Biomedical Engineering Reference
In-Depth Information
TABLE 7.16
Surface Pressure II and Surface Viscosity
of Adsorbed Films of Gum
Arabic After 24 h at the
n
-Hexadecane-Water Interface
a
η
Π
/mN m
-1
10
-3
wt%
10
-3
wt%
pH 7.0
pH 2.3
pH 7.0
pH 7.0
pH 2.3
(I)
A. eriopoda
(5.27% N)
19
16
—
220
200
(II) Commercial sample (0.36% N)
4
4
11
10
1
(III)
A. ampliceps
(0.10% N)
<1
<1
9
<1
<1
a
0.005 M, 25°C.
From Anderson, D. M. W., in
Gums and Stabilizers for the Food Industry,
Phillips, G. O., Wedlock,
D. J., and Williams, P. A., Eds., Elsevier Applied Science, London, 3, 79, 1986. With permission.
good proportion of this protein is bound to the periphery of the gum molecules, then
it may make the major contribution to surface activity. It might be speculated that
the variability in surface activity of commercial gum arabic could be due to the
differing amounts of accessible protein in the various samples.
A strong correlation between surface properties of adsorbed films and the nitro-
gen content of gum arabic samples was observed based on analytical measure-
with a high nitrogen content produced a film with surface pressure and surface
viscosity levels higher than gelatin under the same conditions.
188
It was also shown
that as the temperature of the gum solutions was raised, the proteins present dena-
tured resulting in a significant reduction of emulsification. It was concluded that the
nitrogen content of gum arabic samples was likely an important indicator of the
strength and thickness of gum arabic films adsorbed at the oil-water interface. These
results were consistent with recent studies on the adsorption of gum arabic on
polystyrene latex particles,
189
which showed that only the high molecular weight
proteinaceous component of the gum was capable of stabilizing the particles against
flocculation. Mechanistically, it is easy to visualize that the viscoelasticity of a film
of proteoglycan molecules (e.g., gum arabic) is determined primarily by the polysac-
charide component, with the polypeptide moiety providing an anchor to the interface
(
Figure 7.12
).
Further characterization of the internal product distribution of gum
arabic
190
showed the gum was composed of at least three different glycoprotein
fractions each differing in molecular weight and surface activity. Only one fraction
was amphiphilic and mostly responsible for the adsorption power of the gum.
“Emulsan”— A Microbial Hydrocolloid
Emulsan is a naturally occurring surface active hydrocolloid produced by the oil-
degrading bacterium
Acinetobactercalciaceticus
.
191
It is a microbial bipolymer with
good emulsifying properties and a strong affinity for the oil-water interface.
192,193
In
part, its emulsifying properties arise from the presence of fatty acids (C
12
to C
18
)
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