Chemistry Reference
In-Depth Information
the addition. Milk with a freezing point of
0.5258C or below is usually
presumed to be unadulterated. The selection of
0.5258C as the upper limit
is based on the statistical evaluation of data from a 1968 North American
survey of the freezing point of authentic samples (Henningson, 1969). The
freezing point is generally determined with thermistor or Hortvet-type
cryoscopes; the apparatus and techniques have been described by AOAC
(1995b).
15.7.
Density
Density is defined as mass per unit volume. It has the units kg m
3
and is
commonly indicated by the symbol
(rho). A related dimensionless quantity,
specific gravity (SG), is defined as
product
/
water
, densities being measured at
specified temperatures. Specific gravity has the advantage that its numerical
value is independent of the units used for density, and its temperature depen-
dence is much lower than that of density. Specific gravity is a particular form
of relative density, the ratio of the absolute density of a material to the
absolute density of a reference material. As water is almost always chosen
as the reference material, the terms 'specific gravity' and 'relative density' can
be considered synonymous (Figura and Teixeira, 2007).
Density is identified for a given measurement temperature as, for exam-
ple,
20
, where the superscript is the temperature in 8C (Walstra and Jenness,
1984). Specific gravity is identified in a similar way, e.g., SG
2
20
or SG
2
4
, where
the superscript is the temperature at which milk density was measured and the
subscript the temperature at which the density of water was determined
(Walstra and Jenness, 1984). Density and specific gravity are related by the
equation
1
ΒΌ
SG
1
2
water
(10)
where
is temperature (8C).
20
of whole milk is about 1030 kg m
3
and usually ranges from 1027 to
l033 kg m
3
, depending mainly on fat content (Walstra and Jenness, 1984).
SG
15
:
5
15
:
5
ranges from 1.030 to 1.035 for mixed herd milk, and a commonly
quoted average value is 1.032 (Jenness and Patton, 1959). The equivalent
range and average value for skim milk are 1.0320-1.0365 (Jenness and Patton,
1959) and 1.036 (Sherbon, 1988), respectively. Inter-breed variations in den-
sity and specific gravity are small.
Density data are needed for converting mass to volume and vice versa
and for calculating values of physical properties such as kinematic viscosity
and thermal diffusivity (Walstra and Jenness, 1984). Measurement of the
