11 Physical properties of milk
Milk is a dilute emulsion consisting of an oil/fat dispersed phase and an
aqueous colloidal continuous phase. The physical properties of milk are
similar to those of water but are modified by the presence of various solutes
(proteins, lactose and salts) in the continuous phase and by the degree of
dispersion of the emulsified and colloidal components.
Data on the physical properties of milk are important since such
parameters can influence the design and operation of dairy processing
equipment (e.g. thermal conductivity or viscosity) or can be used to
determine the concentration of specific components in milk (e.g. use of the
elevation in freezing point to estimate added water or specific gravity to
estimate solids-not-fat), or to assess the extent of biochemical changes in the
milk during processing (e.g. acidification by starter or the development of a
rennet coagulum). Some important physical properties of milk are sum-
marized in Table 11.1.Table 11.1 Some physical properties of milk (Walstra and Jenness, 1984; Sherbon, 1988; Singh,
McCarthy and Lucey, 1997)Osmotic pressure - 700 kPa
a, -0.993
Boiling point - 100.15"C
Freezing point -0.522"C (approx.)
Refractive index, np 1.3440-1.3485
Specific refractive index -0.2075
Density (20°C)
Specific gravity (20°C) .. 1.0321
Specific conductance -0.OO50 ohm-' cm-'
Ionic strength -0.08 M
Surface tension (20°C)
Coefficient of viscosity
Thermal conductivity (2.9% fat)
Thermal diffusivity (15-20°C)
Specific heat
pH (at 25°C) - 6.6
Titratable acidity
Coefficient of cubic expansion (273-333 K)
Redox potential (25"C, pH 6.6, in equilibrium with air)
- 1030 kg m-3
-52 N m-'
2.127 mPa s
-0.559 W m-' K-'- 1.25 x lo-' mz s-'
-3.931 kJ kg-' K-'
1.3-2.0 meq OH- per 100 mi
(0.14-0.16% as lactic acid)
0.0008 m3 m-3 K-'
+0.25 to +0.35V