糖——甜蜜的殺手

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Some methods of preserving food are easy to understand.

一些保存食物的方法很容易理解。

For example, it's easy to see that freezing your food, or packing it in salt, would make it inhospitable to the microbes which would otherwise cause it to spoil.

比如，把食物冷藏，或者用鹽腌起來(lái)，都是很常見的方法。通過(guò)這些方法，不給細(xì)菌提供滋生的環(huán)境，以防止食物腐敗變質(zhì)。

You might wonder, however, about jams, jellies, and preserves, all of which are protected from spoiling by a high concentration of sugar.

不過(guò)，果醬，果凍和蜜餞都是用高濃度的糖來(lái)防止其變質(zhì)的，你可能對(duì)此感到疑惑。

Sugar is one of the most basic foods for all life--bacteria and mold like to eat it just as much as we do. On today's Moment of Science we'll learn how too much of a good thing can be deadly, at least for microorganisms.

糖是生物的最基本食物之一-如我們一樣，細(xì)菌和真菌也喜歡糖。在今天的《科學(xué)時(shí)刻》節(jié)目中，我們來(lái)見識(shí)見識(shí)，什么叫物極必反：太多的糖也會(huì)致命-至少對(duì)微生物來(lái)說(shuō)是如此。

Sugar works not by poisoning the food-spoiling microbes, but by causing them to literally die of thirst.

糖之所以能殺滅會(huì)令食物變質(zhì)的細(xì)菌，并不是因?yàn)樗卸?，而是高濃度的糖讓?xì)菌“渴死”了。

This is because sugar attracts water very well; the more sugar there is in any solution, the more water it tries to draw from its surroundings. This is bad news for any microbe that happens to be inside a jar of jam. High concentrations of sugar will suck the microbe's vital water right through its cell wall, causing it to dehydrate. This process is called "osmosis," and it can be deadly for bacteria and mold.

這是因?yàn)?，糖的吸水性較好，在任何溶液中，糖的含量越高，其越能從周圍吸水。這對(duì)恰巧滋生在一瓶果醬里的細(xì)菌來(lái)說(shuō)可不是什么好消息。高濃度的糖會(huì)將細(xì)菌體內(nèi)賴以生存的水分從細(xì)胞壁吸引出來(lái)，造成細(xì)菌脫水。這一過(guò)程被稱為“滲透作用”。對(duì)細(xì)菌和霉菌來(lái)說(shuō)，這可能是致命的。

In order for osmosis to work, the sugar concentration has to be quite high. If any water falls onto the surface of your jam, the sugar concentration at that spot might become low enough to allow mold to grow. That's why it's important to take the back up measure of refrigerating all jams, jellies, and preserves once you've opened them.

為了讓滲透作用發(fā)生，糖的濃度必須達(dá)到很高。如果你的果醬上落了一滴水，那一點(diǎn)的糖的濃度可能就降低了，到了霉菌可以滋生的程度。這就是為何，果醬、果凍和蜜餞一旦開封，就一定要放到冰箱冷藏。

Like an oasis in the desert, condensed water dripping from the jar's lid can give a dehydrating microbe the relief it needs.

如同沙漠中的綠洲一般，凝結(jié)的水滴從瓶蓋上落下，便給正處脫水狀態(tài)的細(xì)菌必要的喘息時(shí)機(jī)。