Adsorption of Macromolecules at Oil
Water Interfaces during Emulsification - Food Colloids: Self-Assembly and Material Science - page 434

The macromolecular emulsifying agent used in food emulsions is commonly
a protein, as proteins are surface-active and are abundant in most foods. Egg
yolk proteins are important in many food applications as they are able to
provide stabilizing effects at low pH, which is uncommon for other proteins
typically used in food dispersions. This is attributed to the fact that egg yolk
contains species with a broad pI range. 5, 6 Proteins in food applications are
often a mixture of different species with varying chemical composition. This in
turn can give rise to competitive adsorption if the interfacial area is small in
comparison to the protein concentration in the bulk solution, which may mean
that ultimately only certain protein species are actually adsorbed. In use as
emulsifiers and emulsion stabilizers, proteins have some limitations at high
ionic strength and also often at low pH. These conditions are, however,
common in many foods. And the increasing demand for heat-treated food
emulsions further limits their applicability.
The other large group of macromolecules abundant in nature are poly-
saccharides. Most natural polysaccharides are not sufficiently surface-active to
act as emulsifiers on their own. 7 But chemically modified polysaccharides, such
as octenyl succinate anhydride starch esters (OSA-starch), do have considerable
surface activity. Owing to the ultra-high molar mass and branched structure,
adsorbed OSA-starch can provide good steric stabilization in oil-in-water
emulsions. 8, 9
There is a large difference in properties, and a huge difference in molar mass,
between egg yolk proteins and OSA-starch. So it is reasonable to assume that
their adsorption behaviour will be different. Knowledge of the adsorption
behaviour and of the composition of the adsorbed layer is beneficial for under-
standing the mechanisms of macromolecular adsorption in emulsions. Thus, the
purpose of the work presented here is to compare the adsorption behaviour of
these two macromolecules with relevance to oil-in-water emulsion formation.

30.2 Experimental
Three OSA-starch samples of waxy barley origin, varying in molar mass and
degree of substitution, were provided by Lyckeby Sta¨ rkelsen (Kristianstad,
Sweden). The properties of the samples are given in Table 1. The protein

Table 1 Properties of OSA-starch samples

Molar mass (10 3 kg mol 1 )
Before
homogenization

r rms a
(nm)

Degree of
substitution

Degree of
branching

After
homogenization Sample

B39-22 0.0224 0.0548 39 12 38

B27-21 0.0213 0.0502 27 8.6 44

B86-10 0.0104 0.0526 86 6.7 41

Source: Modified from ref. 10.
a Root-mean-square radius of homogenized samples.