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Eating out has taught many Americans to be knowledgeable about ordering subtle and complex dishes from around the world, but it has left many of us less knowledgeable about how to cook our own food. That's one reason Russ Parsons has written about the science of cooking. He says the more you know about why meats brown, why sauces emulsify and how frying is different from roasting, the more likely you are to be able to follow a recipe or improvise in the kitchen.

I spoke with Parsons in 2001 after the publication of his book How to Read a French Fry: And Other Stories of Intriguing Kitchen Science. He's a food editor and columnist at the LA Times. He started getting interested in kitchen science when his editor assigned him to write a piece about onions. But his response at the time was...

Mr. RUSS PARSONS (LA Times): Onions? What do you write about onions? So I decided I'd do a piece about all of the kind of folklore about how onions make you cry. And so I rounded up the usual suspects and talked to chefs and things. And what I realized was that nobody really knew, nobody that I talked to really knew why onions made you cry. And so I thought, well, I'll just kind of dabble into this food science thing and see if I can find somebody who can address that question. And it opened up this incredible array of knowledge that I didn't even know was out there. And the people were so anxious to talk.

You know, the first guy I talked to, you know, he's a guy who'd been studying onion chemistry for like 20 years. And what he told me was really interesting. You know, when you have an onion of whatever kind and you smell it before it's been cut, it smells different than an onion that's been cut.

GROSS: Yeah, why is that?

Mr. PARSONS: Well, essentially an onion, essentially most foods, are big bags of water. In the water in the onion there are these little these little vacuoles. They're little pockets of different chemicals. When you cut an onion, all those vacuoles are disrupted, the chemicals empty out, and they begin to combine with each other. You get these chemical reactions. And then those chemicals - the resulting chemicals combine again.

And this guy, I'll never forget when he said this, because it was kind of like key that unlocked this whole thing. He said it was a cascade of chemical reactions that occurred in a blink of an eye. I was just fascinated by that. And so then I started pursuing the whole onion thing further. And it turned out...

GROSS: Well, let me stop you.

Mr. PARSONS: Yeah.

GROSS: So those chemicals are what makes us tear?

Mr. PARSONS: Eventually. You know, the raw chemicals don't but after the fifth or sixth generation of combining and recombining, the result is a kind of a sulfur gas, and actually, it's not clear at this point whether the sulfuric gas goes up your nose or goes directly to the eye. But either way, it irritates you and it makes your eyes tear as a result of that. And the great thing is that the chemical name for those chemicals are lacrimators from the Latin word for tear, lacrima.

GROSS: Well, let's get to the taste. How does the taste of an onion change when it's cut and when it's cooked?

Mr. PARSONS: Now, what happens is that in - you know, you've got the sweet onions that you pay, you know, four or five dollars a pound for. Those onions aren't really any higher in sugar then the brown storage onions that you buy in a big bag for, you know, 95 cents for five pounds. The difference between the two is that the sweet onions are actually lower in the chemicals that produce the tearing effect.

Now, the interesting thing is that those chemicals are heat volatile, which means that as soon as they are heated they evaporate. They go and they, you know, they go off into the air, which is also why you might start tearing when you're frying onions. So when you cook a brown onion, when you cook one of those regular old cheap storage onions, the chemical compounds go away. Those sulfuric compounds, the ones that make you cry, the ones that make the onions taste hot and unpleasant when they're raw, they go away. And what's left is an onion that's sweeter than the so-called sweet onions.

What's also left behind is more onion flavor. If you've ever tried to do, I think when those sweet onions first came out the first thing everybody wanted to do with them, of course, was make onion soup. Well, the result almost universally was an onion soup that didn't really taste like very much, because part of that really delicious oniony flavor is that little bit of the acidity that's left behind, that little bit of those chemicals.

So you're much better - if you're going to even onion raw, like on a sandwich or something like that, well, the sweet onions are wonderful. But if you're going to cook them, you're absolutely wasting your money. Somebody described it - I was talking to a scientist and a lot of those onions, the first of the really popular sweet onions came from Vidalia, Georgia. And I was talking to one of the guys who was raising them, and he was a real good ol' boy. And he said, he said that cooking a Vidalia onion was like mud-bogging in a Rolls-Royce.

(Soundbite of laughter)

GROSS: Do you have a favorite onion soup recipe?

Mr. PARSONS: Probably the classic. You know, that kind of French onion soup that - where you cook the onions very long. You shred them fairly thick, actually. I cook them that way. And you cook them real low, real slow, so that they brown, they come to a very deep brown. And then you add some cognac and you add some beef stock. Cook it for a little bit longer. Then you put it in the big soup plate and cover it with a crouton with some good cheese on it.

GROSS: How long does it take to make...

Mr. PARSONS: That's delicious.

GROSS: ...about?

Mr. PARSONS: Oh, an hour maybe.

GROSS: We're listening back to an interview with Russ Parsons, food editor and a columnist at the LA Times. This interview was recorded after the publication of his book How to Read a French Fry.

We'll hear more after a break. This is FRESH AIR.

(Soundbite of music)

GROSS: Let's get back to our interview with Russ Parsons, food editor and columnist at the LA Times. This interview was recorded after the publication of his book about kitchen science called "How to Read a French Fry."

Let's talk about the principles of frying. Now, we think of frying as being like a really hot form of cooking.

