Chemistry Reference
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appropriate cases, the whole correlation function was analysed in detail to give
the MSD information.
Time-sweep oscillatory measurements were performed at a frequency of 1 Hz
and a stress of 0.02 Pa using a controlled-stress rheometer (AR 1000, TA
Instruments, Newcastle, DE, USA) equipped with a Peltier temperature con-
troller and a Couette device consisting of two concentric cylinders of diameters
28 and 30 mm. The GDL was added to the milk which was then transferred to
the rheometer. Paraffin oil was put on top of the sample to prevent evapora-
tion. Experiments were carried for 12 h at 301C, by which time the sample had
reached pH
ΒΌ
4.8.
17.4 Results
17.4.1 Unheated Milk
The behaviour of the parameters measured by DWS and rheology as a function
of pH during acidification of unheated milk is shown in Figure 1. Several
different types of changes are apparent in the parameters derived from the
DWS experiment. The casein micelles are the only scattering particles present in
the unheated skim milk, and so it is evident that there are two early changes in
the properties of the particles. The apparent radius decreases as the pH
decreases, to a minimum at a pH of about 5.5, after which it increases again,
but relatively slowly, until a point is reached at which there is a rapid increase in
apparent particle size, which presumably arises from the formation of
10
1.1
200
4000
3.2
8
1.0
175
3000
2.8
6
0.9
150
2.4
2000
4
0.8
125
2.0
1000
2
0.7
100
1.6
0
0.6
75
0
6.5
6.0
5.5
5.0
pH
Figure 1 Changes in the parameters derived from DWS and rheology during acidification
of unheated milk with 1.5% GDL. Various quantities are plotted against pH:
K
,
apparent radius from value of
t
calculated from correlation function, and
converted using the Stokes-Einstein relation (left scale);
&
, apparent radius
measured during early part of the reaction (far right scale);
J
, slope of MSD
against time, calculated from correlation functions (middle right scale);
'
, 1/l*
calculated from turbidity (right scale);
,G
0
from bulk rheometry (far left scale)
}
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