This document summarizes the physical and chemical properties of milk. It discusses that milk is a white or yellow-white opaque liquid secreted by mammals for nourishing young. The physical properties discussed include color, flavor, specific gravity, viscosity, surface tension, freezing and boiling points, acidity, and heat stability. The chemical composition of milk is also summarized, including the major components of water, fat, protein, lactose, minerals, vitamins, and enzymes. Factors that can influence the chemical composition are also listed.

1. Physical and chemical properties of milk
Dr. Dhanya KC
Assistant Professor
Department of Microbiology
St. Mary’s College, Thrissur-
680020
Kerala

2. Milk -
Fluid secreted by mammals for nourishment of young
White or yellow-white, opaque liquid
Colostrum –
Concentrated fluid secreted immediately after parturition

3. Physical Properties of Milk
• Colour and optical properties
• Flavor of milk
• Specific gravity and density
• Viscosity
• Surface tension
• Freezing point and boiling point of milk
• Acidity and pH
• Heat stability of milk

4. Physical Properties of Milk
Colour and optical properties
• Turbid and opaque - light scattering by fat globules and protein micelle
• Skim milk - slight blue - higher protein content
• Creamy colour of cow milk - Beta-carotene content
• Greenish tinge in whey - Riboflavin
• Refractive index - ranges from 1.3440 to 1.3485 at 20ºC.
Flavour of milk
• Naturally - sweet flavor
• Off-flavours develop quickly in milk –feed, bacterial growth, enzyme
activities, oxidative reactions, etc.
• Bacterial growth - fruity, barny, malty or acid flavours
• Enzyme activities – rancidity
• Oxidative reactions - cardboard flavour
• Processing of milk - cooked flavours

5. Physical Properties of Milk
Specific gravity and density
• Heavier than water
• Specific gravity varies from 1.018 to 1.036
• Density varies within the range of 1.027 to 1.033 kg/cm3 at 20°C
Viscosity
• Depend on temperature and amount and dispersion of casein and fat
• Viscosity of whole milk at 25°C - about 2.0 cP
• Cooler temp viscosity - voluminosity of casein micelles
• Temp above 65°C viscosity - denaturation of whey proteins
• or in milk pH - viscosity - voluminosity of casein micelles
• Effect of agitation on milk viscosity - not uniform

6. Physical Properties of Milk
Surface tension:
• Surface tension - 50 dyne/cm at 20°C
• Dependent upon proteins, fat, phospholipids and fatty acids present
• Homogenization and heating increase surface tension
Freezing point and boiling point of milk
• Freezing point - vary from -0.512 to -0.572ºC
• Boiling point - 100.17ºC
Acidity and pH
• Freshly drawn milk - pH ranging from 6.5 to 6.7
• Titratable acid - 0.14 to 0.18%
Heat stability of milk - Time required to induce coagulation at a given temp
Temp required to induce coagulation in a given time
• Depends on caseins and salt balance
• Added citrates, phosphates and calcium influence

7. Chemical Properties of Milk - Composition of Milk
Several factors influence
• Age and health of the animal
• Breed of animal
• Seasonal changes
• Weather
• Nature and quality of feed
• Stage of lactation, etc.
Major components - water, fat, protein and lactose minerals,
vitamins and enzymes
Fresh milk from a healthy cow - free from bacteria

8. Water
The principal constituent in milk - 79% to 90%
All other constituents - dissolved or suspended in water
Water activity in milk is high - 0.993
Removal of water increases shelf life of milk
Powdered milk have more shelf life

9. Lipid content
• Most variable feature
• Main fatty acids
• C14 (myristic)
• C16 (palmitic)
• C18 (stearic
• C18:1 (oleic)
• Present in fresh milk as fat globules
• Surrounded by phospholipid layer (milk fat globule
membrane)
• Diameter about 5 mm
• Milk contains about 1012 fat globules per litre
• Upon standing, the fat rises to the surface of the milk and
produce cream line
• Homogenization reduces this tendency

10. Proteins
• About 80–85% of protein in milk – caseins
• 5 main classes of caseins
• Aggregate with calcium phosphate to form micelles
• Average diameter - around 0.2 mm
• Milk contains 1015 casein micelles per liter
• Micelle stabilized by k-casein
• Enzymatic cleavage of k-casein – formation of
coagulum
• Rest of protein in milk - Whey proteins
• β - lactoglobulin and α-lactalbumin
• Milk also contains serum albumin and immunoglobulins

11. Carbohydrates
• Contain Lactose, Glucose, Galactose, Glycoconjugates
• Main carbohydrate is lactose
• Lactose in three forms
• α - lactose monohydrate
• β – lactose
• anhydrous α – lactose

12. Minerals - All essential for human nutrition
• Potassium, Sodium, Calcium, Magnesium, Chloride, phosphate
esters
Enzymes - 20 different enzymes
• Lipoprotein lipase
• Alkaline phosphatase
• Sulfhydryl oxidase
• Lactoperoxidase
• Catalase
• Xanthine oxidase
• Superoxide dismutase
• γ- Glutamyltransferase, etc.