Introduction
An emulsion is a disperse system consisting
of at least two immiscible liquid phases, one
of which is dispersed as globules (the
disperse phase) in the other (the continuous
phase). The dispersion is stabilized by the
presence of an emulsifying agent or
emulgent.(1) Oil-in-water (o/w) emulsion is
formed when the oil phase is dispersed as
globules throughout an aqueous continuous
phase. The opposite type, water-in-oil (w/o)
is also possible depending on the type of
emulgent used.(1,2) Starch mucilage
stabilizes o/w emulsions and it does so
primarily by imparting viscosity to the
dispersion medium. Starch emulsions have
been used as enemas.(3) Starch mucilage
forms coarse emulsions, which are unstable
and are therefore intended as
extemporaneous preparations only. Simple
emulsions of this type serve as primary
emulsions for the more complex multiple
emulsions if they can be suitably stabilized.
Water-in-oil-in-water (w/o/w) multiple
emulsions are systems where small water
droplets are entrapped within larger oil
droplets that are in turn dispersed in a
continuous aqueous phase.(4) Such w/o/w
multiple emulsions have applications in
controlled drug delivery. The viscosities of
the internal and external aqueous phase are
important to the stability of such systems.(5)
Hence, in this present study we have
examined the relationship between the
viscosities of certain starch mucilages and
the globule structure/stability of emulsions
derived from them.
Materials and Methods
The starches used were extracted from the
tubers of cassava plant (Manihot utilissima),
potato tubers (Solanum tuberosum) and maize
grains (Zea mays) following standard
procedures for starch extraction.(3) The
starches were used to form mucilages at a
predetermined concentration of 4%w/v.
Lower concentration of the cassava starch
mucilage (<4%w/v) formed emulsions that
creamed readily while concentration
>4%w/v were too thick and could not form
emulsions. This point 4%w/v was used to
compare the emulsifying property of all
three starch mucilages.
To form the starch mucilages, 4g were
dispersed in 20ml of distilled water. Boiling
water (80ml) was added to cause gelling.
The mucilage was allowed to cool and then
made up to 100ml. The mucilages were
slightly acidic with pH 5.3 (cassava), 6.4
(potato) and 5.7 (maize). Methylparaben
was added to the mucilages 0.2%w/w as
preservative.
The interfacial tension at the oil-mucilage
interface of the different starches was
measured with the Du Nuoy Tensiometer
(Model No 0/36365f, White Elec. Inst. Co.
Ltd, England). The measurement was done
in triplicate and the mean value was
reported.
To form the emulsions, arachis oil (50ml)
was added gradually to the mucilage (50ml),
whilst stirring (1500rev min -1) for 10min
with a silverson mixer/emulsifier fitted
with a dispersator head (Silverson machine
England, No 18214). The resulting emulsion
was identified as o/w type by staining test.
The fresh emulsions were evaluated for
viscosity, globule size and coalescence rate.
Evaluation of the emulsions:
The viscosities of the starch mucilages and
those of the emulsions were determined by
measuring the time of flow of a sample of
the mucilage or emulsion (10ml) through a
plastic tube of stem orifice 1.5mm and
length 11.5cm (6) at room temperature
28±20C. The viscosity of a fluid is directly
proportional to the time (seconds) of
flow(2); since the present determination was
of comparative value only, the time of flow
was taken as the viscosity index. Each
determination was carried out in triplicate
and the mean result presented.
Globule structures of the emulsions were
determined by photomicrocopy at a
magnification x 40. Photomicrographs were
taken and all globules appearing in each