Despite substantial research, it is still difficult to produce high quality reducedfat Cheddar cheese. The objective of this study was to investigate how two coagulants, bovine chymosin (BC) and camel chymosin (CC) having different proteolytic activities and two starter cultures, an O-culture (O) and a thermophilic strain of Lactobacillus plus O-culture (OLb) having different abilities to release amino acids, contribute to the structure and flavour development in reduced-fat Cheddar cheese. Cheeses manufactured using the four combinations of coagulants and cultures were analysed during a 28-week ripening period for the composition of casein, peptides, free amino acids, rheological properties and for sensory properties at end of ripening. Cheeses with CC, showed less extensive primary proteolysis, lower levels of bitterness and higher stress at fracture, which correlates to a harder structure. Whereas cheeses with BC had a higher amount of peptides released by chymosin, e.g. the bitter peptide β-casein (f193-209) or by starter proteases from the chymosin-produced peptide α s1-CN (f1-23). BC cheeses were also judged to be softer by the sensory panel. Cheeses containing the OLb-culture had a higher amount of free amino acids and lower strain at fracture, which correlates to a shorter structure, and the peptide profiles of cheeses produced with BC and CC were rather similar after 28 weeks in contradiction to cheeses with O-culture. Replacing the traditional coagulant BC with CC reduced bitterness but increased hardness of the reduced-fat Cheddar cheese. Replacing O-with OLb-culture also reduced bitterness but resulted in a shorter structure. The results highlight tools
Lactococcus lactis strains depend on a proteolytic system for growth in milk to release essential AA from casein. The cleavage specificities of the cell envelope proteinase (CEP) can vary between strains and environments and whether the enzyme is released or bound to the cell wall. Thirty-eight Lc. lactis strains were grouped according to their CEP AA sequences and according to identified peptides after hydrolysis of milk. Finally, AA positions in the substrate binding region were suggested by the use of a new CEP template based on Streptococcus C5a CEP. Aligning the CEP AA sequences of 38 strains of Lc. lactis showed that 21 strains, which were previously classified as group d, could be subdivided into 3 groups. Independently, similar subgroupings were found based on comparison of the Lc. lactis CEP AA sequences and based on normalized quantity of identified peptides released from αS1-casein and β-casein. A model structure of Lc. lactis CEP based on the crystal structure of Streptococcus C5a CEP was used to investigate the AA positions in the substrate-binding region. New AA positions were suggested, which could be relevant for the cleavage specificity of CEP; however, these could only explain 2 out of 3 found subgroups. The third subgroup could be explained by 1 to 5 AA positions located opposite the substrate binding region.
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