Christmas chemistry

The Physics of Christmas is a very good book by Roger Highland. But there’s more science to Christmas than just physics; there’s chemistry, too.

Consider those popular Christmas spices, ginger and cinnamon.

Ginger has been used for thousands of years. Greek bakers were making gingerbread more than four thousand years ago (though probably not in the shape of little men). Ginger’s name comes from a Sanskrit word meaning horn-shaped (probably referring to the shape of the root), but its distinctive flavor comes from chemistry, from aromatic compounds called–what else?–gingerols.

Cinnamon also derives its flavor from a chemical compound, cinnamic aldehyde. True cinnamon, the bark of a tropical evergreen tree native to Sri Lanka, is one of the oldest known spices–it’s mentioned in the Bible. But most of the cinnamon we buy in North America isn’t really cinnamon at all; it’s actually cassia, which is darker and more strongly flavored, and comes from the bark of a related tree, native to Burma.

One section of The Physics of Christmas deals with the thermodynamics of turkeys. But when it comes to the chemistry of Christmas, other traditional foods are of much more interest.

For Scandinavian people, for example, one traditional Christmas food is lutefisk, made by soaking dried fish in lye for a week, then in water for two more weeks, before finally stewing and serving it.

Lye, a strong base (the opposite of an acid), dissolves protein. A kilogram of dried fish contains about 750 grams of protein; after the lutfisk process, it weighs six kilograms, but has only 350 grams of protein, which is why it’s much softer. The process also creates new compounds, including one that causes reversible kidney damage in rats (which are highly susceptible to it)–so don’t invite any rats to Christmas.

While chemistry is common in Christmas kitchens, it can be found in other rooms of the house, too. Take your Christmas tree, for instance. If it’s an artificial tree, it’s a product of chemistry; if it’s a real tree, its lovely scent is a byproduct of chemistry. Trees of all kinds are complex chemical factories, and the compounds generated by evergreen trees–which, among other things, can be turned into pine oil, turpentine and resin–happen to smell lovely.

Those same compounds make Christmas trees highly flammable, which is why we no longer decorate them with candles. Nevertheless, candles are still very much a part of Christmas–and very interesting, chemically.

Modern candles are generally made of a mixture of hard paraffin and stearin, a substance that raises the melting point of the candle a few degrees and thus prolongs burning time.

Candle wicks are chemically treated so they don’t burn too fast, and plaited flat so the wick will bend as it is charred, which causes it to lean out of the existing flame. The oxygen it finds outside the flame causes it to burn away, and the chemicals its treated with ensure that no ash remains. (If the wick doesn’t lean out of the flame it gets too long, which introduces more fuel into the flame than it can burn. The excess fuel, tiny particles of carbon, is released as smoke.)

When a candle is lit, heat from the flame melts the wax. The liquid wax is pulled up inside the wick by capillary action–the same process by which water gets up into plants–and burned along with the wick.

The candle flame itself is made up of hot, glowing gas. Just above the burning wick there’s a dark cone, topped by a luminous yellow region; at the bottom of the flame there’s a light blue zone (the color comes from the excited molecules of certain compounds). The dark cone is relatively cool, at only 600 degrees Celsius; the edge of the yellow region is the hottest, at 1,400 degrees. The light of the yellow region comes from tiny particles of soot which form at the top of the dark cone and cluster together, then are heated by the gases around them until they glow brightly.

A candle isn’t a terribly efficient light source, since less than half of one percent of the energy it puts out is turned into light (the rest is emitted as heat).

But then, Christmas isn’t about efficiency. Instead, it’s about beauty, good smells, good food, and good friends. Chemistry provides us with the first three; the latter we must cultivate ourselves.

Merry Christmas!

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