Does whole milk spoil faster than skim milk?
BY ALISON COOPER
全脂牛奶是否比脱脂牛奶更易腐坏
全文1675字 预计阅读时间 4分钟
We've all had moments of uncertainty (and, let's face it, paranoia/ˌpærə'nɒɪə/) about the state of the food sitting in our refrigerators. You might be able to eyeball some of the suspicious items, check for expiration dates on others, and with some — milk, usually — you'll probably take a quick whiff and hope for the best. But if you've ever looked at a shelf-life chart to figure out how long your milk might last, you might have noticed that on skim milk is said to last a day or two longer than whole milk. Why is that?
我们每个人都有对冰箱里的食物疑虑的时刻（面对现实吧，就是妄想症）。你或许会打量某些可疑物品，查看他们的保质期，对于某些牛奶，你会迅速地闻闻，寄希望是最佳状态。但如果看过货架期表来查看保质期，你可能会注意到脱脂牛奶的保质期比全脂牛奶长一两天。为什么呢？
Before we get started, we should state that there is by no means universal agreement on this issue. Some dairy scientists say skim lasts longer because certain fat-loving microbes can't develop as quickly in nonfat milk. Others say that whole milk lasts longer because free fatty acids might actually be natural preservatives. Still others say that maybe there isn't a difference in spoilage rate at all — it's just that we notice flavor changes more in skim milk [sources: Murphy, Roberts].
在我们开始之前，需要声明在这个问题上绝没有普遍共识。一些乳品科学家说，脱脂牛奶保质期更长是因为亲脂微生物无法在脱脂牛奶中快速繁殖。另外的人说，全脂保质期更长，因为游离脂肪酸极可能是天然防腐剂。还有些人说，腐败率是不存在差异的——只是我们留意到口味在脱脂牛奶中变化更大罢了。
There's been only one major controlled study on the spoilage rates of whole and skim milk, and it was somewhat inconclusive. Skim milk was found to spoil slightly faster, but the researchers weren't exactly sure why. Psychrotrophic, or cold-resistant, bacteria are what cause spoilage in the fridge, and they multiplied at the same rate in both types of milk.
关于全脂牛奶和脱脂牛奶的变质率只有一项主要的对照研究，某种程度这项研究是无结果的。研究发现，全脂牛奶的腐败速度略快，但研究人员不能确定原因。嗜冷菌、耐寒菌催生了冰箱里的腐化，他们在两种牛奶中繁殖速度是一样的。
When the milk spoiled, both whole and skim contained similar strains of bacteria. There was a pronounced difference in how whole and skim milk reacted when they were injected with the same spoilage/'spɒɪlɪdʒ/ microorganisms, but they affected the milk's taste and smell more than they did the spoilage rate. (Whole milk, for the record, tended to turn sour, and skim milk was on the bitter side [source: Deeth and Khusniati].)
牛奶腐败时，脱脂和全脂牛奶有着相似的菌株。注入相同腐败微生物后，全脂和脱脂牛奶有明显不同的反应，但比起腐败率的变化，他们更多地影响牛奶的风味。（记录发现，全脂牛奶倾向变酸，脱脂牛奶倾向变辛酸）
So for the purposes of your average milk consumer, there's really no hard-and-fast rule about which kind will spoil faster. If whole milk does last longer than skim, the difference is so slight that any given gallon of skim milk could outlast any given gallon of whole milk.
所以对牛奶普通消费者而言，并没有关于哪种腐坏更快的严格规则。若全脂牛奶确实比脱脂牛奶保质期长，差别是如此之小，以至于任何一加仑的脱脂牛奶可能比全脂牛奶更耐喝。
The spoilage rate depends on so many variables — manufacturer, production methods, milk formulation, plant sanitation, storage temperatures, pH and moisture content, to name a few — that a small change in just one of them could give any particular container of milk a slightly longer shelf life than another.
