Bees – The chemistry of honey, pollen baskets and… bee bread? Part One

As the temperatures warmed and plants began to bloom in the UK, it coincides as it always does with the awakening of the bees. As I watched them fly from flower to flower and getting dusted with pollen, I noticed what looked like little saddlebags on their hind legs. Curious, I began researching more on these little pollinators and as I did so this post grew from a relatively short dive about the bags (called corbiculae) and into a deep dive on “bee bread”, how honey is made, what they use it for, and more.

Bees are busy little creatures that carry a huge burden on their collective shoulders. Although this burden is shared with other insects birds and even bats, the western honey bee, Apis mellifera, has been reported by many as one if not “the most important single species of pollinators known”. Although they are effective pollinators, this is a by-product of their mission to sustain their hive through the process of making honey. So how is honey made, is it really “bee vomit”, and what do they do with it?

honey-bee-proboscis
Photo credit: MzePhotos.com

Pollen is an essential part of the bees’ diet, high in protein as well as other vitamins and minerals, this powdery substance is produced by plants as the male gametes (sperm cells) and this is what the bees transfer between plants in pollination. Nectar is the sugar-rich liquid that plants produce to attract insects, which the bee sips with a specialised tongue called a proboscis. The proboscis is a straw-like tongue that is protected by a sheath formed from palps and maxillae, which emerges from between their mandibles, as shown.

These sips of nectar are sent to a collapsible expandable honey stomach, a pre-digestive part of their digestive system which is not used for breaking down food but for storing nectar also called a “crop”. When full the honey stomach can weigh as much as the bee itself, essentially doubling their flying weight. Body-plan-honeybeeThere is a specialised valve between the honey stomach and the rest of their gut called proventriculus, which pulls pollen grains from the honey stomach to digest them whilst keeping the nectar in the honey stomach.
When the nectar reaches the honey stomach, digestive enzymes start to modify the nectar through hydrolysis. This converts the complex polysaccharide sugar sucrose into simpler monosaccharides such as glucose and fructose, which is what honey is primarily composed of. Glucose and fructose have the same chemical formula but their atoms are arranged differently, changing their characteristics in such a way that fructose even tastes different to glucose, perceived as being much sweeter.

Bees produce many enzymes to transform nectar into honey, including:Enzymes

When the forager bee returns to the hive, it passes the contents of its honey stomach into the mouth of another bee, who then passes it to another bee, and so on. With each step in the bee chain, the enzyme content increases, transforming the nectar.
There is some debate whether or not “vomit” is the best word to be used as the nectar is not actually digested but “voluntarily regurgitated” via a process called trophallaxis, vomit or trophallaxis the jury is out and it is up to you.
After around 20 minutes of passing the nectar, the water content of the nectar lowers from 80% to around 20% and is deposited into a honeycomb cell. Here the bees beat their wings to create an air current, which further lowers the water content of the nectar to around 17%, thickening it. The comb is then capped with beeswax so the nectar can continue the conversion to honey.

But why do bees make honey in the first place? Bees store pollen and honey in their active summer months as a food source when the temperatures plummet in the colder winter periods and there are no flowers for foraging to take place. Honey bees cluster together in the hive to conserve heat during the colder months and have been reported to survive when external temperatures are as low as -46C for several weeks.
Before consuming the honey they dilute it back down with a little saliva, then use their proboscis to sip the resulting liquid.
However larvae require a different diet, so what do they eat? And why do some larvae become worker bees while others become queens?
The answer to the second question relies partly on the first.

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Larvae in royal jelly

Initially, all larvae are fed a substance called “royal jelly” for the first few days of their lives. Royal jelly is secreted from mandibular and hypopharyngeal glands in the heads of nurse bees, consisting of mostly water but also various proteins and chemicals. After 3 days the larvae destined to become sterile female worker bees or drones are no longer fed with royal jelly but with bee bread. Bee bread is a mixture of pollen and nectar, but the larvae that are destined to become queens continue with the royal jelly diet throughout not only their development but throughout their life.

It has been shown that changes in regulation of only 14 genes are the difference between a larva becoming a Queen or a worker. A study found that royal jelly didn’t contain any detectable phenolic acids (flavonoids are the largest group of these acids, chemicals that are found in plants), while bee bread did. The researchers found that increasing the flavonoid content of larval diets significantly changed the expression of over half of the genes involved in organ size. It has been believed for some time that denying workers royal jelly was a form of chemical castration, but other research shows that it may be the opposite, with royal jelly chemically protecting the Queens’ ovaries. The mechanism by which some larvae become Queens while others become workers is still not completely characterised, so the debate on bee bread and royal jelly continues.

There are some strange products out there using royal jelly for a plethora of unproven health benefits, but we already take and eat their honey, maybe stealing the phlegm from inside their heads is a step too far?

 

 

References

The worldwide importance of honey bees as pollinators in natural habitats, Keng-Lou, Kingston, Albrecht, Holway & Kohn, 2018

Carbohydrates and the Sweetness of Honey. The National Honey Board. (1995).

The Chemistry of Bees, site by Joel Loveridge, University of Bristol

Wenfu Mao, Mary A. Schuler, and May R. Berenbaum. 2015. A dietary phytochemical alters caste-associated gene expression in honey bees. Science Advances 1(7)

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