Bees are hard at work collecting pollen and nectar to feed themselves and their off-spring. When foraging for food they use the sun for direction, initially using a somewhat irregular path away from the hive to find a suitable source up to 10 km away. The bee will then fly a straight line back to the hive and perform a dancing act to indicate the shortest way to the source for other bees. Many bees will then fly a straight path to the source the original bee found and will repeat the process themselves.
The foraging bees regurgitate nectar and pass it to worker bees in the hive. In order to conserve space and preserve their food, bees transform the nectar into honey by evaporating most of the water from it. Nectar is as much as 70 percent water, while honey is only about 20 percent water. Bees get rid of the extra water by swallowing and regurgitating the nectar over and over. They also fan their wings over the filled cells of the honeycomb. This process retains lots of sugar and the plant’s aromatic oils while adding enzymes from the bees’ mouths.
Honey as human food
Humans have been harvesting honey for more than 6,000 years for our own use. Historically, people have used it to sweeten food and make fermented beverages like mead. Today it is also used in industrial food processing of baked products, confectionary, candy, marmalades, jams, spreads, breakfast cereals, beverages, milk products and many preserved products.
Honey’s high sugar content, flavour and antimicrobial properties make it a useful ingredient. It is also considered to carry health properties.
So far so good making this a good news story. But of course there has to be a negative side to the story as well otherwise it wouldn’t be covered on this food safety blog. And now we come to my own challenge spelling the name of the chemical substance group that can be present in honey and pose a threat to public health. With the dictionary in one hand and typing with the other I want you to be aware of pyrrolizidine alkaloids.
Pyrrolizidine alkaloids are a group of naturally occurring substances that are produced by plants as a defense mechanism against predator attacks.
Pyrrolizidine alkaloids in honey
There are more than 600 different pyrrolizidine alkaloids identified in over 6,000 plants. About half of them are toxic to humans and other animals, mainly damaging the liver or in some cases even causing liver cancer. In an opinion published in 2011, the European Food Safety Authority nominated some pyrrolizidine alkaloids as genotoxic carcinogens. It has been estimated that three percent of the world’s flowering plants contain pyrrolizidine alkaloids, mostly members of the daisy, forget-me-not or borage families as well as the legume family. And unfortunately the pyrrolizidine alkaloids can find their way into honey.
Plants that commonly contribute to pyrrolizidine alkaloid contamination of honey include Echium, Senecio and Borago species whose pollen might be used for honey production by the bees. In Australia, the toxins may get into the honey when bees forage on the flowers of Paterson’s Curse, also known as Salvation Jane. Raw honeys from certain countries in Central and South America, Cuba and Uruguay in particular, and Australia and New Zealand show higher levels of pyrrolizidine alkaloids compared to raw honeys from some European and Asian countries.
For most people who eat small amounts of honey, the levels of pyrrolizidine alkaloids would be well below the tolerable daily intake and not a cause for concern. However, chronic effects cannot be excluded and are often difficult to associate with a particular food source. It is thus recommended that anyone who daily eats more than two tablespoons of honey should be careful in not selecting an exclusive product source.
Most honeys are mixed before retail to reduce the levels of pyrrolizidine alkaloids and this reduces potential risks.
Other sources of pyrrolizidine alkaloids
In addition to honey, there are other potentially more important sources of dietary exposure to pyrrolizidine alkaloids. The majority of reports of acute outbreaks of pyrrolizidine alkaloid poisoning have largely been limited to third world countries. Generally, these have been outbreaks where hundreds, and sometimes thousands, have been poisoned from eating staple foods made from cereal crops contaminated with seeds from pyrrolizidine alkaloid-containing weeds. For example, people were taken ill in Pakistan, India and Afghanistan after eating wheat contaminated with seeds from Heliotropium or Crotalaria species. In Jamaica, cases of poisoning have occurred through so-called bush teas containing Crotalaria and parts of the ragwort plant.
More recently, along with an increasing reliance on unconventional medicine and the use of herbal supplements, notably borage leaf, comfrey and coltsfoot, there has been a rise in the number of poisonings seen in industrialised countries. Also Borage oil and Echium oil marketed as dietary supplements, and salad crops contaminated with ragwort or common groundsel could present a risk to the consumer.
It is believed that the real extent of human poisoning by pyrrolizidine alkaloids has been underreported since the cause of chronic diseases are difficult to establish. So watch your honey consumption and stay away from suspect dietary supplements.
- The Five Plants Bees Love Best (romancingthebee.com)
- A Drop of Honey! (soumyav.wordpress.com)
- Bees use the ‘force’ to choose the best flowers, study finds – Christian Science Monitor (csmonitor.com)
- Decoding the Honeybee’s Dance (other-nations.com)