Chemistry Reference
In-Depth Information
Temperature dependence of viscosity was allowed for by evaluating
s
and
ls
at required temperatures using literature data.
Hallstr om and Dejmek (1988b) used viscosity measurement and
Eilers's equation, with
max
¼
0.79 (Snoeren et al., 1982) and
0
¼
permeate
,
to calculate the
p
of skim milk concentrates prepared using ultrafiltra-
tion. (
p
was equivalent to (
i
) because of the loss of lactose during
ultrafiltration and the absence of fat). Overall protein voluminosity (domi-
nated by casein voluminosity) was then calculated from
p
for various
temperatures, pH values and skim milk preheat treatments. Comparison
with voluminosities measured by sedimentation showed that the latter
were 1.3 times smaller. This was attributed to compression during sedi-
mentation of the hairy
-casein outer layer of casein micelles, and thus
underestimation of voluminosity by this method. Hallstr om and Dejmek
(1988b) tried without success to find a constant value of
max
applicable to
concentrates with different protein contents. This indicated that the casein
micelles in skim milk cannot be regarded as hard spheres when the milk is
(ultrafiltration) concentrated.
Snoeren et al. (1984) investigated the age thickening of skim milk
concentrates at 508C. The apparent viscosity, the rate of age thickening and
the degree of shear thinning all increased with both time and %TS. Even
app
;
g
¼1
increased slightly with storage time (at a given %TS), suggesting that
holding at 508C induced some permanent change in the structure of the
concentrate that could not be reversed by shearing. Binh Trinh et al.
(2007a) found that concentrated reconstituted whole milk (48% TS, held at
658C) showed the same phenomenon.
Snoeren et al. (1983, 1984) showed that the increase with time in the
apparent viscosity of both skim milk and unhomogenized whole milk concen-
trates during age thickening, which is the result of an increase in (
i
) owing to
aggregation, could be predicted by a modified Eilers equation:
2
1
:
25
p
ð
1
þ
t
:
k
p
=
p
Þ
1
p
ð
1
þ
t
:
k
p
=
p
Þ
max
app
;
t
¼
0
1
þ
(45)
where
app, t
is the apparent viscosity at a specified shear rate and tempera-
ture, t is time,
p
¼
p
at t
¼
0andk
p
¼
d
p
=
dt. For a given shear rate and
temperature, k
p
=
p
, called the age thickening constant, was independent of
%TS over the ranges studied (TS
¼
45.4-52.5% for skim milk concentrates
and TS
¼
46.7-58.9% for whole milk concentrates). The average values of the
constant at g
!1
and 508C were 2.82 and 2.60 for skim milk concentrates
and whole milk concentrates, respectively. The rate of age thickening is
somewhat lower for whole milk concentrates than for skim milk concentrates
