2 Physico Chemical Properties

8/17/2019 2 Physico Chemical Properties

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PHYSICO-CHEMICAL PROPERTIES

OF MILK

By:

Epi Taufik, S.Pt., MVPH, M.Si, Ph.D

INSTITUT PERTANIAN BOGOR


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• Color

• Flavor

• Density and SpecificGravity

• Surface Tension

• Foaming

• Viscosity• Specific Heat

Physical Properties


• Electrical

• Conductivity

• Freezing Point• Boiling Point

• Refractivity


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• The color of milk is perceived by consumers to be indicative of

purity and richness. Its white color is due to the scattering of

reflected light by the inherent ultramicroscopic particles, fat

globules, colloidal casein micelles, and calcium phosphate.The intensity of white color is directly proportional to the size

and number of particles in suspension.

• Effect of homogenization increases the surface area of fat

globules significantly as a result of the breakup of larger

globules.• Cow’s milk contains the pigments carotene and xanthophyll,

which tend to give a golden yellow color to the milkfat.

Color



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• Difficult to define > interaction of TASTE and ODOR > sweet

taste of Lactose in balance with salty taste of chloride, both

moderated by protein

• Flavor attributed to lactones, methylketones, aldehydes,dimethyl sulfide, SCFA

• Off flavor > balance disrupted > M.O, processing condition,

chemical and biochemical reactions

Flavor



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• The density of milk with 3 –5% fat averages 1.032 g/cm3.

Accordingly, the weight of 1 L of milk is 1.03 kg. To convert the

weight of milk from kilograms per liter to pounds per gallon, the

number is multiplied by 8.34.• The average specific gravity of milk at 15.5 ° C (60°F) is 1.032. It

varies from 1.028 to 1.035 in commercial milk. The specific

gravities of milkfat (0.93), MSNF (1.62), and water (1.0)

determine the specific gravity of milk.

• Specific gravity is increased by the removal of fat and loweredby the addition of water.

Density and specific gravity



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• Forces causing a reduction in surface area, which is a

characteristic property of a liquid.

• Surface properties are involved in adsorption phenomena and

the formation and stability of emulsions. They are relevant tocreaming, fat globule membrane function, foaming, and

emulsifier use in dairy products. Normal cow’s milk has an

inherent surface activity. Its surface tension approximates 70%

of that of water

• The surface tension of whole milk is usually about 50 –52 mN/m(or dyn/cm) at 20°C. The surface tension of skim milk is 55 –60

mN/m. For cream, it is approximately 46 –47 mN/m (8,9).

Surface tension



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• The formation of stable foam depends upon two main factors.

• First, the lowering of the surface tension allows the gathering and spreading

of the surface-active components into thin films

• Second,the films must be sufficiently elastic and stable to prevent thecoalescence of the gas cells. A stable foam is thus formed when the surface

tension of the liquid is not great enough to withdraw the film from between

the gas cells and when the stabilizing agent has great internal viscosity.

• Foaming of milk is at a minimum at 30 –35°C. At 60°C, the foam volume is

independent of the fat content. Below 20°C and above 30°C, the foaming

tendency appears to increase. Fat tends to stabilize the foam formed below20°C, for instance, during churning. Skim milk produces slightly more stable

foam above 30°C than whole milk or light cream.

Foaming



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• Whole milk and skim milk display viscosities of 2.0 –2.1 and 1.5 –1.8 cP (or

mPa/sec) at 20°C, respectively (4,8). Whey has a viscosity of 1.2 cP.

• The viscosity of milk and cream creates the impression of “richness” to the

consumer. From an organoleptic standpoint, viscosity contributes to

mouthfeel and flavor release.

• The casein micelles of milk contribute more to the viscosity of milk than any

other constituent. Viscosity varies not only with changes in the physical

nature of fat but also with the hydration of proteins. Alterations in the size of

any dispersed constituents result in viscosity changes. The fat contributes

less than casein but more than whey proteins.• An increase of temperature causes a marked reduction of viscosity. For

example, at 20°C, milk is about half as viscous as at 0°C and at 40°C is

approximately one-third of the value at 0°C.

Viscosity



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• Number of calories required to raise the temperature of 1 g of a

substance by 1 degree C..

• The specific heat of milk products is a function of their

composition. The values for whole milk, skim milk, 40% cream,butter, and whey at 15°C are 0.93, 0.95, 0.68, 0.53, and 0.97

BTU/lb•°F, respectively (3.89, 3.97, 3.35, 2.21, and 4.06

kJ/kg•K, respectively).

Specific Heat



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• The reciprocal of electrical resistance exhibited by a 1-cm cube of conductor

(solution containing electrolytes).

• Current passes through the milk by virtue of the activity of its ionic mineral

constituents, of which the chloride ions carry 60 –68% of the current. There is

therefore a close correlation between the electrical conductivity of milk andits chloride content.

