Category Archives: smell of the week

The chirality of smell



As I prepare the first half of my Science of Smell online class, I am having fun looking for various examples of all things biomolecular, biochemical, and genetic related to olfaction. If I were a taste and flavour chemist or a molecular gastronomist, I’d probably be interested in somehow exploiting the chirality of biomolecules in food and drinks!

Chirality refers to the non-symmetrical nature of some molecules. Non-symmetrical molecules are like our right and left hands: they appear the same in reverse but you cannot superimpose the image of one over the other in the mirror nor can a left-handed glove fit a right hand. In the pharmaceutical industry, chirality is very important because the enantiomer of a biomolecule that produces a positive outcome (like reducing morning sickness using thalidomide or hyperactivity using Ritalin) may cause a harmful effect (birth defects) or no effect. The handedness is determined by the stereocenter of a molecular. Those with ‘right’ handed stereocenters are ‘R’ or + enantiomers and those with ‘left’ handed stereocenters are ‘S’ or – enantiomers (the S comes from sinister, Latin for left).

Our olfactory receptors are clever things: they can tell the difference between the right and left hands! The two most common examples are of carvone and limonene.  The R-carvone/(-) carvone is recognized as mint and its enantiomer as carraway. R-limonene/(+) limonene smells ‘orange’ and its enantiomer is lemon (see image to the right). So next time you smell oranges and lemons at the same time, recognize the power of your nose to be ambidextrous by distinguishing between the two biomolecules!

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Black diamonds

The genome for the Périgord truffle was published in 2010. Considering that these special truffles go for 1000-2000 ($1300-2700) an ounce, the genome has been under-exploited by culinary scientists and molecular gastronomists…until recently when specialists in bioinformatics and proteomics got together to mine the secrets of Brillat-Savarin‘s “diamond of the kitchen”. The resulting paper, released today in the Journal of Proteome Research, seeks to unlock the “puzzling biology of the Black Périgord truffle Tuber melanosporum“.

As ectomycorrhizal fungi, truffles exist in symbiotic relationships with tree roots (black truffles like both oak and hazelnuts). Cultivation first occurred successfully in France when phylloxera destroyed viticulture in France (temporarily) and made available huge tracts of land for the creation of truffle groves. Much of that knowledge was lost in the wars that followed and the trees eventually reached the end of their truffle-producing life cycle. Attempts to start again today are rife with price control politics.

I was amazed to find that I LOVED LOVED LOVED truffles because I dislike mushrooms (something about their texture turns me off). I had truffles for the first time in a town called Stenay in Lorraine region of France. They were tender and aromatic and meaty without being meat. There wasn’t anything that I had eaten that compared to them and they filled a distinct niche that nothing else had in my vegetarian diet.

The smell. The lovely smell of good truffles–so good that Peter Mayle describes a truffle breakfast eaten with one’s head under the napkin to better capture the aroma. Amazingly, the new article reports that only nine enzymes are responsible for creating over 90 different odorants associated with the unique aroma of the black diamonds of Périgord! The authors note:

It is hardly surprising that such low numbers of enzymes were shown to be involved in the production of over 90 volatiles considering over 70% of the proteome is yet to be annotated. The potential to discover novel enzymes that could be of economic, medicinal, or other uses remains a tantalizing possibility.

Attempts have been made to capture this aroma to cheaply produce ‘truffled’ food. There is even a truffle-scented vodkaThe food industry uses two  chemicals (DMS and 2-methyl-1-butanol) predominant in the odor profile in order to produce a truffle flavour. A chemical called 2,4-dithiapentane is used to simulate the aroma in olive oils. Both DMS and 2,4-dithiapentane are derived from methanethiol, which is the key odor produced in human urine after metabolism of asparagus. The ability to detect this compound has a genetic component. I wonder if this influences one’s preference for truffles…something I am exploring in my on-going research on food preference, odors, and genes! 

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Smell of the week: Clove

In a continuing series on the spices of mince pies, cloves (Syzygium aromaticum) pick up where we left off last week with the genus Myrtaceae, from which we get our nutmeg and mace. Cloves are also native to Indonesia (the Maluku islands to be specific.

