## 3 Results and Discussion  
### Milk, Retentate, and IF Chemical-Physical Composition  
As shown in Figure 1, three ingredients derived from sheep's milk were utilized in the production of the liquid milk-derived formulations (whole milk, skimmed milk and retentate from skimmed whey). Retentate is an ingredient of primary importance to raise the WP content of FM up to that of human milk, this latter being richer in WP and poorerof CN protein, especially in early lactation stage [2]. The chemico-physical composition of milk and milk-derived ingredients is reported in Table 1.  
The three milk ingredients showed similar pH values (6.57-6.63; \(p>0.05\)) indicating that the processing conditions, applied in the whey pre-treatment stages and during ultrafiltration, limit the possible alteration of pH parameters, essentially due to spontaneous acidification processes. As expected, the compositional differences between whole and skimmed milk were mainly due to the lowering of the fat content in the latter which leads to a slightly increased percentage of all the other macro-components. Retentate, whole and skim milk showed statistically different DM values (13.9%, 16.8% and 11.7%, respectively \(p<0.05\)). Since retentate was produced from skim whey concentrated by ultrafiltration, it showed by far the highest value of whey protein compared to whole and skim milk (7.7%, 0.9% and 0.9%, respectively) accounting for more than half of the DM. This whey protein content is in agreement with that expected for the VCR (-7) selected during the whey concentration process. Retentate and skimmed milk showed similar fat content (0.6% and 0.4%, respectively; \(p>0.05\)), while the lactose content showed comparable values for all samples. The ash percentage, indicative of the mineral content, was lower in retentate with respect to skim milk (0.70% vs. 0.96%; \(p<0.05\)) but did not differ significantly compared to whole milk (0.88%; \(p>0.05\)). The above-described ingredients were then mixed with water and lactose powder to obtain two liquid milk-derived formulations, one intended for infants of 0-6 months of age (first stage, IF1) and the second one for follow-on infant formulae (from 6 months on, IF2). It has been reported that the CN:WP ratio, in human milk, fluctuates between 30:70 and 20:80 in early lactation and decreases in late lactation [2]; therefore, the exact amount of the dairy ingredients was determined to obtain a final CN:WP ratio of approximately 30:70 for IF1 and of 60:40 for IF2, respectively. The IF1 and IF2 samples were obtained by mixing different percentages of the three ingredients with water and lactose, as shown in Table 2. \begin{table}
\begin{tabular}{l c c c} \hline  & **Whole Milk** & **Skimmed Milk** & **Retentate \({}^{*}\)** \\  & **(_n = 6_)** & **(_n = 6_)** & **(_n = 6_)** \\ \hline pH (UpH) & 6.61 \(\pm\) 0.03 & 6.63 \(\pm\) 0.04 & 6.57 \(\pm\) 0.13 \\ Dry matter (\%) & 16.8 \(\pm\) 0.4 \({}^{\rm a}\) & 11.7 \(\pm\) 0.2 \({}^{\rm c}\) & 13.9 \(\pm\) 1.7 \({}^{\rm b}\) \\ Fat (\%) & 5.9 \(\pm\) 0.2 \({}^{\rm a}\) & 0.4 \(\pm\) 0.1 \({}^{\rm b}\) & 0.6 \(\pm\) 0.1 \({}^{\rm b}\) \\ Protein (\%) & 5.4 \(\pm\) 0.2 \({}^{\rm b}\) & 5.7 \(\pm\) 0.2 \({}^{\rm b}\) & 8.0 \(\pm\) 1.9 \({}^{\rm a}\) \\ Fat/Protein (-) & 1.10 \(\pm\) 0.04 \({}^{\rm a}\) & 0.06 \(\pm\) 0.01 \({}^{\rm b}\) & 0.08 \(\pm\) 0.01 \({}^{\rm b}\) \\ Whey protein (\%) & 0.9 \(\pm\) 0.1 \({}^{\rm b}\) & 0.9 \(\pm\) 0.1 \({}^{\rm b}\) & 7.7 \(\pm\) 1.8 \({}^{\rm a}\) \\ Lactose (\%) & 4.4 \(\pm\) 0.1 \({}^{\rm b}\) & 4.8 \(\pm\) 0.1 \({}^{\rm a}\) & 4.5 \(\pm\) 0.1 \({}^{\rm ab}\) \\ Ash (\%) & 0.88 \(\pm\) 0.15 \({}^{\rm ab}\) & 0.96 \(\pm\) 0.08 \({}^{\rm a}\) & 0.70 \(\pm\) 0.01 \({}^{\rm b}\) \\ \hline \end{tabular} \({}^{\rm a}\) Retentate from ultrafiltration of skimmed whey. Different letters in a row indicate mean differences statistically significant at \(p<0.05\).  
\end{table} 
Table 1: Chemico-physical composition of the dairy ingredients used to produce the liquid milk-derived formulations (mean \(\pm\) SD).