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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
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• 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
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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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