Natural acidity of milk: Freshly drawn milk is slightly acidic in nature. This is due to the presence of natural constituents of milk. These include various salts such as phosphate, citrate, carbonate, etc. Also milk constituents like casein, albumin, non-protein nitrogenous compounds and various acids which contribute towards the natural acidity.
Developed acidity of milk: During storage of milk acidity develops due to the fermentation of milk lactose to lactic acid and other acidic components, primarily due to microbial effect. Beyond an acidity of 0.18% as lactic acid milk coagulates on boiling.
COB Test for detection of coagulation of milk: The developed acidity along with natural acidity, which is refered as total acidity, gives positive reaction on boiling milk. A platform test named COB test called clot-on-boiling test is used to detect coagulation of milk. Formation of clot on boiling is an index for acidic abnormal milk.
Measurement of acidity: Acidity of milk can be determined against a standard alkali such as 0.09 N NaOH or 0.1N NaOH solution in the presence of phenolphthalein indicator. On complete neutralization of acidity phenolphthalein indicator changes its colour in milk to faint light pink. The following reaction occurs during titration.
CH 3 CHOH COOH + NaOH → CH 3 CHOH COONa + H 2 O
Lactic acid Sodium lactate
The titratable acidity is expressed as lactic acid per 100 ml of milk.
TA = 0 . 9 × V 1 × N 1
Where V 1 = Volume in ml of the standard
NaOH solution required for titration
N 1 -= Actual normality of the NaOH solution
TA = Titratable acidity
Note: The calculation becomes simpler when 0.09N (N/9)NaOH is used for titration as titre value is direct reading of acidity.
Developed acidity of milk: During storage of milk acidity develops due to the fermentation of milk lactose to lactic acid and other acidic components, primarily due to microbial effect. Beyond an acidity of 0.18% as lactic acid milk coagulates on boiling.
COB Test for detection of coagulation of milk: The developed acidity along with natural acidity, which is refered as total acidity, gives positive reaction on boiling milk. A platform test named COB test called clot-on-boiling test is used to detect coagulation of milk. Formation of clot on boiling is an index for acidic abnormal milk.
Measurement of acidity: Acidity of milk can be determined against a standard alkali such as 0.09 N NaOH or 0.1N NaOH solution in the presence of phenolphthalein indicator. On complete neutralization of acidity phenolphthalein indicator changes its colour in milk to faint light pink. The following reaction occurs during titration.
CH 3 CHOH COOH + NaOH → CH 3 CHOH COONa + H 2 O
Lactic acid Sodium lactate
The titratable acidity is expressed as lactic acid per 100 ml of milk.
TA = 0 . 9 × V 1 × N 1
Where V 1 = Volume in ml of the standard
NaOH solution required for titration
N 1 -= Actual normality of the NaOH solution
TA = Titratable acidity
Note: The calculation becomes simpler when 0.09N (N/9)NaOH is used for titration as titre value is direct reading of acidity.
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