Chemistry Reference
In-Depth Information
Table 15.2
Surface tension of skim milk and whole milk (3.5%
fat) at 10, 25 and 408C after a 300 s measuring time
a
Surface tension (mN m
1
)
Product temperature, 8C
Mean
Standard deviation
Skim milk
10
25
40
Whole milk (3.5% fat)
10
25
40
49.72
47.29
42.22
0.86
1.21
2.87
49.87
41.89
41.56
1.32
1.05
1.01
a
Adapted from Kristensen et al. (1997).
fat contents (Watson, 1958; Michalski and Briard, 2003). The effect of
homogenization is dependent on the pressure; at low homogenization pres-
sure (5 MPa), the surface tension of homogenized milk has been found to be
lower than that of whole milk, which is due to the release of surface-active
components of the milk fat globule membrane as a consequence of fat globule
disruption (Michalski and Briard, 2003) or could be due to the release of free
fatty acids because of activation of lipase during homogenization (Whitnah,
1959). Homogenization of raw milk at higher pressures, however, can lead to
an increase in surface tension (Michalski and Briard, 2003). The surface
tension of homogenized milk is higher than that of unhomogenized milk, if
the milk has been pasteurized before homogenization (Webb, 1933; Watson,
1958). The surface tension of milk held at 58C and brought to 208C is lower
than that of milk cooled to 208C and measured immediately (Mohr and
Brockmann, 1930; Sharp and Krukovsky, 1939). Heat treatment of milk at
sterilization temperatures can cause a small increase in surface tension
(Nelson, 1949), which is probably related to the denaturation of whey pro-
teins, which reduces their surface activity.
15.5.
Optical Properties
15.5.1.
Light Absorption and Scattering
When electromagnetic radiation encounters chemical species (atoms,
ions or molecules), it can be absorbed (i.e., its energy is transferred to the
chemical species), scattered (i.e., its direction of propagation changes) or can
excite fluorescence. Absorption occurs when specific frequencies of a beam of
