Sweet science

Novice beekeeper Nathan Kilah isn’t just in it for the honey. With a complex social structure governed entirely by chemistry, bees are the scientists of the natural world, and getting to know them is well worth the occasional sting.

There’s a bee buzzing about 10 centimetres from my eyes, back and forth, distracting me from looking for her royal mother. She wants me to leave, and will give her life to scare me away. Her plunging stinger can deliver a biochemical attack that will cause me pain and discomfort for days (fortunately not everyone reacts as strongly as me), and will rally her sisters to sting as well to defend their hive.

The pheromone signal that draws worker bees to attack is part of a network of chemical products and transformations that govern a colony’s success. I got into bee-keeping for the honey and through my interest in being somewhat self-sufficient, but along the way I’ve become fascinated with bees’ social structure and what amaz­ing chemists they are.

The bee most keepers exploit is the Western (or European) honey bee, Apis mellifera, which is now spread across the globe. Highly social, bees have evolved to make massive amounts of stored food to help the colony survive winter. The most common methods of managed bee-keeping encourage them to fill “super” boxes with as much honey as possible. The bees’ instincts direct them to store available resources while they can.

Honey lore and (thermodynamic) order

Honey is the stuff of myth and legend, and no wonder. Our modern tastebuds are satu­rated with sweet flavours, but to the earliest of brave bee robbers honey must have been an intoxicatingly precious substance.

At its simplest, honey is concentrated nectar – sugary water – but a number of chemical transformations take place on its journey between flowers and a piece of but­tered toast.

 Flower nectar is a solution of a small quantity of sucrose (the same sugar you’d find in your kitchen cupboard), along with some other sugars, minerals and other unique chemicals. The bee drinks the nectar, and the enzyme invertase in its saliva immediately commences breaking down the sucrose (a disaccharide) into glucose and fructose (both monosaccharides). So rather than fully digesting the sucrose, the bee transforms the nectar to a form better suited for storage in the hive.

The foraging bee flies back to the hive and regurgitates the content of its crop to a waiting, younger worker bee, which has not yet graduated to foraging. The forager leaves to gather more nectar, while the hive bees continue back-and-forward regurgitation (best not to think about this when eating your next crumpet) to further transform the sucrose while lowering the nectar’s water content from around 80% to under 20%.

Once suitably concentrated, the sugar syrup is no longer nectar, but honey. The moisture content of the honey is crucial: without it, it would ferment with natural yeast or bacteria.

 The honey’s preservation is enhanced by another enzyme – glucose oxidase, which speeds up the reaction of glucose and oxygen to make a glucose derivative – and hydrogen peroxide, an antiseptic. Glucose oxidase remains active in stored honey, pro­ducing more hydrogen peroxide. Honey is also mildly acidic, with a pH of around four.

These properties combine to make honey an antibiotic with diverse applications, in­cluding for propagating plant cuttings and as a wound dressing. But it’s not a sterile product – it can contain nasties like botu­lism spores – so medical-grade honeys are sterilised by irradiation.

Honey stored in the hive typically has 18% water, 40% fructose, 28% glucose and 9% other sugars, with the remainder made up of minerals, protein, acids, trace vitamins and phenolic compounds. The concentration of sugars in this mixture is very high, forming a “supersaturated” solution. In the hive, this mixture normally stays liquid, as the bees work hard to keep the temperature around 30°C. Heat is produced as they metabolise nectar and honey and vibrate their abdomens and wing muscles. But once the honey’s temperature drops, glucose in the solution will start to crystallise. This is an inevitable process: thermodynamics dictate that the glucose from the supersaturated solution will form crystals at room temperature. The crystals grow from a “seed”, a small piece of wax or a pollen grain.

Honey’s origin species of flowering plant can also play a role in the speed of crystallisation, as nectars with a greater glucose-to-fructose ratio are more likely to crystallisation. A great example is the deli­cious honey from prickly box (Bursaria spinosa), which starts crystallising inside the hive. Other honeys are intentionally crystallised (creamed) through the addition of fine glucose crystals as seeds, giving a lovely smooth-in-the-mouth feel. Highly processed honey is filtered, which reduces the number of seed particles and allows the honey to remain liquid for longer.

The forage plant species can make a big difference to the flavour and other charac­teristics of the honey. The most sought after honey at the moment is manuka, made from the nectar of Leptospermum scoparium, but many other honeys are desirable for their unique flavour components. For example, citrus honey is noted for the pres­ence of methyl anthranilate (the flavour compound in grape-flavoured soft drinks and sweets); clover and canola honey contain tart-flavoured formaldehyde and acetaldehyde; eucalypt honey is rich in a range of diketones, and is described as having herbal or medicinal flavours.

