"There are more than 1,000 different chemicals that elicit a bitter response," says Jennifer McLagan, author of Bitter: A Taste of the World's Most Dangerous Flavor, with Recipes. Found in foods from celery leaves to chocolate, bitterness is something McLagan says we can feel "with our fingers, our lips, our teeth and our tongue."

Lynne Rossetto Kasper: "The world's most dangerous flavor" -- what are we talking about?

Jennifer McLagan Jennifer McLagan

Jennifer McLagan: Bitter can be dangerous; bitter can signal a poison or a toxin. We have this innate reaction of negativity to bitterness. The reason for me writing the book is to try to help people overcome that negative reaction to bitter and try to see it as a positive taste. Now we're not really worried about eating anything poisonous that we buy in the supermarket or the grocery store.

LRK: The basic flavors -- salty, sweet, sour, I would add hot as in chili -- they're pretty easy to nail down. But bitter is more elusive. Why is that?

JML: It's interesting because I thought I knew what bitterness was. I asked friends to give me some suggestions to put in the book. They came back with what I expected, things like rapini, beer, coffee and citrus zest.

But some of them suggested rhubarb, pickled onions or sorrel, which for me aren't bitter at all; they're actually sour. That's when I thought, "I'm really going to have to think about what bitterness means."

I talked to a couple of food scientists who specialize in taste. They said bitter and sour are often confused. In large amounts we have a negative reaction to both of them, so people get those confused in their head. But only acids give you a sour response, whereas there are more than 1,000 different chemicals that elicit a bitter response. Bitter is much more complex, I think, than all the other flavors. It ranges from a mild bitterness like celery leaves all the way through to something like bitter gourd, which is very bitter.

LRK: This must mean that it's registering in the brain in a lot of different ways.

JML: It's our brain that's creating flavor, and all the other senses come into play -- our taste buds, there's our sense of smell. I would argue that you can smell bitterness. Think about when you smell dandelions, maybe burned toast or roasting coffee, that will give your brain an impression of bitterness.

We can see taste. I didn't realize this before working on the book that actually the biggest influence on how we taste is what we see. People always say you eat with your eyes. Usually they're referring to pretty photographs in cookbooks. But we actually do eat with our eyes. Half our brain is devoted to processing visual information, so it uses all these shortcuts to do it quickly. Color is one way.

That happens with food. If we see something that's red, we assume, especially in Western culture, that it's going to be sweet. It's like Campari -- it's bitter, but I think we perceive it as being less bitter or more sweet than it is because it's red.

Heston Blumenthal, the molecular chef, had on his menu a beetroot and orange jelly, which of course was orange and dark purple. But not until people closed their eyes did they understand that the dark purple was blood orange and the orange side was beetroot, orange beets. Because their brains just said, "OK, that's the orange side; that tastes like orange ..." But it's totally reversed. Their eyes were tricking them into perceiving that.

Beer Jelly McLagan's recipe: Beer Jelly

LRK: Bitter sounds like it's more complicated than the other flavors.

JML: I think so. I think because it has a bigger range. We can feel bitter with our fingers, our lips, our teeth and our tongue. There's a nerve that runs down the side of your tongue, the trigeminal nerve, and that's the one responsible that gives you a brain freeze when you have really cold ice cream.

But it's also the same thing when you have a mint -- it feels cold even though it's not cold. A cognac feels warm even though it's room temperature. That also gives us pungency and tannins, so you sense bitterness in arugula from its pungency, or in horseradish, or the astringency in celery, or the tannins in tea. Cooked apricots are another one for me; I think they're bitter in a tannic sense.

LRK: What are some of the things to know about using bitter when cooking?

Bitter Bitter

JML: I think one of the most important things is not to try to eliminate it. People say, "Oh it's bitter," and they'll put some sugar in there. Try to balance bitterness with something else. Something salty -- salt will often cut the bitterness -- but also something fatty. I should have known this straight away, fat and bitter are perfect partners. If you have something rich and you match it with something bitter, they play off each other and make each other taste even better.

LRK: So horseradish in whipped cream as a sauce?

JML: Exactly. A magret de canard with a side of rapini.

LRK: The duck breast is almost sweet, it's so rich.

JML: Yes. I think it's interesting to use bitterness in something like a truffle. If you take your truffle and you roll it in cocoa powder instead of in confectioner's sugar, you get a much more interesting, complex flavor. You get that bitterness on your tongue with the cocoa, and then afterward you get the chocolate. It balances. It's not just sweet, sweet, sweet, one tone, it's many different things in your mouth. You're getting all those different kinds of complex, intriguing flavors together.

Or a salad is the same thing. Maybe you don't want all bitter greens, but if you mix a regular salad and you add some celery leaves, some escarole, Belgian endive and radicchio for color, you get a really interesting salad.

LRK: That sounds wonderful.

JML: Embrace bitterness.

Lynne Rossetto Kasper
Lynne Rossetto Kasper has won numerous awards as host of The Splendid Table, including two James Beard Foundation Awards (1998, 2008) for Best National Radio Show on Food, five Clarion Awards (2007, 2008, 2009, 2010, 2014) from Women in Communication, and a Gracie Allen Award in 2000 for Best Syndicated Talk Show.