Chemistry Reference
In-Depth Information
Milks in general: 1.25
10
7
m
2
s
1
in the temperature range 15-208C.
Milk and cream: 9.7
10
8
m
2
s
1
in the temperature range 0-228C.
Thermal diffusivity can be measured directly; alternatively, a value can
be calculated readily using Equation 67 and appropriate values of thermal
conductivity, density and specific heat capacity.
15.9.4.
Measurement of Thermal Properties
The accurate measurement of the thermal properties of foods requires
careful design and operation of suitable equipment, which is often purpose-
built. Proprietary equipment is available for some types of measurement;
differential scanning calorimetry (DSC) is an example. DSC, a form of differ-
ential thermal analysis (DTA), is used to investigate phase changes in foods.
The measurements obtained can be converted to enthalpy-temperature data
(Lewis, 1987). DTA and calorimetry are discussed in detail by Wunderlich
(1990). Measurement of the thermal properties of foods is discussed and
reviewed by Ohlsson (1983), Lewis (1987), Rahman (1995), Sahin and
Sumnu (2006) and Figura and Teixeira (2007).
15.10.
Electrical and Dielectric Properties
15.10.1.
Electrical Conductivity
Electrical conductivity (EC),
, is a measure of a material's ability to
carry an electrical current. It ranges in value from 10
18
to 10
7
Sm
1
, depend-
ing on the material. The conductivity of aqueous systems such as milk lies
between these extremes (Mabrook and Petty, 2003). The EC of normal whole
milk is about 0.460 S m
1
(Hogeveen and Ouweltjes, 2003).
EC is easily measured by applying a known voltage across a pair of
parallel electrodes immersed in the sample, measuring the current produced,
and calculating the resistance of the specimen (the volume bounded by the
electrodes):
¼
1
l
A
¼
G
:
l
A
ð
Sm
1
Þ
R
:
(68)
where R
¼
resistance ();
G
¼
conductance (S);
l
¼
distance between the electrodes (m);
A
¼
electrode area (m
2
).
