## 3 Results and Discussion
### Milk, Retentate, and IF Mineral Composition  
In breast milk, minerals contribute to a variety of physiological functions, forming essential parts of many enzymes and are of biological importance to molecules and structures [2]. Ruminant milks contain a great amount of minerals, but data regarding their bioavailability are scanty. Minerals such as iron, zinc and copper of ruminant milks are mainly associated with casein, in contrast to human milk (linked to soluble proteins), implying lower assimilation [24]. The mineral composition of milk ingredients and liquid milk-derived formulations IF1 and IF2 is reported in Table 4.  
\begin{table}
\begin{tabular}{l c c} \hline  & **IF1** & **IF2** \\  & **(_n = 6_)** & **(_n = 6_)** \\ \hline pH (UpH) & 6.74 \(\pm\) 0.10 & 6.71 \(\pm\) 0.03 \\ Dry matter (\%) & 16.2 \(\pm\) 0.1 & 16.1 \(\pm\) 0.1 \\ Fat (\%) & 1.5 \(\pm\) 0.1 b & 3.1 \(\pm\) 0.1 a \\ Protein (\%) & 3.82 \(\pm\) 0.08 b & 4.32 \(\pm\) 0.02 a \\ Fat/Protein (-) & 0.38 \(\pm\) 0.01 b & 0.72 \(\pm\) 0.01 a \\ Casein (\%) & 1.3 \(\pm\) 0.1 b & 2.8 \(\pm\) 0.1 a \\ Whey protein (\%) & 2.6 \(\pm\) 0.1 a & 1.6 \(\pm\) 0.1 b \\ Casein/Whey protein (-) & 0.49 \(\pm\) 0.06 b & 1.67 \(\pm\) 0.05 a \\ Lactose (\%) & 10.16 \(\pm\) 0.03 a & 7.88 \(\pm\) 0.04 b \\ Ash (\%) & 0.46 \(\pm\) 0.03 b & 0.69 \(\pm\) 0.11 a \\ \hline \end{tabular} Different letters in a row indicate mean differences statistically significant at \(p<0.05\).  
\end{table}
Table 3: Chemico-physical composition of liquid milk-derived formulations IF1 and IF2, to be used in the production of first stage (from 0 to 6 months of age) and follow-on (from 6 to 12 months of age) infant formulae, respectively (mean \(\pm\) SD).  
In ruminant's milk the two most abundant mineral components are calcium and phosphorus; they are 1/3 and 1/2 in the colloidal form, respectively, linked to caseins. Approximately 1/3 of magnesium is also found associated with caseins [25]. In this regard, in Table 4 it is interesting to note that the mineral profile of retentate differed significantly from that of the milk in terms of calcium, phosphorus and magnesium contents (\(p<0.05\)). In fact, the retentate is produced by ultrafiltration of rennet whey, deriving from cheese making; therefore, since caseins are almost absent in retentate, the mineral component associated with them is also missing (calcium, phosphorus and magnesium). The minerals and trace elements composition of sheep's milk appeared to be in line with data reported in the literature [24; 26], whereas no information regarding sheep whey retentate is available. IF1, compared to IF2, showed significantly lower levels of calcium, magnesium and phosphorus, (\(p<0.05\)). This aspect was consistent with the fact that the IF2 formulation was obtained by blending higher quantities of milk (whole and skimmed), compared to IF1 formulation, and therefore, the level of these minerals increased with the casein content of the IF2 formulation. To compensate for the lower bioavailability of the minerals present in the liquid milk-derived formulations compared to human milk, a mix of minerals and trace elements is usually added to the final FM, so that the content of these chemical elements in the final infant formulae is higher than the levels present in human milk [27; 28]. In commercial infant formulae the content of calcium and magnesium ranges between 250-600 mg 100 g\({}^{-1}\) and 40-150 mg 100 g\({}^{-1}\), respectively, while that of trace elements zinc and iron ranges between 5-15 mg 100 g\({}^{-1}\) and 8-15 mg 100 g\({}^{-1}\), respectively [27].