Some people like it on their bread, others in their tea, cake, or even cocktail. Honey is not only versatile, it is delicious. Except to me; I’m not a huge fan of the taste of pure honey. But put it in a cocktail and I am in! Yet even if honey is a total showstopper for you, there is something here for you too. Because honey is one of these innocuous everyday items which hide an utterly insane array of scientific curiosities. Just like the non-Newtonian fluid made from cornstarch (called Oobleck) or the thixotropic properties of ketchup (it becoming less viscous when force is applied), honey has hidden depths. Who knew that honey is actually a supercooled liquid for instance? Well, you will, in a few minutes.
Honey, in most cases, is basically nectar which was regurgitated and partially digested by honey bees (yet for some obscure reason we deem eating the bees to be more gross than eating the honey). I say “in most cases” because there are examples of wasps producing honey, specifically the Mexican honey wasp, Brachygastra mellifica. Which sounds terrifying to me. Hell, there is even the stranger-than-fiction honeypot ant which uses its body as a living storage unit for honey. Luckily, there is also the wondrous stingless bee, mostly present in Australia and Brazil, which forms smaller hives but still rocks in terms of honey productivity. This one is even more desirable if you uncover the suppressed and horrifying truth that most bee species actually don’t die when they sting you. In fact, even for a bee to make honey is exquisitely rare. Earth harbors around 20,000 bee species, yet only a small fraction of these species are known to produce honey.
I also say “in most cases” because honey doesn’t always stem from nectar. Sometimes it is produced from the sweet excretions of aphids (the slave insects of ants) which is potentially even more gross but euphemistically called “honeydew honey.” But hey, it does have a stronger flavor than regular honey so who’s to complain. After ingesting their sweet payload, bees make their arduous way back to the hive. Well, after digesting some of the nectar before they reach home sweet home. Because, you see, collecting the material for honey is a tough and demanding business. For 1 kg of honey, more than four million flowers need to be harvested. Which, incidentally, means a trip around the world for the bees. Twice. The average bee, busy as the cliché purports, can take care of up to 100 flowers per trip. And they do about 12 trips per day. Yet still, at the end of their six-week-life, an average honey bee can only look back on a paltry 0.8 g of honey. Even worse, bees need about 9 kg of honey for one kilogram of beeswax. At least bees are efficient, getting 29 kcal out of every kcal invested in gathering nectar for honey. Humans, monsters that we are, consume close to an entire kilogram of honey per year per person. And that despite the fact that, per teaspoon, honey contains more calories than pure sugar (21 vs. 16). Though, to be fair, we do produce about 100 times more sugar than honey.
But back to the bees. The “honey” that was digested and brought back from our busy foragers still contains far too much water to be useful for the colony. In this form, natural yeast will start fermenting all the delicious sugars in no time. So what do you do to get rid of excess moisture? You crank up the heat and provide efficient air circulation! That’s exactly what bees do: they flap their wings & generate body heat to bring the water content of honey down to about 17%. Now it truly is long-lasting, in the range of thousands of years (potentially edible honey was found in ancient Egyptian tombs!). Though only if the honey container is closed, as honey is hygroscopic and will grab all the air moisture it can get. Once its water content surpasses ~25%, the merry dance of fermentation can commence.
Seen on a colony-level, honey usually originates from an amalgam of different nectars. Whatever flowering plants are in reach for a hive will be harvested. Yet on a bee-level, the flow of amber liquid stems from a single source. Bees are flower-monogamous. Clever humans have, long ago, combined these two factoids and placed hives in areas in which all surrounding nectar comes from a single source. By that, the resulting honey will be pure. And there is a wide array of different options for producing single-origin honey. Buckwheat honey, eucalyptus honey, chestnut honey, you name it. Of course the origin of a pot of honey will determine its flavor profile, including its roughly 100 volatile organic molecules. The bland & ubiquitous vanilla of honeys is clover honey, whereas eucalyptus honey makes a name for itself with a hint of a menthol-like taste, and orange blossom honey is as citrusy as its name suggests. And then there’s buckwheat honey: dark, full of protein, characterized by a malty flavor thanks to the presence of methylbutanal.
The provenance of a type of honey not only greatly influences its taste but also its properties as a physical substance. Here are some things about honey that no one in their right mind should know: honey is technically a supercooled liquid at room temperature, just waiting for nuclei/particles to crystallize around. And it stays a liquid for an impressively long time. Even at -20°C (a.k.a. your freezer), honey may appear to be solid while actually flowing, just with an incredibly high viscosity. You really have to crank up the cool, all the way to -50°C, to coax honey into its glass transition, in which it forms an amorphous solid.
