PHYSICO-CHEMICAL PROPERTIES OF
MILK
 Physical state
 Acidity & pH
 Density & specific gravity
 Colour
 Flavour
 Viscosity
 Surface tension
Refractive index
 Specific heat
 Electrical conductivity
 Oxidation-reduction potential
 Boiling point
 Freezing point
Importance of properties
Helps in detection of adulteration.
Helps in determining quality of milk.
Helps in processing of milk & milk products.
Helps in evaluating physical changes in milk & milk
products during processing.
Physical state of milk
 In milk water is present as continuous phase in
which other constituents are either dissolved or
suspended.
 Lactose and Portion of mineral salts form Solution.
 Protein and Remainder of minerals form Colloida.l
 Fat forms Emulsion.
Acidity and pH of milk
(a) Natural or apparent acidity
(b) Developed acidity or real acidity
Titratable acidity = DA
0.13 to 0.14 % Cow milk
0.14 to 0.15 % Buffalo milk
Casein, acid phosphate, citrates, whey proteins, CO2 etc of milk givesNA
Produced Lactic acid from Lactose by LAB gives- DA
TA is measured in terms of present % Lactic acid
pH of fresh milk
6.4 to 6.6 - Cow milk
6.7 to 6.8 - Buffalo milk
Density and specific gravity
• D = Mass (Weight) / Volume
• SG is the ratio of density of the substance to density of a standard substance(Water). SG of
milk is usually expressed at 600F(15.60C). Average SG of milk at 600F ranges from 1.028 to
1.030 for cow milk and 1.030 to 1.032 for buffalo milk. For skim milk it ranges from 1.035 –
1.037
• Specific gravity of milk is lowered by addition of water and cream and increased by addition of
skim milk or removal of fat.
• Although buffalo milk contains more fat than cow milk, its specific gravity is higher than the
cow milk; this is because buffalo milk contains more SNF with fat which results ultimately
results in higher specific gravity. Percentage of TS or SNF in milk is calculated by formula
• % TS = 0.25D + 1.22 F +0.72
• % SNF = 0.25D + 0.22 F +0.72
• D = 100 (d-1)
• d= density of sample of milk at 200 C (680 F)
• F= fat percentage of sample
Color of milk
• Color of milk is a blend of individual effects produced by
• Colloidal calcium caseinate/phosphate particles and dispersed/emulsified fat
globules, both of which scatter light.
• Carotene (to some extent xanthophylls), which imparts a yellowish color.
• Color ranges from yellowish creamy white (cow milk) to creamy white (buffalo
milk). Intensity of yellow color of cow milk depends on various factors such as
breed, feeds, size of fat globules, fat percentage etc.
• The greater intake of green feed, results in deeper yellow color of cow milk.
• Larger fat globules and higher fat percentage also results in increased intensity of
yellow color.
• Upon heating whiteness increases due to increased reflection of light by coagulate.
• Skim milk has a bluish and whey a greenish yellow color (due to presence of
riboflavin)
Flavor of milk
• Flavor is composed of small (odor) and taste. Flavor of milk is a blend of the
sweet taste of lactose and salty taste of minerals.
• Phospholipids, fatty acids and fat of milk also contribute to the flavor.
• Changes in milk flavor may occur due to type of feed, season, stage of lactation,
condition of udder, sanitation during milking and subsequent handling during
processing. A pronounced flavor of any kind is considered abnormal, source of
it may be
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Bacterial growth
Feed
Absorbed
Chemical composition
Processing & handling
Chemical changes
Addition for foreign material
Viscosity
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Viscosity is the resistance to flow
Milk : 1.5 – 2.0 cp at 20 0 C
Increase in temperature results in decrease in viscosity.
Casein contributes more to viscosity than any other
constituents.
Surface tension
• Surface tension affected by fat content i.e. addition of fat
lower down ST.
• ST of water : 72 dynes /cm2 at 200 C
• Milk : 50 dynes /cm2 at 200 C
• Skim milk : 52 – 52.5 dynes /cm2 at 200 C
Refractive Index
• Refractive Index is the measure of change in direction of
light beam in a medium.
• γ= Sin i/ Sin r
• For water ® 1.33
• Milk ® 1.344 to 1.348
• RI is affected by protein, lactose & minerals, not by fat.
• Instrument ® Abbe – Refractometer
Specific Heat
• Helps in fabrication of equipment and calculating heat
requirements to process milk.
• specific heat milk ® 0.938 cal at 150 C
Electrical conductivity
• Measured in terms of specific resistance or specific
conductance.
• It depends on ions present in milk, mostly Na+ , K+, Cl- ions
are responsible.
• Sp. conductivity = sp. conductance
•
sp. resistance
• For milk (sp. conductance) ® 4.2 to 6.9
Oxi- Red potential
• Oxi - red potential concerned with the balance in between
oxidized and reduced forms of the chemical substances.
• Milk ® + 0.2 to 0.3 ev
• Fat, sugar, protein-no affect on ORP
• Ascorbic acid, lactic acid ® influences ORP
Boiling point
• Boiling point increases with increase in TS
• Pure water ® 1000C
• Milk ® 100.170
Freezing point
• Presence of soluble constituents lower or depress freezing
point.
• For milk ® - 0.525 0C to 0.565 0C
• Lactose & minerals affects FP.
• Fat & proteins have no effect on FP. Boiling & sterilization
increase the value of FP depression but pasteurization has
no effect.
Collegative properties
• Properties, which depend on number of solute particles in
solution i.e. FP, BP, ORP, VP.