Characteristics of Milk Res. Assist, DVM Bahar ONARAN Ankara University, Faculty of Veterinary Medicine Department of Food Hygiene and Technology

Physical and chemical properties of milk; Density Non-fat Dry Matter pH Acidity Freezing point Boiling point Surface tension Viscosity Electrical conductivity Optical properties Flavour

Density Density is defined as an object’s mass divided by it’s volume. The density of milk products can be used to convert volume into mass and mass into volume, to estimate the amount of solids present in milk, and to calculate other physical properties. The density of milk varies within the range of 1028 to 1032 kg /m3 at 20°C.

Density is dependent on: The temperature at the time of measurement Temperature history of the material Composition of the milk (fat,solid and water content) İnclusion of air (a complication with more viscous products)

NON-FAT DRY MATTER DRY MATTER  THE RESIDUE left when water and gases are removed is called the dry matter (DM) or total solids content of the milk. DRY MATTER (DM) 12.5 % FAT 3.5 % NON-FAT DRY MATTER (NFDM) 9 %

Acid-base equilibria Both titratable acidity and pH are used to measure milk acidity. There are many components in milk which provide a buffering action. The major buffering groups of milk are caseins and phosphate.

pH Is expressed as the concentration of hydrogen ions. it is a measure of ionized acids present in milk and it reveals the intensity of acidity. Fresh milk has pH of 6.6-6.8. The pH can be measured by using pH meter or colorimetric methods. The casein in milk forms into a curd at a pH of 4.6.

pH of 4.6 ?? As the isoelectric point (IEP) of the protein and is generally the pH at which the protein is least soluble. For casein, the IEP is approximately 4.6 and it is the pH value at which acid casein is precipitated. In milk, which has a pH of about 6.6, the casein micelles have a net negative charge and are quite stable.

pH of 4.6 ?? (milk products) During the addition of acid to milk, the negative charges on the outer surface of the micelle are neutralized (the phosphate groups are protonated), and the neutral protein precipitates. The same principle applies when milk is fermented to curd. The lactic acid bacteria produces lactic acid as the major metabolic end-product of carbohydrate [lactose in milk] fermentation. The lactic acid production lowers the  the pH of  milk to the IEP of casein. At this pH, casein precipitates.

Titratable acidity It is a measure of acids present in 100 ml of milk. It is the amount of alkali required to bring the pH to neutrality. This property of milk is used to determine bacterial growth during fermentations, such as cheese and yogurt making, as well as compliance with cleanliness standards. Naturally, there is no lactic acid in fresh bovine milk, however, lactic acid can be produced by bacterial contamination. The titratable acidity forms due to the casein and phosphates.

Developed acidity It is lactic acid contents due to lactic acid fermentation. The organism causing the reaction is Lactococcus lactis. The normal TA% ranges from 0.16-0.18 % LA. Higher acidity i.e. ˃ 0.18 % indicates lower keeping quality of milk.

Significants of the TA% test: Milk with a high percent of acid means  it contains a large number of bacteria which get into the milk through either: A- Poor sanitary conditions i.e lack of care and cleanliness. B- Milk has been subjected to improper cooling (if the storage temperature exceeds 4◦C). TA% is valuable for use as guide in the manufacturing operations and for measuring the quality of dairy products.

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 usually in an average of about -0.555° C. When water is added to the milk, the salts dissolved in the serum are diluted and the F.P is raised.

Factors affecting the Freezing point in milk: Acidity of the milk: an increase in the acidity of the milk results in a lower freezing point, because the colloidal minerals dissolved by the acid and this increase the solutes in the milk. Preservatives: Preservatives added to the milk will increase the materials held in solution which tends to lower F.P. Species of animal: The F.P. of milk differs between species of animal and this depends upon the amount of lactose and ash in the milk of each species.

Boiling point Milk is slightly heavier than water, and since the boiling point of a liquid is influenced by factors responsible for its specific gravity, milk boils at a temperature slightly above that of water. Water boils at 100ºC at sea level and average milk boils at 100.17ºC.

Surface tension At the surface of liquids there is a force which tends to contract that surface. The force is seen in falling drops which assume a spherical shape thus bringing the ratio of surface to volume to a mininum value. This force is known as surface tension. The surface tension is measured in terms of the force per unit length and generally is reported in dynes per centimeter. Surface tension of milk is about 52 dyne/cm. The addition of substances to water may cause an increase or a decrease in surface tension.

Viscosity Viscosity is the quantity that describes a fluid's resistance to flow. Viscosity of water is 1.0 and the viscosity of milk is 1.5-4.2. The increase in viscosity is due to the fat and the protein primarily the casein. Changing in viscosity depending on the fat rate is due to the fat globules have different sizes.

Viscosity Temperature: cooler temperatures increase viscosity due to the increased voluminosity of casein micelles temperatures above 65° C increase viscosity due to the denaturation of whey proteins pH: an increase or decreases in pH of milk also causes an increase in casein micelle voluminosity.

Electrical conductivity is mainly due to its soluble salt fraction. Lactose does not conduct current, and fat decreases conductivity. The contribution of proteins and peptides is of minor importance.

Electrical conductivity pH decrease causes hydrogenation of monohydrogen phosphate ions to dihydrogen phosphate ions, which have lower molar conductivity. Thus, addition of lactic acid to a phosphate buffer solution (pH 6.5) decreases conductivity. Fermentation of lactose to lactic acid sharply increases conductivity.

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.

Optical properties 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 ‘opaque' colour of milk. Riboflavin imparts a greenish colour to whey.

Appearance 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.

Flavor of milk: The term flavor means a combination of both taste and odour of milk. Odour: Freshly drawn milk has a characteristic although not pronounced odor. The odor of milk disappears when the milk is allowed to stand a few hours or following cooling or following aeration. Smelling of the milk is important for detection of any abnormal odor or any spoilage.

Flavor of milk: Taste: Freshly drawn milk tastes slightly sweet to most people and the pleasing taste of milk may be correlated with a high lactose and relatively low chloride content. A low lactose and high chloride content probably would mean milk with salty taste. At the end of the lactation period the milk produced by a cow often has such a salty taste, also mastitic milk has a salty taste.

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.