Clove oil can come from the leaves, the stem, or buds. This results in tremendous variability across the type of oil, variation in the spice (time of harvest etc), and extraction methods. But, like nutmeg and mace, eguenol ranks high on the volatile list. Since the buds are most commonly used in cooking and baking (and what we use in mince pies), we’ll focus on them. In addition to eugenol, other significant volatiles include eugenyl acetate and beta caryophyllene. Methylamlketone, methylsalicylate, alpha and beta humulene, benzaldehyde, beta ylangene, and chavicol are found in smaller amounts but impart the pleasant aroma of cloves to the nose.

Cloves are actually quite nutritious and provide manganese, dietary fiber, vitamin C,  omega-3 fatty acids, calcium, and magnesium. Cloves have health benefits from the eugenol as anti-toxin,anti-inflammatory, and mild anti-baterial agent (I love my clove and fennel toothpaste and foot deodorizer!). As with the other spices, cloves are best when ground fresh as the powdered form lose flavour fast. When pressure is applied to the clove bud, a small amount of oil is released if the clove is fresh. Another freshness test is floating the clove in water–the oil provides buoyancy. However you use them, keep them dark and dry and airtight and enjoy!

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Smell of the week: nutmeg

In a continuing short series on mince pies, today’s spice is nutmeg. Nutmeg and it’s aromatically ‘lighter’ sister mace both come from trees in the genus Myristica. On the left is nutmeg in fresh form: the seed is nutmeg and the aril is mace. Myristica fragrans, the most common source, is an Indonesian flowering evergreens that is sometimes classified with magnolias. These trees take several years to mature (7-9) but have a long period of productivity–up to 90 years.

Nutmeg has a storied economic history. Romans travelled to the spice islands to trade for nutmeg and other spices. Arabs took over this spice trade in the 6th century and, at one point, nutmeg was used for warding off the plague. Nutmeg even has an illicit history as a cheap high during a drug drought–it was Nostredamous’ drug of choice!

Nutmeg is used ground in baking (though fresh grated is best), as an essential oil, and as nut butter. The oil is the most commonly used variety in the taste and fragrance industries (including Coca Cola). Its major compounds are: 50% sabinene OR camphene (and derivatives), 20% d-Penine, and the remaining 30% a mix of chemicals including terpenes, esters,  myristin (the purported hallcinogenic agent) and eugenol. Sabinene confers the spicy quality to nutmeg and camphene has tremendous industrial value (for itself and as a precursor to other chemicals). D-pinene can be used to manufacture camphor (a previous Smell of the Week). Eugenol, though found in smaller quantities in nutmeg, is one of the major volatile components of the seed and is used for making vanillin and also as a substitute for cloves (next week’s smell!) in perfume-making.

Ground nutmeg loses flavour rapidly. So, when cooking or baking with nutmeg (as with cinnamon) always grate from the seed, which can be kept indefinitely.

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Smell of the Week: Cinnamon

Now that I have a ticket out of Alaska with an upcoming mid-December departure, I am thinking about holiday baking. In particular, I am thinking about my favorite holiday goodie–mince pies. So, the next few posts will feature the spices that I use in making those little pies of perfection. Today is cinnamon, not a favorite of mine because many bakers are heavy-handed on this fascinating spice but, when used lightly, cinnamon is a wonderful flavour-enhancer. Both sweet and savoury at the same time, I use it  in Mexican and Indian food as well as baking.

Cinnamon comes from the bark of many Cinnamomum trees from SE Asia. Chinese cinnamon or cassia (Cinnamomum aromaticum) is often sold as cinnamon (in the US) but ceylon cinnamon (Cinnamomum verum) is the true cinnamon. Cassia has a rougher bark (as opposed to the layered bark of true cinnamon–see above left for cinnamon and right for cassia bark). Cassia is often preferred in curries.

The key volatile  in the essential oil is eugenol and in the bark, cinnamaldehyde. Of these, cinnamaldehyde is the odor compound we associate most with cinnamon. Eugenol is the clove-like odor component: eugenol is named after the clove genus, Eugenia. In addition to fragrance, eugenol may be used for antiseptic purposes.