At room temperature however, the two main sugars in honey, glucose and fructose, work together to form this viscous amalgam of a fructose solution with precipitated glucose. That means, the higher the glucose content the more precipitated material in your honey. Rapeseed honey, exceptionally skewed in its glucose/fructose ratio, thus is considerably more likely to crystallize than, say, chestnut honey. Now is the time to let you in on the best-kept secret of honey. Ready to flip some tables? Here we go: While it may appear that honey crystallizes in your pantry when you don’t use it for a while, honey crystals in fact form when you stir the honey. Yes, you were the villain all along. Granted, these crystals can grow in size after forming during their stay in your pantry. Their optimal growth temperature is around 15°C, so if you really want to avoid crystal growth, put your honey into the fridge, at 4°C, where crystal growth subsides. If it’s already too late however, and you stirred your honey into a crystalline grave, you can always melt the crystals. Above 50°C, all honey crystals will disintegrate and it can flow freely yet again. Not only is the viscosity of honey greatly increased by warming, above 50°C the water content of honey does not even impact its viscosity anymore.
Speaking of the flow of honey, I want to briefly come back to Oobleck & ketchup. While most types of honey are righteous, upstanding Newtonian liquids, heather- and manuka-derived honey are tempted by the dark side and exhibit thixotropic properties. Like ketchup, their viscosity decreases when agitated. The weirdness of honey certainly doesn’t end here. For one, honey is electrically conductive. Though infinitely more interesting, honey ages & matures. The Maillard reaction, of browning fame, is doing its amino acid & sugar-combining thing even at room temperature. Honey, composed of both protein as well as sugar, visibly darkens after a few months as the Maillard reaction schleps itself forward. Of course, this process can be dramatically accelerated by an increase in temperature. But be careful as honey has a poor thermal conductivity for a liquid, so applying heat can actually result in localized caramelization as the heat is not properly distributed among the whole batch of honey. Not that that’s necessarily a bad thing though.
There is (at least) one more curious property of honey that most people would never have guessed: honey is acidic. With an average pH value of 3.9, the sickly sweet liquid is more acidic than tomato juice for instance. So acidic in fact that it would be a bad idea to store honey in metal containers for a prolonged time, as it would corrode the metal. A silver lining of this acidity is that it constitutes one of the many ways honey exerts its antimicrobial effects, by preventing the growth of microorganisms. The most intuitive antimicrobial mechanism of honey is its high osmolarity because of the high concentration of solutes, the reason for its hygroscopicity, drawing all available water from potential microbial tenants. Yet even when diluted with water, honey has been shown to be more effective against microbes than simple sugar water. Indeed, honey does have other microbe-defeating arrows in its quiver. One of them would be bleach. Or, better put, hydrogen peroxide, the molecule at the center of bleach. Yes, your breakfast spread basically contains bleach. An enzyme in the stomach of bees, glucose oxidase, transforms glucose into gluconic acid (which incidentally leads to the acidity in honey) and hydrogen peroxide, doubly protecting the liquid gold in a bee hive. Interestingly, not all types of honey contain hydrogen peroxide. Both manuka and jelly bush honey, for instance, are devoid of any detectable hydrogen peroxide. Finally, there have been reports of antimicrobial peptides and proteins in honey, including honey bee venom-like proteins, further ensuring the sanctity of this important source of calories.
Under certain circumstances, honey can not only be perilous for our distant, single-celled cousins but also for ourselves. Consider the case of mad honey for instance. Stemming from rhododendron nectar, this wild concoction contains grayanotoxins, neurotoxins which can cause cardiac issues in humans. Despite these undesirable outcomes, countries such as Nepal or Turkey deliberately produce mad honey for its hallucinogenic properties. Another issue for humans can be introduced by tutin, another powerful neurotoxin, from the tutu bushes in New Zealand. For this, you need to recall the honeydew honey, produced by gathering excretions from aphids. Only in this case, the aphids (passionvine hoppers) gorge themselves on tutu bushes and pass the toxin tutin onto their excretions. Finally, though admittedly very rarely, the enjoyment of honey can also be impeded by cases of honey allergy.
The aforementioned misanthropic tendencies of honey notwithstanding, humanity benefits from honey. Next to its delectable properties, it also exhibits some desirable medicinal properties. Though most claims of healing-by-honey are vastly overblown, it can for instance be topically applied to help with post-operative infections. One exceedingly common application of honey, the soothing of coughs, was however not conclusively supported by data. According to a Cochrane review, no strong evidence pointed to the purported beneficial effects of honey on acute as well as chronic coughs. Take that, grandparents of the world. But if you want to nevertheless pour honey into your tea or milk, be my guest. After all, despite the abounding richness of science around the topic of honey, in the end most of us consume honey because of its flavor. But the next time you do so, think about the craziness contained in each and every tiny droplet of that amber fluid.
Engineering Life 