PHYSICALL PROPERTIES OF MILK

Dr. M. Ashraf Paul

Division of Livestock Products Technology

Faculty of Veterinary Sciences & Animal Husbandry SKUAST-K Alusteng, Ganderbal Kashmir

Significance of studying physico- Chemical properties of milk

• Design of dairy equipments e.g. heat transfer/ heat conductivity, viscosity etc.• Determination of conc. of one or more components e.g. Sp.gravity to estimate SNF; FP to estimate added H2O.•Assessment of a chemical or physical change e.g. TA to follow microbial activity; viscosity to assess aggregation of protein micelles or fat globule

Physical Properties of Milk Milk is the natural physiological secretion

from normally functioning mammary gland of a mammal intended to nourish the young ones.

All female mammals can secrete milk, the properties of which are similar to those of cows milk in general but there are considerable differences still existing with respect to their physical and chemical nature

Physical properties1. Colour Milk is a liquid of yellow white colour . colour varies from bluish white to light yellow,

depending upon the breed of the cow, the feed fed to the cow, and the quantity of fat and other solids present in it

Cows milk is yellow white that of buffloe sheep, goat and other Spp. is white.

yellow colour of the milk is due to a pigment known as carotene which is synthesized from the green feed fed to the cow

• conversion of carotenes into Vit.A chiefly occures in liver.

In case of buffaloe milk this change is complete and thus buffaloe milk is white.

In case of cows this conversion of carotene into Vit.A is partial so cows milk is yellow in colour.

The white colour (apolescence) of milk is due to reflection of light by the fat globules and the colloidal protein, calcium caseinate & phosphate

The bluish Colour of separated milk or whey is due to another pigment known as Riboflavin (Vit.B2) or Lactochrome.

2. Taste Milk is slightly sweet in taste. This is due

to the presence of lactose (Milk Sugar) in it.

The Sweet taste of lactose is balanced against the salty taste of chloride in Milk.

3. Smell Milk has got a characteristic odour of its

own, when it is drawn from the udder. Freshly drawn milk has a “cowey” odour which

disappears when kept exposed for some time milk has got the capacity to acquire odour from the

surrounding and also from the feed etc. but these odours are abnormal.

Milk has develops odour due to bacterial action and change in its chemical composition

Certain metals may have an adverse effect on the flavour of the milk which comes in contact with them the metals are like copper, and copper alloys, as nickel, brass, bronze etc.

• Rusty cans or other rusty surfaces may prove harmful producing a metallic or tallowy flavors.

4. Acid base Equilibrium Freshly drawn milk has got “Amphoteric Reaction” i.e. it

changes red litmus blue and blue litmus red. Its average pH value is 6.7 on titrating it with an alkali it is found to contain 0.1 to

0.17% acidity. This acidity is not due to lactic acid (Developed) but due

to phosphates of milk proteins Citrates and carbon dioxide present in milk (Natural).

5. Specific Gravity of milk The specific gravity of freshly drawn milk is lower

than sp. gravity obtained, after an hour or later. The rise in sp. gr. is regular ,more rapid at low

temperature than at higher ones and amounts on an average to 0.001. This is called “Recknagels phenomenon” and is attributed to:

i. Change in the sp.gr. Of fat due to partial cooling and solidification.

ii. Hydration of the proteins.iii. Loss of carbon dioxide.iv. Presence of air bubbles. The sp. gravity .of a fluid varies with its temperature. Water reaches its maximum sp. gravity at 390F or

3.90c ,while milk does not attain its maximum Sp. gravity until a temperature of 31.010F or – 0.550c, the freezing point of milk, is reached.

As the milk fat is the lightest constituent of milk, the more that is present the lower the sp. gravity and the greater the percentage of SNF the heavier the milk will be

• The Sp.gravity ranges from 1.025 to1.032 at 600F or 15.50c

• skim milk is heavier than whole milk, the Sp.gravity varying from 1.032 to 1.037

• variations in Sp.gravity are due to variation in amount of various constituents.