Another compound found in cinnamon (among a few other plants) is coumarin, which is moderately toxic to humans and very toxic to our (primates) close evolutionary relatives, rodents. Coumarin imparts a fresh grassy odor to fragrances. High concentrations of coumarin impart a bitter taste in plants that deter foragers. Interestingly, humans are sensitive to bitter taste, an evolutionary adaptation to avoid plants such as these that may be toxic. In the case of coumarin, the liver and kidneys are affected, but only in high doses.

Not only are there many trees that may produce cinnamon, synthetic versions are be made with powdered beechnut husk laced with cinnamaldehyde. So as we approach the holidays, bakers and chefs beware that you buy the true cinnamon (or cassia depending on your culinary needs) to grind yourself and not a cheap substitute!

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Smell of the week: peppermint (Mentha piperita)

With cold winter days and waning daylight, I love flavouring my soy latte with peppermint from time to time or having a hot chocolate laced with mint schnapps by the evening fire. This is an odd choice in the winter since the menthol in peppermint provokes a ‘cool’ feeling via cold-sensitive receptors in the skin (much the same way chili peppers provoke a ‘hot’ feeling without changes in temperature).

Indigenous to Europe, peppermint is actually a hybrid of Mentha aquatica andMentha spicata (spearmint). As a hybrid, it is sterile and spreads via it’s rhizomes (budding new plants off the roots). Medicinally, peppermint is used as an anti-spasmodic (digestive distress) and/or an anti-nausea (when inhaled). Menthol from mint plants is commonly used in cold medicines (and cigarettes) for soothing sore throats due to its anti-irritant properties. The menthone in peppermint is a natural pesticide (mainly for honey bees).

Major odor compounds are menthol (a primary compound), menthone, methyl acetate, menthofuran, and trace amounts of eucalyptol. Isolated menthol looks just like it smells–like icy shards (see left in the bowl).  Menthol comes from any member of the mint family but the one most commonly used for extraction is Mentha arvensis. Non-natural sources of menthol supplement those from the mint family of plants. Derived from menthol, menthyl esters are used in perfumery to improve, enhance, or modify fragrances. Peppermint oil odor can also be used to detect plumbing leaks!

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Smell of the week: Coffee

I infrequently drink more than one cup of coffee a day (and always black)–usually because my enjoyment increasingly diminishes to the point that I don’t finish my cup. I have always perceived my coffee to be primarily an olfactory experience with secondary pleasures directed towards either the warmth or the flavour complement to some other food.  And, since the olfactory system becomes desensitized to most odors (excluding ones that serve as warnings), my lessening enjoyment makes sense. The exception is my weekend morning cup which I usually do finish.

A study published in the Journal of Agriculture and Food Chemistry found that the volatiles in coffee berries themselves (the study used Coffea arabica, or Columbia) are dominated by high levels of ethanol, except overripe berries which are dominated by esters (as might be expected with any overripe fruit). Interestingly, the largest number of chemical compounds were detected in the overripe berries.

Another study published in 1960 on the volatiles of roasted and ground coffee found 158 compounds. When added to previously identified volatiles, the number jumped to 318, suggesting that coffee aroma is too complex to be fully captured by a few key odors. Half a century later, the problem of finding key volatiles persists. Two studies published in 2010 (in Chemometrics and Intelligent Laboratory Systems and in Analytical Chemistry) noted that the trouble is the high number of volatile peaks that occur when analyzing the compounds–roughly 300 volatiles in green beans to over 1000 in roasted beans. compiled data for what might be the strongest contribuuters to aroma, noting that furans have a dominant role and contribute the caramel-like note in many coffees.

“A coffee’s aroma will vary as a function of changes in soil, microclimate, altitude, types and species of bean used, the roasting process, and the preparation of the coffee. These various conditions affect the concentration and composition of the various aroma volatiles, which include carboxylic acids, alcohols, aldehydes, alkanes, alkenes, aromatics, esters, furans, ketones, lactones, oxazoles, phenols, pyridines, pyrazines, pyrroles, thiazoles, and thiophenes.”–from the 2010 Analytical Chemistry publication above by Susslick

Such a complex odor that creates a person-specific experience…however and whenever you like your coffee, take a moment to appreciate it’s complex nature that has stymied normal methods of detecting and synthetically creating odors.

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