Manipulation of Adsorption Behaviour at Liquid Interfaces by Changing Protein–Polysaccharide Electrostatic Interactions - Food Colloids: Self-Assembly and Material Science - page 199

containing the sample solution. Temperature was controlled at 22 11C. The
surface tension was determined by axisymmetric bubble shape analysis and
the results are presented in terms of surface pressure, P ¼ g 0 - g ,where g 0 is the
air-water tension (72 mN m 1 ) or oil-water tension (30 mN m 1 )and g is the
measured solution tension. Surface rheology measurements were performed with
the same apparatus. By subsequent expansion and compression of the interfacial
area A (amplitude of area oscillation ¼ 4%, frequency ¼ 0.1 Hz), and recording
the resulting change in surface tension g , the dilatational modulus e ¼ d g /d ln
A was followed in time. Periods of five oscillations were alternated with equally
long resting periods. Average dilatational moduli were measured using the last
three oscillations of each period. All experiments were performed in duplicate.

13.3 Adsorption Kinetics
In our previous work, 17 the b-Lg-pectin interaction in bulk solution was char-
acterized as a function of mixing ratio by dynamic light scattering. Figure 2
shows that, with a small addition of b-Lg to LMP, the scattered light intensity is
found to increase, indicating formation of soluble protein-polysaccharide com-
plexes as suggested previously. 5 On further increasing the mixing ratio, the
scattered light intensity slightly increases while the hydrodynamic radius remains
fairly constant, indicating that the soluble complexes are gradually filled with
protein. From the point where the positive charge on the protein compensates the
negative charge on the polysaccharides and the net charge of the complexes
approaches zero, 34 the soluble complexes aggregate and phase separate. This is
shown in Figure 2 by the strongly increasing hydrodynamic radius at a mixing

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0 0

0 12 3 4 5 6 8 10 12 15

Protein / pectin w / w ratio

Figure 2 Dynamic light scattering at various b -Lg/pectin mixing ratios for solutions (pH
¼ 4.5, ionic strength ¼ 2 mM) with protein concentration kept constant at 0.1 g
L 1 . The plot shows hydrodynamic radius ( ' ) and intensity-scattered light ( )
for mixtures with LMP, and hydrodynamic radius ( & ) and intensity-scattered
light ( ) for mixtures with HMP