本文选自雅思听力Section 4，难度较大

Summary:味精是许多人在烹调的时候会使用的调料，但却很少有人了解它的历史。实际上，许多种调料在进入人类的厨房之前都经历了一个复杂而又曲折的过程。味精也是一样，它最先是由人们在一种还槽中提炼出来的。

HINTS
MSG
glutamate
amino
Kikunae Ikeda
MSG
John Prescott
the University of Chicago

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So, how exactly does MSG work? Well, in the Western world, we commonly talk of four "tastes", and I'm sure you're all familiar with the concepts of sweet, sour, bitter and salt. Well, in 1908, Kikunae Ikeda identified a fifth "taste". And it is thought that MSG intensifies this naturally occurring "taste" in some food. It does make perfect evolutionary sense that we should have the ability to detect or taste glutamate because it is the amino acid which is most common in natural foods. John Prescott, an associate professor at the University of Chicago, suggests that this fifth taste serves a purpose just as the other tastes do. He suggests that it signals to us the presence of protein in food, in the same way that sweetness indicates that a food contains energy-giving carbohydrates. Bitterness, he says, alerts us of toxins in the food, while sourness warns us of spoilage and saltiness signals the presence of minerals.
那么味精究竟是怎么起作用的呢？在西方国家，人们经常谈论四种味道。我相信人们都非常熟悉酸甜苦辣四种味道。1908年池田菊苗发现了一种新味道-味精。味精大大提高了一些食品中自身含有的味道。我们因该能发现或尝出谷氨酸盐因为谷氨酸盐就是未经加工的食物中常见的氨基酸。 芝加哥大学副教授约翰普瑞斯科特认为这第五种味道与其他四种味道起着同样的作用。味精对人们来说意味着食物中含有蛋白质。同样的，甜味表明食物中含有给予人能量的碳水化合物；苦味让人们警觉食物中的毒素；而酸味警告人们食物变质，咸味则说明含有矿物质。 ——译文来自: sweetyqun