【为什么隔夜水会变味?水也会有“保质期”】

【为什么隔夜水会变味?水也会有“保质期”】研究表明水的变质原理与一般食物不同,水中不含有蛋白质或糖类,所以不会受到微生物破坏。但水也的确会过期,长期暴露在空气中的水会吸收二氧化碳,产生碳酸等酸性物质,所以隔夜水会有一股酸涩味道。但这种化学改变不会对人有明显伤害

Does water 'go off'? Video reveals why your bedside glass of water tastes strange in the morning

  • Chemical changes that happen overnight can change the taste of water
  • Carbon dioxide absorbed into the water makes it slightly more acidic
  • This is not dangerous to humans but it explains the change in taste 
  • Plastic bottles can release a hormone disruptor when heated 

 

If you've ever left a glass of water on your bedside table overnight, you may have noticed that it tastes a little strange in the morning.

It turns out this effect is not all in your head - subtle chemical changes that happen overnight can actually change the composition of the water, making it ever so slightly acidic.

But don't worry, the changes are not dangerous, unless you happen to be a crab.

Water does not have proteins or sugars, so microbes don't break it down like they do with food. But its chemical composition does change as it is left open to the air, absorbing some carbon dioxide making it slightly more acidic (file image)

The science of this change, as well as the reason why bottled water has an expiry date, has been explained in a video published by Discovery News.

The water itself does not have proteins or sugars, so microbes are not breaking it down like they do with food, so water does not 'go off' in the same way.

But its chemical composition does change - as it is left open to the air overnight in the glass, it absorbs some carbon dioxide.

A tiny fraction of this is converted into carbonic acid.

'Some carbonic acid will then lose a proton or two forming carbonate or bicarbonate respectively,' Discovery News explains.

'This lowers the PH of water making it slightly acidic, which changes the taste. So does that mean it's unsafe? Probably not - unless you are a shellfish.'

Carbonic acid forms four million more times bicarbonate than carbonate, and shellfish use carbonate to build their shells, so they would have less carbonate to work with.

The slightly higher acidity can dissolve the shells of crabs and other sea creatures - which has implications beyond the glass of water on your bedside table.

Global levels of acidity in the world's oceans have risen since the industrial revolution, due to the extra carbon in the air.

The slight change in acidity in your glass of water won't hurt you however, but water that is exposed to air for prolonged periods of time - days at a time - will be exposed to bacteria if left out.

This can lead to the growth of algae and mosquito larvae too if left for too long.

This is the reason, the video reveals, why sailors used to take beer and rum on long journeys, with the alcohol protecting the liquid from micro-organisms that might grow in fresh water.

As for bottled water, it should be safe indefinitely as long as it is stored correctly.

The plastic itself is slightly permeable, meaning that it should not be stored near to dangerous chemicals.

Some plastics also release a hormone disruptor called bisphenol A into the water when heated, which is why it should not be stored in direct sunlight.

So while water in bottles doesn't 'go-off' as such, the plastic might slightly change the taste over time, which is why manufacturers provide an expiry date.

However in the UK, the type of plastic used for water bottles, known as PET (Polyethylene Terephthalate), does not contain bisphenol A.

文章来源:http://www.dailymail.co.uk/sciencetech/article-3167957/Does-water-Video-reveals-bedside-glass-water-tastes-strange-morning.html

 


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