Casein micelle structure: susceptibility of various casein systems to proteolysis

The somatic cell count (SCC) in milk is associated with increasing proteolytic degradation of caseins and it has been suggested that enzymes derived from somatic cells contribute to a lower yield and poorer quality of cheese. It is essential to increase the knowledge on naturally occurring milk proteinase activities to better understand how to improve the technological quality of milk. The aim of this work was to identify peptides actually present in milk as a result of proteolysis at different levels of SCC and to assign these peptides to potential responsible proteases where possible. Peptide fractions were prepared from acid whey by ultrafiltration at a molecular cut-off value of 10 000 Da. The peptides were separated using capillary reversed phase high performance liquid chromatography (RP-HPLC) and identified by matrix-assisted laser desorption/ionization-time of flight tandem mass spectrometry (MALDI-TOF MS/MS). Peptides identified ranged in mass from 1023 to 2000 Da, and originated from alphaS1-, alphaS2- or beta-casein. Possible responsible proteases that could be suggested when examining the peptide cleavage sites included plasmin, cathepsin B, D and leukocyte elastase. The results indicated that plasmin was primarily responsible for the observed proteolysis in milk at low cell count, whereas the cathepsins and elastase became implicated at elevated cell count. Specificity and activity of cathepsins and elastase has earlier mainly been studied in model systems, whereas less is known about their activities in milk itself. This is also the first indication of involvement of elastase in milk proteolysis through the unequivocal determination of cleavage sites.

Section: Discussionmentioning

confidence: 98%

Identification of peptides in milk as a result of proteolysis at different levels of somatic cell counts using LC MALDI MS/MS detection

Wedholm

Møller²,

Lindmark-Månsson³

et al. 2008

“…8-1 -43-6 lower than that of y 3 -casein, the most important from a quantitative point of view in all cheeses with the exception of those of the Bleu de Bresse variety. Regarding the products with the highest electrophoretic mobility, all the varieties except Roquefort contained the component a sl -I, the major product in the degradation (hydrolysis of the 23-24 peptide bond; Fox & Guiney, 1973) of a sl -easein by chymosin. The presence of this C-end polypeptide of a sl -casein in Cabrales cheese indicates that it was made from cows' milk.…”

Section: Hydrolytic State Of Caseinsmentioning

confidence: 99%

Proteolysis of Cabrales cheese and other European blue vein cheese varieties

Fernández-Salguero

Marcos

Alcalá

et al. 1989

SummaryThe extent and level of proteolysis were evaluated in Cabrales and five other European blue vein cheese varieties, Bleu de Bresse, Danablu, Edelpilzkäse, Gorgonzola and Roquefort, by analysing their different N-containing constituents and determining the degree of casein hydrolysis. The electrophoretic patterns of the caseins in the cheeses confirmed their strong proteolysis, components featuring the lowest electrophoretic mobility offering the greatest resistance to subsequent hydrolysis. Component αs1 — I was detected in all varieties studied with the exception of Roquefort cheese.

“…However, in those experiments, a sl -casein was hydrolysed at a significantly slower rate in disrupted than in native micelles, as observed in the present work for ^-casein. In Fox & Guiney's (1973) study, it appears from the data that /?-casein hydrolysis by rennet proceeded further in milk than in CPF milk during 48 h at 2 °C. However, those results must have been influenced by extensive dissociation of /?-casein from the micelle and exchange with that in the serum during the incubation period (Creamer et al 1977).…”

Section: Discussionmentioning

confidence: 74%

Probing the location of casein fractions in the casein micelle using enzymes and enzyme–dextran conjugates

Chaplin¹,

Green²

1982

The rates of milk clotting and of formation of para-/c-casein in milk, colloidal phosphate-free milk and isolated /c-casein by pepsin and by its soluble size-fractionated conjugates with dextran were determined. Milk clotting with all the enzyme derivatives was dependent on the rate of the enzymic phase and required essentially complete /c-casein hydrolysis at 30 °C and throughout the range of pH 5-6-6-7. K-Casein hydrolysis by pepsin at pH 6-6 was fastest in milk and slowest in isolated /c-casein, but the rate decreased as the enzyme size increased, especially with milk. When corrected for the changes in pepsin activity, the rates of /c-casein hydrolysis in all substrates were identical at 30 and 5 °C, but increased with decrease in pH, especially with the larger enzyme conjugates. Hydrolysis of the C-terminal bonds of /?-and /c-casein in native and disrupted casein micelles by carboxypeptidase A and soluble conjugates of it were also investigated. /c-Casein was hydrolysed much faster, and /^-casein slightly faster, in native than in disrupted micelles by the native enzyme. Increase in the size of carboxypeptidase A increased the rate of hydrolysis of /c-casein in disrupted micelles and also induced lag periods before hydrolysis commenced, especially with disrupted micelles. The results are compatible with a model for the casein micelle in which the /c-casein is on the outside and the casein components are in a more ordered arrangement than in the casein complexes formed on micelle disruption. They also indicate that immobilized coagulants would be unable to clot milk.