雅思聽力:味精的歷史 (4/4)

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.