b-Lactoglobulin Aggregates from Heating with Charged Cosolutes: Formation, Characterization and Foaming - Food Colloids: Self-Assembly and Material Science - page 182

stirring at 801C for 10 min. After the heat treatment, the samples were cooled in
ice water to 41C. The solutions were filtered using 0.22 mm filters (Millipore) to
eliminate dust and other insoluble particles. The filtered solutions were placed
in the cell and analysed using the Nanosizer ZS (Malvern Instruments, UK).
The effective voltage in the measurement cell was between 50 and 150 V
depending on the ionic strength of the samples. The electrophoretic mobility
experiments were performed at room temperature and recorded values are
averages of two runs of 20 measurements.

12.2.7 Determination of Protein Aggregate Interfacial Properties
Dynamic surface tension measurements of the heated b-LG+cosolute solu-
tions at pH 7.0 and protein concentration 10 g L 1 were carried out using a
dynamic pendant-drop Tracker tensiometer (IT Concept, Longessaigne,
France). The filtered b-LG+cosolute solutions (0.45 mm filter) were placed in
a square optical quartz cuvette (volume 7 mL). The tip of the aluminium needle
(0.9 mm o.d., 0.55 mm i.d., Popper, Milian, Switzerland) of a syringe (0.25 mL,
Micro Syringe, Milian Switzerland) was put in contact with the liquid in the
cuvette. An axisymmetric air bubble was formed in the solution with a constant
area of 16 mm 2 . The image of the bubble was recorded by a CCD camera and
the interfacial tension g was calculated by application of the Laplace equation
to the profile of the bubble image. Once equilibrium of the surface tension was
reached, oscillations at frequency 0.01-0.1 Hz and amplitude 10% were per-
formed. From these measurements the dynamic elastic modulus E was calcu-
lated: 30,31

|E| ¼ A(D g /DA) ¼ E 0 +iE 00 . (7)

Experiments were performed at 25 11C. In the presence of too many
insoluble aggregates or too strong aggregation, the formation of a stable
bubble was not possible due to film rupture or lack of sufficient surface-active
protein due to particle sedimentation.

12.2.8 Determination of Protein Aggregate Foaming and Foam-
Stabilizing Properties
Foaming properties of b-LG+cosolute solutions (pH 7.0, protein concentra-
tion 10 g L 1 , cosolute concentration 0-200 mM) have been determined by the
method of Guillerme and co-workers. 32 The principle of the method is to foam
a defined quantity of protein dispersion by sparging gas through a glass frit
with controlled porosity and gas flow. The generated foam rises along a glass
column, where the foam volume evolution is followed by image analysis using a
CCD camera. A second CCD camera equipped with a 'macro' objective was
used to image the variation of the air bubble size at a foam height of about 10
cm (corresponding to half of the foam height). Image analysis was performed
on a foam area of 0.38 cm 2 using software developed in-house based on