Partition of essential trace elements in donkey milk fractions

The aim of this trial was to study the concentration of Zn, Cu, Mn, Se, Mo and Co in donkey milk and their partition among fat, casein, whey proteins and aqueous phase. Individual milk samples were collected mechanically from 16 lactating donkeys (6 Amiata and 10 Ragusana breed). Subsequent centrifugation, ultracentrifugation, and ultrafiltration (cut-off 3 kDa) were carried out to remove fat, casein and whey protein, to obtain skimmed milk, a supernatant whey fraction and the aqueous phase of milk, respectively. The concentration of the mentioned elements was measured by inductively coupled plasma-mass spectrometry. The concentration of elements associated with fat, casein and whey proteins was then calculated. The effect of fat, casein, and whey proteins removal was determined by analysis of variance for repeated measures. The milk concentration of Zn (2,729 μg/l), Cu (77.2 μg/l), Mn (4.7 μg/l), Se (4.1 μg/l), Mo (3.0 μg/l) and Co (0.37 μg/l) was within the range previously published for donkey milk. The effect of milk fractionation was significant for all the investigated elements. The percentage of samples below the limit of detection was between 60 and 80% for Mn in ultracentrifuged, and for Mn and Se in ultrafiltered samples (aqueous phase), therefore the lower and the upper bound were calculated for such elements in these fractions. The effect of fat removal was not significant for Zn, Cu, Mn and Se but a significant, although small, amount of Mo (approximately 14%) and of Co (7%) was associated with fat. The large majority of milk Zn (95%) and Cu (82%) was associated with casein, as well as the majority of Mn (min. 73%) and Mo (46%). Significant amount of Se (38%) and Co (28%) were also associated with casein. The majority of Co (37%) and of Se (min. 41%) was associated with whey proteins, as well as 13% of Cu and 8% of Mo. The aqueous phase contained 33% of Mo, 27% of Co and between 10% (lower bound) and 25% (upper bound) of Mn and Se.