Physicochemical properties

PHYSICOCHEMICAL PROPERTIES OF MILK

Milk is the physiological secretion of normally functioning udder. It is a very complex substance having many characteristics and physical properties, the knowledge of which, is essential since it will be useful when milk is used to prepare products and when it is subjected to many processing to make it safe.

Water is the dispersion media for other solids. It is in continuous phase of liquid to which the constituents are dispersed. Solids exist in 3 states of dispersion. They are,

  • Coarse dispersion [particles greater than 0.001 mm]
  • Colloidal dispersion [particles between 0.001 mm and 0.000001 mm]
  • Molecular dispersion [particles Less than 0.000001 mm].

Materials in solution

  • Materials in solution are called materials in molecular dispersion. These include most of salts, lactose, part of albumin, globulin. A true solution is defined as one in which the molecules are dispersed individually. Some of the albumin and globulin, which are large in size, also exist in colloidal state.

Materials in colloidal dispersion

  • Here the particles are microscopic but they are large enough to be held by an ultra filter. E.g. Calcium caseinate, part of albumin and globulin and calcium phosphate. These can be separated by dialyzes through appropriate membrane [semi permeable membrane]. For the suspension of these colloids, certain materials are required and these are called as stabilizers. In milk, the colloids themselves function as stabilizers to each other.

Materials in coarse dispersion

  • Butterfat is suspended as tiny spherical particles called fat globules. Cellular constituents also come under this category [E.g. In mastitis, leukocyte count of milk exceed 5 lakhs]. Fat particles vary in size from 0.1 m to 10 m in diameter, with an average of 3 m .1 ml of milk will contain approximately 2-4 million fat globules.

PHYSICAL PROPERTIES OF MILK

Milk acidity

  • pH of milk is approximately 6.6, which lies on the acid side of neutral (7). Fresh milk drawn from the udder possesses a certain acidity termed as natural acidity, which has to be distinguished from developed acidity in the form of lactic acid. Normal acidity or natural acidity is due to casein, acid phosphates and citrate and to a lesser extent by albumin, globulin and CO2. Natural acidity varies from one cow to another. It ranges from 0.08-0.30%. Colostrum has high natural acidity because of its high protein content. Acidity is higher than normal during the early lactation period. It falls to normal in about 2 months of lactation.
  • Titrable acidity or total acidity is equal to natural acidity and developed acidity. Natural acidity is also called as apparent acidity. Developed acidity is also called as real or true acidity. Titrable acidity is used as rapid platform test for accepting or rejecting milk and to find whether the milk is suitable for heat processing, because the developed acidity lowers the temperature of heat coagulation of casein. 0.18 to 0.19% is taken as a deciding line between satisfactory and unsatisfactory milk samples.
  • Milk of high natural acidity is usually high in their total solids content. They have a fair quantity of buffering substances like proteins, phosphates and citrate, which resist changes in pH when acids or alkali are added. So milk of high natural acidity is highly buffered. Such milk requires more lactic acid to develop before the pH reaches the isoelectric point of casein [pH 4.6] which means that a longer time will be required before such samples curdle at ordinary temperature. High natural acidity increases the keeping quality.

Colour

  • Characteristic white colour or white opalescence of the milk is due to scattering of light by the colloidal particles. Yellow colour of the milk is due to the carotene. The intensity of yellow colour increases in cow milk when they are fed with green fodder. Buffalo milk is white in colour due to the absence of carotene which is efficiently converted to vitamin A. Dilute acid or rennet when added results in coagulation of casein and fat and the separated whey will be having a distinct greenish yellow colour due to the pigment riboflavin.
  • The colour of the opaque object is the colour it results. The colours of the visible spectrum are absorbed. Thus an object is yellow because more yellow light is reflected to the eye than any other colour. (A white object reflects all the colours of the light that fall on it while a black object absorbs all of them).

