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01.31.2009

The Science of Scent

How exactly it is that we smell (the verb, not the adjective) continues to be somewhat of a mystery, even despite the "advanced" state of modern biochemistry and neurology. The more research we do, the more we continue to be amazed at how sophisticated our equipment is for detecting and appreciating aromas. So sophisticated, in fact, that we've started moving away from chemistry and biology to explain it, and into the realm of quantum mechanics.

One of the world's leading scientists in the field of scent is a guy named Luca Turin. Dr. Turin is a biophysicist that got interested in the biomechanics of aroma perception many years ago, and has become one of the foremost world experts on aromas, in particular when it comes to the multi-billion dollar perfume industry, in whose employ he continues to do research. His most recent book The Secret of Scent: Adventures in Perfume and the Science of Smell has been on my reading list for months, ever since I heard about it.

Turin has a quite controversial theory about the mechanics of how we smell different things. It's fascinating, especially for anyone even with a passing interest in the idea of how it is that we can smell the things we do.

And thankfully, the folks at TED have put a video of him giving a talk about perfume that explains his theory up on the web:

The idea that our noses are all nano-scale spectrometers is pretty cool. But just think about the fact that these smart people are still working out the mechanics of smell, which is only PART of the mechanics of flavor perception.

It's going to be some time before the world figures out how some fermented grape juice can taste like chocolate and mint.

Thanks to Hector for sending me a link to the video!

Comments (12)

Arthur wrote:
02.01.09 at 10:20 AM

I read about this in a recent issue of Discover magazine. Interesting concepts.

Alfonso wrote:
02.01.09 at 1:07 PM

The Secret of Scent makes for fascinating reading.
Luca Turin is an amazing character.

Good 'on ya for the post, amico!

Sam Spencer wrote:
02.01.09 at 2:22 PM

Alder this topic is fascinating. I read an acount of Turin's nascent theory of olfcation and its actual mechanics in 2001 or 2002 and ever since I have told anyone who would listen that his is the most important new concept related to wine. The text doesn't touch on wine. The book is called The Emperor of Scent by Chandler Burr. Interestingly Turin has a wine project that would be worth runniog down for a review.

Weston wrote:
02.01.09 at 8:55 PM

that was cool reminds me of the Fat Duck Cookbook (science essays in the back) and the perfume guy hah that cookbook rocks

Don Clemens wrote:
02.02.09 at 9:00 AM

Alder: Thanks so much for posting this. Absolutely fascinating stuff!

Dylan wrote:
02.02.09 at 11:33 AM

Wow, this was very neat. The theory of vibrations leads me to think of the frequencies of molecules made audible. What if musical concert on a summer night in August was made up of the frequencies of all the molecules present in the area? What would nature "sound" like then?

Rajiv wrote:
02.02.09 at 3:14 PM

Luca's theory is certainly novel, but more concerned with identifying new aromatic compounds than analyzing compounds in a given substance, such as wine.

Using the cutting edge of bioseparations technology (developed largely for the now-crumbling big pharma industry) a number of researchers, notably Denis Dubourdieu in Bordeaux, have been working to link the smells of wine with specific compounds. While we are a long way from complete understanding, there have been some helpful results which (in my opinion) can enlighten the taster...

Brettanomyces:

4-Ethylphenol was linked to the "Bretty smell" produced by Brettanomyces bruxellensis. While other microbes are capable of producing 4-EP, only Brett was able to produce 4-EP in a red-wine environment (can't recall the source, sorry).

Varietal Aroma in Sauvignon Blanc:

Tominaga, Dubourdieu, et al. (1998) showed that certain components of Sauvignon Blanc varietal character can be amplified during fermentation, as the yeast metabolize certain precursors to flavor-active volatile thiols, in particular the instigators of box tree, broom, and passion fruit flavors. This confirms that there is no simple answer to the question "where do the flavors of wine come from?" since clearly fermentation is partly responsible for varietal flavor.

IBMP, varietal, and vineyard:

Isobutyl methoxypyrazine is a textbook example of a compound humans are extremely sensitive to (.015ppb threshold ) (Reineccius, Flavor Chemistry and Technology). Though originally identified in actual green bell pepper (Buttery et al. 1969), it has long been known to exist in wines, particularly sauvignon blanc and red Bdx varietals.

A paper by Dubourdieu et al. (2000) largely rehashed what tasters have known: IBMP's are found in unripe Bdx varietals, and are responsible for green bell pepper aromas. However I found several parts of the article interesting:

1) They established that in Bdx, merlot wines have lower IBMP concentrations than Cab and Cab franc (statistically significant). In fact, only a minority of merlot wines had IBMP concentrations above typical threshold values. They also established that this matches with the degree of perception by skilled tasters. In other words, they confirmed that the reason Merlot usually doesn't display green bell pepper tones is not because greater fruit masks their presence, but because the grapes physiologically produce less IBMP.

2) concentrations in the must and wine matched - in other words, IBMP can be considered mainly a function of the grapes, not the vinification.

3) in 1996 several plots of vines on different soils were compared (cab and merlot). At the end of veraison, grapes from sandy-silt soil displayed significantly more IBMP than grapes grown on gravel. in addition, IBMP degradation proceeded gradually on sandy-silt soil, but had a quick dip in gravel soil.

4) they showed a high correlation between malic acid content (used as a metric for ripening) and IBMP content (R^2 values from .9 to .99). In other words, malic acid and IBMP drop together during ripening.

02.02.09 at 3:58 PM

As I sit here with a cold & can't smell anything, Turin's theory makes great sense. There is no way a plugged up nose can vibrate. I mean this seriously. It is a simple comment, but it does make me wonder if my lack of smell is as simple as that.

Arthur wrote:
02.02.09 at 4:07 PM

Laurie

Medical data estimates the incidence of true anosmia and hyposmia at 1%-2%.

The remainder of cases where people have diminished acuity of smell is usually attributable to either environmental factors (smoke filled house, excessive use scented products) and to other medical causes like congestion due to allergies or chronic rhinitis or sinusitis.

Arthur wrote:
02.02.09 at 5:37 PM

By my last comment I intended to validate your suspicions, Laurie.

Rajiv wrote:
02.03.09 at 5:32 PM

Dylan and Laurie -

Concert "A" on the piano is 440 Hertz, or 440 cycles per second (cps). Even the highest notes on the piano do not exceed a few thousand cps, and the upper limit of human hearing is around 2x10^4 cps. By contrast, molecular vibrations occur on the order of 10^13 cps, so we are talking about totally different length scales here.

In order to smell molecules in the air, these molecules have to first dissolve in your mucous nasal membranes, then diffuse to the G-protein receptors. A build-up of mucous, or excessive dryness, or indeed anything that interferes with your nasal epithelium will physically prevent the aroma molecules from reaching your scent sensors, regardless of the precise mechanism by which they work.

On the other hand, mucous buildup is irrelevant to dampening of vibrations on the molecular scale.

Hope this helps
-Rajiv

Kathleen wrote:
02.05.09 at 8:34 AM

Thanks for blogging about this. Interesting stuff!

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