Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 378

1.0

0.5

0.0

5 6 7

pH

Figure 5 Fraction of coalescence (F c ) in single bubble layer experiments with 1 wt%
protein as a function of pH: b -L ( K ); b -L + oil droplets ( b ¼ 0.25%) ( J );
sodium caseinate ( m ); and sodium caseinate + oil droplets ( f ¼ 0.25%) ( n )

The most obvious interpretation of these trends is as follows. The isoelectric
pH (pI)ofb-L and SC is pH ¼ 5.3 0.1. This is in the pH region where there is
the maximum stability (minimum F c ). Reduction of the electrostatic repulsion
between proteins as the pI is approached generally increases the adsorbed
amount, and it can also strengthen the net attractive interaction between
adsorbed proteins, mechanically strengthening the films. 19 Whereas the b-L
remains soluble at or below pI, the SC precipitates. Indeed, in this pH range,
the SC solutions became cloudy, and after an hour or so individual precipitates
of protein were visible to the naked eye. When the protein becomes so
aggregated, the films are likely to be more liable to fracture, and there is not
enough free protein to adsorb to freshly formed interfaces; hence the bubbles
become very unstable to pressure change. The slight decrease in stability
between pH ¼ 6 and 6.5 may be due to reduced electrostatic repulsion between
adjacent air-water interfaces, which is not compensated for by any increase in
intermolecular strengthening of the film at this intermediate pH. Therefore, this
reduced repulsion between interfaces enhances the probability of coalescence.
Figure 5 also shows the results of the corresponding experiments with
protein-stabilized n-tetradecane emulsion droplets present ( f ¼ 0.25%). There
are two main features associated with the inclusion of oil droplets. Firstly, the
stability is markedly increased for both proteins, but particularly for b-L, so
that at pH values approximately just below pI the bubbles were found to be
completely stable (F c ¼ 0). So far we have never observed such high stability
under these expansion conditions with any protein on its own at any concen-
tration. Secondly, the added oil droplets have the effect of broadening the
stability curves so that the variation with pH is not so marked. For example,
with SC there is still a significant increase in F c at low pH, but this occurs at a
lower pH than without droplets and the onset of increased stability occurs at a
higher pH than without droplets.