I have a confession: I hate raw tomatoes. Really hate them. Really, really hate them. It's a positively visceral reaction, beyond my conscious control. Even the smell makes me slightly nauseous. Once, as a kid, my mom got fed up with watching me shove tomatoes to the side of my plate, and insisted I couldn't leave the dinner table until I ate them. I put it off as long as possible, but finally, desperate to get away and lose myself in the book du jour, I shoved the offending food into my mouth... and promptly gagged and spit it up. My mother, to her credit, threw up her hands in resignation. Her daughter would never eat raw tomatoes.

This intense hatred of raw tomatoes was incomprehensible to my mother; she loves them as much as I despise them. I suppose it's equally incomprehensible to all the rest of your tomato-lovers out there. Frankly, it's even a little incomprehensible to me, seeing as how I love ketchup and spaghetti sauce, provided there are no huge chunks of tomatoes -- boil and puree those suckers, and season with tons of garlic, olive oil, basil, thyme, and oregano, and it overcomes even my rebellious palate. All my life, I've been vaguely ashamed of my dislike, probably because it was such a profound disappointment to my mother, and naturally I craved her approval.

But no more! I just discovered that I am not alone in the blogosphere when it comes to hating raw tomatoes. Kylee Baumle of Our Little Acre just came out of the closet as a "mater hater." [UPDATE: Apologies to Kylee for spelling her last name wrong originally -- and then getting her blog name wrong.] So did Steve Bender, a.k.a., the Grumpy Gardener, and Chris Tidrick, who blogs at From the Soil. They recently discovered their mutual dislike while at a Garden2Blog event in Arkansas. Like Chris, I, too, carefully remove all bits of tomato from food and leave it on the side of the plate. Solidarity!

We mater-haters have to stick together. As Grumpy notes, "Telling people you hate fresh tomatoes is like saying you hate giggling babies or that you loathe the prospect of world peace."

I've wondered about this strange aversion for years. Why raw tomatoes? It might have something to do with the chemistry, and how it changes when tomatoes are cooked.

Oh, yes, cooking can change a foodstuff's chemistry. For instance, there is an antioxidant called lycopene, found not just in tomatoes, but also watermelon, pink guava, red bell peppers, and papaya. In tomatoes, it seems to be affected by heat. I found a 2002 study in the Journal of Agriculture and Food Chemistry that determined levels of cis-lycopene rise 35% after tomatoes are cooked for 30 minutes at 190.4 Fahrenheit.

Rui Hai Liu, a food scientist at Cornell and the author of the study, thinks it's because the heat breaks down the thick cells walls, and this makes it easier for the human body to absorb the nutrients bound up in those walls -- including lycopene. This seems to be confirmed by a 2008 study in the British Journal of Nutrition, which found that 198 test subjects who followed a raw food diet had low levels of lycopene, compared to other nutrients (Vitamin A, beta-carotene).

The same journal published a 2007 study by Steven Schwartz, who analyzed the structure of lycopene molecules and concluded that while the basic chemical properties of the molecule are the same whether cooked, processed, or raw, the form of lycopene that shows up in the human body has a bent molecular form, while the form in your standard raw tomato is more linear -- limiting how much can be absorbed into the bloodstream.

Schwartz developed a technique that subjects tomatoes to intense heat, and then combines them with fat (like olive oil), that changes the form from linear to bent. And that makes it more likely to be absorbed into the bloodstream. A small test study (12 subjects) did, indeed, find higher levels of lycopene in the bloodstream after consuming tomato sauces made via this process. Traditional home cooking techniques -- long, slow simmering times, reheating sauces day after day -- can have a similar effect. So cooking tomatoes does seem to change them in a chemically significant way.

Perhaps lycopene isn't the culprit, however. I like watermelon, pink guava, red bell peppers and papaya, after all. Maybe it's something to do with the smell, or one of the myriad other flavor compounds that's at fault. People like me just lack certain key taste receptors, preventing us from appreciating the rich, sweet, meaty flavor of raw tomatoes that the rest of you are always rhapsodizing about. The problem is that tomatoes have something on the order of 400 volatile compounds and who knows which one of those (or combination thereof) might be responsible for the harsh reaction many of us experience in response to raw tomatoes?