变质率关乎许多变量——制造厂、生产方式、配方、卫生、贮存温度、酸碱度、湿度，其中任意一个的细微变化都可以使一罐牛奶比另一罐保存更长。
A couple of other factors make things even more ambiguous. For one, it's pretty much impossible to pinpoint the exact moment of spoilage. Depending on your sense of smell and taste (and your tolerance for changes in milk flavor), you might turn up your nose at a gallon of milk that someone else might readily swig.
另外一些因素使问题变得更加含糊。比如说，几乎不可能查明从哪一刻开始腐败。考虑到你的嗅觉和味觉（和你对于牛奶风味的耐受力），你可能对别人很轻易喝下的一加仑牛奶嗤之以鼻。
And there's no federal regulation of milk expiration dates in the United States — only 20 states have standards, and they vary widely. One state might mandate a "sell by" date of a certain number of days after pasteurizatio /pæstəraɪ'zeɪʃən/ , and milk jugs in another state will be printed with a "use by" date [source: Cormier].
在美国，是没有关于牛奶保质期的联邦法律规定的——只有20个州有标准，他们相差甚远。一个州可能强制要求杀菌后牛奶的“最迟销售”日期，而另一个州的牛奶壶会印上“之前使用”日期。
The upshot: Don't base your milk purchases on which type might last longer. If you're concerned about shelf life, you'd be better off following a few simple steps to slow down milk spoilage, whether you're a whole or skim drinker.
总之，不要根据哪种类型的牛奶期限更长来购买。无论是全脂还是脱脂牛奶，若你担心他们的保质期，最好遵守些简单的步骤来延缓腐败。
First, make sure your refrigerator is the correct temperature: It should be set at 40 degrees Fahrenheit/'færən'haɪt/(4.4 degrees Celsius/'selsiəs/）[source: FDA]. Store your milk on an interior shelf instead of on the door, which fluctuates more in temperature. And put your milk back in the fridge as soon as possible — leaving it out on the counter for even a few minutes exposes it to light and heat, giving bacteria a chance to spring into action.
首先确保你的冰箱处于合适温度：调到40华氏度（4.4摄氏度）。在内置架上而不是门边贮存牛奶。并且尽快将牛奶放入冰箱——哪怕放在外面几分钟，当暴露在光线和热量下，细菌也会趁虚而入。
Sources
· Cormier, Roger. "When Is the Actual Expiration Date on Your Milk?" Mental Floss. Sept. 2, 2014. (April 1, 2015) http://mentalfloss.com/article/58358/when-actual-expiration-date-your-milk
· Deeth, Hilton, T Khusniati et al. "Spoilage patterns of skim and whole milks." Journal of Dairy Research. May 2002. (March 27, 2015) http://www.ncbi.nlm.nih.gov/pubmed/12222801
· Food and Drug Administration. "Refrigerator Thermometers: Cold Facts about Food Safety." March 10, 2015. (April 1, 2015) http://www.fda.gov/Food/ResourcesForYou/Consumers/ucm253954.htm
· Ledenbach, Loralyn H and Robert T Marshall. "Microbiological Spoilage of Dairy Products." Compendium of the Microbiological Spoilage of Foods and Beverages. 2010. (April 1, 2015) http://link.springer.com/chapter/10.1007%2F978-1-4419-0826-1_2
· Murphy, Steven C. "Shelf-Life of Fluid Milk Products: Microbial Spoilage." Cornell University. June 30, 2009. (April 1, 2015) https://foodsafety.foodscience.cornell.edu/sites/foodsafety.foodscience.cornell.edu/files/shared/documents/CU-DFScience-Notes-Bacteria-Milk-Shelf-Life-Evaluaton-06-09.pdf
· Roberts, Bob. "Sour Milk." Penn State College of Agricultural Sciences. (April 1, 2015) http://extension.psu.edu/food/preservation/faq/sour-milk
