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Total rennet-whey-N

a When cheese is made from whole or part skimmed milk, the proteins of the fat globule membrane will be incorporated in the network of the precipitated caseins.

b In milk, a part of h-casein may be hydrolyzed by the action of the milk enzyme plasmin, resulting in g-caseins (the more hydrophobic parts of h-casein) and proteose-peptones (the more hydrophilic parts). c A part of the proteose-peptones, which all are soluble at pH 4.6, is precipitated with the caseins at pH 6.6. The partition of proteose-peptones on the soluble and the nonsoluble parts, after precipitation by rennet at pH 6.6, which in the table is set to 1:1, may be somewhat lower or higher (3,4).

a When cheese is made from whole or part skimmed milk, the proteins of the fat globule membrane will be incorporated in the network of the precipitated caseins.

b In milk, a part of h-casein may be hydrolyzed by the action of the milk enzyme plasmin, resulting in g-caseins (the more hydrophobic parts of h-casein) and proteose-peptones (the more hydrophilic parts). c A part of the proteose-peptones, which all are soluble at pH 4.6, is precipitated with the caseins at pH 6.6. The partition of proteose-peptones on the soluble and the nonsoluble parts, after precipitation by rennet at pH 6.6, which in the table is set to 1:1, may be somewhat lower or higher (3,4).

coagulation takes place at about pH 6.6. The coagulation is brought about by enzyme-catalyzed hydrolysis of a single peptide bond of k -casein:

k - casein ! para - k - casein + glyco - macro - peptide (GMP).

The glyco-macro-peptide (GMP) constitutes about one-third of the mass of the k-casein. The para-n-casein component of the k-casein remains as a part of the para-casein in the curd. The GMP is soluble and separates with the other soluble compounds (whey proteins, lactose, water-soluble vitamins, and the soluble salts) into the whey. Typical figures for the amount of the various groups of milk proteins as percent of total nitrogen, and their distribution on the nonsoluble part (in the cheese curd) and the soluble part (in the whey) after precipitation of casein with acid at pH 4.6 or with rennet at pH 6.6, are listed in Table 3; 76% is given as a typical value for retention of protein for rennet cheese. Variations in the relative amounts of the various protein fractions can cause actual protein retention values to vary from about 75 to 78% of total protein (N x 6.38).

Heat treatment of the milk can cause denaturation af the whey proteins. Heat-denatured whey proteins precipitate together with the caseins giving higher protein retention. Higher retention of protein can also be achieved by concentrating the protein by ultrafiltration. Lower retention of protein may be caused by inflammation of the udder or by growth of psychotropic, proteolytic bacteria during cold storage of cheese milk.

2. Retention of Fat and Bacteria

Fat globules with diameters of 0.5 to 10 Am will be incorporated into the casein network. Consequently, the retention of fat is high (85-95%).

For bacteria, the retention is of the same magnitude as for the fat globules.

3. Retention of Lactose

Lactose is dissolved in the water phase of milk and follows the whey out of the curd; hence, retention of lactose in curd is low, usually in the range of 3 to 5%, depending on the final moisture content of the cheese and on amount of water added.

4. Retention of Ash Compounds

Some of the minerals (K + , Na + , Cl_) in milk are dissolved in the aqueous phase and these minerals separate with the whey. Others are present as colloidal particles integrated in the casein micelles (Ca2+, Mg2 + , PO43~). At the pH of fresh milk, these salts remain in the curd, but as the pH decreases during the syneresis these salts will gradually become dissolved.

Some of the phosphate is covalently bound in the casein molecules as serin-phosphate. This phosphate, which accounts for about 2 /2% of the mass of casein, will be included in the ash in analysis of both milk and cheese, a fact that should be taken into consideration in calculations on cheese yield. The figures found for retention of ash typically are about 3540% for hard and semihard rennet cheese. For Cheddar cheese, with a higher degree of acidification during syneresis, retention of ash is lower—30-35%. For cheeses with acid precipitation of the casein at pH around 4.6, the retention of ash is low—10-15% of total ash.

5. Retention of Citrate

Most of the citrate in milk is in aqueous solution with 10% linked to the caseins together with calcium in the same way as with inorganic phophate. The retention of citrate will, therefore, depend partly on the whey content of the cheese and partly on the acidification during syneresis. For semihard cheeses, the retention of citrate is typically around 10%.

B. Calculation of Composition and Yield of Cheese

Table 4 shows an example of the calculation of yield of cheese and of composition of cheese. The example is based on typical figures for production of a semihard cheese (e.g., Danbo or Gouda).

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