63 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.
油和水
为理解乳化过程的发生，我们必须首先认识到这样一条科学原理：水和油不能自 然混合，它们实实在在互相排斥。 家制的油醋沙拉调料就是一个好例子。 当你搅拌沙拉调 料时，你不仅使油扩散到醋里，而且使油滴被搅得很小可以悬浮在醋中(自此我们改称它为 水，因为事实上这种酸性调味品的主要成份就是水)。 一旦你停止了搅动，油滴就会凝聚， 大到再也不能悬浮在水中时，它们就向上滑动出来而与水分离了。 因为油的密度小，所以 油会浮在水上。 如果想得到稳定的乳化，就需要一种乳化剂。 它可以不让油形成大的油滴。 自然界中，很多动物体中含有乳化剂，如蛋黄和牛奶。 乳化剂可以从以下三方面阻止油滴 凝聚。 一、 乳化剂包裹住油滴，在油滴间形成一道物质屏障。 二、 乳化剂降低水的表面 张力，从而降低了水排斥油的能力。 三、 乳化剂使油滴表面带上了同种电荷，因为同种电 荷互相排斥，油滴之间也相互排斥。
64 Salt and Metabolism
Just how salt became so crucial to our metabolism is a mystery; one appealing theory traces our dependence on it to the chemist ry of the late Cambrian seas. It was there, a half billion years ago, that tiny metazoan organisms first evolved systems for sequestering and circulating fluids. The water of the early oceans might thus have become the chemical prototype for the fluids of all animal life - the medium in which cellular operations could continue no matter how the external environment changed. This speculation is based on the fact that, even today, the blood serums of radically divergent species are remarkably similar. Lizards, platypuses, sheep, and humans could hardly be more different in anatomy or eating habits, yet the salt content in the fluid surrounding their blood cells is virtually identical. As early marine specics made their way to fresh water and eventually to dry land, sodium remained a key ingredient of their interior, if not their exterior,
milieu. The most successful mammalian species would have been those that developed efficient hormonal systems for maintaining the needed sodium concentrations. The human body, for example, uses the hormones renin, angiotensin, and aldosterone to retain or release tissue fluids and blood plasma. The result, under favorable conditions, is a dynamic equilibrium i [1] [2] 下一页 |
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