Per a recent Wired article, Harry Klee, a plant molecular biologist, has managed to narrow things down a bit, identifying around two dozen compounds that seem to be involved in the tasty appeal (for people who are not me) of tomatoes. And it's not the most likely suspects: a class called C6 volatiles. Instead, three compounds in particular -- geranial, 2-methylbutanal, and 3-methyl-1-butanol -- contribute to the sweetness of tomatoes, at least. Or maybe it has something to do with furaneol, another compound found in tomatoes that contributes to sweetness.

Frankly, the scientific community has been sadly remiss in getting to the bottom of the mystery of why raw tomatoes make some of us gag, despite a few scattered flavor studies. But they're hot on the case of cilantro, an even more polarizing herb. I love cilantro. To me, it tastes fresh and citrusy with just a tinge of an herbal edge to it. But to some people, it just tastes like soap. Or worse. They have as strong a visceral reaction to cilantro as I have to fresh raw tomatoes.

Celebrity chef Ina Garten (a.k.a. the Barefoot Contessa) admits to disliking cilantro, and Julia Child memorably confessed in a 2002 Larry King interview that if she spotted cilantro in her food, she would pick it out and "throw it on the floor." There are Facebook groups devoted to the haters. Sometimes people post haiku about how much they hate cilantro. ("The curry sickens/ It looked good on the menu/ Alas, cilantro.")

At least cilantro haters can point to a smidgen of scientific evidence that there is a genetic component to their dislike. A couple of years ago, behavioral neuroscientist Charles J. Wysocki (who works at the Monell Chemical Senses Center in Philadelphia) attended a twins festival in Ohio. He asked 41 pairs of identical twins and 12 pairs of fraternal twins to rate their reactions to the taste of cilantro, on a scale of plus 11 (yummy!) to minus 11 (gross!), with 0 indicating a neutral response.

More than 80% of the identical twins (who share 100% of their DNA) rated the taste of cilantro on a par with their siblings, compared to just 42% of the fraternal twins. A new study in the journal Flavour breaks this down into ethnic groups, finding that 21% of those with East Asian roots hate cilantro, compared to just 3% of those with Middle Eastern roots.

Wysocki thinks those who hate cilantro are reacting to its odor more than its flavor, and that the haters can't detect certain pleasing chemicals in the leaf -- instead, they just detect that soapy aspect. From a chemical compound perspective, cilantro is less complex than raw tomatoes.

According to food chemistry expert Harold McGee, there are around six substances that contribute to the telltale aroma of cilantro, mostly fat molecules known as aldehydes. And yes, similar aldehydes can also be found in soaps and lotions -- and bugs, which make use of aldehyde-drenched body fluids as either an attractant or a repellant. In contrast, Wysocki says that the fresh, flavorful, pleasantly herbal compound derives from dodecenal.

Wysocki conducted an experiment involving gas chromatography, a device that uses heat to separate the various molecules in something like cilantro, so subjects can take a whiff of each separate compound. People who like cilantro first detected the soapy scent, followed by the stronger citrus-y herbal scent we savor; but cilantro haters couldn't smell the latter. At all.

"The person who hates cilantro is, in fact, detecting the soapy odor. But what they seem to be missing is the nice, aromatic green component," Wysocki told MSNBC in 2011. "It's possible that they have a mutated or even an absent receptor gene for the receptor protein that would interact with the very pleasant smelling compound."

Broccoli and Brussels sprouts also have Facebook groups for the haters. And here there is even stronger evidence of a genetic link -- in this case, the ability (or lack thereof) to taste bitterness, or certain bitter chemical compounds. I share the ability to taste bitterness with roughly two-thirds of the population.

In 1931, a chemist named Arthur L. Fox accidentally released the powdered form of phenylthiocarbamide (PTC) in his lab. He didn’t notice anything unusual, but his lab mate sensed a bitter taste. Subsequent experiments confirmed that this variation existed in the broader population, and that not being able to taste bitterness was a recessive genetic trait.

Perhaps, as Faye Flam mused last year, it is a side effect of some other gene or genes that adapted so we’d be sensitive to the acrid bitter taste of dangerous toxins, although this is still in the realm of speculation. One could argue that those without the bitter taste ability should have died off long ago as they blissfully munched on toxic plants – mass extinction through poison. Yet people still inherit that recessive trait.

