托福閱讀資料之What Causes A Can Of Food To Swell?

　　If you’re munching grapes, you’ll probably skip one that’s all shriveled and discolored in favor of the round, healthy ones. Likewise, when you choose a can of food at the supermarket, you’ll likely choose one that’s exactly can-shaped, not one that’s misshapen and swollen.

　　There's a good reason for this.

　　A can of food can swell for two separate reasons. The first is what food scientists call “hydrogen swelling,” and it only happens to cans of acidic food such as tomatoes or citrus fruit.

　　The acids in the food begin to attack and dissolve the metal lining of the can, and one of the by-products of this chemical reaction is hydrogen gas. Because the can is sealed, the gas builds up, causing the can to swell.

　　The second reason a can swells is the result of poorly processed food. Before it’s canned, food needs to be sterilized to remove all the potentially dangerous microorganisms– especially those that can live inside a can without oxygen.

　　If some of these organisms are left behind, they can thrive in the can, spoiling the food and giving off carbon dioxide gas. In this case, carbon dioxide causes the can to swell.

　　Standing in the supermarket aisle, there’s no way you can tell whether a can is swollen because of hydrogen or carbon dioxide gas, but that shouldn’t really matter.

　　Assume that the food has spoiled, inform the store manager, and choose a healthy, can-shaped can instead.

　　大多數(shù)人對吃的東西都很謹(jǐn)慎。

　　比如在吃葡萄時(shí)，你會選擇圓圓的，健康的葡萄而不是那些枯萎變色的。同樣地，當(dāng)你在超市挑選罐裝食品時(shí)，會選擇包裝完好的罐頭，而不是那些畸形或膨脹的。

　　這樣做是有原因的。

　　罐裝食品膨脹有兩種不同的原因。第一種就是食品科學(xué)家所說的“氫膨脹”，只有裝有酸性食品的罐頭才會發(fā)生這種現(xiàn)象，比如西紅柿或柑橘類的水果。

　　食品中的酸性物質(zhì)開始溶解罐的金屬內(nèi)層，這種化學(xué)反應(yīng)會產(chǎn)生氫氣。由于罐是密封的，氣體逐漸積聚起來，使罐頭膨脹。

　　第二種原因是糟糕加工的結(jié)果。在裝罐之前，食品需要消毒，消除所有潛在的不安全微生物，特別是那些能夠在無氧條件下生存的微生物。

　　如果這些生物留在罐頭內(nèi)，它們會大量繁殖，使食物變質(zhì)產(chǎn)生二氧化碳?xì)怏w。在這種情況下，就會是罐頭膨脹。

　　站在超市的走道上，我們沒有辦法辨別罐頭膨脹是由于氫氣還是二氧化碳，但也沒關(guān)系。假如食品已經(jīng)變質(zhì)，告訴商店的經(jīng)理，然后選擇一罐好的就可以了。