Flavour

  • A property very difficult to define. It is a combination of taste and smell. Milk has a characteristic mild, pleasant flavour. Sweet taste of lactose is balanced against the salty taste of chlorides. Some research workers attribute the rich flavour of milk to the fat present in milk. As lactation advances, lactose declines while chloride increases; the taste is deflected towards salty. A similar effect is caused by udder infections. When odouriferous substances like garlic are fed, milk gets the characteristic taste, by its passage to the milk from the blood. Milk also absorb the volatile vapors present in the atmosphere. Both these types can be avoided by feeding such feeds immediately after milking. Feeding molasses and beet by-products sometimes causes fishy flavour. Metals like copper acts as a catalyst to develop oily taints if it is present in a concentration of 3 ppm; it imparts a metallic taste. Cows suffering from ketosis produce milk with cowy odour due to entry of ketone bodies from blood to milk. As lactic acid develops, the flavour of milk changes towards characteristic sour odour. This is due to the production of various by products like butyric acid, diacetyl, etc.

Specific gravity

  • The term specific gravity as applied to milk means the weight of the given volume of milk compared with the weight of same volume of water at the same temperature
  • Average specific gravity of milk and its components at 60 ° F (15.6 ° C)
    • Cow milk : 1.028-1.030
    • Skim milk : 1.035-1.037
    • Buffalo milk : 1.030-1.032
    • Water : 1
    • Fat : 0.93
    • Protein : 1.346
    • Lactose : 1.666
    • Salts : 4.12
    • SNF : 1.616
  • The variation in specific gravity of different individuals milk is due to the flocculation in water, fat, protein, lactose and minerals of milk. There will be difference in specific gravity of liquid fat and solid fat. One of the peculiarities of milk fat is that there is appreciable time lag in adjusting its physical condition to a change in temperature. Addition of water lowers the specific gravity. High fat milk has low density. Removal of fat results in separated milk or skim milk or defatted milk, which has a higher specific gravity, can be restored by adding water.
  • Specific gravity is lowered by addition of water, addition of cream or by increasing the temperature, while the contrary effect is caused by addition of separated milk or skim milk, removal of fat or lowering the temperature.

Recknagel phenomenon

The specific gravity of freshly drawn milk is low. It increases by 0.001 as time advances. This is due to

  • Partial cooling and solidification of fat
  • Hydration of protein
  • Loss of CO2
  • Escape of air bubbles.

Surface tension

  • Surface tension is due to the force of attraction between molecules. When compared to water surface tension of milk is low. The surface tension of milk at 20 ° C is 54.5 dynes/cm. It decreases as the temperature is raised (at 60 ° C it is about 40-45 dynes/cm). The presence of fat lowers the surface tension. Whole milk has a slightly lower surface tension than skim milk and that of cream is still lower. Milk and cream on aging undergo a slight decrease in surface tension. Colloidal constituents like proteins also lower the surface tension along with fat globules. The substances, which lower the surface tension, will get concentrated at the liquid air interspace. When milk is warmed, calcium caseinate gather at the liquid air interspace together with small amount of fat globules, albumin & globulin. If milk is agitated similar concentration occurs around the air bubbles & the phenomenon of frothing or foaming occurs.
  • Recknagel found that the specific gravity of freshly drawn milk was lower than the specific gravity subsequently obtained, after an hour or later. He found that the rise in specific gravity to be regular, more rapid at lower temperatures than at higher ones and to amount on an average of 0.001. This is called Recknagel's phenomenon.

Viscosity

  • The viscosity of a substance refers to its resistance to flow. It is a measure of friction between molecules as they slide. Milk is considerably more viscous than water mainly on account of fat emulsion and colloidal particles. Homogenization increases viscosity. Increase in temperature causes reduction in viscosity. At 20 ° C, milk will be half viscous as it is at 0 ° C and at 40 ° C, it will be 1/3rd viscous as it is at 0 ° C.

