Friday, December 19, 2014

The Chemistry of Christmas

What are the sights, the sounds, the smells, and the textures that you associate with Christmas? Perhaps it is Christmas trees with their lovely green shape, color and wonderful pine smell. Maybe it’s the smells of cooking, the savory smells of turkey or the sweet smell of warm cookies. Or what about all of the cozy feelings you get with big sweaters or a roaring fire? Did you know that there is a lot of chemistry that goes into all of the senses we associate with this holiday?

I was browsing through holiday-related articles, looking for something different from the usual psychology or sociology centered holiday study. That’s when I came across an article from 2012 published in the Journal of Chemical Education about the five senses of Christmas chemistry. The authors look through the lens of organic chemistry and take five “Christmas compounds” to examine in the context of the five senses.

Sound: Silver Fulminate

English CrackersIn 1824, Justus Liebig correctly determined the molecular formula of silver fulminate (AgCNO), and, around the same time, Friedrich Wohler identified the molecular formula for silver cyanate (AgOCN). Now, these might look the same, but they are in fact very different. Soon after, these scientists collaborated and rationalized these differences by introducing the notion of isomerism – molecules that have the same kinds of atoms but because these atoms have different arrangements in shape they differ in their chemical and physical properties. Silver fulminate is highly unstable, and even a small amount of friction leads to its violent decomposition. That makes it very useful when you want to make things go boom.

Christmas crackers are items that are a more traditional favorite in the UK. The Christmas crackers used today are short cardboard tubes wrapped in colorful paper. When they are pulled – bang! – out comes a colorful hat (usually looking like a crown), a small toy or a joke. The sound is made from the rapid breakdown of silver fulminate present in small quantities in the paper. Two thin strips of cardboard are glued together, one containing silver fulminate and the other a rough surface. When the cracker is pulled, the surfaces rub together to produce friction and facilitate the reaction. The compound goes through a redox reaction followed by a release of nitrogen gas and carbon monoxide. This sudden production of gases is what produces the distinctive popping sounds.

Sight: α-pinene

Different varieties of real Christmas treesA beautifully lit and decorated Christmas tree is one of the most common sights of this holiday. For the purposes of this section, we’ll assume that you bought a real tree rather than an artificial one. Geographic region can play a big part in the species of tree that is available to you, but they are very likely all evergreens (fir, spruce, pine, and cedars). In their resin, these conifers release terpene hydrocarbons, specifically monoterpenoids that is composed of two isoprene building blocks. This word should sound familiar as it is a derivative of turpentine, which you may know for its distinctive pine scent. Pine oil contains two monoterpenoids, α-pinene and β-pinene, which are both liquid at room temperature. This is another case of isomerism; although both have the typical C10H16 molecular formula there are four stereoisomers of each where the bonded atoms differing in their 3-D orientations. α-pinene is one of the most common volatiles in nature and is directly linked to the Christmas tree’s smell.

Touch: Sodium Acetate 

I don’t live in what most people would term a cold climate. Sure, we get cold weather, but we’re not talking blizzards. However, in my days as a field ecologist I spent many a winter day outside taking measurements. On those days, I was ever-so-grateful for one little invention: the hand warmer. Squish around the contents of the packet to get it to heat up to keep your pockets, and hands, toasty all day long. Bliss.

Many hand warmers are based on a simple chemical reaction – the crystallization of a supersaturated sodium acetate solution. When you squish around the contents of the hand warmer, you are triggering a chemical process. A nucleation site, usually a metal disk with small seed crystals, causes rapid crystallization of the super saturated solution. This is a highly exothermic process that releases energy to its surroundings as heat. These types of hand warmers are often reusable because of their physical mode of action. Other hand warmers rely on the exothermic oxidation of iron when exposed to air. Activated charcoal is used to catalyze the reaction, along with vermiculite and salt as additives. However, this chemical mode of action means that these are one use only products.

Taste: Tryptophan

/If you are a fan of a big turkey dinner then you have probably heard of tryptophan. It is a common misconception that the tryptophan from your turkey binge brings on sleepiness. It’s true that tryptophan is involved in sleep and mood control. However, turkey doesn’t contain enough of the compound (only 350 mg per 115 g) to have a soporific effect. It’s more likely that the sheer volume of food that you eat (and probably the wine you drank with it) on these occasions decreases blood flow and oxygenation, inducing the drowsiness.

Tryptophan is an essential aromatic amino acid that is commonly found in proteins. This compound has two enantiomers, chiral molecules that are mirror images of one another (kinda like left and right hands). L-tryptophan exists in nature and has a pronounced bitter taste, while D-tryptophan is synthetic and has a very sweet taste. Once you consume tryptophan, it goes through a series of metabolic reactions, one of which ends with melatonin. This final product is a neurohormone that is naturally secreted by the pineal gland, is involved in regulating circadian rhythms and may also have strong antioxidant effects.

Smell: Gingerol

Gingersnap cookies and gingerbread houses are common sights around the holidays, and with them come their wonderful ginger scent. Ginger products usually contain fresh or powdered bits from the rhizomes (rootstalk, or modified underground stem) of the ginger plant (Zingiber officinale). One of the organic compounds produced by this plant is gingerol, an aromatic vanilloid compound containing a β-hydroxyketone functionality. Interestingly, the taste of this compound can be modified via laboratory synthesis. Shogaol is derived by either refluxing gingerol with concentrated sulfuric acid or allowing a dehydration reaction can occur on gingerol to give the aldol condensation product. Shogaol has a more pungent flavor than gingerol. Conversely, ginergol can completely break down into zingerone after refluxing in strong aqueous base. Zingerone is considered to have less pungency than gingerol.

Considering this chemistry, you can alter the flavor of the ginger you use in your cooking. For example, if you cook ginger extensively, particularly in the absence of acid, then you produce the mildest tasting vanilloids, zingerone. But also keep in mind that your kitchen conditions are not laboratory conditions. This means that you are likely to end up with a mixture of all three compounds in your cookies.

That’s all for our journey through the senses of Christmas. If you are a chemistry teacher, or simply a chemistry nut, then I recommend reading through the paper. It has all sorts of skeletal formulas that you’ll love.

ResearchBlogging.orgJackson, D., & Dicks, A. (2012). The Five Senses of Christmas Chemistry Journal of Chemical Education, 89 (10), 1267-1273 DOI: 10.1021/ed300231z

p.s. In the interest of equal opportunity holiday science, does anyone recommend a study on Hanukka, Kwanzaa, etc. My academic database search is coming up with nil.

(images via Science NotesOld English Crackers, Buzzle, ThinkGeek, NYTimes Blogs, spydersen, respectively)
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