Those who can sense bitterness are probably responding to compounds called glucosinolates, present in most cruciferous vegetables, like broccoli, Brussels sprouts, and cauliflower. I happen to dislike all three, although not on the same visceral gag-worthy level as raw tomatoes. (I can, and have, eaten all in a pinch.) But on this front, at least, I can claim that part of that aversion lies in my genes.

About 25% of the population can’t taste propylthiouracil (PROP), a chemical that is similar to the bitter compounds found in cabbage, raw broccoli, coffee, tonic water, and dark beers. They are, in essence, “taste blind.” I do not fall into that 25%; I’m sensitive to this particular bitter taste, thanks to a gene called TAS2R38 that encodes taste receptors on the tongue.

I have the (GG) variant; that and the (CG) version both result in being able to taste bitterness, since (C) is the dominant allele. The (CC) allele is the taste-blind version, although even then, there is a 20% chance that one still might be able to sense some bitterness, depending on what other genes they inherit.

There are 25 “bitterness” genes known thus far; different bitter foods act through different receptors, and people can be high or low responders for one but not another. This is probably why I don’t mind grapefruit or tonic water, but balk at cruciferous vegetables.

Of course, genes are only half the story when it comes to, well, just about any trait, including taste preferences. No one gene could ever be responsible for whether we love or hate specific foods. There are so many different compounds related to flavor and aroma, not to mention texture (mouth feel), all of which contribute to how much we savor or loathe a food.

Furthermore, those tastes are subject to change. There are lots of foods I disliked as a child that I've come to tolerate, even savor. Raw tomatoes, though, still make me gag; that preference has withstood the test of time. Consider this a plea for everyone to be less quick to pass judgment on those who like things we loathe, or loathe things we love. It's not like you're going to shame anyone out of their dislike. Some of us just can't help it.

Images: (top) Via Mrs. Dull's Nourished Kitchen. (center) Thamizhpparithi Maari, Creative Commons/Wikimedia. (bottom) Eric Hunt, Creative Commons/Wikimedia.

References:

(Don't judge me! Behold teh SCIENCE!)

Bufe et al. (2005) “The molecular basis of individual differences in phenylthiocarbamide and propylthiouracil bitterness perception,” Current Biology 15:322-327.

Dewento, Veronica et al. (2002) "Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity," Journal of Agricultural and Food Chemistry 50(10), 3010-3014.

Drayna (2005) “Human taste genetics,” Annu Rev Genomics Hum Genet 6:217-35.

Duffy et al. (2004) “Bitter receptor gene (TAS2R38), 6-n-propylthiouracil (PROP) bitterness and alcohol intake,” Alcohol Clin Exp Res 28(11):1629-37.

Garcia, A.L. et al. (2008) "Long-term strict raw food diet is associated with favorable plasma beta-carotene and low plasma lycopene concentrations in Germans," British Journal of Nutrition 99(6), 1293-300.

Hayes, John et al. (2010) “Allelic Variation in TAS2R Bitter Receptor Genes Associates with Variation in Sensations from and Ingestive Behaviors toward Common Bitter Beverages in Adults,” Chemical Senses, 36(3), 311-319.

Mauer, Lilli and El-Soheny, Ahmed. (2012) "Prevalence of Cilantro (Coriandrum sativum) Disliking Among Different Ethnocultural Groups," Flavour 1(8).

Onsekiz, Canakkale. (2001) "The Chemistry of Fresh Tomato Flavor," Turkish Journal of Agricultural For. 25, 149-155.

Prodi et al. (2004) “Bitter taste study in a sardinian genetic isolate supports the association of phenylthiocarbamide sensitivity to the TAS2R38 bitter receptor gene,” Chemical Senses 29(8):397-702.

Tepper (1998) “6-n-propylthiouracil: a genetic marker for taste, with implications for food preference and dietary habits,” American Journal of Human Genetics 63:1271-1276.

Tieman, Denise et al. (2012) "The Chemical Interactions Underlying Tomato Flavor Preferences," Current Biology 22(1), May 24, 2012.

Unlu, Nuray Z. et al. (2007) "Lycopene from heat-induced cis-isomer-rich tomato sauce is more bioavailable than from all-trans-rich tomato sauce in human subjects," British Journal of Nutrition 98(1), 140-146.