Chemical, Physical, and Functional Characteristics of Dairy Ingredients 45A physical equilibrium in milk exists
between the colloidal dispersion phase and
the salts. However, it is infl uenced by several
factors including the concentration of serum
solids, addition of ionizable salts or alcohol,
heat, and pH changes that all infl uence the
technological behavior of milk during
processing.Electrical Conductivity
Electrical conductivity is a measure of the
electrical resistance of solution in reciprocal
ohms (mho/cm) that is used to assess the total
ionic content of milk. The electrical conduc-
tivity of bovine milk at 25 ° C ranges from
0.004 to 0.005 mho/cm.
Electrical conductivity measurements are
used to screen for the presence of diseases in
milk, particularly mastitis, and to detect
residual cleaning agents including sanitizers.
The ions that contribute the greatest to elec-
trical conductivity are sodium (Na), potas-
sium (K), and chloride (Cl). Sodium and
chloride ions increase when mastitis is
present, leading to an increase in electrical
conductivity.
Skim milk has lower conductivity com-
pared to whole milk because it is increased
by the presence of fat. Furthermore, the fer-
mentation of milk increases conductivity due
to conversion of calcium and magnesium to
ionic forms. Conversely, it decreases during
the demineralization of whey proteins due to
the loss of ionic minerals.Oxidation - Reduction Potential
The oxidation - reduction potential (Eh), also
known as the redox potential, is a measure of
the capacity of a chemical species to acquire
electrons, which is expressed in volts (V). If
a chemical species is oxidized it loses elec-
trons and if it is reduced it gains electrons.
The oxidation - reduction potential of milk at
30 ° C is between + 0.2V and + 0.3V, which is
determined mainly by the amount of dis-
solved oxygen.nifi cantly higher in organic milk compared to
conventional milk, which has been shown to
prevent asthma, cardiovascular disease, and
allergies (Bloskma et al., 2008 ; Lavrencic
et al., 2007 ; Bergamo et al., 2003 ; Anon.,
2008 ).
Physical Properties of Milk
The physical properties of milk greatly infl u-
ence the quality of milk, milk products,
and the operations during processing, includ-
ing the fl uid fl ow, heat transfer processes,
and emulsifi cation. A summary of the physi-
cal properties of bovine milk are shown in
Table 2.6.
Physical Structure
The structure of milk can be segregated into
three different phases: a dilute emulsion, col-
loidal dispersion, and a solution. All of the
particles in milk exhibit Brownian motion
because they have a negative electrostatic
charge. The dilute emulsion phase consists of
fat globules; the colloidal dispersion phase
consists of the casein micelles and colloidal
calcium phosphate (salts); and the solution
consists of the whey proteins, water, lactose,
and dissolved minerals.
Table 2.6. Physical Properties of Bovine Milk
Property Value
Density (g/cm^3 at 20 ° C) 1.032
Freezing point ( ° H) − 0.540 ( − 0.521 ° C)
Boiling point ( ° C) 100.17
Electrical conductivity
(mho/cm at 25 ° C)0.004 – 0.005
Specifi c heat
(kJ/kg/K at 15 ° C)3.92
Viscosity (cP) 1.9
Surface tension (dynes/cm) 50 – 52
pH (25 ° C) 6.5 – 6.7
Thermal conductivity
(J/msK at 37 ° C)193
Refractivity index (20 ° C) 1.3440 – 1.3485
Osmotic pressure 700 kPa
Specifi c gravity 1.032
Titratable acidity (mmol/L) 13 – 20Adapted from Francis (2000) , Kailasapathy (2008) ,
Neville (1995) , Sherbon (1988) , Walstra et al. (1999)