Physico- Chemical properties of milk :
Physico- Chemical properties of milk :
A. Physical States of Milk: Water is the continous phase in which other constituents are either dissolved or suspended.Lactose and a portion of the minerals salts are found in solution , proteins and the remainder of the minerals in colloidal suspension and fat as an emulsion.
B.Acidity and pH of milk:
(a) Acidity:
Freshly –drawn milk is amphoteric to litmus, i.e. it turns red litmus blue and blue litmus red.
However it shows a certain acidity as determined by titration wit h an alkali ( sodium hydroxide) in the presence of an indicator ( phenolphthalein) .
This acidity also called Titratable Acidity (T.A.)as it is determined by titration, is know as ‘natural’ or ‘apparent’ acidity and is caused by the presence of casein, acid-phosphates, citrates, etc.,in milk.
The natural acidity of individual milk varies considerably depending on species, breed, individuality, stage of lactation, physiological condition of the udder etc,. but the natural acidity of fresh, herd milk is much more uniform.
higher the solids-not-fat content in milk , the higher the natural acidity ( N.A.) and vice versa.
The titrable acidity of cow milk varies on an average from 0.13 to 0.14 % and buffalo milk from 0.14 to 0.15 % .’Developed’ or ‘real’ acidity is due to lactic acid, formed as a result of bacterial action on lactose in the milk.
Hence the titrable acidity of stored milk is equal to the sum of natural acidity and developed acidty.
titrable acidity is usually expressed as a ‘ percentage’ of lactic acid.
(b) pH;
The pH of normal , fresh, sweet usually varies from 6.4 to 6.6 for cow milk and 6.7 to 6.8 for buffalo milk .
Higher pH values for fresh milk indicate udder infection ( Mastities) and lower values , bacterial action.
Note The acitidy and pH of fresh milk vary with:
- species
- Breed
- Individuality
- Stage of lactation
- Health of animals etc
Whereas density of a substance is its mass( weight) per unit volume, specific gravity is the ratio of density of the substance to density of a standard substance (water) .
Since the density of a substance varies with temperature , it is necessary to specify the temperature when reporting densites or specific gravity.
The specific gravity of a substance ( when referred to water at 4⁰ C ) is numerically equal to the density of that substance in the metric system.
The specific gravity of milk is usually expressed at 60.⁰ F.(15.6.⁰ C).
The specific gravity of milk may be determined by either determining the weight of a know volume or the volume of a know weight.
The weight of a know volume may be determined either with a pycnometer or with a hydrostatic balance;while the volume of a know weight is determined by using lactometers, the scale of which is calibrated not in term of volume but as a function of either density or specific gravity.
The common type of lactometer are Zeal, Quevenne, etc. Milk is heavier than water .
The average specific gravity range ( At 60.⁰ F) from 1.028 to 1.030 for cow milk, 1.030 to 1.032 for buffalo milk and 1.035 to 1.037 for skim milk.
The specific gravity of milk is influenced by the proportion of its constituents (i.e. composition) , each of which has different specific gravity approximately as fallows: water-1.000, fat-0.93, protein-1.346, lactose-1.666, and salts-4.12 ( solid-not –fat-1.616).
As milk fat is the lightest constituents, the more there is of it the lower the specific gravity will be and vice versa.
However , although buffalo milk contains more fate than cow milk , its specific gravity is higher than the latter’s this is because buffalo milk contains more solid 0not-fat as well, which ultimately results in a higher specific gravity.
The
specific gravity of milk is lowered by addition of water and cream, and
increased by addition of skim milk or removal of fat.
Note; The specific gravity of milk should not be determined for at
least one hour after it is drawn from the animal; else a lower- than normal
value will be obtained (due to the Recknagel
phenomenon).
D.Freezing point of milk:
Milk freez at temperature slightly lower than water due to the presence of soluble constituents such as lactose, soluble salts, etc. which lower or depress the freezing point.
