Proteolysis is one of the major biochemical events which takes place during cheese ripening and its degradation products, amino acids and peptides, have a considerable influence on the sensory characteristics of cheese (Urbach, 1993). Primary proteolysis leads to the formation of large water-insoluble peptides and smaller water-soluble peptides. Several peptides from bovine milk cheeses have been isolated and identified, particularly from Cheddar cheese (e.g., McSweeney et al. 1994; Singh et al. 1994, 1995, 1997; Gouldsworthy et al. 1996; Fernández et al. 1998). However, there are few data available on the identification of peptides from ewes’ milk cheeses, although Michaelidou et al. (1998) identified some major peptides in the water-soluble fraction of Feta cheese (ewe's milk cheese).

Changes in nitrogen compounds during ripening of 40 batches of Manchego cheese made from raw milk (24 batches) or pasteurized milk (16 batches) at five different dairies throughout the year were investigated. After ripening for six months, degradation of p-κ- and β-caseins was more intense in raw milk cheese and degradation of αs2-casein in pasteurized milk cheese. Milk pasteurization had no significant effect on breakdown of αs1-casein. Hydrophobic peptide content did not differ between raw and pasteurized milk cheese, whereas hydrophilic peptide content was higher in raw milk cheese. There were no significant differences between seasons for residual caseins, but hydrophobic peptides were at a higher level in cheese made in autumn and winter and hydrophilic peptides in cheese made in winter and spring. Raw milk cheese had a higher content of total free amino acids and of most individual free amino acids than pasteurized milk cheese. The relative percentages of the individual free amino acids were significantly different for raw milk and pasteurized milk cheeses. The relative percentages of Lys and Ile increased, while those of Val, Leu and Phe decreased during ripening. There were also seasonal variations within the relative percentages of free amino acids. In raw milk cheeses, Asp and Cys were relatively more abundant in those made in autumn, Glu and Arg in cheeses made in winter, and Lys and Ile in cheeses made in spring and summer. Biogenic amines were detected only in raw milk cheese, with the highest levels of histamine, tryptamine and tyramine in cheeses made in spring, winter and spring, respectively.

An automatic purge and trap apparatus, coupled to a GC-MS was used to study the seasonal variability of the volatile fraction of raw milk Manchego cheese. Both season and dairy significantly affected abundance of most volatile compounds. Most aldehydes, methyl ketones, n-alcohols, and secondary alcohols reached significantly higher concentrations in spring cheeses. Branched chain alcohols showed significantly higher concentrations in autumn and winter cheeses, while significantly higher amounts of diketones were found in summer cheeses. Most ethyl esters reached higher concentrations in spring and winter cheeses and lower in autumn cheeses. Lower concentrations of α-pinene were found in spring cheeses, and higher amounts of limonene were observed in winter cheeses. Heptane and octane were significantly more abundant in summer cheeses. No significant seasonal differences were found either for quality or intensity scores.