Chemistry Reference
In-Depth Information
Table 11.1.
Intrinsic viscosity of milk proteins (voluminosity, V
e,
can be calculated
from V
e
¼
[
] / 2.5 where [
] is intrinsic viscosity (Mulvihill and Fox, 1989))
Temperature
8C
Intrinsic viscosity,
ml/g
Proteins
pH
Minerals
s1
-Casein
37
7.02
0.01 M KCl
10.2
4
7.02
0.01 M KCl
11.6
20
7.08
0.1 M KCl
7.7
4
7.08
0.1 M KCl
9.3
20
7
0.1 M KCl
6 M Gu
HCl
19
25
12
19.2
s2
-Casein
7
0.1 M NaCl
11.3
-Casein
4
7
I
¼
0.2
23
-Casein
25
7
0.1 M NaCl
9.5
12
15.1
7
5 M Gu
HCl
31
- Lactoglobulin
5.2
0.1 M NaCl
2.9
- Lactoglobulin
6
I
¼
0.2
3
The increase of V
e
with decreasing temperature (Table 11.1) is assigned
to the release of
-casein from the micelle, due to a direct effect of the high [
]
of
-casein, a loosening of the micelle structure (Walstra, 1979) and a decrease
in hydrophobic interactions (Snoeren et al., 1984). Changes in pH induce
strong variations in V
e
, particularly for micellar casein. As expected from the
minimum in electrostatic repulsion, V
e
was found to be minimal at pH 4.6, the
isoelectric point of casein, for proteins in bulk milk. A maximum in V
e
was
found at pH 5.4, which may be due partly to the release of
-casein (Snoeren
et al., 1984). With isoelectric-precipitated casein, however, V
e
was reported to
be maximal ([
]
71 ml/g) close to the isoelectric point. This very high V
e
value was interpreted as representing water occluded in aggregates (Colas
et al., 1988a). An inverse relationship between V
e
of casein and whey proteins
and ionic strength (0.005-0.07 M NaCl, pH 6-8) (see also Table 11.1) was
explained by the shielding effect of neutral salts on the electrostatic charges of
the proteins (Boulet et al., 1998).
Table 11.2 illustrates the voluminosity increase for casein modified by
reductive alkylation with reducing sugars. The decrease in the parameter k
0
of
the Huggins equation (
red
¼
[
]+k
0
.[
]
2
.c where
red
and [
] are reduced and
intrinsic viscosity, respectively, and c the protein concentration) confirmed
that the modification had resulted in a better dispersion of the casein mole-
cules (Colas et al., 1988b). Moreover, ESR measurements showed that the
