Colour:- Milk ranges in colour from a bluish white to golden yellow, depending upon the breed, species, kind of feed, amount of fat and solids present. E.g. Mares milk is slightly bluish range due to low total solids in milk, buffalo milk is rich white and cow milk is golden yellow.
The white colour of milk is due to reflection of light by dispersed fat globules, calcium caseinate, the colloidal phosphate.
The yellow color of milk is due to the pigment carotene derived from the green plants. The content of yellow colour in the milk depends upon the kind of food given to the animal.
The greenish tint in whey or separated milk is due to the presence of riboflavin.
Reddish yellow color of milk indicates the udder infection.
Taste & Odour:- Normal freshly drawn milk is slightly sweeter & it has a characteristic flavor. The pleasing flavour of milk is due to high lactose content & low chloride content. High chloride content result in salty flavour in milk. Towards the end of lactation period or when animal is suffering with udder disease, the milk produced by a cow often has a salty taste. Abnormal odour in milk may be due to feeding certain aromatic feeds such as onion, garlic, silage etc. Abnormal growth of bacteria also causes changes in flavour. Certain metal such as iron or copper contribute metallic flavour in milk.
Acidity & pH :- Freshly drawn milk shows an amphoteric reaction i.e. it turns red litmus blue & blue litmus red. Normal fresh milk has a pH of above 6.5-6.7 which indicates that milk is slightly acid. When normal milk is titrated with an alkali solution using phenophthalin as a indication, it appears acid, showing from 0.10 to 0.13 % acid. The acidity of fresh milk is due to certain constituents such as phosphates, proteins (casein & albumin), CO2 & citrates. This acidity is known as natural acidity. The acidity more than 0.2 is known as developed acidity which is due to conversion of lactose to lactic acid. The average titrable acidity of cow milk ranges b/w 0.13-0.14 & for buffalo milk 0.14-0.15.
Specific gravity:- Milk contains salt on sugar in solution. It is therefore heavier than water. The average specific gravity ranges from 1.027-1.035 at 15.60C. The specific gravity of milk is influenced by the relative proportion of its constituents, each of which has a different specific gravity approx. as follows- fat 0.93, lactose 1.666, protein 1.346, casein 1.31, and salts 4.12.
Viscosity:-it is the resistance of liquid to its flow on pour. The viscosity of milk is influenced by increase or decrease in temp. of milk. It may be accurately determined in the laboratory and expressed in terms of the centipoises, a unit of force. Cold milk has greater viscosity than warm milk. The absolute viscosity of water at 200 C is 1.005 centipoise while that of milk is 1.5 to 2.0 centipoise.
Boiling point:- milk is slightly heavier than water. Boiling point of milk is greater than that of water due to the presence of certain dissolved substances. The boiling point of milk is 100.170C.
Freezing point:- water freezes at 00C while milk freezes at temp. slightly lower, the average being –O.550C. The range is b/w -0.50 to -0.610C.
Refractive index:- refractive index of milk is higher than that of water. RI of water is 1.333 & milk is 1.344-1.348.
Surface tension:- S.T. of water at 200C is 72-75 dynes/cm while that of milk ranges b/w 40-60 dynes/cm. high fat content in milk increase surface tension.
Thermal Conductivity:- it ranges from 4.2 to 6.9 at 250C. High value indicates milk from mastitis in which the concentration of sodium & chloride is higher. Dilution of milk with water lower the conductivity, temp. & increase the conductivity.
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