## 4 Discussion
[73] reported higher levels of aminopeptidase activities in Cheddar cheese with increasing salt concentrations. The lowest aminopeptidase activity (pro-pNA) detected in Pecorino could be ascribed to the dominance of _S. thermophilus_, as remarked by the negative correlation between this species and enzyme activity. _S. thermophilus_ does not harbor the proline iminopeptidase within its enzymatic portfolio [74; 70]. Enzymes involved in the catabolism of FAA and esterase showed the highest activity in Pecorino cheese. In particular, the highest GDH activity could be ascribed to _S. thermophilus_, as evidenced by a positive correlation between these variables, and in agreement with [PERSON] et al. [75] Caprino and Vaccino cheeses were characterized by a relatively high concentration of FAA, consistently with their high peptidase activities. The low concentration of FAA and total area and number of pH 4.6-soluble peptides found in Cacioricotta cheese reflected its low peptidase activities, as well as the minor hydrolysis degree of \(\beta\)-casein estimated through urea-PAGE. Salt inhibits hydrolysis of \(\beta\)-casein [73]. Vaccino cheese showed hydrolysis products from \(\alpha_{\text{S1}}\)-casein, probably because the proteolytic enzymes of microbial renenene could have a higher affinity towards cow \(\alpha_{\text{S1}}\)-casein, compared to the same fraction of other milk species [76]. Besides microbiological, enzymatic and proteolytic traits, we compared the VOC profiles of the four TAP cheeses, using the solid phase micro-extraction followed by GC-MS, well known for two decades as a reliable tool to detect some flavor compounds that, although at low concentrations, strongly impact on aroma and odor of cheese [77]. VOC profiling highlighted that carboxylic acids (free fatty acids, FFA), resulting from hydrolysis of triacylglycerlecols, were the VOC found at the highest concentrations. Although they (especially those with less than 12 carbon atoms) affect the odor and taste of cheese, ketones, alcohols, and esters, produced upon catabolism of FFA, have an even greater influence on sensory traits, due to their lower flavor threshold [78]. Methyl ketones (alkan-2-ones), the VOC class found at the highest (after FFA) concentration in our cheeses, derive from \(\beta\)-oxidation and decarboxylation of carboxylic acids [78]. For instance, 2-heptanone and 2-nonanone derive from octanoic and decanoic acids, respectively. They confer fruity, floral, musty, or \"blue cheese\" notes [79]. Among alcohols, our cheeses were rich in ethanol (from lactose fermentation and/or amino acid metabolism) [80; 81] and some secondary alcohols. The latter may result from the reduction of methyl ketones [82]. For instance, 2-pentanol could derive from 2-pentanone. Other secondary alcohols, such as 1-butanol,3-methyl, and phenylethyl alcohol (_alias_ 2-phenylethanol), could derive from the microbial driven Ehrlich pathway of FAA [83]. Esters may derive from either esterification of FFA with ethanol or the transfer of fatty acid from glycerides (especially mono- and di-glycerides) to ethanol (alcoholysis) [84]. They have a 10-fold lower flavor threshold than their alcohol precursor, often ethanol [85], and confer sweet, fruity, and floral notes to cheese [86]. All the cheeses subjected to this study showed distinct VOC profiles. This depends on milk type and quality, microbial activity [87; 88] and processing operations. Specific dairy processing affects the VOC profile of a cheese so much that this distinctive trait can be used as marker of product and process [89]. In a recent study, several VOC characterizing the profile of model cheeses were associated, through linear discriminant analysis, to cheese milk obtained from different breeding systems [90]. In the present study, Pecorino was the richest cheese in VOC related to lipolysis, i.e., FFA, ketones, and esters.