托福official61听力lecture2 A method of decaffeinating coffee while preserving its flavor原文解析+翻译音频

[00:00.00]Listen to part of a lecture in a food science and technology class.
[00:05.95]One of the most popular beverages worldwide is coffee. [00:10.07]You’re probably not surprised to hear that. [00:12.49]Here in the United States, for example, more than half of the adult population drinks at least one cup of coffee everyday. [00:20.79]Now for various reasons, some of those who drink coffee choose to drink decaffeinated coffee either some of the time or all of the time. [00:29.63]So the coffee industry had to face this sort of need or demand for decaffeinated coffee. [00:35.77]Taking caffeine out wasn’t the problem. [00:38.84]It was actually taking the caffeine out without changing the flavor of the coffee or without increasing the health risk to drinking the coffee. [00:47.14]And although coffee has been decaffeinated for probably just about a century, at this point, there is one relatively new method that’s fairly commonly used these days to decaffeinate coffee. [00:59.82]And that’s supercritical fluid extraction.
[01:03.44]Supercritical fluid, they were, it’s on the board, we can’t understand the process of extracting the caffeine from the coffee if we don’t get this concept down pat. [01:14.94]In order to do that, let’s take a look at the phase diagram. This should be a review. [01:20.39]So, the phase diagram, it shows the conditions of pressure and temperature, which a substance exists in 1 of 3 phases, a solid, a liquid and a gas. [01:31.46]So we’ve got pressure on the vertical axis, temperature on the horizontal. [01:36.28]And as you know, a substance will change its phase if you start changing temperature or pressure. [01:42.13]For example, if we choose sort of an arbitrary pressure right about here, and keep the pressure constant and increase the temperature, we start as a solid, but as you can see, just follow the line going to the right. [01:58.15]As we increase the temperature, the solid changes to a liquid. And then changes to gas. [02:05.88]And that’s what we would expect. [02:08.45]We certainly know that happens with ice. [02:10.73]It goes from a solid to a liquid and to a gas, if we keep the pressure constant.
[02:16.26]Now, let’s do it the other way and keep the temperature constant. [02:20.72]Let’s take a couple of arbitrary temperatures and increase the pressure. [02:24.87]We start with gas. And we go up from the X axis and we see that gas will change either to a solid or to a liquid, depending on what temperature we are keeping constant. [02:37.87]There is, however, a temperature at which if we exceed that temperature, regardless of how much we increase the pressure, this gas will not change into a liquid or a solid. [02:49.35]Let’s call this critical temperature point TC, so there won’t be a full phase change. [02:56.90]There will be a change in its properties, however. [02:59.85]It won’t behave like a gas anymore. [03:02.12]It will have the properties intermediate between the liquid and the gas. [03:06.43]And we refer to it as a supercritical fluid at that point. [03:10.08]It’s in the upper right-hand corner here. [03:12.66]So that means it only exists at high pressure and high temperature. [03:17.73]So that’s what a supercritical fluid is.
[03:21.11]Now, what about these properties? Listen carefully, a supercritical fluid diffuses like a gas and yet, it’s dense like a liquid. [03:33.00]The significance of this is that a supercritical fluid can go through, can permeate a solid substance, for example, a coffee bean, because it diffuses like a gas and it can also dissolve other substances like caffeine, much like a liquid can, because of its density. [03:51.67]These particular properties or the values of these properties can be fine-tuned or adjusted by changing the pressure and the temperature.
[04:00.65]Now the way this is used in decaffeinating coffee is if we take the coffee beans and put them in a tank that can withstand extremely high pressures, and then we take supercritical carbon dioxide and pass it through the tank. [04:15.93]So it’s passing over the coffee beans. It actually permeates the coffee beans. [04:21.50]And if we have the pressure and temperature just right, the caffeine will selectively dissolve into the supercritical carbon dioxide. [04:30.38]Together the supercritical carbon dioxide and dissolved caffein will pass out of that tank and into a second tank leaving decaffeinated coffee beans behind. [04:43.32]Then we have a pressure relief valve on that second tank. [04:47.59]The pressure decreases and the supercritical carbon dioxide changes back into a gas, which then releases the caffeine in a crystallized form.