• The electrical conductivity of normal milk corresponds to that of

approximately 0.25% sodium chloride solution (w/w) and ranges from 45 to

55 A•v–1m –-1 (8) or 45 –55 x 10 –4 mho (1). Conductivity of milk is the basis

of the new ohmic process for sterilizing milk.• Effect of fermentation > increases conductivity

Electrical Conductivity



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• The freezing point of milk is generally expressed as degrees Hortvet (H),

reflecting the commonly used Hortvet crysoscope procedure.

• Determination of freezing point is a parameter widely used in the industry for

detection of adulteration of milk with water. The freezing point of milk has a

relatively narrow range (from –0.520 to –0.560°C, the average being –0.540°C). It corresponds to the freezing point of an 0.85% sodium chloride

solution (w/w).

• Addition of water raises the freezing point, and readings above –0.520

support strong suspicion of watering the milk. As little as 3% water added to

milk can be detected by this method.• Lactose and chloride are the major milk constituents responsible for 70 –80%

of the overall depression in the freezing point of milk.

Freezing Point



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• A solution boils at a higher temperature than does the pure solvent,

according to the concentration of the dissolved substance.

• The boiling point of milk is 100.17°C. The milk constituents in true solution

are mainly responsible for the elevation of the boiling point above 100°C.

• Elevation of the boiling point is based on the same principles as depression

of freezing point. However, for detecting added water, the freezing point

method is far superior on the grounds of accuracy and convenience.

Boiling Point



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• A physical property of a substance that relates to how light is refracted from

the material. Usually used to indirectly measure some other property such as

concentration.

• The refraction of light by a solution is a function of the molecular

concentration of the solute in solution. Each solute maintains its ownrefractivity, and the refractive index of a mixture is that of the total of the

refractive indices of the substances plus that of the solvent.

• The components of milk contributing to its refractive index in descending

order of importance are water, proteins, lactose, and minor constituents.

Whey proteins are more important than casein.• The refractive index of milk at 20°C is 1.3440 –1.3485.

• The refractive index of butterfat is distinct enough to indicate its authenticity.

Refractivity



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• Acidity

• pH

• Buffering action

• Oxidation-reduction

potential

Chemical Properties


Freshly drawn milk normally shows an

amphoteric reaction, i.c., it turns red

litmus blue, and blue litmus red.

This property in milk is due lo thepresence of different amino acids.

Amino acids consists of NH2++ as well

as COOH- groups which show alkaline

as well as acidic reactions.

But, most common chemical properties

are:


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• When normal fresh milk is titrated with an alkali solution using

phenolphthalein as an indicator, it appears acidic though fresh milk does not

contain any acid.

• This acidic nature of freshly drawn milk is due to the presence of

phosphates, proteins, citrates and dissolved carbondioxide. This acidity inmilk is called natural acidity.

• The natural acidity in milk may range from 0.13 to 0.21 per cent. This is

always expressed as percent lactic acid. Natural acidity in milk is considered

important from the heat stability point of view.

Acidity



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• Normal fresh milk has a hydrogen ion concentration of approximately pH 6.5

to 6.7, which indicates that the milk is slightly acidic. If the animal is suffering

from udder disease mainly mastitis, then the milk obtained from such animal

may have pH above 7 suggesting that it is alkaline.

• Such milk sample will have abnormal odour, taste and poor heat stability. If

the pH value is less than 6.6 then it is considered to be added with colostrum

or bacterial deterioration has taken place in milk.

pH



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• Fresh milk acts as a complex buffer because it

contains several substances which contributes to this

property of milk.

• Certain constituents viz. carbondi-oxide, proteins,phosphates, citrates and a number of minor

constituents are responsible for buffering capacity of

milk. This property of milk is considered important from

the curdling and heat stability point of view.

Buffering Action



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• In case of organic materials, oxidation is defined as the uptake of oxygen or loss of

hydrogen, in the same manner, reduction may be defined as a process of loosing

oxygen or gaining hydrogen. However, in broader sense the processes of oxidation

and reduction are not necessarily limited to the gain or loss of oxygen and hydrogen.

In ionic systems, it can be demonstrated that phenomenon may involve loss or gain

of electrons.

• The voltage measured under these conditions reflect the oxidizing or reducing

capacity of the solution. This potential is called the oxidation reduction potential or

redox potential. It is designated by the symbol 'Eh'. It normally falls within the range

of +0.2 to +0.3 volt. Certain factors such as heat treatment, bacterial activity and

contamination with trace metal, particularly with copper, affect the 'Eh' of milk.Methylene blue reduction test of milk is based on available oxygen by the

microorganisms. By this test we can test the effect of metals used in dairy industry

and also the level of microbial activity.

Oxidation-reduction potential


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2 Physico Chemical Properties