Pyrrolizidine alkaloids – how contaminated are honey and tea?
From the perspective of a plant they play a quite helpful, even lifesaving role: Produced by more than 6.000 plant species, pyrrolizidine alkaloids (PAs) appear as an effective defence mechanism against herbivores. Considered from the view of a human being or an animal, the situation looks quite different. If consumed, 1,2-unsaturated PAs can not only cause liver damages but they are also suspected to cause cancer – therefore, they have been the object of special scientific attention during the last decades and still remain under observation. After all, it is a legitimate question if plant-based food like honey or tea gets contaminated by pyrrolizidine alkaloids and if so to what extent. Quality Services International (QSI) from Bremen, Germany, focused on this specific issue during several studies.
About Pyrrolizidine alkaloids
Pyrrolizidine alkaloids are a group of more than 350 naturally occurring plant substances, approximately half of them toxic. Horses and cattle normally avoid to eat PA containing plants as they taste bitter. This is not always the case if those plants get co-harvested and processed into hay or silage – this is how PAs can enter the food chain. Concerning honey, the way leads over bees collecting a bigger amount of nectar and pollen from PA containing plants.
So far, there are no existing limits relating to the occurrence of pyrrolizidine alkaloids in food in the European Union and Switzerland, yet they exist for some herbal remedies. In Germany, The Federal Ministry of Health (Bundesministerium für Gesundheit) limited the consumption of herbal remedies to 1 µg per day. If the period of taking the medication takes more than six weeks, the limit is 0.1 µg per day. Assuming that honey is being consumed in portions of 20 g daily, the maximum concentration of pyrrolizidine alkaloids would be 5 and 50 µg per kilogram of honey, respectively.
Detecting pyrrolizidine alkaloids in honey
But which are the most common PA containing plant species? Worldwide, they mainly belong to the families Asteraceae, Boraginaceae and Fabaceae. In the context of a joint project between Quality Services International GmbH (Bremen, Germany), the Swiss Bee Research Centre Agroscope (Bern, Switzerland), the Institute for Pollen Analysis (Kehrsatz, Switzerland) and the Federal Office of Public Health (Bern, Switzerland) a number of honey samples from different locations within Switzerland with distinct climatic conditions and botanical settings were obtained. The focus was put on a variety of PA containing plants commonly found in Switzerland, e. g. Senecio spp., Eupatorium spp., Petasites spp., Tussilago farfara, Senecio jacobaea and Senecio aquaticus, Echium vulgare, Borago officinalis, Symphytum spp., Cynoglossum and Myosotis spp.
From 2009 to 2010, the team collected 69 honeys while each production year included honeys from the five different biogeographical regions “Jura Mountains”, “Swiss Plateau”, “Northern flank of the Alps”, “Central Alps” and “Southern flank of the Alps”. Two further samples were collected in 2011. In order to determine the PA concentration, an HPLC-MS/MS-system for target analysis of 18 different PAs was used – as a result, a number of different PAs and PA-N-oxides typical for the genera Echium, Eupatorium and Senecio could be found. The team determined the botanical origin of the honeys by qualitative as well as quantitative pollen analysis and additional sensorial analysis and chemical characteristics.
PAs in honey: results
Pyrrolizidine alkaloids were found in 54 % of the samples, in 46 % the amount of PA was below the limit of quantitation. The mean (arithmetic average) PA concentration of the positive samples was 6.7 μg/kg and the mean concentration of PAs in all honeys was 3.6 μg/kg – almost all of them below 18 µg/kg. Only one honey from the southern flank of the Alps contained 55 µg/kg of PAs.
Meanwhile, the team could determine that the occurrence of PAs in honeys varies according to the climatic regions of Switzerland: PA-contamination was most frequently observed in honey from the Central Alps (92 %), the northern flank of the Alps (75 %) and the southern flank of the Alps (71 %), while occurrence of PAs was only 35 % in honeys from the Swiss Plateau and 9 % in honeys from the Jura Mountains. They found that honey from the north of the Alps contained less frequently PAs compared to those from the Alpine regions which can probably attributed to the different floristic patterns found in the five defined regions.
