Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 371

Some systems were acidified by addition of GDL. To 200 ml of protein
solution, or protein solution + emulsion droplets, 0.2 g of GDL was added.
The solution was then split into two parts: one was used for the coalescence
stability measurements, while the pH of the other was measured simultaneously
in the same laboratory (and therefore at approximately the same temperature
(18-231C). At this concentration and temperature, the GDL slowly hydrolyses
to lower the pH homogeneously throughout the sample. As the rate of pH
lowering was much slower than the timescale of the coalescence stability
measurements, in an individual stability experiment the pH could be considered
constant ( 0.05 pH).

25.2.3 Single Bubble Layer Experiment
Two techniques were used to measure bubble stability with respect to interfacial
expansion. One of these, referred to here as the 'single bubble layer experiment',
has been described fully elsewhere. 1
The basis of the technique is schematically illustrated in Figure 1. Briefly, the
equipment consists of a stainless-steel cell containing a flexible square barrier
positioned at the air-water interface of the aqueous solution contained within
the cell. The barrier sides are 4.2 mm long when undeformed. The barrier shape
is maintained by four hinged pins at each corner; these can be moved symmet-
rically in the directions of the diagonals of the square in order to compress and
expand the interface contained within the barrier. The air gap above the
interface can also be increased and decreased, to lower and raise the air
pressure, by vertical movement of the outer sleeve of the cell. The movement
of the sleeve and pins is synchronized, and the geometry of the different parts is
such that the relative increase in area of a bubble at the interface due to a fall in
pressure is equal to the relative increase in area of the planar interface
contained within the barrier. In this way, the individual bubbles at the interface

(a) (b)

F

A

A

E

C

B

D

Figure 1 Schematic representation of the single bubble layer experiments: (a) before
expansion; and (b) after expansion. Key: A ¼ flexible square barrier;
B ¼ bubbles; C ¼ air-water interface; D ¼ bulk aqueous phase; E ¼ sliding
wall of cell, showing direction of motion to increase air gap above the interface;
and F ¼ microscope.