Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 370

coalescence of bubbles that would otherwise be completely stable. Further
work suggested that fracture and breakage of interfacial films at critical extents
of deformation may also be important in determining stability. Susceptibility to
fracture and breakage may be related to the degree of aggregation and cross-
linking within the film. Here we outline improved techniques that can be used
to examine the behaviour of single bubbles and bulk foams undergoing the
same types of expansion. We illustrate the effects of different proteins, mixtures
of protein + oil droplets and changes in the interactions between them, on the
stability of systems of direct relevance to aerated food products.
25.2 Materials and Methods
25.2.1 Materials
Bovine b-lactoglobulin (b-L, three times crystallized, lyophilized, desiccated, lot
no. 21K7079, containing variants A and B), ovalbumin (OA, three times crys-
tallized, lyophilized, desiccated, lot no. 127H7037), glucono-d-lactone (GDL)
(99.0%) and n-tetradecane (99%) were purchased from Sigma-Aldrich (Poole,
UK). The commercial whey protein isolate (WPI, BiPro) from Davisco Foods
(MN, USA) contained 97.7% protein, 0.3% fat, 1.9% ash and 4.8% moisture.
Spray-dried sodium caseinate (SC) (4 82 wt% dry protein, o 6 wt% moisture,
o 6 wt% fat and ash, 0.05 wt% calcium) was obtained from DMV International
(Veghel, the Netherlands). Water from a Milli-Q system (Millipore, Watford,
UK), free from surface-active impurities and with a conductivity of less than
10 7 Scm 1 , was used throughout. Solutions of pH ¼ 7.0 consisted of 0.05 mol
dm 3 imidazole buffer + 0.05 mol dm 3 NaCl, adjusted to pH ¼ 7.0 by addition
of HCl. Invert sugar syrup (82% total solids, 77 1 wt% invert solids, 0.4 wt%
ash) was provided by Cadbury Trebor Bassett (Bournville, UK). A 70% corn
syrup (LF9, C*Sweet F017Y4) was obtained from Cerestar (Manchester, UK).
25.2.2 Emulsion Preparation and Characterization
In some bubble experiments, a protein-stabilized emulsion was added to the
protein solution used to stabilize the bubbles. Oil-in-water emulsions (1 wt%
protein, 30 vol% oil) were prepared using a Shields S-500 high-pressure homog-
enizer. Droplet size-distributions of emulsions were determined using a Master-
sizer 2000 multi-angle static light-scattering instrument (Malvern Instruments,
UK). The average droplet diameter for each freshly prepared emulsion was d 43 ¼
0.4 0.05 mm. In the case of OA, however, it was difficult to obtain stable
emulsions in the same droplet size range. It was therefore decided to add some of
the SC-stabilized emulsion to the OA solution. Since the volume fraction of
emulsion added to the protein solution was low (10 3 -1%), and the concentra-
tion of the protein solution was typically 1 wt%, it was assumed that the SC
added along with the emulsion did not change the bulk protein composition
significantly, nor the interfacial properties of the adsorbed OA film.