PHYSIO - CHEMICAL PROPERTIES OF MILK

 

In order to understand the nature of milk, it's several chemical and physical properties are to be studied.

CHEMICAL PROPERTIES OF MILK:-

ACIDITY:-

• Freshly dawned milk shows an amphoteric reaction. It is due to the presence of phosphate, nitrate, carbon dioxide, amino acid, etc., in the milk protein.

(Amphoteric reaction – turns blue litmus paper into red).

• The acidity of milk varies from 0.13 to 0.16.

Acidity is of two types;

• Natural Acidity
• Developed Acidity

1. NATURAL ACIDITY:-

• When normal fresh milk is titrated with a standard alkali solution using phenolphthalein as an indicator then it appears acidic although fresh milk doesn’t contain any acid.

• This acidic nature of freshly dawned milk is due to the natural acidity.

• It is present in the milk in the form of percentage lactic acid.

• This acidity also called as titratable acidity.

• The titratable acidity of cow & buffalo milk;

  Cow milk      -  0.13 to 0.14

                               Buffalo milk  -  0.14 to 0.15

2. DEVELOPED ACIDITY:-

• When milk is kept under atmospheric conditions an increase in the acidity of milk is noted.

• This type of increase in acidity known as developed acidity.

• It is due to the production as a result of microbial action on lactose in milk.

• The microorganism after it leaves the udder of animal converts lactose into lactic acid.

• If the growth of microorganisms is not checked properly, the production of lactic acid continues and a sufficient amount of lactic acid is produced in milk.

pH OF MILK:-

• Normal fresh milk has hydrogen ion constituent 6.5 - 6.7 which indicates that the milk is slightly acidic.

• Lower pH value for the fresh milk indicates bacterial action.

• Higher pH value for the fresh milk indicates udder infection.

• If the animal is suffered from the udder disease called mastitis, then the milk obtained from such animal may have pH above 7 suggesting that the milk is alkaline.

• pH of cow & buffalo milk;

Cow milk       -  6.4 to 6.6

                            Buffalo milk  -  6.7 to 6.8

NOTE:-

• If the acidity of milk increases, pH decreases, and vice – versa.
• The acidity and pH of milk vary with:

Ø  Species

  Ø  Breed

  Ø  Individuality

  Ø  Stage of lactation

  Ø   Health of the animal

PHYSICAL PROPERTIES OF MILK:-

TASTE & ODOUR:-

• To most people, milk is slightly sweet and has a mild aromatic flavor.

• The sweet taste in milk came from the milk sugar – lactose.

• The flavor and the aroma were imparted by butterfat.

• When the milk is produced under unclean surroundings, the taste and aroma are adversely affected as the milk is acceptable of observing all flavors.

• The sources of abnormal flavor may be;

Ø    Bacterial growth

    Ø    Feed

    Ø    Chemical composition

    Ø    Processing and handling

    Ø   Addition of foreign material

COLOUR:-

• The milk contains several types of pigments that impart the color.

• COW MILK:-

• Cow milk is always yellow in color due to the presence of fat-soluble pigment “Carotene”.




• The intensity of yellow color increases in cow milk while feeding green fodder.

• BUFFALO MILK:-
• Buffalo milk is white in color due to the absence of Carotene pigment.


• Buffaloes have the ability to synthesize the Carotene into Vitamin A (which is colorless) inside the stomach.

SPECIFIC GRAVITY:-

Density of milk

SPECIFIC    =                                       

GRAVITY       Density of water 

• Lactometer is used to determine the specific gravity of milk.

• The temperature at which the specific gravity determined is 15.6°C / 60 F

• The most common lactometer used are “Zeal” & “Quevenne”.

• Milk contains many constituents so the specific gravity of milk is slightly higher than that of water.

• Specific gravity of milk is always more than except for the fat.

• Fat is the only constituent that has a specific gravity less than 1.

• The specific gravity of different milk constituents are

Water        –  1

                                                       Fat     –  0.93

                                              Protein      –  1.346

                                                      Lactose     –  1.666

                                              Minerals   –  4.12

• The specific gravity of milk is influenced by the proportion of its constituents.

• Milk fat has been the most variable component has maximum influence on the specific gravity.

• An increase in the fat content decreases the specific gravity and vice-versa.

• Specific gravity of milk normally ranges from 1.028 to 1.034

Cow milk       - 1.028 to 1.032

    Buffalo milk   - 1.032 to 1.034

                        Skim milk       - 1.036

(The specific gravity of skim milk should always be higher than the whole milk from which it is separated).

• The specific gravity varies with temperature.

• At high temperature, lactometer shows lower reading and vice – versa.

• Addition of water decreases the specific gravity.

• Addition of solids increases the specific gravity.

   

                                                           CLR

         SPECIFIC GRAVITY  = 1 +                

                        OF MILK                          1000 

BOILING POINT:-

• The boiling point of any liquid is at the temperature at which the given pressure of the material is in equilibrium, both in liquid and gas.

• This is the temperature at which the liquid phase ‘vaporizes’ and the gas phase ‘condenses/liquify’ according to the heat supply.

• Water boils at 100◦ C at normal atmospheric temperature and pressure.

• The presence of dissolved substances increases the boiling point of the solid-state.

• Milk contains several dissolved substances. Hence it has a higher boiling point than water.

• The boiling point of milk is 100.15°C to 100.17°C

FREEZING POINT:-

• Pure distilled water freezes at 0°C

• Milk freezes at a temperature slightly lower than that of water because milk contains certain suspended solids.

• The freezing point of milk is -0.535°C to -0.555°C

REFRACTIVE INDEX:-

• The refractive index is used to detect adulteration in milk.

• Refractive index of milk is 1.3440 to 1.3480

ELECTRICAL CONDUCTIVITY:-

• The electrical conductivity of milk increases with an increase in temperature.

• The average value at 30°C,

  Cow milk     -  4.67 Milli hoes/cm

  Buffalo milk  -  3.67 Milli hoes/cm

SPECIFIC HEAT:-

• The specific heat of milk ranges from 0.92 to 0.93

VISCOSITY:-

• The viscosity of milk is determined by Ostwald Viscometer.

• At normal temperature, viscosity of milk is 1.52 cp/mPas

SURFACE TENSION:-

• It is the force of attraction between the milk.

• Surface Tension of milk is,

  54.5 dynes / cm at 20°C

45 dynes / cm at 60°C