changes its chemical composition. When this carbonic acid releases one or two protons and transforms into carbonate or bicarbonate, it reduces the water pH value thus changing its taste. Basically, after water has been out too long, you’re drinking very mild acid rain.
Secondly, when you drink from a glass, you introduce microbes from your lips, mouth and saliva into the water and its container. These microbes produce more microbes in environments with mild temperatures. Thus, the colder the room the longer the bottle or glass of water will keep.
For one thing, most tap and bottled water also contains low levels of chlorine compounds intended to keep away microorganisms. But, according to Kellogg Schwab, an environmental microbiologist at Johns Hopkins University, these chemicals can only last for a day or two.
This is what happens when water is exposed to the outside world, but what if it’s sealed in a bottle? Bottled water itself doesn’t go bad, though it does have an expiration date. If nothing else, plastic and environmental chemicals can do the harm.
But, don’t start panicking about a-day old water. In Kellogg’s words, it’s almost certainly safe to drink. But, the thing you will want to avoid is reusing dirty glasses, sharing glasses with others or touching the rim of your glass with dirty fingers.