The average freezing point depression of Indian cow milk may be taken as 0.547 C (31.02.⁰F) and buffalo milk 0.549.⁰C(31.01.⁰F).
Most bulk milk samples have a freezing point depression of 0.530 (31.05.⁰ F) ‘a freezing point depression lower than this value indicates added water.
Mastites milk shows a normal freezing point .
The freezing point test of milk is a highly sensitive one and even up to 3 % of watering can be detected.
While the freezing point of normal fresh milk is remarkably constant and employed mainly for detection of adulteration of milk with water, souring result in a lowering of the freezing p[oint due to increase in the amount of soluble molecules.
Hence the freezing point should be determined on unsoured samples for greatest accuracy.
Boiling and sterilization increase the vale of freezing point depression, but pasteurization has not effect.
The fat and protein content of milk have no direct effect on the freezing point of milk.
The drawbacks of the freezing point test are;
(i) it does not detect the addition of skim milk or removal of fat from the milk sample and
(ii) watered milk , which has subsequently soured
, may pass the test.
E. Colour of milk:
The colour is a blend of the individual effect produced by
(i) The colloidal casein particles and the dispersed fat globules, both of which scatter light, and
(ii) the carotene ( to some extent xanthophylls) which impart a yellowish tint.
Milk range in colour from yellowish creamy white ( cow milk) to creamy white ( buffalo milk).
The intensity of the yellow colour of cow milk depend on various factors such as breed, feeds, size of fat globules, fat percentage of milk, etc.
Certain breeds of cow impart a deeper yellow tint to their milk than other.
The greater the intake of green feed, The deeper yellow the colour of cow milk.
The larger the fat globules and the higher the fat percentage ,
The greater the intensity of the yellow colour.
Skim milk has a bliish , and whey a greenish yellow colour (which is milk is masked by the other constituents present).
Note:
(i) The colour of food is an important aspect of their marketability . colour has three aspects, viz. tint , intensity and uniformity. Variation in intensity is tolerated as it occurs in practice.
(ii) The colour of an opaque object is the colur its reflects, the colur of the visible spectrum are absorbed. Thus an object is yellow because more yellow light is reflected to the eyes than any other colour. ( A white objects reflecta all the colour of light that fall on it, ahile a black object absorbe them all.
Milk is slightly heavier than water , and since the boiling point of a liquid is influenced by factors responsible for its specific gravity, milk boil at a temperature slightly above that of water.
Water boils at 212.⁰F (100.⁰C) at sea level and average milk at 212.3.⁰F (100.17.⁰C).
Variation in the boiling point of milk are so slightly that they are of little practical important.
G. Effect of Enzymes on milk:
Milk is subjected to many changes.
The complexity of its composition make milk subject to the action of all types of enzymes which are either present in milk at the time of production , or produced by microorganism which find milk an excellent medium in which they may grow.
The extent to which milk may be acted upon by enzyme may be realized when it is started that The protein , fat, and sugar of milk may be decomposed completely by the action of enzymes.
So many by-products may be formed through the decomposition of these products that it would be difficult to attempt to name them.
The protein of milk may be reduced to simpler forms , sugar may become the source of lactic acid, acetic acid , butyric acid, formic acid, alcohol, etc. f
at may be broken down into the fatty acids and glycerol, carbon dioxide and water.
H.Flavour:
This is composed of smell (odour) and taste.
The flavor of milk is a blend of the sweet taste of lactose and salty taste of minerals, both of which are damped down by protein.
The phospholidids , fatty acids, and fats of milk also contributing tho the flavor.
Changes in the flavor of milk occur due to type of feed , season, stage of lactation, condition of udder, sanitation during milking and subsequent handling of milk during storage.
The sulfydryl compounds significantly contributes to the cooked flavor of heated milk.
Note: A prounced flour of any kind is considered abnormal of milk:
The source of abnormal flavor may be;Bacterial growth, feed, absorbed, chemical composition, processing and handeling, chemical changes, addition of frogein material.
References:
Comments
Post a Comment