Pyrrolizidine alkaloids in honey: Primarily originate from nectar
They can cause liver damages and are suspected to cause cancer: Pyrrolizidine alkaloids (PAs) in honey are a potential health risk for consumers what makes them a serious issue for research. But where do PAs in honey originate from – is it from pollen or the floral nectar? Previous studies identified pollen as the major source of PA contamination. Up to now now: A joint study between QSI International GmbH (Bremen, Germany), Agroscope – Swiss Bee Research Centre (Bern, Switzerland) and the University of Neuchâtel (Institute of Biology, Laboratory of Fundamental and Applied Research in Chemical Ecology & Neuchâtel Platform of Analytical Chemistry, Neuchâtel, Switzerland) now revealed that nectar is the primary source for PAs in honey.
About Pyrrolizidine alkaloids
Pyrrolizidine Alkaloids are plant substances that help plants to defend against herbivores. Most of the plants which contain PAs belong to the families of the Asteraceae, the Boraginaceae and the Fabaceae – and they are widespread. So far, there is no limitation concerning the permitted quantity of PAs in in food in the European Union and Switzerland. However, there is a recommendation given by the German Federal Institute for Risk Assessment that a person should not consume more than 0.007 µg of 1,2-unsaturated PAs per day and kg bodyweight.
You may read more about PAs and their occurrence in honey (and tea) here:
The study: origin of PAs in honey
But how are PAs transferred into honey and by that entering our food chain? In order to find out if it is the pollen or the nectar, QSI and their colleagues from Switzerland chose Echium vulgare (Boraginaceae) as a model. It was found previously that PAs from Echium vulgare are the most frequent PAs in European honeys.
For the study, 4 honeys from north of the Alps near Basel and 6 honeys from south of the Alps in the Verzasca valley were included. Additionally, plant material like nectar, pollen and petals (including stamens) from Echium vulgare from the two observation sites.
The task now was to analyse the PA concentration of the honeys and bee collected pollen. This was done by target analysis using an HPLC-MS/MS-system which allowed the detection of 6 different PAs and PA-N-oxides in Echium vulgare. Meanwhile, 10 alkaloids were detected in small sized nectar and pollen samples by using non-targeted analysis (LC-HR-MS system).
Limits of quantification, based on echimidine standard:
- for bee collected pollen: 10 µg/kg
- for honeys: 1 µg/kg
- for floral nectar or plant pollen: 1.5 ng/mL
The Result: Nectar as primary source
Results were used to compare the PA pattern in floral nectar and pollen with honey produced at the two observation sites. In the plant pollen, total Echium-type PAs were detected in a concentration range between 0.3 – 95 µg/g in nectar and between 0.5 – 34.9 mg/g. It was found that echimidine and its -N-oxide was the main alkaloid detected in honey and floral nectar samples, while echivulgarine (and its -N-oxide) was the main PA found in bee-collected and plant pollen.
Although pollen contain a far higher amount of PAs it is nectar that contributes substantially more PAs to honey. This is due to the strong decrease of water content during processing of nectar by honey bees on the one hand and relatively low amount of pollen present in honey on the other hand.
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