Olobayo O. Kunlea, Yakubu E. lbrahimc, Martins O. Emejea, Sam Shabac, Yemisi Kunleb
a
Dept. of Pharmaceutical Technology and Raw Materials Development, National Institute Pharmaceutical Research and Development (NIPRD), Abuja, Nigeria
b
Dept. of Medicinal Plant Research and Traditional Medicine, National Institute of Pharmaceutical
Research and Development (NIPRD), Abuja, Nigeria
c
Dept. of Pharmaceutics and Pharmaceutical Microbiology, Ahmadu Bello University, Zaria,
Nigeria
Extraction, Physicochemical and Compaction Properties of Tacca Starch - a Potential
Pharmaceutical Excipient
Some physicochemical properties of tacca starch (Tacca leontopetaloides, Taccaceae) have been
examined and the results compared to those of maize and potato starches. Tacca starch was found to
have a higher amylose content than maize starch but a lower content than potato starch. The starch
granules were small (average particle size 3.5 urn) relative to maize and potato starches and were
predominantly polyhedral with edges. The gelatinisation characteristics except from the temperature
were similar to those of maize starch but much higher than those of potato starch. Tacca starch had
relatively higher swelling power and solubility than the other starches. Its features in the formation
of compacts (tablets) were comparable to those of maize starch with tacca starch being more
resistant to deformation.
Keywords: Tacca starch; Physicochemical properties; Compaction; Tabletting
1 Introduction
Starch finds a wide application in the production of phar-maceuticals. Its use is based on its
adhesive, thickening, gelling, swelling and film-forming properties as well as its ready availability,
low cost and controlled quality [1].
In the last 25 years, the general tendency in the starch industry has been in the utilization of raw
materials other than maize and potato [2, 3]. Nigeria has many native species, which can be used as
a source of starch for pharmaceutical purposes and some of these species have been investigated [48]. Tacca leontopetaloides (L), Taccacea, an annual herb, which grows upright up to 30 cm,
resembles the maize plant. The tacca tuber, which resembles cocoyam and is more or less starchy, is
edible.
Since the chemical composition and physical characteristics of a starch are essentially typical of its
biological origin, we have examined some of the physicochemical properties of tacca starch that
may influence its role in tablet formulations. In addition, we have studied the compaction
characteristics of the starch using the Heckel
Correspondence: Olobayo O. Kunle, Depl. of Pharmaceutical Technology and Raw Materials
Development, National Institute Pharmaceutical Research and Development (NIPRD), P.M.B. 21,
Abuja, Nigeria. E-mail: kunleoo@hotmail.com.
equation [9] to provide an insight into how it will perform in tablet formulations. The Heckel
equation is given by
where D is the relative density of the powder bed during compression, P is the applied pressure; and
K, which is the slope of the straight-line portion of the curve, is the reciprocal of the mean yield
pressure, Py, of the material. From the intercept, A, the relative density, DA, can be calculated using
the following equation
D0 which is the relative density of the powder at the point when the applied pressure equals zero is
described by the equation
D0 = DB - DA
DB describes the phase of rearrangement.
These properties are compared to those of maize and potato starches.
Potato starch was extracted in our laboratories using the same procedure as was used for tacca
starch to validate the extraction process.
2 Materials and Methods
2.1 Materials
Tacca starch was isolated from the tubers of T. leon-topetaloides which were collected from Suleja,
Niger State, Nigeria and the starch extracted as described below, using sodium metabisulphite
purchased from Sigma Chemicals Ltd. (St. Louis, MN, USA). All other materials used were either
of analytical or reagent grade.
2.2 Methods
2.2.1 Starch extraction
Fresh tubers of T.leontopetaloides were washed and peeled, with the eyes and all bruises pitted out.
Immediately after peeling the tubers were immersed in water containing 0.075% (w/v) of sodium
metabisulphite. The fresh tubers were then pulverised with a grinding mill into a homogenous fine
pulp. Another portion of the sodium metabisulphite solution was added, mixed thoroughly, the
slurry filtered through fine muslin and then allowed to settle, after which the supernatant was
decanted. The starch layer was repeatedly treated with sodium metabisulphite solution and finally
washed several times with water. The resulting starch was air dried at room temperature and then
pulverised into fine powder.
2.2.2 Chemical composition and density
Ash content was estimated by measurement of the residue left after combustion in a furnace at 550
°C and moisture by oven drying according to the corresponding British Pharmacopoeia (B.P.)
method [10]. The true density of the starch was determined using a 50 mL specific gravity bottle
with xylene as the displacement liquid. Soluble reducing sugars (expressed as glucose) were determined by thin layer chromatography (TLC). The pH value was determined electrometrically on a
1% (w/v) suspension of starch in distilled water.
2.2.3 Particle size determination
Particle size analysis was carried out using a light microscope with a micrometer .A small quantity
of the starch was dispersed in a drop of glycerol on a microscope slide and covered with a slip. The
particle size was determined at 400 x magnification, and the photomicrograph recorded.
