Conversion of αs1-Casein-(24-199)-Fragment and β-Casein under Cheese Conditions by Chymosin and Starter Peptidases

Camel and bovine chymosin hydrolysis of bovine α(S1)- and β-caseins studied by comparative peptide mapping

In many cheese varieties, the general proteolytic activity of the coagulant is of great importance to the development of flavor and texture during ripening. This study used capillary electrophoresis and LC-MS/MS to compare the in vitro proteolytic behavior of camel and bovine chymosin (CC/BC) on bovine α(S1)- and β-casein (CN) at pH 6.5 and 30 °C. β-CN hydrolysis was also studied at pH 5.2 and in the presence of 0, 2, and 5% (w/v) NaCl. A total of 25 α(S1)- and 80 β-CN peptides were identified, and initial rates of early peptide formation were determined. The modes of proteolytic action of CC and BC shared a high degree of similarity generally. However, except for a few peptide bonds, CC was markedly less active, the magnitude of which varied widely with cleavage site. Preferential α(S1)-CN (Phe23-Phe24) and β-CN (Leu192-Tyr193) hydrolysis by CC proceeded at an estimated 36 and 7% of the initial rate of BC, respectively. The latter rate difference was largely pH and NaCl independent. Several cleavage sites appeared to be unique to CC and especially BC action, but qualitative differences were often predetermined by quantitative effects. In particular, negligible CC affinity to α(S1)-CN₁₆₄/₁₆₅ and β-CN₁₈₉/₁₉₀ prevented further exposure of the N-terminal products. β-CN hydrolysis by either enzyme was always stimulated at the lower pH, yet either inhibited or stimulated by the presence of NaCl, depending mainly on the predominating type of molecular substrate interactions involved at the specific site of cleavage. The potential impact of this proteolytic behavior on cheese quality is discussed.

Altered specificity of lactococcal proteinase p(i) (lactocepin I) in humectant systems reflecting the water activity and salt content of cheddar cheese

By using various humectant systems, the specificity of hydrolysis of alpha(s1)-, beta-, and kappa-caseins by the cell envelope-associated proteinase (lactocepin; EC 3.4.21.96) with type P(1) specificity (i.e., lactocepin I) from Lactococcus lactis subsp. lactis BN1 was investigated at water activities (a(w)) and salt concentrations reflecting those in cheddar type cheese. In the presence of polyethylene glycol 20000 (PEG 20000)-NaCl (a(w) = 0.95), hydrolysis of beta-casein resulted in production of the peptides comprising residues 1 to 6 and 47 to 52, which are characteristic of type P(III) enzyme activity (lactocepin III) in buffer. The fragment comprising residues 1 through 166, inclusive (fragment 1-166), which is typical of lactocepin I activity in buffer systems, was not produced. Similarly, peptide 152-160 from kappa-casein, which is usually produced in aqueous buffers exclusively by lactocepin III, was a major product of lactocepin I. Most of the specificity differences obtained in the presence of PEG 20000-NaCl were also obtained in the presence of PEG 20000 alone (a(w) = 0.99). In addition, alpha(s1)-casein, which normally is resistant to lactocepin I activity, was rapidly hydrolyzed in the presence of PEG 20000 alone. Hydrolysis of casein in the presence of PEG 300-NaCl or glycerol-NaCl (both having an a(w) of 0.95) was generally as expected for lactocepin I activity except that beta-casein peptide 47-52 and kappa-casein fragment 1-160 were produced; both of these are normally formed by lactocepin III in buffer. The differences in lactocepin specificity obtained in the humectant systems can be attributed to a combination of a(w) and humectant hydrophobicity, both of which are parameters that are potentially relevant to the cheese-ripening environment.

Contribution of Lactococcus lactis cell envelope proteinase specificity to peptide accumulation and bitterness in reduced-fat Cheddar cheese

Bitterness is a flavor defect in Cheddar cheese that limits consumer acceptance, and specificity of the Lactococcus lactis extracellular proteinase (lactocepin) is widely believed to be a key factor in the development of bitter cheese. To better define the contribution of this enzyme to bitterness, we investigated peptide accumulation and bitterness in 50% reduced-fat Cheddar cheese manufactured with single isogenic strains of Lactococcus lactis as the only starter. Four isogens were developed for the study; one was lactocepin negative, and the others produced a lactocepin with group a, e, or h specificity. Analysis of cheese aqueous extracts by reversed-phase high-pressure liquid chromatography confirmed that accumulation of alpha(S1)-casein (f 1-23)-derived peptides f 1-9, f 1-13, f 1-16, and f 1-17 in cheese was directly influenced by lactocepin specificity. Trained sensory panelists demonstrated that Cheddar cheese made with isogenic starters that produced group a, e, or h lactocepin was significantly more bitter than cheese made with a proteinase-negative isogen and that propensity for bitterness was highest in cells that produced group h lactocepin. These results confirm the role of starter proteinase in bitterness and suggest that the propensity of some industrial strains for production of the bitter flavor defect in cheese could be altered by proteinase gene exchange or gene replacement.

Acceleration of cheese ripening

The characteristic aroma, flavour and texture of cheese develop during ripening of the cheese curd through the action of numerous enzymes derived from the cheese milk, the coagulant, starter and non-starter bacteria. Ripening is a slow and consequently an expensive process that is not fully predictable or controllable. Consequently, there are economic and possibly technological incentives to accelerate ripening. The principal methods by which this may be achieved are: an elevated ripening temperature, modified starters, exogenous enzymes and cheese slurries. The advantages, limitations, technical feasibility and commercial potential of these methods are discussed and compared.