Mr. PARSONS: Mm-hmm.

GROSS: But compare oven heat to frying heat and how it affects the foods that are being cooked.

Mr. PARSONS: Well, that's one of those really great paradoxes. Why can you stick your hand in a 500 degree oven but you can't stick your hand in 200 degree water or in 200 degree oil? It's because the way we measure heat is very imperfect. Temperature is what we usually refer to when we're talking about heat. But temperature is a measure of how fast these molecules are moving. You know, heat is motion, kind of.

What temperature doesn't measure is how quickly the heat is being transmitted. Now, in an oven for example, you're cooking in air and air is not very dense. You know, there are very few molecules per square inch. It takes a while for that heat to transmit itself to whatever is being put in there. With water it's more dense, and so the water transmits the heat much more quickly.

Now, water has this interesting thing too, though, that it's not as dense as oil, so that when water gets to a certain point, those molecules begin to break loose and they go up in the air - you know, there's steam. That point it's 212 degrees. With oil, the magic of oil, is that it's denser than water and so the molecules stick together better. And so you can get oil up to 400 degrees where you can't get water any higher than 212.

That's important in cooking because the kinds of browning reactions, the things that makes food brown, they don't really start happening until you get to about oh, you know, between 300 and 350 degrees. So there's lesson one right there. No matter if you cook something when there's any moisture present, no matter how long you cook it, it's never going to turn brown.

GROSS: Why does frying crisp the outside of whatever you're cooking?

Mr. PARSONS: The special thing about frying is that in most kinds of cooking, whether you're talking about roasting in an oven or boiling in water, the cooking medium doesn't change very much. The water stays essentially the same, the air stays essentially the same. I mean there are fine differences but essentially they're the same. With frying, both the oil that this food is being fried in and the food are changing all of the time.

For example, one of the common problems people have when they're deep frying things is that the first batch never turns out. Well, there's a good reason why it never turns out. With really fresh oil, again, most of the foods that we cook are made up primarily of water. Oil and water don't mix. So when you put a piece of food in really hot oil, all of the moisture comes to the surface and it forms this - a moisture barrier that the oil can't penetrate so the oil can't really touch the food that's being fried.

Now, as the frying progresses, as the oil is heated and more things are added to it, the oil begins to break down. One of the byproducts of this breakdown is soaps, or chemical soaps. These aren't the same kinds of things that you wash your hands with, but they're very close. And you know, when you wash your hands, what soap does is it allows the water to penetrate the grease that's on your hand. The soaps in frying do exactly the same thing. They allow the cooking oil to penetrate that water barrier so that the cooking oil comes in direct contact with the food that's being fried so that it browns it better and it cooks it through more thoroughly.

GROSS: So the more you use oil, the more efficient it's going to be.

Mr. PARSONS: Up to a point, and then the oil begins to break down to a point where, you know, you get these kinds of - the chemicals come off of it. You get these nasty kinds of odors, and it's no longer working as well as it could. That's kind of where the title "How to Read a French Fry" came from. When I was talking to one of the scientists who was studying oil chemistry, and he started explaining all of this stuff to me, I said, no, no, give me an example, give me an example. He said, well, look at a French fry. You know, the next time you go to a fast food place, look at a French fry and you can see at what stage the oil was.

I mean it's usually - there's usually four stages of oil. There's, you know, pure oil, then there's oil beginning to break down, then there's oil at the perfect frying stage, and then there's what they call runaway oil, when it's broken down to the point where it no longer functions. A French fry fried in really pure oil, really fresh oil, it won't be as brown. It probably won't be cooked all the way through. There would probably be a little bit of a raw part at the center of the French fry.

When it begins to break down, that kind of first stage when it's really good for frying, the oil the French fry will be browner and you'll start to see, it'll be cooked all the way through. When it's really perfect, when the oil is perfect for frying, the French fry will be really well browned. The corners will be really crisp. It'll be cooked all the way through so that you have that kind of - that perfect French fry combination of really crisp exterior and almost a steamy inside. And you'll have all of these delicious flavors from the oil because good oil is essentially delicious, which is why we fry so many foods in it.

Then when it gets into the runaway stage what will happen is you'll see that it'll be brown very quickly. They'll be really dark spots on it, kind of black where it started to burn a little bit, and the French fry will have started to collapse in on itself. It won't be attractive anymore. And you'll also get the flavor, there's that, there is an old oil flavor that's not very attractive.

GROSS: Everything you're saying sounds very familiar. I don't mean the information, I mean the descriptions of the French fries.

(Soundbite of laughter)

Mr. PARSONS: Well, see, that's one of the really fascinating things about this, is that this is all stuff that we, that when you cook, that you recognize it down the road. What this does is kind of explains to you why it's happening in a way kind of summing up 20 years of making cooking mistakes and explaining, oh, here's what went wrong, so that it's a little bit like it's kind of a cooking course.

GROSS: Well, Russ Parsons, thank you so much for talking with us.

Mr. PARSONS: Wonderful to be here. Thank you.

GROSS: Russ Parsons is the food editor and a columnist at the LA Times. Our interview was recorded in 2001 after the publication of his book "How to Read a French Fry: And Other Stories of Intriguing Kitchen Science." Our All You Can Eat Week continues tomorrow.

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