2.2.4 Paste clarity, swelling power and solubility
The paste clarity was determined using the method of On-art and Bristol [2]. Accurately measured
concentrations of the starch - 0.1563, 0.3125, 0.6250, 1.2500 and 2.5000%
(w/v) - were taken in different boiling tubes, placed in a water bath and observed until gelatinisation
was complete. The transmittance was recorded at 580 nm using Zea mays and Solanum tuberosum
as reference standards on a Shimadzu UV 160A spectrophotometer (Shi-madzu, Tokyo, Japan). The
experiment was performed in duplicate.
The method of Chalmers [11] was used to determine swelling power and solubility. Essentially the
weight of moisture-swollen sediment per gram of dry starch was determined after heating the starch
sample in water at different temperatures and centrifugation at 4500 rpm for 30 min. The
supernatant was dried and weighed as a measure of the dissolved starch. This was carried out in
duplicate.
The browning and charring temperatures were determined in the Electro thermal melting point
apparatus (Electrothermal Engineering Ltd., Southend, England).
2.2.5 Estimation of amylose content
Amylose content was determined by complexing amylose with amyl alcohol [11] and collecting
amylose by centrifugation [12].
2.2.6 Compaction studies
2.2.6.1 Preparation of granules
The wet granulation method of massing and screening was used, with distilled water as binder. In
each case, 60 g of starch was weighed and transferred into a mortar to which 26 mL of water was
added in four aliquots and mixed with a pestle after each addition. The moistened mass was then
pressed through a 599 urn screen on an Erweka Type FGS wet granulator (Erweka GmbH,
Heusenstamm, Germany). The mass was then dried at 60 °C to constant weight in a hot air oven.
2.2.6.2 Test of granules
The bulk and tapped densities were determined by pouring 30 g of each granulate into a 1000 mL
calibrated glass cylinder through a short-stemmed glass funnel and the volume occupied by the
granules was read and used to calculate the bulk density. The cylinder was then tapped using a Stav
20003 Stampfvolumeter (J. Engelsmann AG, Ludwigshafen, Germany) until a constant volume was
obtained; the reading was used to calculate the tapped density. Both densities were expressed in
g/mL and the means of two readings are reported.
2.2.6.3 Preparation and analysis of compacts
Compacts (250 mg) of the starches were produced by compressing the granules for 1 min at various
compaction pressures using a hydraulic hand press (Model C, Carver Inc., Savannah, GA, USA).
The dimensions of the compacts were determined with a Mitutoyo model IDC - 1012 EB (Mitutoyo
Corporation, Tokyo, Japan) thickness gauge to the nearest 0.01 mm. The data was analysed using
the Heckel equation.
3 Results and Discussion 3.1 Chemical composition
The chemical composition of tacca starch obtained by the extraction method described above is
shown in Tab. 1, corresponding values for maize and potato starches determined simultaneously are
given for comparison.
The results show that the total ash, acid-insoluble ash and water-soluble ash values of tacca starch is
identical to that of maize starch, an indication that their chemical compositions are similar. The true
density of tacca starch was 1.58, which is lower than those of maize and potato starches.
3.2 Physicochemical properties
Tacca starch is off-white in colour, without clumps and relatively non-sticky. Fig. 1 shows the
photomicrograph of tacca and maize starches. The granules of tacca starch are tiny with polyhedral
edges (Tab. 2), while the granules of maize starch are round in shape. This probably explains the
observation during extraction that it took over 24 h for reasonable sedimentation of tacca starch to
be achieved and the difficulty in obtaining starch after cen-trifuging at 4500 rpm for 1 h. Unlike
maize starch, tacca starch is made up of tiny simple granules with striation but no visible hilum.
Fig. 2 shows the swelling pattern of tacca starch compared to maize and potato starches. For tacca
and maize starches, swelling power increased with temperature, while for potato starch it decreased
at temperatures above 70 °C, even though there was a slight increase between 90 and 95 °C. The
starches derived form underground storage organs, e. g. tacca and potato tubers, have a much higher
swelling power than maize starch derived from a cereal. This is in agreement with results of Okafor
et al. [13], who found that tubers generally have higher swelling power than cereals and attributed
this to the relatively higher moisture content of the starch granules on formulation. The results also
indicate that there is a lesser degree of associative forces in the granules of tacca starch relative to
the other starches.
Tab. 1. Chemical composition of tacca starch compared to maize and potato starches.
Tab. 2. Physicochemical properties of investigated starches.
Fig. 1. Photomicrographs of (A) tacca starch and (B) maize starch.
The solubility pattern of the starches (Fig. 3) shows the same trend for all three starches - increased
solubility with increased temperature up to a maximum of about 80 °C and then a sharp drop as
temperature increased further. In the case of potato starch, however, the peak was attained much
earlier at about 65 °C, swelling power decreasing to zero at 80 °C with gradual increase subsequently. Again tacca starch had the highest solubility of all three starches at all the temperatures
used, a confirmation that the associative forces between the molecules were rather weak.
