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Concentrations of heavy metals in the whole raw milk of dairy cows under different management systems and country of origin: A meta-analytical study
The objective of this meta-analytical study was to investigate concentrations of toxic heavy metals including aluminium (Al), arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), nickel (Ni), and vanadium (Va) in whole raw bovine milk from different production systems (conventional vs organic) and countries of origin. Data from 44 studies and 19 countries were used to create a dataset, which was used for statistical analyses. The average concentrations of Al, As, Cr, Va in the raw milk from the conventional system were greater (32.49, 0.27, 2.19, and 0.82, respectively) than those coming from (P < 0.05) organic farms (2.13, 0.14, 0.28, and 0.32 μmol/L, respectively). In addition, there were greater concentrations of Cd (0.25 μmol/L) and Pb (0.31 μmol/L) in the raw milk from conventional system vs organic system (0.02 μmol/L and μmol/L 0.08, respectively) (P < 0.05). Lower amounts of Ni were reported for milk from conventional system (0.57 vs 0.67 μmol/L; P < 0.05). Country of origin had significant effect on concentrations of toxic elements (P < 0.05). Turkey had greatest Al levels (283 μmol/L), whereas USA had the smallest means for this element (0.89 μmol/L) (P < 0.05). The lowermost concentrations of As were reported in Spain (0.01 μmol/L), whereas the greatest concentrations were found in Croatia (13.60 μmol/L) (P < 0.05). Average Cd level was 0.11 μmol/L; however, the greatest concentrations of Cd were reported in milk samples from Slovakia (6.49 μmol/L) (P < 0.05). In Nigeria concentrations of Cr in the milk were greater compared with other countries (38.04 μmol/L) (P < 0.05). Moreover the greatest levels of lead (Pb) contamination were reported in milk samples from Egypt (4.63 μmol/L), whereas, the lowest means were observed in Poland (0.01 μmol/L) (P < 0.05). Regarding Ni, the lowest amounts were found in Brazil (0.03 μmol/L), whereas the greatest levels (50.25 μmol/L) were found in milk samples from Nigeria. Concentrations of Va varied from below 0.01 μmol/L (Spain) to 0.72 μmol/L (Poland). In conclusion, data from this meta-analytical study indicated that organic farms were characterized by lower concentrations of toxic heavy metals compared to the conventional system of management. This study also showed high variability in concentrations of heavy metals in raw milk with regards to the country of origin.
Keywords: whole raw milk, heavy metals, meta-analysis