Properties of Milk

1. Properties of Milk

2. Milk is a complex colloidal dispersion containing fat globules, casein micelles and whey proteins in an aqueous solution of lactose, minerals and a few other minor compounds. Its physical and chemical properties depend on intrinsic compositional and structural factors, extrinsic factors such as temperature and post-milking treatments. An understanding of these properties is important in the technological and engineering design and operation of milk processes and processing equipment, the design of modern methods of milk analysis, the determination of milk microstructures and the elucidation of complex chemical reactions that occur in milk. Measurement of some of the physico-chemical properties is used to assess milk quality.

3. Basic Properties Physical Properties Chemical Properties Microbiological properties Physico-Chemical Properties

4. Significance of studying physico- Chemicalproperties 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

5. Basic Properties of Milk/ Physical State of Milk Emulsion Colloidal Solution Emulsion: •An emulsion is a suspension of droplets of one liquid into another liquid. Milk is an emulsion of fat in water. Butter is an emulsion of water in fat. The solute is known as the dispersed phase and the solvent is known as the continuous phase. Other examples of emulsions include margarine, mayonaise, cream, and salad dressing. Colloidal solution: •A colloidal solution is when matter exists in a state of division in between a true solution, which is sugar in water, and a suspension, which is chalk in water. The characteristics of a colloid are small particle size, electrical charge, and affinity of the particles for water molecules. •In milk, the whey proteins are in colloidal solution. The casein proteins are in colloidal suspension.

6. Physico-chemical Properties of Milk Color and appearance Taste Smell Density Specific Gravity of Milk Acid-base Equilibria Boiling Point Freezing Point Viscosity Refractive index Surface Tension O/R Potential Electrical Conductivity Adhesiveness of milk Cream Rising Foaming

7. Colour and Appearance/Optical properties Optical properties provide the basis for many rapid, indirect methods of analysis such as proximate analysis by infrared absorbency or light scattering. Optical properties also determine the appearance of milk and milk products. Light scattering by fat globules and casein micelles causes milk to appear turbid and opaque. Light scattering occurs when the wave length of light is near the same magnitude as the particle. Thus, smaller particles scatter light of shorter wavelengths. Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more than the red. The carotenoid precursor of vitamin A, ß -carotene, contained in milk fat, is responsible for the 'creamy' colour of milk. Riboflavin imparts a greenish colour to whey. Refractive index (RI) is normally determined at 20° C with the D line of the sodium spectrum. The refractive index of milk is 1.3440 to 1.3485 and can be used to estimate total solids.

8. The opacity of milk is due to its content of suspended particles of • fat, proteins, and minerals. The color varies from white to yellow depending on • the carotene content of the fat. Skim milk is more transparent and has a • slightly bluish color. • 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 whereas milk of ofbuffloe, sheep, goat and other species 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.B ) or Lactochrome.

9. 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. Smell • Milk has got a characteristic odour of itsown, when it is drawn from the udder. • Freshly drawn milk has a “cowey” odourwhichdisappears 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 odoursare abnormal. • Milk has develops odour due to bacterial action and change in its chemical composition • Certain metals may have an adverse effect on the flavourof the milk which comes in contact with them the metals are like copper, and copper alloys, asnickel, brass, bronze etc. • Rusty cans or other rusty surfaces may prove harmful producing a metallic or tallowy flavors.

10. Density of Milk • Density is defined as an object’s mass divided by it’s volume. It depends on the temperature of the object, composition of the material, and whether or not the object contains air. • The density of milk and milk products is used for the following; • to convert volume into mass and vice versa • to estimate the solids content • to calculate other physical properties (e.g. kinematic viscosity) • Density, the mass of a certain quantity of material divided by its volume, is dependant on the following: • temperature at the time of measurement • temperature history of the material • composition of the material (especially the fat content) • inclusion of air (a complication with more viscous products) • With all of this in mind, the density of milk varies within the range of 1027 to 1033 kg /m3 at 20° C.

