Influence of Mucilage Viscosity On The Globule Structure And Stability Of Certain Starch Emulsions

micrograph were counted and sized with
the aid of a photomicrograph of a calibrated
stage slide, also taken at magnification x40.

Mean globule size (ʌ) was calculated from
the expression:

(1)

where f is the frequency of each size □ . For
each emulsion sample, 3 representative
photomicrographs were used in the size
analysis. These parameters (i.e. viscosity and
globule structure) were evaluated at selected
time intervals for 1 week. These emulsions
were stored at room temperature 28±2⁰C.
The average rate of increase in globule size
per day was taken as a measure of the
globule coalescence rate and hence a
measure of instability of the emulsions.

Results And Discussion

The results on starch mucilage/emulsion
viscosity (Table 1) showed that cassava
starch mucilage or emulsion were
considerably more viscous than those of the
other two starches.

Table 1: Viscosity index (seconds) of
freshly made mucilages and emulsions

Photomicrographs of the emulsion globules
revealed that cassava starch formed a more
homogenous emulsion characterized by
small size globules of even distribution (Fig
1a). By comparison potato or maize starch
formed coarse emulsions characterized by
large globules (Fig 1a) and uneven size
distribution. The mean globule sizes in the
starch emulsions were (μm) 28±6 (cassava
starch), 42±6 (Potato starch) and 45±5 (Maize
starch). The interfacial tension (at the oil-
mucilage boundary) were (N/m) 0.058
(cassava) 0.032 (Potato) and 0.028 (Maize
starch). Cassava starch mucilage thus
displayed the higher interfacial tension than
the two other mucilages.

A lower interfacial tension will be expected
to facilitate dispersion of the oil globules in
the continuous phase to produce fine
emulsions. The results of the present study
however showed that this was not the likely
determinant factor because cassava starch
mucilage with the highest interfacial tension
produced the finest emulsion. More
probably the difference in globule structure
of the three emulsions relate to the
difference in their viscosities. It is thought
that during emulsification all three
emulsions formed small size oil globules of
a homogenous distribution initially, but
upon withdrawal of the mixing and
shearing forces, globules in the system with
the lower viscosities (i.e. potato and maize
starch emulsions) coalesced very rapidly to
produce coarse emulsions, while in the case
of the cassava starch emulsions, the high
viscosity discouraged rapid coalescence.
Thus, the difference in viscosity accounted
for the observed difference in the globule
structure of the fresh emulsions. On the
other hand, the oil phase was difficult to
disperse when the mucilage viscosity was
excessive (>510 seconds, viscosity index)
resulting in coarse and unstable emulsion.
The application of this finding is that the
viscosity of the aqueous phase can be
optimized to obtain the desirable globule
size distribution and hence stability of the
emulsion. Thus, certain multiple water-in-
oil-in-water (w/o/w) emulsions have been
stabilized by polymerization of the aqueous
phase (5) to increase the viscosity of the
aqueous phase in order to discourage
globule coalescence.

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