Oxidation-reduction potential

  • Incase of organic materials, oxidation is defined as the uptake of oxygen or loss of hydrogen. In the same manner, reduction may be defined as the process of loosing oxygen or gaining hydrogen. In ionic system, it can be demonstrated that phenomenon may involve loss or gain of electrons. In practice, the potential difference created by platinum electrode in a solution of an oxidant or reductant is measured by completing a circuit through calomel half-cells and a potentiometer. The voltage measured under these conditions reflects the oxidizing or reducing capacity of the solution. This potential is called as oxidation-reduction potential or O-R potential (or) redox potential and is designated by Eh.
  • The O-R potential of milk normally falls within the range of +0.2 to +0.3 volts. In milk, the O-R potential is controlled by the following substances.
    • Dissolved O2
    • Ascorbic acid
    • Riboflavin
    • Lactose
    • Cysteine
    • Cystine
  • Milk under anaerobic conditions as in udder has O-R potential of 0.13 volts. It increases to +0.3 volts on contact with air in the atmosphere. The bacterial action reduces the O-R potential. Methylene Blue dye Reduction Test (MBRT) is based on the principle of lowering of the O-R potential through use of available oxygen by the microorganisms.

Refractive index

  • Milk has a refractive index of about 1.35 and that of water is 1.33. So addition of water lowers the refractive index.

Boiling point

  • Boiling point of any liquid is the temperature at which at the given pressure the material is in equilibrium both as a liquid and as a gas. This is the temperature at which the liquid phase will vaporize and the gas phase condense or liquefy according to the heat supply.
  • Water boils at 100 ° C under normal atmospheric temperature and pressure. The presence of dissolved substances increases the boiling point of a solution. Since milk contains several dissolved substances, it has higher boiling temperature than that of water. Because there is variation in the dissolved substances, the boiling point of milk also varies between 100.15-100.17 ° C [100.2-101.02].

Freezing point

  • The freezing point is the temperature at which, at a given pressure, a material is in equilibrium as both a solid and as a liquid. This is the temperature at which the liquid phase may freeze or crystallize and the solid phase may melt or liquefy.
  • Pure distilled water freezes at 0 ° C under normal atmospheric pressure. Milk freezes at a temperature slightly lower than that of water due to soluble constituents like lactose, soluble salts [chlorides] which lower and depress the freezing point [to an extent of 75% total depression]. Fat and protein, two variable constituents of milk will have very little influence on the freezing point depression. The range of value is –0.525 to –0.565 ° C [-0.55 ° C]. Season has no influence on freezing point. By determining the freezing point in milk, it can be ascertained whether water is added or not. The equipment used to determine the freezing point of milk is Hortvet cryoscope.

Percentage of water

added to milk = T-T'  / T X 100  

Where,

T = normal freezing point [-0.550c]

T' = observed freezing point of the given sample.

The addition of 1% water to milk will raise the freezing point by 0.0060c.

Limitations of freezing point estimation

  • It is unable to find out addition of separated milk.
  • It is unable to detect the removal of fat in milk.
  • Freezing point is seriously affected by the developed acidity.

Electrical conductivity

  • In a pure solution, the conductivity is a function of the ionic concentration. In an heterogeneous system such as exists in milk, the fat and the colloidally dispensed substances obstruct the ions in their migration and decrease the conductivity. The electrical conductivity increases with increase in temperature. About 80% of current in milk is carried by chloride ions [chloride content increases during mastitis]. The electrical conductivity value of cow milk is 0.005 ohm-1 cm-1 at 25 ° C.

Adhesiveness of milk

  • When a piece of paper is moistened with milk, it sticks to surface of wood or glass or metal due to casein glue.

Cream Raising

  • When milk is allowed to stand, fat rises to the top and eventually forms a layer packed with fat globules called cream. This is due to the difference in the specific gravity of serum and fat. One drop of milk contains about one lakh globules with the diameter varying from 0.3 to 10 m .

Foaming

  • It is due to materials that lower the surface tension like milk protein and fat. Milk fat not only increases the foaming, but also increases the stability of the foam.
Last modified: Thursday, 12 April 2012, 7:00 AM