11. Specific Gravity of Milk The Specific Gravity of a liquid is a dimensionless unit defined as the ratio of density of the liquid to the density of water at a specified temperature. Specific Gravity of a liquid can be expressed SG = ρ / ρH2O (3) where SG = specific gravity ρ = density of fluid or substance (kg/m3) ρH2O = density of water (kg/m3) It is common to use the density of water at 4 oC (39oF) as reference - at this point the density of water is at the highest - 1000 kg/m3or 62.4 lb/ft3.

12. 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: • Change in the sp.gr. of fat due to partial cooling and solidification. • Hydration of the proteins. • Loss of carbon dioxide. • Presence of air bubbles. • The sp. gravity .of a fluid varies with its temperature. • Water reaches its maximum sp. gravity at 39 0F or 3.90C ,while milk does not attain its maximum Sp. gravity until a temperature of 31.01 0F 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 to1.032at 60 0F or 15.5 0c • skim milk is heavier than whole milk, the Sp.gravityvarying from 1.032 to 1.037 • variations in Sp.gravity are due to variationin 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.

13. Acid base Equilibrium • Freshly drawn milk has got “AmphotericReaction” 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 contain0.1 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) pH The pH of milk is higher, or more alkaline, outside of the cow than inside the cow due to loss of carbon dioxide to the air. The pH of milk is never determined immediately after milking because the processing milk goes through removes dissolved gasses. The pH is determined after processing the milk to assure that lactic acid is being produced at the desired rate by added microorganisms during the preparation of cheeses and fermented milk. The casein in milk forms into a curd or a gel at a pH of 4.6.

14. Titratableacidity Milk is amphoteric in reaction which turns blue litmus to Red and Red litmus to blue. The acidity of milk is of two kinds.i. Apparent or natural acidity which is due to citrates and phosphates present in the milk and dissolved CO2 during the process of milking and thereafter.ii. Real acidity or developed acidity which is due to lactic acid produced by the action of bacteria on lactose in milk.Generally the acidity of milk means the total acidity (Natural + developed) or tirratable acidity. It is determine by titrating a know volume of milk with standard alkali to the point of an indicator line phenolphthalein. Acidity is expressed as percent lactic acid. Since 1 ml of 0.1 N lactic acid contains 0.009 grams of lactic acid, the number of ml. of 0.1 N NaOH required to neutralize the lactic acid in the sample, multiplied by 0.009 will give the amount of lactic acid (grams) in the sample, when the result is divided by weight of milk sample and multiplied by 100 the percent lactic acid will be obtained.

15. 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) • Freezing Point • The freezing point of milk is lower than the freezing point of water because of the dissolved components in milk. • Measuring the freezing point is used as a legal standard to determine if milk has been diluted with water. The freezing point of milk is -0.552oC or 31oF. • Milk freezes at -0.55 0C to -0.560C (31.0 to30.960F). Skim, whole milk or cream have same FP. • Milk has lower freezing point is than water due to the presence of lactose and salts inaqueous phase. • The freezing point is affected by :~ Increased acidity (Decrease FP)~addition of preservatives (Decrease FP) ~addition of water

16. 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 O0C 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~microbialgrowth~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.

17. Refractive index • Milk has a R.I. of about 1.35 and for water being 1.33 • Addition of water would therefore lower the refractveindex of milk • But since considerable variation is found invalues 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 20 0C) • Some what higher values are shown bys eparatedmilk 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

18. O/R Potential • Fresh milk exhibits a potential of +0.20to +0.30 v at noble metal electrodes. • The dissolved 02plays 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-1cm-1) at 250CRange- 0.0040 - 0.0055 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 tocasein, which is used in large quantities inthe manufacture of casein glue, one of thestrongest glues made.

19. 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 dependanton the factors that affect Clumping • The fat particles are held together in the clumps by the mucin - like material surrounding each fat particle . Foaming • Milk has the property of forming formon 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.

20. References http://www.scribd.com/doc/22879742/Physical-Properties-of-Milk http://www4.ncsu.edu/~adpierce/u03_characteristics_milk.pdf