Chemistry Reference
In-Depth Information
Fig. 9.1
Dynamic interfacial tension data of an oil-water interface stabilised by sodium
caseinate. The change in the interfacial tension with time relates to the rate of protein
adsorption and protein conformational changes at the interface.
stabilised film (Srinivasan, 2005), which serves to initially 'protect'
the formed emulsion droplets against immediate re-coalescence (during
the emulsification process) but also provides long-term stability (post-
production) against droplet coalescence and potentially flocculation.
The rate of protein adsorption at the oil-water interface is also crucial
in terms of emulsion stability during emulsification. Under these condi-
tions, there is a sudden increase in the total surface of the newly formed
emulsion droplets due to the decrease of their size and increase of their
number. It is therefore the rate of protein adsorption that will determine
the stability of the newly formed droplets against re-coalescence events.
Information on the kinetics of protein adsorption at an oil-water
interface can be obtained from dynamic interfacial tension measure-
ments (Beverung
et al.
, 1999). Protein adsorption displays three distinct
regimes. Initially, there is an induction period where interfacial tension
is only marginally reduced (Fig. 9.1) and the duration of which is con-
trolled by diffusion and protein interfacial affinity. The second regime is
defined by the continual rearrangement of the proteins at the interface.
During this stage, a sudden reduction in interfacial tension takes place
(Fig. 9.1), as more protein molecules adsorb at the interface and the num-
ber of interfacial contacts per protein molecule is increased. The final
regime occurs upon monolayer coverage and is attributed to the contin-
ued relaxation of the adsorbed layer and the possible build-up of multi-
layers towards the latter stages. It is at this stage, and after monolayer
coverage, that equilibrium interfacial conformation can be achieved.
