نطق سوم

Lecture 3:

Listen to part of a lecture in a food science and technology class.

One of the most popular beverages worldwide is coffee. You’re probably not surprised to hear that. here in the united states, for example, more than half of the adult population drinks at least one cup of coffee every day. for various reasons.

Some of those who drink coffee choose to drink decaffeinated coffee either some of the time or all of the time. so the coffee industry had to face this sort of need or demand for decaffeinated coffee. taking caffeine out wasn’t a problem. It was actually taking the caffeine out without changing the flavor of the coffee or without increasing the health risk to drinking the coffee. and although coffee has been decaffeinated for probably just about a century at this point, and there’s one relatively new method that’s fairly commonly used these days to decaffeinate coffee, and that’s super critical, fluid extraction, super critical fluid, big word.

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, in order to do that, let’s take a look at the phase diagram. This should be a review. so the phase diagram, it shows the conditions of pressure and temperature at which a substance exists in 1 of 3 faces, a solid, a liquid, and a gas. so we’ve got pressure on the vertical axis, temperature on the horizontal. and as you know, a substance will change its phase.

If you start changing temperature or pressure, 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. as we increase the temperature, the solid changes to a liquid, and then changes to a gas. and that’s what we would expect. We certainly know that happens with ice. It goes from a solid to a liquid and to a gas. if we keep the pressure constant, now let’s do it the other way and keep the temperature constant.

Let’s pick a couple of arbitrary temperatures and increase the pressure. we start as gas, and we go up from the x axis. And we see that that gas will change either to a solid or to a liquid, depending on what temperature we’re keeping constant. 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. let’s call this critical temperature point t c so there won’t be a full phase change. There will be a change in its properties. However, it won’t behave like a gas anymore. it will have the properties intermediate between a liquid and a gas.

and we refer to it as a super critical fluid. At that point. it’s in the upper right hand corner here. So that means it only exists at high pressure and high temperature. so that’s what a super critical fluid is. now, what about these properties? Listen carefully. A super critical fluid diffuses like a gas, and yet it’s dense like a liquid.

the significance of this is that a super critical 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. these particular properties, or the values of these properties, can be fine tuned, or adjusted by changing the pressure and the temperature. 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 super critical carbon dioxide and pass it through the tank. So it’s passing over the coffee beans. it actually permeates the coffee beans. and if we have the pressure and temperature just right, the caffeine will selectively dissolve into the super critical carbon dioxide. together, the super critical carbon dioxide and dissolved caffeine will pass out of that tank and into a second tank, leaving decaffeinated coffee beans behind. then we have a pressure relief valve. On that second tank, the pressure decreases. And the super critical carbon dioxide changes back into a gas, uh, which then releases the caffeine in a crystallized form.

go out for 12 hours in Quebec, Canada.