All of the tested honeys showed a relatively low level or even no contamination with PAs (except one). This leads to the conclusion, that Swiss honey usually does not pose a risk for consumers.
Detecting pyrrolizidine alkaloids in tea
Contamination with pyrrolizidine alkaloids is not a honey-exclusive issue – there are as well known cases of intoxications due to the consumption of contaminated herbal teas. Therefore, Shimon Barel and Jakob Avi Shimshoni (Kimron Veterinary Institute, Department of Toxicology, Bet Dagan, Israel), Arne Duebecke and colleagues carried out another study in 2015 concerning 70 pre-packed teabags of herbal and non-herbal tea types sold in supermarkets in Israel.
While no PA contamination could be determined in fennel and melissa herbal teas, the concentration in chamomile, peppermint and rooibos herbal teas was relatively high, lower amounts could be detected in black and green teas.
Fennel and Melissa do not seem to be the problem. Still it has to be kept in mind, that only relatively small number of samples were analysed. Green and black tea remain below the recommended maximum daily intake of PAs, when only one or two cups a day are consumed. In case of higher consumption, the recommended intake will be exceeded. Peppermint, Chamomile, Rooibos and mixed herbal teas all exceed the maximum recommended daily intake already when only one cup is consumed. This clearly indicates that these products need to be monitored in order to better evaluate a potential risk for the consumers.
Pyrrolizidine alkaloids in the future
As the studies showed, the problem of pyrrolizidine alkaloids in food is existent, though usually there is no risk for acute poisoning. However, as PAs are potential carcinogens which might exert negative effects even at lower PA contents.
Beekeepers may try to place their hives in regions with low occurrence of PA-containing plants. By harvesting honey before the bloom of large populations of PA plants, the incorporation of PAs into honey could be prevented. Although this sound straightforward, it may not always be possible to implement these measures.
For the tea industry the “CODE OF PRACTICE FOR WEED CONTROL TO PREVENT AND REDUCE PYRROLIZIDINE ALKALOID CONTAMINATION IN FOOD AND FEED” of the Codex Alimentarius is a useful document, as it describes a number of measures that could be taken to reduce PAs in the final product.
PAs have been an issue of the public interest in the past and it can be assumed that it will be in the future. Only recently (20.01.2016) “The Sydney Morning Herald” published a report titled “Australian honey could be making us sick”, referring to high concentrations of PAs found in Australian honey. This clearly shows that PAs are a worldwide topic and not just restricted to Germany or Europe. Considering the vast number of different plants used for human consumption as well as the large variety of harvesting and processing practices used in different countries it can be expected to encounter PAs again in other products.
Schweizerische Bienen-Zeitung Ausgabe 10/2010, S. 14 ff
DLR| September 2013, S. 481 ff
Journal of Apicultural Research 53(1): 75-83 (2014) DOI 10.3896/IBRA.1.53.1.07 – Analysis of Swiss honeys for pyrrolizidine alkaloids – Christina Kast, Arne Dübecke, Verena Kilchenmann, Katharina Bieri, Michael Böhlen, Otmar Zoller, Gudrun Beckh and Cord Lüllmann
Food Additives and Contaminants, Vol. 28, No. 3, March 2011, 348–358 – Pyrrolizidine alkaloids in honey and bee pollen – A. Dübecke, G. Beckh and C. Lüllmann
Jakob Avi Shimshoni, Arne Duebecke, Patrick P.J. Mulder, Olga Cuneah & Shimon Barel (2015): Pyrrolizidine and tropane alkaloids in teas and the herbal teas peppermint, rooibos and chamomile in the Israeli market, Food Additives & Contaminants: Part A, DOI: 10.1080/19440049.2015.1087651
Code of Practice for Weed Control to prevent and reduce Pyrrolizidine Alkaloid Contamination in Food and Feed
The Sydney Morning Herald , “Australian honey could be making us sick“, http://www.smh.com.au/national/australian-honey-could-be-making-us-sick-20160120-gm9u6f.html – last accessed 2nd February 2016