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The new molecular gastronomy, or, a gustatory tour of network analysis
 
Jeffrey M. Perkel, Ph.D.
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Pelchat basically agrees. Many food ingredients, she says, are like cloth fabrics laced with different colored pinstripes: They can be paired with a wide range of things.

But that's only part of the story, she says. Food isn't just about chemicals; there's also visual appearance, aroma, and texture. “People in our culture like contrast as well,” she notes. Like the classic BLT, with the warm bacon and cold tomato, the soft bread and the crunchy lettuce.

Genomics in the kitchen

Of course, there's also the possibility that the West-vs-East pairing differences Barabási and colleagues uncovered really do have an actual biological basis, some genetic quirk that influences how different subpopulations perceive food combinations? In other words, could the differences be traced back to variations in odor or taste receptors?

It certainly wouldn't be unprecedented. The ability to detect that characteristic sulfur-like odor in urine following ingestion of asparagus? There's a SNP for that (2).

Donald Katz, associate professor of psychology and neuroscience at Brandeis University, studies neural networks underlying taste, and authored a 2006 review on the gustatory processing (3). One figure in that review details the presumptive circuitry underlying how humans taste and see. The visual system is displayed as a highly interconnected, dizzying circuit diagram. The taste system appears a bit sparser by comparison (taste, he says, is “the red-headed stepchild of the sensory systems”). But neither representation even comes close to the complexity of the actual neural circuitry involved, says Katz.



Fundamentally, humans taste because chemicals in food interact with receptors on sensory neurons in the tongue, producing electrical signals that transduce the chemicals into brain activity. “But that's a little bit like saying, how does a TV work? Well, you plug it in,” notes Katz.

Recently Charles Zuker of Columbia University and colleagues took a stab at the problem, using in vivo two-photon calcium imaging to produce what they call a “gustotopic map” of the mouse brain, in which sweet, bitter, umami, and salty tastes are wired to neurons in discrete “cortical folds” in the gustatory cortex (4).

Zuker's findings, says Katz, are controversial. “No one else, using any other technique, has ever seen what looks like a gustatopic map in cortex,” he says. (Zuker could not be reached for comment.) Indeed, it is likely that taste is “hideously complex,” involving as it does more than just taste receptors, but also smell, touch, and vision.

But in any event, the Barabási results are “almost certainly” more cultural than biological, he says. Why? Well, rodents told him so—more or less.

Katz is looking for something of a 30,000-foot overview on taste in rats, observing their behavior and recording their neural activity in response to different foods. Rats, it seems, do not much care for cocoa. But, if they smell the flavor on another animal's breath, they will readily eat it.

In one experiment, his team forced one rat—Katz calls him “Ralphie”—to eat cocoa-laced chow. They then allowed that rat to interact with another animal— “Don” —and watched what happened. “Even if he's never had it before—and mind you, it's a bitter, bitter taste—when [Don] gets his first chance to taste [cocoa], he won't reject it. He'll actually like it,” says Katz. If Katz records Don's brain activity before and after this experience, he can detect changes from one associated with an unpleasant taste, to one linked to something like sugar.

And that, says Katz, suggests that taste preference is essentially a cultural phenomenon, rather than a biological one. After all, people tend to eat what others around them eat, as well. “When you're sitting in a high chair and your parents are enjoying a certain kind of food, that actually has a strong impact on you,” he notes. Conversely, when humans see someone getting sick on a certain food—or if they themselves get ill—that forms a psychological association that is likewise hard to break.

“It is so easy, from a social and cultural perspective, to cause differences in taste profiles, that it strikes me as a little lame to go running to the genome and saying, there's got to be a genetic difference that explains it,” says Katz. “You don't need to go to the genome. It's the easiest thing in the world to provide a half-hour experience and change what an animal likes and doesn't like.”

That's probably true. But I'm still not going to like that papaya.

References
1.) Ahn, Y.Y., S.E. Ahnert, J.P. Bagrow, and A.L. Barabási. 2011. Flavor network and the principles of food pairing. Sci Rep. Epub 2011 Dec 15 1:196.

2.) Eriksson, N., J.M. Macpherson, J.Y. Tung, L.S. Hon, B. Naughton, S. Saxonov. 2010. Web-based, participant-driven studies yield novel genetic associations for common traits. PloS Genet. 6:e1000993.

3.) Jones, L.M., A. Fontanini, and D.B. Katz. 2006. Gustatory processing: A dynamic systems approach. Curr Opin Neurobiol. Epub 2006 Jul 13 16:420-8.

4.) Chen, X., M. Gabitto, Y. Peng, N.J. Ryba, and C.S. Zuker. 2011. A gustotopic map of taste qualities in the mammalian brain. Science. 333:1262-6.

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