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
- 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
- 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,
45 dynes / cm at 60°C
