Milk has several important physical and chemical properties. It is a colloidal solution with water as the continuous phase and fat, protein, and minerals dispersed throughout. The acidity, pH, density, freezing point, color, and flavor of milk are all indicators of its composition and quality. Fresh cow milk has a titratable acidity between 0.13-0.14%, pH of 6.4-6.6, density of 1.028-1.030, and freezing point of 0.54°C. These properties are affected by the milk's fat, protein, mineral, and lactose content as well as any bacterial contamination.
2. Physico- chemical properties of milk
• 1. Physical state of milk-
water is the continuous phase in which other
constituents are dissolved or suspended. Protein and
minerals are found as colloidal suspension and fat as
emulsion.
• 2. Acidity of milk-
Freshly drawn milk is amphoteric to litmus i.e. It turns
red litmus blue and vice versa.
However milk shows a certain acidity as determined by
Titration with an alkali (NaoH) in presence of an indicator
(phenolphthalein) . This acidity is known as titratable acidity
(T.A.).
3. • Natural or apparent acidity (N.A.)-
• due to presence of casein, acid- phosphates , citrates
etc. in milk.
• The higher the SNF content of milk higher is the N.A. of milk
and vice versa.
• T.A. of cow milk varies from 0.13-0.14 %.
• Buffalo milk = 0.14-0.15%.
• Developed or Real acidity-
• due to lactic acid formed in the milk as a result of
bacterial action on lactose in the milk.
• Titrable acidity= N.A.+ developed acidity
4. • 3. pH of milk-
• normal fresh cow milk =6.4-6.6
• buffalo milk = 6.7-6.8
• Higher pH values indicate udder infection (mastitis)
• While lower pH values indicate bacterial action.
• 4.Density and specific gravity -
• D = mass/ volume
• Sp.gr= density of substance/ density of water
• The density or sp.gr. of milk may be determined by Pycnometer
or Hydrometer or Lactometer.
• Milk is heavier than water.
• Av. Sp.gr. of cow milk (60 F) = 1.028 to 1.030
• Av. sp.gr. of buffalo milk = 1.030 to 1.032
• The sp gr. of milk is lowered by addition of water & cream and
increased by addition of skim milk or removal of fat.
5. • 5. Freezing point of milk
• Milk freezes at temp. slightly lower than water due
to presence of soluble constituents.
• Av. F.pt. of cow milk= 0.54 C
• buff. Milk = 0.55 C
• Mastitis milk shows normal F.pt.
• Lowering of f.pt indicates addition of water.
• 6. Colour of milk-
• casein , fat globules and carotene.
• Buffalo milk = white
• Cow mik= light yellow
• Skim milk = light bluish
• Whey = greenish yellow
6. • 7. Flavour-
• odour(smell) + taste
• Sweetish taste due to lactose.
• Salty taste due to more minerals.
• The fat, fatty acids and phospholipids also contribute to the
flavour.