Bees and being

Nectar is the carbohydrate of a hive’s diet, pollen its protein and fat. Bees collect pollen as they forage from flowers (pollinating crops along the way), then pack it into hive cells with saliva and nectar and cap it with honey. This mixture ferments due to yeast and bacteria to form “bee bread”. The fer­mentation increases the availability of amino acids and nutrients from the pollen, and in a form that can be stored in the hive. Nurse bees that care for eggs and young larvae are able to take pollen and nectar that they con­sume and turn it into a special product called royal jelly. This viscous, white, protein-and-sugar-rich substance is excreted from the hypopharyngeal gland on the worker bees’ heads. All developing larvae are exclusively fed royal jelly for the first three days after hatching, but a larva destined to be a queen continues to be fed royal jelly for the rest of her life.

The benefit of collected pollen greatly depends on its floral source. Not all pollens offer a complete diet, and bees collecting from a single species may not get sufficient nutrition. The diversity of bees’ forage plants is reflected in the stored pollen: more flower species equals more colours. Incidentally, there’s little evidence to sup­port the many stories that suggest eating honey containing local pollen will lower hay-fever symptoms. This is in part because the types of pollen that cause hay fever are typically from wind-pollinated grasses and trees, not from the flowers bees favour.

The mass storage of honey and bee bread wouldn’t be possible without bees building their wax home. A lot of chemical structures contain hexagonal rings, which are dead ringers for the hexagonal pat­tern found in the bees’ comb. Bees exude white wax from their abdomens as a mixture of organic molecules, including hydrocarbons, esters and fatty acids. The molecules’ common structures are long, zig-zag chains of carbon atoms that can interact with one another but are resistant to water. Bees mould the wax, and it hardens over time into a robust home in which they raise young and store honey or pollen.

The hexagonal pattern is strong and a very efficient way to fit cells into the hive. The comb provides the bees with a nursery, bedroom and pantry, and its surface is used for waggle dances to communicate information about the direction and distance to plentiful nectar.

The less visible com­munication pathways are the chemical signals essential to the functioning of the hive. Bee pher­omones are typically small, volatile chemicals that travel through the air to other bees. For example, when a bee stings a beekeeper, the alarm pheromone is released from a gland near the stinger as a complex mixture of esters and alcohols. This stimulates other bees to rush to the site of their sister’s demise to also start stinging. One easily identified molecule is isoamyl acetate – which is also the molecule responsible for the smell of a ripe banana. Puffing smoke into the hive (or onto a sting site) can mask the alarm pheromone, while also tricking the bees into feeding on their stored honey in preparation to flee the hive.

Her chemical majesty

The tremendous organisation of the hive is governed by the queen’s pheromones. The main chemical controller, 9-oxo-2-decenoic acid, is produced from a gland on her man­dible. Its levels change over the queen’s lifecycle. It acts as a potent sex pheromone for drones when the queen flies out to mate, and causes young worker bees to act in ret­inue to the queen, grooming, feeding and passing around her pheromones to other worker bees in the hive.

A high concentration of queen pher­omone in the hive ensures workers remain devoted to caring for the queen and conducting the work of the colony. But the strength of the pheromone drops as she ages, and less devoted workers will raise a new queen. Beekeepers often want to introduce new queens, either to replace an ageing monarch or to improve some aspect of the hive, such as lowering bees’ aggression, or to select for hygienic behaviour – a cleanliness trait where workers can detect dead bees and remove them from the hive, which lowers infections from fungus, bacteria or the presence of parasites.

Introducing a new queen can be tricky. Her distinct pheromone can trigger aggression in the hive, and worker bees will seek to kill her. The transition to a new queen is typi­cally performed over a few days using a special cage, sealed with a wedding-cake-icing-like fondant plug. The worker bees slowly eat the plug to release the new queen, giving them enough time to get used to her phero­mone signature.

Worker bees produce their own pheromone mix, known as the Nasonov pheromone, in a gland of the same name near the tip of their abdomen. It’s a multipurpose scent, capable of directing forag­ing workers to weakly scented nectar sources, identifying the hive entrance for bees learning to fly, and directing swarms of bees to a new home. It has many chemicals in common with lemongrass, with constituent compounds including citral (citrusy smell) and geraniol (floral smell). Beekeepers try to tempt swarming bees into their empty hives by baiting them with lemongrass oil or other artificial swarm lures, chemically con­vincing them that their box is the optimum new home.

My nose isn’t as sensitive as a bee’s antenna, but I always like to start my hive in­spection by smelling it (before letting loose with the smoke!) to try to learn something of the bees’ activity. I’ve still got a long way to go in my beekeeper education, but I’m thankful to have such wonderful and pro­ductive little chemists as teachers – albeit with the occasional sting in the tail.

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This article was first published in Cosmos Issue 88, Spring 2020.