## 3 Results and Discussion  
### T2 Relaxation  
Due to the stretching process, the mozzarella cheese structure is characterised by channels filled with serum and fat globules dispersed between the casein fibres constituting the gel network [15]. This particular structure led from Equation (1) to a multi-exponential decay, which in the case of \"Mozzanella di Bufala Campana\" PDO is characterised by four relaxation components, in agreement with [PERSON] et al. (2007) [16]. The \({}^{1}\)H population with shorter relaxation times (T2\({}_{1}\)) can be attributed to water protons in chemical exchange with hydroxyl and aminic groups at the protein gel surface [16]. Another population with relaxation time (T2\({}_{2}\)) ranging between 33 and 39 ms is represented by water protons diffusing in small channels, experiencing slow diffusion exchange. The third component with a relaxation time between 113 and 135 ms (T2\({}_{3}\)) is constituted by the superimposition of signals originated by water protons in medium sized channels and the protons of the liquid fat phase [17]. The last population is made of water protons in channels with a dimension bigger than the threshold of slow diffusion exchange, with relaxation time (T2\({}_{4}\)) influenced by the presence of whey proteins and saccharides in solution (serum). No clear trend in the discrete T2 coefficients of these four components as a function of the FC content is observed in Table 2, indicating that the addition of FC does not significantly change the proton relaxation mechanisms in the system. Differently, some population intensities (e.g., I\({}_{0,1}\) at day 6) show some trend as a function of FC addition. However, in most cases these trends are not followed by statistical significance. On the other hand, differences in T2 relaxation times of mozzarella samples during storage are due to water migration processes [9] and documented through T2 measurements in mozzarella di bufala cheese [16], and therefore are not treated in this work. As a matter of fact, experiments were performed at two different storage days for evaluating whether the time has an effect on the discriminative power associated to the relaxation data of cheese with different curd additions, not for studying the influence of the storage. In T2 distributions (Figure 1), it is however possible to see that at a longer T2 (water molecules in large compartments experiencing fast diffusive exchange) there is a signal reduction and shift of the population toward shorter T2 times, when comparing 0% and 50% FC distributions. A different scenario emerges for low and moderate FC concentrations: One distribution (15% FC) is only experiencing signal reduction while the other one (30% FC) is characterised by a shift of T2 times. Moreover, the \({}^{1}\)H distribution around 30 ms tends to spread for higher FC percentages. This behaviour highlights the increase in dispersion of compartment dimensions where water molecules are subjected to a slow diffusive exchange, as well as an increase in the fraction of water molecules experiencing fast chemical exchange at the protein gel surface. Generally, an increase in dispersion of T2 distributions can be seen as a function of FC addition during the production process. Since the total signal remains constant between the formulations, this phenomenon can be explained by changes in the protein gel structure, which in turns determines the redistribution of water proton between the different relaxation sites. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Class** & **0\% FC** & **15\% FC** & **30\% FC** & **50\% FC** & **Total** \\ \hline \hline \multicolumn{6}{c}{**Number of Observations**} \\ \hline
**Training Set** & 11 & 13 & 11 & 13 & 48 \\
**Test Set** & 3 & 3 & 3 & 3 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Training and test set composition: Frozen curd (FC). Continuous fitting of T2 data resulted in more accurate information about the effect of FC content in mozzarella cheese.