FLORA LICHTMAN, BYLINE: Up next, another mover and shaker in the alimentary canal - coffee. Whether you're a home brewer or a latte devotee, whether you take it light and sweet or on ice, your coffee is guaranteed to be chock full of chemistry. It starts in the bean, which is actually not a bean at all.
It's a seed, according Harold McGee, author of "On Food and Cooking: The Science and Lore of the Kitchen" along with other books on science and food. And we caught up with Harold, to hear more about how coffee gets its signature taste.
HAROLD MCGEE: If you don't mind, maybe what I'll do is back up and just talk about what's in the coffee seed, and how it got there. You start out with a raw coffee bean, which is very hard. If you try to bite into it, it's like biting into a pinto bean. You'll - you might break your tooth before you actually get into it.
And it has no flavor - or if any flavor at all, it's kind of like a stale green pea with kind of a stringent woody flavor to it. Not much aroma at all, just kind of irritating. So if you make an infusion of green coffee beans, that's mostly what you get - is acidity and bitterness from the caffeine.
And then what happens is that you roast that bean and generate hundreds and hundreds and hundreds of new chemical compounds that are locked into that bean and give it all kinds of flavor qualities: taste, aroma, mouth feel, body. It's really a miraculous kind of thing. You then end up after roasting with this bean that's full of hundreds and hundreds of chemicals that you want to get into the water that's going to form the bulk of the drink. So you grind up the bean to increase the surface area that's going to get exposed to the water and then you heat the water up because extraction of molecules from a solid happens much faster at higher temperatures.
Everything is just jumping around a lot faster and with more energy, and so you're able to knock out the desirable flavors from the bean much more readily at high temperatures than you are at low temperatures. And then you expose the bean to that water and leave the two materials in contact with each other long enough to extract the really good stuff, but not so long that you extract the not so good stuff, because there are things in the bean that are not so pleasant.
LICHTMAN: This is SCIENCE FRIDAY from NPR. I'm Flora Lichtman, chatting with Harold McGee. Is just luck that they come out later?
MCGEE: Yeah. I would say. Basically the things that come out early are small molecules and very water-soluble molecules, so acids. The things come out very late are not so water soluble and they're larger molecules. And it turns out that those are the ones that do tend to be bitter and do tend to be astringent.
LICHTMAN: Will we be familiar with any of these chemicals? Do we, you know, or are they things we've never heard of before?
MCGEE: No. They're found in all other roasted foods or most other roasted foods. And that's one of the amazing things about the Maillard reactions. Those are the - that's the general term for these reactions that at high temperatures turn pale, tasteless things into brown tasty things. So a lot of the things that you find in coffee you also find in other toasted, roasted foods. The few that make coffee coffee tend to be, as far as we can tell, and we're still learning about this, tend to be very peculiar sulfur-containing aromatic molecules that you just don't find other places.
LICHTMAN: Sulfur. I mean, we know that from things like skunk spray.
LICHTMAN: Maybe different ones.
MCGEE: It's true. Yes. They are different, although you do find some overlap between defensive sprays of animals and foods that we love. You have to get the proportions right. So if there's too much of it, then of course you're not going to like it. But a little bit gives a kind of depth and interest to food flavors.
LICHTMAN: Do you think that caffeine is bitter? Does that mean caffeine has a flavor itself?
MCGEE: Yes. Yes, indeed. Caffeine is - it's an alkaloid. And we find, humans, that is, find almost all alkaloids bitter. And the biological thinking about that is that almost alkaloids are also toxic, and our bitter taste, which is an intrinsically unpleasant experience, bitterness is way of warning us that there is something in whatever we've put in our mouth that is likely to be bad for us. Now, you might say, well, but we love caffeine because it does wonderful things for our attentiveness and wakefulness and so on. And that's true to a certain point.
But if you have too much caffeine, it will, in fact, be toxic. So bitterness is a flavor that we learned to enjoy in the course of our lives because we can associate it with desirable things like the delicious flavors of coffee and the stimulating effects of coffee. But initially it's an unpleasant experience.
LICHTMAN: It's kind of an upper or downer or about the caffeine, right? Anyway, that was Harold McGee, author of "On Food and Cooking: The Science and Lore of the Kitchen." And we chatted with him from KQED in San Francisco. That's about all we have time for this hour. If you want to learn more about the science of coffee, visit our website. We have a video produced by Jenny Woodward featuring Harold McGee and overflowing with fun facts on coffee. It's on our website at sciencefriday.com. Transcript provided by NPR, Copyright NPR.