The results of the paste clarity determination show that the percent transmittance declined rapidly
with increase in concentration (Fig. 4). While potato starch had the highest transmittance at all
concentration levels used, tacca starch consistently had the lowest.
The amylose content of tacca starch was found to be 22.5%, which is in the same range as the
amylose content of potato, cassava and some other root starches [14]. It was much higher than that
of maize starch.
The gelatinisation temperatures of tacca and potato starches were lower than that of maize starch
inspite of their higher amylose contents. Although it has been reported that high amylose content is
associated with high gelatinisation temperature [15], the lower gelatinisation
temperature of tacca starch could be due to the interplay of factors other than amylose content. For
example, the degree and type of molecular association in starch is known to influence the strength
and character of the mi-cellar network within the granule, with the order of decrease given as:
cereal, root and tuber. It would therefore seem that the source of the tuber starches (7. lenontopetaloides and S. tuberosum) is a contributory factor to the results obtained. Other factors which
affect this degree of association, include the ratio of amylose to amy-
lopectin; the characteristics of each fraction in terms of molecular weight; molecular weight
distribution; degree of branching and length of branching in the amylopectin [11].
3.3 Compaction characteristics of tacca starch
The Heckel plots for tacca and maize starches are similar and most closely resembles a type B
compaction profile [16], an indication that both starches deform mainly by plastic flow. At all the
applied compression pressures, the plots indicate that tacca starch was most resistant to deformation.
To explain the deformation characteristic of starch, Heckel constants were derived from the plots
(Tab. 3). The yield point (Py) [17] of the starches, which is an important indication of granule
compressibility and describes the tendency of the material to deform either by plastic flow or
fragmentation, was obtained from the initial portion of the plot [18, 19]. It is inversely related to the
ability of the material to deform plastically under pressure. The results (Tab. 3) therefore indicate
that maize starch underwent plastic deformation more easily _and rapidly than tacca starch. This
also confirms that tacca starch is quite resistant to deformation.
It has been reported by Paronen and Jusl/n [21] that starches under pressure do not fragment even
though they change their shapes. The results would therefore imply that tacca and maize starches
mainly underwent elastic deformation resulting in the low tensile strength of the
Tab. 3. Heckel constants of tacca and maize starches.
compacts. Comparatively, tacca starch underwent elastic deformation to a larger extent than maize
starch (Tab. 3), resulting in tacca starch compacts with relatively lower tensile strength (Fig. 5).
The values of A (Tab. 3), which is a function of the original volume of the compact and relates to
the movement of the particles during the initial stages of compression, was determined from the
intercept of the plots and shows that maize starch particles were more mobile than tacca starch. This
is an indication that there was more consolidation by the maize starch particles either due to the initial relative density of the powder or as a result of densifi-cation by particle rearrangement or both
[22].
This relationship is supported by the higher value of DA for maize starch than tacca starch. Since DA
is a measure of the densification of the material due to die filling and rearrangement in the early
stages of downward movement of the upper punch into the die (i.e. low pressures), the results
indicate that maize starch became relatively denser in the precompression stage.
Tacca starch had higher D0 values than maize starch. D0 is the packing fraction of the starches, and
is indicative of the initial rearrangement phase of die filling at zero pressure with a high value
showing very dense packing. The order obtained is attributable to the small size of the tacca starch
granules as well as its varied shape, which would tend to ensure a much closer packing and filling
of the voids.
DB which describes the phase of rearrangement of the particles, the extent of which depends on the
theoretical point of densification at which particles deformation begins, was also determined for the
starches. Maize starch had a higher value than tacca starch. Tacca starch particles were therefore
more resistant to movement once the initial phase of packing as a result of die filling had been
completed. This was probably due to the high cohesive forces present as a result of the very small
particle size.
As expected, the tensile strength of the compacts increased with increase in compression pressure
indicating the formation of more and stronger bonds (Fig. 5). The extent of increase was higher at
the lower pressures as more new bonds would be formed at lower pressures. As pressure increased,
the tensile strength was virtually constant with tacca starch compacts exhibiting a slight decrease
probably due to the destruction of some bonds as a result of excessive pressure or break up after
removal of pressure. Maize starch compacts were relatively harder than those of tacca starch. This
is not unexpected as the Heckel plot and constants pointed to a higher resistance to deformation by
the tacca starch.
4 Conclusion
The physicochemical tests show that the properties of tacca starch are similar to those of potato and
maize starches. It can be seen from the results of the compaction studies that the compaction
characteristics of tacca starch can be studied using the Heckel equation, which is useful in
explaining the processes involved in compact (tablets) formation. The characteristics of tacca starch
were similar to those of maize starch even though tacca starch was relatively more resistant to
compression. It is concluded that tacca starch can be used as a pharmaceutical excipient comparable
to maize starch in tablet formulation.
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(Received: August 22, 2002) (Revised: December 19, 2002) (Accepted: December 20, 2002)