## 4 Materials and Methods
### Milk and Mozzarella Volatile Organic Compound Analysis  
The milk and cheese samples were separately analyzed by group and by day of production. The extraction of VOCs was performed using Headspace-SPME according to the procedure described by [PERSON] et al. [64] for milk, and [PERSON] et al. [65], for mozzarella cheese. Briefly, 22.5 g of milk was transferred in a 50-mL bottle, then 30 \(\upmu\)L of 2-methyl-3-heptanone (99% purity, Sigma-Aldrich, St. Louis, MO, USA), as internal standard (408 mg L\({}^{-1}\), in water solution), and 2.75 g of sodium phosphate (NaH\({}_{2}\)PO\({}_{4}\)) (Sigma-Aldrich) were added. For the mozzarella cheese analysis, the samples at \(-20^{\circ}\)C were finely grated and 22.5 g was transferred in a 100-mL bottle and suspended with 25 mL of distilled water. Thereafter, 2.75 g sodium phosphate and 50 \(\upmu\)L of 2-methyl-3-heptanone (99% purity; Sigma-Aldrich), as the internal standard (408 mg L\({}^{-1}\), in water solution), were added. The bottle was magnetically stirred for 5 min at 55 \({}^{\circ}\)C to homogenize the sample and accelerate the equilibrium of headspace volatile compounds between the milk matrix and the headspace. The SPME fiber was inserted through the Teflon septum in the bottle and exposed to the sample headspace 60 min at 55 \({}^{\circ}\)C during stirring. The SPME device (Supelco Co., Belleofonte, PA, USA) was equipped with a 50/3-\(\upmu\)m thickness divinylbenzene/carboux/ polydimethylsiloxane (DVB/CAR/PDMS) fiber coated with a 2-cm length stationary phase. The same type of fiber, highly efficient for cheese aroma analysis [66], was used for milk and cheese samples. Volatile compounds were analyzed by GC coupled with a mass spectrometer using a GC/MS Hewlett-Packard 6890N (Agilent Technologies, Palo Alto, CA, USA) equipped with a J&W HP-5 MS capillary column (30 m \(\times\) 0.25 mm i.d. \(\times\) 0.25 \(\upmu\)m Film Thickness; J&W Scientific, Folsom, CA, USA). The temperature was set at 40 \({}^{\circ}\)C for 2 min and increased from 40 to 160 \({}^{\circ}\)C at the rate of 6 \({}^{\circ}\)C/min, and from 160 to 210 \({}^{\circ}\)C at 10 \({}^{\circ}\)C min\({}^{-1}\). The injector was kept at 250 \({}^{\circ}\)C. Helium was used as a carrier gas (0.9 mL min\({}^{-1}\)). The volatile compound thermal desorption was carried out by exposing the SPME fiber in the injector in splitless mode for 10 min. The compound identification was performed by comparing retention times and mass spectra obtained by analyzing pure reference compounds in the same conditions. Moreover, the identification was confirmed by comparing the mass spectra with those of the NIST data base. All chemical standards were supplied by Sigma-Aldrich. In a few cases, the pure chemical standard was not available, and the compound identification was labeled as tentative. Mass spectra were recorded at 70 eV. The source temperature was 230 \({}^{\circ}\)C and the interface temperature was 250 \({}^{\circ}\)C. Before using it, the fiber was conditioned at 270 \({}^{\circ}\)C for 1 h for the analysis. A blank test was performed before each analysis to prevent the release of undesirable compounds. Semi-quantitative data of milk volatile compounds was obtained by normalizing the peak areas of each compound with respect to the area of the internal standard peak.  
Peak area data were processed by the software Chemstation (Agilent Technologies, Palo Alto, CA, USA). For the analysis of milk, the coefficients of variation (%) of the chemical classes of acids, aldehydes, alcohols, esters, hydrocarbons, and ketones were 10.4, 19.2, 15.3, 13.6, 19.1, and 11.1, respectively. For the analysis of mozzarella cheese, the coefficients of variation (%) of acids, aldehydes, alcohols, esters, hydrocarbons, and ketones were 11.6, 7.7, 5.3, 7.5, 8.9, and 6.1, respectively. The analyses of volatile compounds were performed in triplicate and were repeated on three different samplings (n = 9).