Macro minerals distribution in different fractions of donkey milk

It is known that nutritional and technological properties of milk are also influenced by mineral composition and distribution. The aim of this trial was to study the concentration of Ca, P, S, Mg, K, and Na and their distribution in fat, casein, whey proteins and aqueous phase of donkey milk. Sixteen lactating donkeys (6 Amiata and 10 Ragusana breed) were used to provide individual milk samples, collected mechanically. Milk samples were analysed for casein content. Fat, casein and whey proteins were removed from whole milk samples by subsequent centrifugation, ultracentrifugation and ultrafiltration, respectively. Skimmed milk, a supernatant whey (soluble) fraction and the aqueous phase of donkey milk were therefore obtained. The concentration of the aforementioned elements was measured by inductively coupled plasma-mass spectrometry in whole milk and fractions. The concentration of elements associated with fat, casein (colloidal) and whey proteins was then calculated. The effect of fat, casein, and whey proteins removal was determined by analysis of variance for repeated measures. Results on casein percentage (6.2 g/l), and on milk concentration of Ca (799 mg/l), P (484 mg/l), Mg (78 mg/l), K (707 mg/l), and Na (143 mg/l) were within the range available in literature. The concentration of elements in whole and skimmed milk did not differ, the fat fraction carrying no or very little amount of the investigated elements. Sodium was entirely contained in the aqueous phase of milk. Approximately 63% of total Ca, 53% P, 33% Mg and no K were associated with the casein fraction. The molar ratio of colloidal Ca, P and Mg to casein was 2.04, 1.36 and 1.07 mmol/g casein, respectively. A small but significant amount (P<5%) of Ca, P and K, and 9% of Mg were associated with whey proteins. The aqueous phase of milk contained 32% of total Ca, 43% P, 58% Mg and 97% K. The milk concentration of S was 142 mg/l, of which 64% was associated with whey proteins and only 25% with casein, indicating a higher content of sulphurcontaining amino acids in donkey whey proteins than casein