PHYSICO-CHEMICAL PROPERTIES OF MILK

PHYSICO-CHEMICAL PROPERTIES OF MILK Physical stateAcidity & pHDensity & specific gravityColourFlavourViscositySurface tensionRefractive indexSpecific heatElectrical conductivityOxidation-reduction potentialBoiling pointFreezing pointImportance of propertiesHelps in detection of adulteration.Helps in determining quality of milk.Helps in processing of milk & milk products.Helps in evaluating physical changes in milk & milk products during processing.

Physical state of milk In milk water is present as continuous phase in which other constituents are either dissolved or suspended. Lactose and Portion of mineral salts form Solution. Protein and Remainder of minerals form Colloida.l Fat forms Emulsion.

Acidity and pH of milk(a) Natural or apparent acidity(b) Developed acidity or real acidity Titratable acidity = DA 0.13 to 0.14 % Cow milk 0.14 to 0.15 % Buffalo milkCasein, acid phosphate, citrates, whey proteins, CO2 etc of milk givesNA Produced Lactic acid from Lactose by LAB gives- DA TA is measured in terms of present % Lactic acid pH of fresh milk 6.4 to 6.6 - Cow milk 6.7 to 6.8 - Buffalo milk

Density and specific gravityD = Mass (Weight) / VolumeSG is the ratio of density of the substance to density of a standard substance(Water). SG of milk is usually expressed at 600F(15.60C). Average SG of milk at 600F ranges from 1.028 to 1.030 for cow milk and 1.030 to 1.032 for buffalo milk. For skim milk it ranges from 1.035 1.037Specific gravity of milk is lowered by addition of water and cream and increased by addition of skim milk or removal of fat.Although buffalo milk contains more fat than cow milk, its specific gravity is higher than the cow milk; this is because buffalo milk contains more SNF with fat which results ultimately results in higher specific gravity. Percentage of TS or SNF in milk is calculated by formula% TS = 0.25D + 1.22 F +0.72% SNF = 0.25D + 0.22 F +0.72D = 100 (d-1)d= density of sample of milk at 200 C (680 F)F= fat percentage of sampleColor of milkColor of milk is a blend of individual effects produced byColloidal calcium caseinate/phosphate particles and dispersed/emulsified fat globules, both of which scatter light.Carotene (to some extent xanthophylls), which imparts a yellowish color.Color ranges from yellowish creamy white (cow milk) to creamy white (buffalo milk). Intensity of yellow color of cow milk depends on various factors such as breed, feeds, size of fat globules, fat percentage etc.The greater intake of green feed, results in deeper yellow color of cow milk.Larger fat globules and higher fat percentage also results in increased intensity of yellow color.Upon heating whiteness increases due to increased reflection of light by coagulate.Skim milk has a bluish and whey a greenish yellow color (due to presence of riboflavin)

Flavor of milkFlavor is composed of small (odor) and taste. Flavor of milk is a blend of the sweet taste of lactose and salty taste of minerals.Phospholipids, fatty acids and fat of milk also contribute to the flavor.Changes in milk flavor may occur due to type of feed, season, stage of lactation, condition of udder, sanitation during milking and subsequent handling during processing. A pronounced flavor of any kind is considered abnormal, source of it may beBacterial growth Feed Absorbed Chemical composition Processing & handling Chemical changes Addition for foreign material

ViscosityViscosity is the resistance to flowMilk: 1.5 2.0 cp at 20 0 CIncrease in temperature results in decrease in viscosity.Casein contributes more to viscosity than any other constituents.

Surface tensionSurface tension affected by fat content i.e. addition of fat lower down ST.ST of water : 72 dynes /cm2 at 200 CMilk : 50 dynes /cm2 at 200 CSkim milk : 52 52.5 dynes /cm2 at 200 C

Refractive IndexRefractive Index is the measure of change in direction of light beam in a medium.= Sin i/ Sin rFor water 1.33Milk 1.344 to 1.348RI is affected by protein, lactose & minerals, not by fat.Instrument Abbe Refractometer

Specific HeatHelps in fabrication of equipment and calculating heat requirements to process milk.specific heat milk 0.938 cal at 150 C

Electrical conductivityMeasured in terms of specific resistance or specific conductance.It depends on ions present in milk, mostly Na+ , K+, Cl- ions are responsible.Sp. conductivity = sp. conductance sp. resistanceFor milk (sp. conductance) 4.2 to 6.9

Oxi- Red potentialOxi - red potential concerned with the balance in between oxidized and reduced forms of the chemical substances.Milk + 0.2 to 0.3 evFat, sugar, protein-no affect on ORPAscorbic acid, lactic acid influences ORP

Boiling pointBoiling point increases with increase in TSPure water 1000CMilk 100.170

Freezing pointPresence of soluble constituents lower or depress freezing point.For milk - 0.525 0C to 0.565 0CLactose & minerals affects FP.Fat & proteins have no effect on FP. Boiling & sterilization increase the value of FP depression but pasteurization has no effect.

Collegative propertiesProperties, which depend on number of solute particles in solution i.e. FP, BP, ORP, VP.