Cities provide an excellent food source for bees, offering a variety of nectar and pollen throughout the long flowering period. They tend to be more ecologically diverse than the surrounding rural environments (Fox et al., 2022). This diverse nectar is the main raw material for creating city honey with unusual taste qualities. The city's bee products are free of pesticides, but are they free of other contaminants?
One of the most harmful contaminants that find their way into bee products are heavy metals. We include among others cadmium and lead. The documents that define the maximum permissible limits for the content of heavy metals in food products are: COMMISSION REGULATION (EU) 2021/1317 of 9 August 2021 amending Regulation (EC) No. 1881/2006 as regards the maximum levels of lead in certain foodstuffs and Commission Regulation (EU) 2021/1323 of 10 August 2021 amending Regulation (EC) No. 1881/2006 as regards maximum levels of cadmium in certain foodstuffs (here the data were adopted for milk chocolate, as the regulation does not specify safe amount of cadmium contained in honey). The European Commission agrees that the safe limit is 0.10 mg per kg of fresh weight (both for Pb - data for honey and Cd - data for chocolate).
Bees collect nectar from flowers, drink water from a puddle, and their hairy bodies collect aerosol particles. According to various estimates, honey bees collect nectar from an area of about 10 square kilometers. Assuming that each hive contains at least 1,000 worker bees, and that each of them collects nectar from 1,000 flowers per day, the honey produced can be considered the result of at least 1,000,000 interactions. The consequence of this fact is that honey presents probably the best composed sample and the most representative values for the average concentrations of bio-available elements in a given environment (Barišić et al., 1999). For this reason, it was natural to use honeybees to determine the state of the natural environment (Rashed et al., 2009). In theory, it looks promising, but does the state of the natural environment really translate into the quality of honey?
Anthropogenic and natural origin of pollutants
Metals present in the natural environment can come from both natural and anthropogenic sources. Natural sources of metals include weathering of rocks, volcanic eruptions and soil formation processes. Anthropogenic sources of metals include discharges of municipal and industrial sewage, mining and metallurgical industries (Bojakowska 1995), road transport (Kaniuczak et al., 2003), agriculture and dust from city streets (Niedźwiedzki et al. 2000). Sources related to human activities are the main cause of pollution and the presence of metals in the environment.
Research results presented by many researchers (e.g. Rashed et al., 2009; Przybyłowski and Wilczyńska, 2001; Skorbilowicz et al., 2018) show that honey from polluted areas contains significantly more heavy metals than honey from non-polluted areas . Research by Barišić et al. (1999) showed that the accumulation of heavy metals in soil largely depends on its type. Research by Barišić et al. (1999) also showed a difference in the dynamics of uptake of inorganic chemical compounds from soils into flower nectar in relation to their uptake into the phloem of fir and spruce conifers; the above studies confirm the results of analyzes of Pomeranian honeys carried out by Przybyłowski and Wilczyńska (2001), who found an increased content of heavy metals in honeydew honey compared to nectar honeys.
Leita et al. (1996) found a linear relationship between the amount of Cd in honey and the amount found in other biological markers, such as flowers of red clover ( Trifolium pratense ). The above comparisons show the high complexity of the dependencies that occur during the transfer of impurities to honey. It is worth emphasizing that the degree of saturation of heavy metals on the path soil - plant - honey decreases. The ability of plants to take up elements depends on many factors: plant species, sorption and desorption processes in the soil, soil mineral composition, humidity and intensity of precipitation, grain size and surface area, pH, content of organic matter and content of competing cations. In turn, the heavy metal content in flowers is much higher than the heavy metal content in honey (Rashed et al., 2009). The reason for this phenomenon is the repeated purification of the nectar by the bee in the honey goiter during its processing into honey, and the visible effect of this process is the concentration of heavy metals in the bee's body (Roman, 2003). An additional factor affecting the metal content in honey is seasonality. Skorbilowicz et al. (2018) analyzed the content of heavy metals in bee samples and showed a seasonal relationship between the concentration of heavy metals and the activity of bees.
Research on the content of heavy metals in honey
The most commonly used method to determine the content of heavy metals in honey is atomic absorption spectrometry (ASA). Atomic absorption spectrometry (ASA) is a method of chemical analysis that uses the phenomenon of absorption of electromagnetic radiation by atoms. The sample is irradiated with light of a certain wavelength, and then the amount of light absorbed by the sample is measured. Thanks to this, it is possible to determine what elements and in what amounts are present in the tested sample. ASA is a very sensitive and selective method that is often used to analyze the composition of metals and other elements in various types of samples, such as water, soils, food and drugs.
Bees - the key to reducing pollution
The research results of the team from the University of Life Sciences in Lublin (Borsuk et al., 2021) clearly confirm that bees have the ability to effectively clean the nectar from impurities, especially heavy metals. Removal of undesirable substances occurs during the processing of nectar into honey. Even though honey loses about 80% of its water during this process, making it four times thicker, the heavy metal content of honey is much lower than that of nectar. The results of the study of urban honeys from Poznań and Lublin showed that the Cd content in honey was 75% and 88% lower, respectively, than in the corresponding nectar.
Cadmium content in soil, plants, bees and bee products from Lublin and Poznań. Small letters (ac) indicate differences between soil, plants, bees and products (p < 0.05; Kruskal-Wallis test). Error bars represent the standard deviation of the data (Borsuk et al., 2021)
The Pb removal process was less effective and its amount in honey remained the same as in nectar. However, it should be taken into account that honey is four times concentrated in relation to nectar.
Lead content in soil, plants, bees and bee products from Lublin and Poznań). Small letters (ac) indicate differences between soil, plants, bees and products (p < 0.05; Kruskal-Wallis test). Error bars represent the standard deviation of the data (Borsuk et al., 2021)
Where does the pollution go?
Bees use various bioelements that are crucial for the proper development of their organisms. Among the metals, Fe plays an important role, helping them read the Earth's magnetic field — vital for navigation. The excess of unnecessary elements is removed with the feces, while some of them are permanently stored in the fat body. The pollutants filtered out of the nectar also accumulate in all parts of the bees' body: in the head, thorax and abdomen. Particles that get there in the form of biocomplexes are transported from the digestive tract. According to Dżugan et al. (2018) bees is a biofilter that prevents the penetration of elements into bee products, especially honey.
Research on Warsaw meads
Research conducted on city honeys in Warsaw confirmed that the content of cadmium and lead in these products is very low. The average value for lead was 0.005 mg/kg and for cadmium the result was below 0.02. The highest amount of lead was detected in honeys with the addition of honeydew — on average 0.013 mg/kg.
Test results of lead content in Warsaw meads. Tests carried out by PiWet on behalf of Pszczelarium.
These results are in line with the assumptions of scientists from the University of Life Sciences in Lublin (Borsuk et al., 2021), who found that bees can effectively remove heavy metals from nectar during honey production. Coming to the conclusion, honeys (including urban honeys) are free of heavy metals, and all this is due to the bees, which perfectly clean the honey of harmful substances that may pose a threat to the next generations of worker bees and drones.
Results of tests of cadmium and lead content in Warsaw city honeys. Tests carried out by the National Institute of Public Health - National Institute of Hygiene on behalf of the Pszczelarium.
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