Caracterización tecnológica, química, bioquímica y sensorial del queso de Valdeón con I.G.P. durante la maduración

Laburpena

In recent years, it has conducted several studies to characterize cheeses in Spain, encompassing almost all Protected Designation of Origin (PDO) and Protected Geographical Indication (PGI) currently in existence. However, there are some protected varieties for which only a few studies have been reported such as PGI Valdeón cheese. This Doctoral Thesis Report arises with the main aim of characterize PGI Valdeón cheese studying the influence of ripening time, season of manufacture and heat treatment applied to milk on the final quality of this variety. For this, 12 batches of Valdeón cheese were elaborated, 8 of them from pasteurized milk (2 of them in each season of the year) and 4 from raw milk in Quesería Picos de Europa S.L. (Posada de Valdeón, León, Spain). The study of the first part of the main aim, that is, the influence of ripening time was dealt with in four of the papers of this Doctoral Thesis. Firstly, it was found that Lactic acid bacteria, mainly lactococci, were the predominant microbial groups during the early stages of ripening, gradually being replaced by moulds and yeasts. Enterococci and Enterobacteriaceae counts were very low or zero. This variety was characterised by a total solids content of 61.80 g per 100 g-1 of cheese, a salt/moisture ratio of 8.92 g salt per 100 g-1 moisture, a pH of 6.4–7.6 and a water activity of 0.917. At the end of ripening, primary and secondary proteolysis were very high, which resulted in an almost total degradation of αs1- and β-casein. In addition, as consequence of this fact, the peptide profile of the aqueous soluble extracts at pH 4.6 showed great complexity during ripening and it was observed an increase in free amino acids content, especially during the first two months at which a slowing down was observed coinciding with the huge biogenic amines increase. At the end of ripening, Glu, Ala, Pro, Tyr, Lys, Asp, Leu and Phe were the predominant amino acids, while spermine and tyramine were the majority biogenic amines. Free fatty acids content study showed a significantly increase during ripening, reaching values of 6370.15 mg 100 g-1 cheese at 120 days. Except at 2 days, oleic and palmitic acids were the predominant free fatty acids. The ratios of short, medium and long chain fatty acids to total FFAs were indicative of ripening time. Furthermore, the principal components analysis showed that oleic, palmitic, butyric and myristic acids presented the best correlation with the variables as a function of ripening time. The rheological and textural study showed that 120-day-old cheese was characterized by higher G’, G’’ and G* values, showing a more elastic disposition, as well as, lower fracturability, gumminess and chewiness values and higher adhesiveness values. Moreover, as time increased cheeses were characterized by a decrease in L* and a* values. Finally, the sensory attributes that most influenced the perception of Valdeón cheese were: saltiness and pungenty tastes; sour and pungent odours; adhesiveness and granularity; and taste strength. The second part of the main aim, the influence of season of manufacture, was dealt with in three of the papers that constitute this work. Cheese elaborated in summer and autumn presented lower total mesophilic counts as result of the high total solids and salt/moisture ratio and low water activity. Nitrogen fractions determination showed a great extent and profound of proteolysis in autumn batches, which agreed with the high casein degradation observed. On the other hand, spring cheeses showed the lowest extent and profound of proteolysis. As consequence of proteolytic variations, it was observed significant differences in both, total and individual free amino acids content allowing characterize cheeses according to amino acids profile. However, these differences were not reflected in biogenic amines content. For its part, free fatty acids profile also was influenced by season of manufacture showing the highest concentration in cheeses made in summer and allowing classified the cheese as function of short, medium and long chain fatty acids to total FFAs ratios. Meanwhile, cheeses made in autumn and summer showed higher rheological parameters values, thus they were characterized by presented a more elastic disposition with better capacity to resisting deformation and a more firmness matrix. These results agreed with those observed in texture profile analysis in which autumn and summer batches presented greater hardness. Finally, cheeses made in summer were given higher overall scores, although no significant differences were observed between cheeses elaborated throughout the year. The influence of heat treatment applied to milk on Valdeón cheese characteristics third part of the main aim of this Thesis, was dealt with in three papers. In general, cheeses made from pasteurized milk showed with respect to those elaborated from raw milk lower microbiological counts (5.30 versus 7.57 log units in Rogosa agar), higher total solids content (60.87 g 100 g-1 of cheese versus 51.49 g 100 g-1 of cheese), higher salt/moisture ratio (7.60 versus 6.57), pH values (7.08 versus 6.57) and lower aw (0.937 versus 0.957). It was observed greater extent proteolysis in pasteurized-milk cheeses while raw-milk cheeses showed higher deeper proteolysis. These differences in proteolytic activity were reflected in free amino acids profile, although they were not reflected in total amino acids content. However, it was found significant differences in both, total and individual, biogenic amines content, especially in tyramine content. Similarly, cheeses made from raw milk showed higher free fatty acids content although short, medium and long chain fatty acids to total FFAs ratios were not affected by heat treatment applied to milk. On the other hand, it was observed a more elastic with better capacity to resisting deformation and a more compact matrix in batches elaborated from pasteurized milk, as well as, higher fracturability and hardness values. Finally, cheeses made from raw milk showed overall scores slightly higher. Finally, in the last study the characterization of the peptidome of a Spanish blue cheese, Valdeón, has been conducted before and after gastrointestinal digestion. In addition, the digest were compared to those obtained, in the same conditions, from pasteurized skimmed milk powder using a bioinformatics platform. Peptidomic profiling of digests revealed several regions that are especially resistant to digestion (among them β-casein 60-93, 128-140, and 193-209). Some of them correspond to wellconserved regions between species (human, cow, sheep, and goat) and include peptide sequences with reported bioactivity. The great peptide homology found between both digests, cheese and SMP, suggests that the gastrointestinal digestion could bring closer the profile of products with different proteolytic state. Although most of the biologically active peptides found in cheese after digestion were also present in SMP digest, there were some exceptions that can be attributed to the absence of the relevant precursor peptide before digestion.