Smells Like Satiety

The NYT Well Blog recently covered a fascinating study out of TUM* in Munich, Germany: olive oil may cause you to feel satiated faster and eat less food. Apparently, study volunteers fed yogurt with olive oil mixed in showed a spike in serotonin levels, and ate about 175 calories less each day than experimental groups given different fats (rapeseed, butterfat, or lard).

But was it the fat, or was it the flavor? The researchers isolated aroma compounds from the olive oil, specifically hexanal and 2E-hexenal, and mixed them in with yogurt. Same effects! In the press release, Prof. Peter Schieberle comments "Our findings show that aroma is capable of regulating satiety."

Smells rule neurochemistry. It wasn't that long ago, really, that Axel and Buck won the 
Nobel Prize for their research into olfactory organization. Two years later, another unsolved mystery, that of "blood smell," came to light in ACIEE. Turns out, both blood smell and the new research on olive oil aroma depend partially on medium-chain aldehydes (hexanal-decanal) and unsaturated ketones. You can even smell ketosis, an alternative metabolism found in diabetic crises and Atkins dieters, by the tangy, astringent odor on the breath. 

Amazing how sensitive the human nose is for volatile carbonyls! Perhaps this harks back to a rather strange notion of odotopes, or "weak-shape" theory. This suggests that, rather than the "lock-and-key" model observed for enzymatic binding, smell may result from a collection of weak interactions with multiple receptors, based predominantly on molecular shape. 

*Bonus note: I knew I recognized the TUM group from somewhere! Remember this abstract, from J. Agric. Food Chem. last year? Based on my (single, not to be repeated) experience with durian fruit candy, I do wonder how they convinced this poor scientist to be the GC smell-port 'volunteer.'

You couldn't pay me enough.
Source: Steinhaus, TUM | J. Agric. Food Chem 2012

Super Tasters and Smells in Space

Back in Junior High School¹, my first chemistry teacher ran a fascinating experiment. He dabbed a small amount of dilute capsaicin on the back of a piece of bread, and handed it to each class member. Of course, most of us felt the burn, but not one girl - she was genetically insensitive to capsaicin’s painful effects.

The science of taste and smell fascinate me, since they not only underlie many of our daily chemical interactions, but provide clear survival benefit to the species. The ability to smell smoke or taste bitter alkaloids might allow you to detect fires or avoid poisonous foods. In 2006, Glindemann and coworkers reported the source of the “iron smell,” the dark, musky odor produced when you touch coins or metal handles. His group traced the smell back not to compounds in the metal itself, but to chemical changes of body oils spontaneously produced upon contact (one of the stronger odorants, 1-octen-3-one, shown at left). The chemists theorize that the ability to smell these metallic odors traces far back in our evolutionary history, to when “blood smell” helped early human hunters track their prey through the woods.

Well, if smells are so critical, pity the poor astronauts. Space missions show that after a few days in weightless conditions, astronauts can no longer smell or taste their food properly, and begin to crave hot spices and bold flavors. NPR reporter Joe Palca explored this condition, called “stuffy-nose effect” or the “Charlie Brown syndrome” (due to the enlarged appearance of your head in space when your facial tissues swell with fluid, an effect of lessened gravity).

Charlie Brown and Snoopy
Source: The Telegraph | Charles M. Schultz

Want to help test the phenomenon? A joint Cornell /U.Hawaii study is now recruiting applicants. You’ll live in a simulated Mars mission habitat in Hawaii, where researchers will test food preparation with limited ingredients. They’ll induce the “stuffy-nose effect” with specialized beds that keep the head slightly lower than the rest of the body, and test various spices and food replacements for space palatability.

Maybe these scientists shouldn’t worry about smell deprivation, and just recruit a bunch of supertasters. These genetically gifted sense superstars possess more taste buds (papillae) on the tongue surface than others, which can be observed by simply staining the tongue blue and looking in a mirror. Supertasters report heightened sensitivity to subtle changes in food or drink, and naturally cluster in occupations such as chefs or sommeliers.

Count the taste buds!
Source: NPR | Maggie Starbard

The physiological prowess of supertasters was first noted in back-to-back PNAS papers in the 1930s. Amazingly, the trend started when two chemists – Drs. Fox and Noller, of DuPont – exhibited markedly different reactions to the taste of phenyl thiocarbamide dust in the air. The two scientists then prepared several analogues of the parent thiocarbamide, and ran around asking people to taste them!

The second paper, from geneticist Albert Blakeslee at the Carnegie Institution in Cold Spring Harbor, tested the effect of dilution on the taste sensations of a few hundred people of different racial and ethnic backgrounds. On the menu, in addition to the DuPont compound, were hydrochloric acid, picric acid, salt, and saccharine. Blakeslee found that there were distinct groups of people who could taste at certain thresholds, the “tasters” and the “non-tasters.” Further research, as explained by food scientist John Hayes in a broadcast of the WNPR Colin McEnroe show a few nights back, has shown that humans actually fit into three distinct categories: “non-tasters,” “medium-tasters,” and “supertasters.” The updated test involves placing a small crystal of propylthiouracil (PTU, see above) on the tongue and recording the intensity of the each person’s reaction; supertasters recoil at the bitterness, while “non-tasters” barely notice a difference.

1.No, it wasn’t ‘middle school’ back then, and yes, I walked uphill both ways, in the snow, carrying my books.