2018年11月10日托福阅读真题原文+题目+答案：Atwater Food Energy Value System

2018年11月10日托福阅读真题原文+题目+答案：Atwater Food Energy Value System

The Atwater system of ascertaining energy values in foods was developed by Wilbur Olin Atwater, a college chemistry professor, around the end of the nineteenth century. ▊Atwater knew that food contained three main substances the body uses for energy: protein, fat, and carbohydrate. ▊Using a simple laboratory device called a bomb calorimeter, he recorded how much heat was released when typical proteins, fats, and carbohydrates were completely burned.▊ He found that there was not a lot of variation among the different types of each item.▊ For example, all proteins tended to produce a little more than four kilocalories of heat per gram (kcal/gram).

1.The word “ascertaining” in the passage is closest in meaning to

A. describing

B. determining

C. maintaining

D. maximizing

After that, Atwater needed to know two more things. First was how much of the major macronutrients-protein, fat, and carbohydrate-a food contains. Fat was easy, because unlike protein and carbohydrates, fat dissolves in ether (a liquid used to dissolve other substances). So Atwater chopped foods finely, shook them up with ether, and weighed how much material was dissolved in the ether. That gave him a food's fat content (or, more strictly, lipid content: lipids include both fats, which are solid at room temperature, and oils which are liquid). The same method is used today. Protein was harder to index because no test identifies proteins in general. However, Atwater knew that about 16 percent of the weight of an average protein was nitrogen. So he found a way to measure the amount of nitrogen, which gave him the concentration of protein.

2.According to paragraph 2, why is it easy to find out a food's lipid content?

A.Lipids remain when fats are removed from the oils found in a food.

B.Lipids melt easily and can be removed from the food by heating it above room temperature.

C.Lipids are the part of the food that remains after the proteins have been dissolved in ether.

D.Lipids are the only part of the food that dissolves in ether.

Carbohydrates were the hardest. There was no test then, nor is there now, for identifying the concentration of carbohydrates in general. But Atwater knew the main organic matter in foods was the three big items, protein, fat, and carbohydrate. He also knew how to calculate the total amount of organic matter. He simply burned the food completely, leaving only the mineral ash that did not burn and was therefore the inorganic part. Knowing how much organic matter the food contained and how much fat and protein it held, he obtained the amount of carbohydrate by subtraction: the weight of the carbohydrate was what was left when the weights of the fat, protein, and mineral ash had been subtracted from the total weight of the original food item.

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