There are several constituents of milk which act as a buffer i.e. they give resistance towards change in pH. These include carbon dioxide, proteins, phosphate, citrate and a number of minor constituents. Due to microbial action additional components are introduced in milk, which include lactate and many other organic anions. Titration of milk in the pH range of 4.8 to 8.3 has been used to assess the behaviour of milk during titration in the presence of phenolphthalein indicator. When milk is titrated against an alkali maximum buffering occurs at a pH range of 4.8 to 6.8 but at pH 8.0 to 8.3 very little alkali is required resulting in lower buffering.
Titration of fresh milk over pH 6.6 to pH 8.3 requires 13 to 20 ml of 0.1 NaoH per 100 ml (1.3 to 2.0 meq. per 100 ml). Most fresh milk samples fall in the range of 1.5 to 1.8 meq. per 100 ml.
Buffering of milk varies between samples and breeds. Some of the constituents responsible for buffering of milk are given here.
i)Carbon dioxide: Milk contains 20 mg CO 2 per 100 ml of milk or 10% by volume. CO 2 behave like an acid in the form of carbonic acid. This on titration offers a titration value of 0.5 meq/100 ml between pH 6.6 and 8.3 out of the total value of 1.3 to 2.0
ii) Proteins: Milk contain around 2.5% casein and 0.6% whey proteins. Casein contributes to a titration value of 0.8 meq/100 ml and whey proteins 0.1 to 0.2 meq/100ml.
iii) Phosphate: Presence of phosphate in milk has three buffering ranges. In the presence of calcium it forms a complex of calcium phosphate. During titration precipitation of calcium phosphate occurs as the pH is raised e.g. pH 6.0 About 0.6 meq of alkali is needed around pH 6.0
Titration of fresh milk over pH 6.6 to pH 8.3 requires 13 to 20 ml of 0.1 NaoH per 100 ml (1.3 to 2.0 meq. per 100 ml). Most fresh milk samples fall in the range of 1.5 to 1.8 meq. per 100 ml.
Buffering of milk varies between samples and breeds. Some of the constituents responsible for buffering of milk are given here.
i)Carbon dioxide: Milk contains 20 mg CO 2 per 100 ml of milk or 10% by volume. CO 2 behave like an acid in the form of carbonic acid. This on titration offers a titration value of 0.5 meq/100 ml between pH 6.6 and 8.3 out of the total value of 1.3 to 2.0
ii) Proteins: Milk contain around 2.5% casein and 0.6% whey proteins. Casein contributes to a titration value of 0.8 meq/100 ml and whey proteins 0.1 to 0.2 meq/100ml.
iii) Phosphate: Presence of phosphate in milk has three buffering ranges. In the presence of calcium it forms a complex of calcium phosphate. During titration precipitation of calcium phosphate occurs as the pH is raised e.g. pH 6.0 About 0.6 meq of alkali is needed around pH 6.0
iv) Citrate: Citric acid as citrate ions has very little buffering capacity of 0.1 meq per 100 ml. However, it complexes with calcium as calcium citrate. This delays the precipitation of calcium phosphate and affect the titration indirectly.
v) Lactate: Lactic acid also delays the titration as it also forms a complex with phosphate ion.
CH 3 CHOH COOH + HPO 4 ̄ ̄ = D H 2 PO ̄ 4 + CH 3 CHOHCOO ̄
Lactic acid Lactate ions
v) Lactate: Lactic acid also delays the titration as it also forms a complex with phosphate ion.
CH 3 CHOH COOH + HPO 4 ̄ ̄ = D H 2 PO ̄ 4 + CH 3 CHOHCOO ̄
Lactic acid Lactate ions
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