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 Oil and Water

　　To understand the emulsifying process, we must first accept the scientific principle that oil and water do not naturally mix. Quite literally, they find each other's presence repulsive. A good illustration of  this aversion  is homemade  oil and vinegar salad dressing.When you shake or beat your salad dressing, you do more than disperse the oil throughout the vinegar: you also break down the oil into droplets minute enough to remain temporarily suspended in the vinegar (which from now on we will call water, because that tart condiment is in effect mainly water). The second you  stop  agitating  the  dressing,  the  oil  droplets  start  to  combine  into  units  too  large  to  be suspended in the water, and thus slither their way upward, separating from the water in the process. The oil rises to the top and the water sinks because oil has a lower specific density than water. If you want a stable emulsion, you need an emulsifying agent which prevents the oil droplets from combining  into  larger  units.  Emulsifying  agents  occur  naturally  in  many  animal  substances including egg yolks and milk. An emulsifying agent helps to keep the oil particles from combining in three basic ways. First, the agent coats the oil, serving as a physical barrier between the droplets. Second, it reduces the water's surface tension, which, in turn, reduces the water's ability to repulse oil. Third, the agent gives the surfaces of the oil droplets identical electrical charges; since like charges repel each other the
droplets repel each other.
　　全文翻譯：油和水為理解乳化過程的發(fā)生，我們必須首先認識到這樣一條科學原理：水和油不能自 然混合，它們實實在在互相排斥。 家制的油醋沙拉調料就是一個好例子。 當你攪拌沙拉調 料時，你不僅使油擴散到醋里，而且使油滴被攪得很小可以懸浮在醋中(自此我們改稱它為 水，因為事實上這種酸性調味品的主要成份就是水)。 一旦你停止了攪動，油滴就會凝聚， 大到再也不能懸浮在水中時，它們就向上滑動出來而與水分離了。  因為油的密度小，所以 油會浮在水上。 如果想得到穩(wěn)定的乳化，就需要一種乳化劑。 它可以不讓油形成大的油滴。 自然界中，很多動物體中含有乳化劑，如蛋黃和牛奶。乳化劑可以從以下三方面阻止油滴 凝聚。一、  乳化劑包裹住油滴，在油滴間形成一道物質屏障。二、乳化劑降低水的表面 張力，從而降低了水排斥油的能力。  三、  乳化劑使油滴表面帶上了同種電荷，因為同種電 荷互相排斥，油滴之間也相互排斥。