b-Lactoglobulin Aggregates from Heating with Charged Cosolutes: Formation, Characterization and Foaming - Food Colloids: Self-Assembly and Material Science - page 191

The ability of the b-LG aggregates to prevent coarsening of the foam has
been evaluated by following the evolution of the normalized mean bubble
diameter over time. It should scale as t a with a ¼ 1 for purely diffusive foam
coarsening (Figure 9). 41 Heated b-LG solutions (10 or 18 g L 1 , the latter
corresponding to an elevated content of dry matter due to presence of cosol-
utes, e.g., N140) showed low foam stability (t { 3 ks) and high gas diffusion
rates. Calculation of the coarsening exponent resulted in values of 2.4-2.5.
Disproportionation in the foams decreased significantly with increasing cosol-
ute concentration to a values from 1.7 to 2.0. In the presence of soluble and
insoluble aggregates, and no matter what type of cosolute was present, the
foams exhibited significantly higher stability against gas diffusion with a values
from 1.3 to 1.7. However, the presence of insoluble particles was in some cases
(e.g., A70) detrimental to the gas diffusion properties, whereas several samples
(G50, N140, N150 and N200) showed very high resistance against gas diffusion
(not all the results are shown in Figure 9). This might be explained 9,10,12 by
specific particle and surface properties like size, smoothness, shape, surface
hydrophobicity and surface charge. In general, for lower cosolute concentra-
tions (and the protein controls), the coarsening exponent reflects the fact that,
besides disproportionation, additional destabilization mechanisms also partic-
ipate in bubble growth. This might be due to the formation of insufficiently
stable interfacial networks, leading to film thinning, film rupture and bubble
coalescence or collapse, which is in agreement with observations made for foam
volume stability under these same conditions (results not shown). In conditions
where soluble aggregates of size of 4100 to 130 nm were present in the heated
protein+cosolute mixture, and in some cases also insoluble particles, the foams
demonstrated significantly higher stability and lower gas diffusion rates. This
might be explained 6,8 by the formation of highly viscoelastic interfacial films
and gel-like interfacial layers, which entrap air bubbles and control drainage.

250

N60

200

150

A30

100

β -LG

10 g/l

β -LG

18 g/l

A60
N140
G50

50

G10

0

0 2000 4000 6000 8000

Time [s]

Figure 9 Time evolution of normalized cube of average air bubble radius as calculated
from image analysis of foams stabilized by b -LG aggregates at pH 7.0