Milk fat has a Sp.gravity of 0.935 to 0.945, milk sugar 1.67; salts about 4.0; and proteins 1.31 to 1.346.

6. Freezing Point Milk freezes at -0.550C to -0.560C (31.0 to

30.960F). Skim, whole milk or cream have same FP

Milk has lower freezing point is than water due to the presence of lactose and salts in aqueous phase.

The freezing point is affected by : ~ Increased acidity (Decrease FP) ~addition of preservatives (Decrease FP) ~addition of water

7. Boiling Point Milk is slightly heavier than water because of its

solute content and boiling point of a liquid is influenced by factors responsible for its Sp.gravity.

Milk boils at a temperature slightly higher than water.

water boils at 2120F (1000C) at sea level, while average milk boils at 212.30F (100.170C)

8. Viscosity It is the resistance to flow and is the reverse of

fluidity. Viscosity is the property of all fluids. It can be expressed in only relative terms and for convenience the relative viscosity of any fluid is compared with water.

Water flows with ease .Syrup and honey pour much more slowly and posses greater viscosity.

Milk is 1.5to 1.7 times more viscous than water owing to the presence of solids in milk.

Factors affecting viscosity

~Temperature (At O 0C milk has a fluidity of 0.233 and water has a fluidity of 0.558 & At 200C, these values change to 0.473 and 1.00 )

~fat content ~homogenization

~souring

~ageing

~microbial growth

~high heating followed by cooling

Representative values of viscisity of milk @200c

~whole milk-2.0 cp ~ skim milk-1.5 cp ~whey-1.2 cp

salient contributors

~ caseinate micelles ~ Fat globules

• Heating the milk to pasteurization temperature or agitating it lowers the viscosity.

9. Adhesiveness of milk A piece of paper moistened with milk

sticks to a flat surface of wood, glass or metal. This property is undoubtedly due to casein, which is used in large quantities in the manufacture of casein glue, one of the strongest glues made.

10. Refractive index Milk has a R.I. of about 1.35. that of water

being 1.33 Addition of water would therefore lower the

refractve index of milk But since considerable variation is found in

values for genuine milk, it is not possible to use this property alone as a criterion for the genuineness of milk samples

• Surface Tension • Compared with water the surface tension of milk

is low(Milk-50 ;water-72.75 dynes/cm 200 C)• Some what higher values are shown by separated

milk while cream has a lower surface tension• The colloidal constituents particularly the

proteins are responsible for this lowering of surface tension –have a tendency to get concentrated at liquid/air interface.

• Decreased ST with Increase in temp./fat content• 12. Cream Rising• When whole milk is permitted to stand, the fat

rises to the top and eventually

• forms a layer packed with fat globules called cream.

• The difference in Sp. Gravity between the milk serum and milk fat is one of the most important factors responsible for cream rising .

• At least for the rapid and complete rising of the milk fat, the fat globules must aggregate or clump together.

• The rate of creaming ,then, is dependant on the factors that affect Clumping

• The fat particles are held together in the clumps by the mucin - like material surrounding each fat particle .

• 13. Foaming• Milk has the property of forming form on

agitation.• Foam is due to the formation of a physical

phase in which air becomes incorporated in the milk with thin layers of milk separating the air bubblers from one-another .

• The capacity for foaming is due to materials lowering the surface tension

• The milk protein and fat reduce surface tension and therefore are the causes of the foaming capacity.

• Milk fat not only increases the foaming capacity but also increases the stability of the foam. Milk foam is unstable and breaks down when allowed to stand.

• O/R Potential-• Fresh milk exhibits a potential of +0.20 to

+0.30 v at noble metal electrodes.• The dissolved 02 plays a major role.• Ascorbate, lactates , riboflavin are principal

contributors

Electrical Conductivity:- Considered a possible index of :~ Mastitis infection~ Added water~ Added neutralizes. ~Means of controlling solid conc. The specific conductance of milk reflects its

conc. & activity ions and is of the order of 0.005 siemens (ohms -1 cm -1) at 250 C

Range- 0.0040 - 0.0055