Physical and chemical properties of milk

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.