Interactions during the Acidification of Native and Heated Milks Studied by Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 260

throughout a process, a change in the value of l* will indicate some degree of
increase in the interactions between particles via the change in S(q). Therefore,
changes in this parameter offer the possibility of interpreting changing organ-
ization within the suspension arising from the interactions between the particles.
A further parameter is available from the analysis of the correlation func-
tions. As the motion of particles in a suspension begins to be restricted, the
motion of the particles is no longer freely diffusive, and Equation (1) is no
longer completely descriptive of the system. We must distinguish here between
aggregates that diffuse freely (although more slowly than the original particles)
and aggregates that are part of, or contained within, a gel and have restricted
motion inside the gel framework. In the latter case, the correlation function can
be related to the mean-squared displacement (MSD) of the particles,
o Dr 2 (t)4, and the correlation function (Equation (1)) can be analysed to
determine the MSD at any time by replacing t/ t by k 0 2 o Dr 2 (t)4. The MSD is
an indication of whether the movements of the particles are being constrained.
Once a gel starts to form, the use of the Stokes-Einstein equation is invalid
because the particles are probably non-spherical and are certainly not free-
diffusing. It has been shown that, for a fractal type of gel, the slope of MSD
against time decreases from a value of 1 for freely diffusing particles to 0.7 for a
gelled material. 18
This study of the gelation of milk will make use of all of these parameters
derived from the DWS experiment, namely the apparent particle size, the slope
of the MSD against time, and the quantity 1/l*.

17.3 Materials and Methods
Fresh milk, obtained from the University of Guelph dairy farm, was skimmed
by centrifugation in the laboratory, followed by filtration through glass-fibre
filters. Heated milks were prepared by placing 10 mL tubes of milk in a water
bath at 851C and leaving for 10 min before removing and cooling rapidly
to room temperature in an ice bath. The milk was acidified by the addition
of glucono-d-lactone (GDL, 1 and 1.5% w/w). The GDL was added to the
milk at a temperature of 301C, and it was agitated briefly to dissolve the
acidulant. The part of the sample used for DWS or rheological measurement
was placed in the measuring equipment. The rest of the sample was used for
monitoring the pH value, which was followed continuously until

it had

decreased to pH ¼ 4.8.
The DWS equipment has been described elsewhere. 20 Light from a laser
(532 nm, 100 mW) was passed through 5 mm path length cuvettes filled with
milk or mixtures maintained at a temperature of 301C. The transmitted light
was detected and analysed by photomultipliers and a correlator to give the
measured intensity correlation function. The value of l* was measured from the
total intensity of the scattered light, and by comparison with the light scattered
by a latex of known size and scattering properties. The correlation function
was corrected using the values of l* and analysed to give the value of t .In