托福阅读资料之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.

　　大多数人对吃的东西都很谨慎。

　　比如在吃葡萄时，你会选择圆圆的，健康的葡萄而不是那些枯萎变色的。同样地，当你在超市挑选罐装食品时，会选择包装完好的罐头，而不是那些畸形或膨胀的。

　　这样做是有原因的。

　　罐装食品膨胀有两种不同的原因。第一种就是食品科学家所说的“氢膨胀”，只有装有酸性食品的罐头才会发生这种现象，比如西红柿或柑橘类的水果。

　　食品中的酸性物质开始溶解罐的金属内层，这种化学反应会产生氢气。由于罐是密封的，气体逐渐积聚起来，使罐头膨胀。

　　第二种原因是糟糕加工的结果。在装罐之前，食品需要消毒，消除所有潜在的不安全微生物，特别是那些能够在无氧条件下生存的微生物。

　　如果这些生物留在罐头内，它们会大量繁殖，使食物变质产生二氧化碳气体。在这种情况下，就会是罐头膨胀。

　　站在超市的走道上，我们没有办法辨别罐头膨胀是由于氢气还是二氧化碳，但也没关系。假如食品已经变质，告诉商店的经理，然后选择一罐好的就可以了。