A JASSA comparative study of wild yam starch.Starch was extracted from the tubers of Dioscorea schimperiana and the following physicochemical properties were determined: viscosity, gelation temperature, solubility, water-binding capacity, grain size and refractive index. Elemental, phosphorus, protein, amylase, oil and ash contents were also determined. The analyses results of the above parameters were compared to those of the starch derived from Dioscorea dumetorum which had been the subject of an earlier investigation

Sibanda, S.

Sychawska, B.

JASSA Journal of Applied Science in Southern Africa

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JASSA
Journal of Applied Science in Southern Africa 
The Journal of the University of Zimbabwe
Volume 6 • Number 2 • 2000 ISSN 1019-7788
CONTENTS
Editorial
Random walk and the Zimbabwe capital markets
R Dhlakama
A comparative study of wild yam starch from Diosconea  
schim periana
S Sibanda, S Sibanda, and B Sychawska
Effects of a digestive modifier Browse Plus on ruminal and intestinal 
breakdown of forages containing high tanning levels
T E Halimani, C Mutisi and N T Kusina
Susceptibility of selected commercial yeast to autolysis for yeast 
extract production
R N Okagbua, M Siwela and G Takurukura
Predicting the chemical composition of herbaceous legumes using 
Near Infrared Reflectance Spectroscopy
J F Mupangwa, N Berardo, N T Ngongoni, J H Topps,
H Hamudikuwanda and M Ordoardi
Evaluation of community participation in water quality monitoring 
by the use of water quality descriptors as a preliminary approach to 
assessing water quality
T T Mukono and T Tongesayi
JASSA
Journal of Applied Science in Southern Africa Vol. 6 No. 2 • 2000
The Journal of the University of Zimbabwe ISSN 1019-7788
CONTENTS
Random walk and the Zimbabwe 
capital markets...................................
A comparative study of wild yam 
starch from Diosconea 
schimperiana.....................................
Effects of a digestive modifier 
Browse Plus on ruminal and 
intestinal breakdown of forages 
containing high tanning levels.......
Susceptibility of selected commercial 
yeast to autolysis for yeast extract 
production..........................................
Predicting the chemical composition 
of herbaceous legumes using Near 
Infrared Reflectance Spectroscopy
Evaluation of community 
participation in water quality 
monitoring by the use of water 
quality descriptors a preliminary 
approach to assessing water 
quality
R Dhlakama............................................ 71
S Sibanda, S Sibanda and B 
Sychawska..........................................79
T  E Halimani, C Mutisi and N T Kusina... 87
R N Okagbua, M Siwela and G 
Takurukura.......................................... 99
J F Mupangwa, N Berardo, N T 
Ngongoni, J H Topps, H 
Hamudikuwanda and M Ordoardi .... 107
I *
i ;
■- * ■ ;
’l ’ •- •
1 ’  • *
, * » ' j
T T Mukono anp T Tongesayi.......11*5
© University of Zimbabwe 2000
Published by University of Zimbabwe Publications 
P.O. Box MP203, Mount Pleasant, Harare, Zimbabwe
Typeset by University of Zimbabwe Publications 
Printed by Mazongororo Paper Converters
Journal of Applied Science in Southern Africa • Voi. 6 No. 2 • 2000
A comparative study of wild yam starch 
from Dioscorea schimperiana
S Sibanda1, *S Sibanda1 and B Sychawska2
1Department ot Chemistry, University of Zimbabwe, P 0  Box MP 167,
Mount Pleasant, Harare. Zimbabwe.
2Academy of Agriculture, Miekiewicz Avenue, 24-23, 30059 Kracow, Poland.
'Corresponding author
Starch was extracted from the tubers of Dioscorea schimperiana and 
the following physicochemical properties were determined: viscosity, 
gelation temperature, solubility, water-binding capacity, grain size and 
refractive index. Elemental, phosphorus, protein, amylase, oil and ash 
contents were also determined. The analyses results of the above 
parameters were compared to those of the starch derived from Dioscorea 
dumetorum which had been the subject of an earlier investigation.
Keywords: Dioscorea dumetorum, Dioscorea schimperiana, 
dioscoreaceae, gelatinization, retrogradability, International Organization 
for Standardization (ISO), solubility and starch.
Introduction
The genus Dioscorea includes an important group of tropical food yams. Included 
in the family are some of the following Dioscoreaceaes: schimperiana (Kunth) (under 
analysis), dumetorum (K im tli)(N'kala, 1992), cyanesis and tdata. This type of plant 
grows throughout equatorial and tropical Africa and is found in fertile, hilly or 
well-drained veld areas of Zimbabwe. Starch constitutes about 75 percent of the dry 
matter content of the tubers (Watt and Breyer-Brandwijk, 1962). Fresh tubers of 
Dioscoreaceae varieties have also been found to contain two classes of metabolites, 
namely alkaloids and steroids (Watt and Breyer-Brandwijk, 1962; Iwu, 1982). These 
metabolites are thought to be responsible for the herb-like taste of the tubers, the 
toxic components mainly being saponins. The yam tubers are generally used for 
nutritional purposes by rural people (Hegnauer, 1963), but this is only after a 
preliminary intensive washing to free the tubers of the toxic principles. Failure to 
do so usually results in a quick knock-down effect in humans, zebras, birds and fish 
(Khumaloef a i,  1992), if consumed. Some of the Dioscoreaceae tubers have been used 
in the treatment of tropical diseases, such as leprosy and tumours (Corley and 
Tempesta, 1985). They have also been used as phytotherapeutic agents for the 
treatment of diabetes and as anaesthetics. Disabilities such as infertility have also 
been minimised by use of the tubers (Watt and Breyer-Brandwijk, 1962) and the
p79-86
80 Sibanda, Sibanda and Sychawska •  JASSA •  Vol. 6 No. 2 • 2000
latter have also been recommended in certain preparations for piles and dysentery 
(Soni et al., 1985).
The main thrust of this investigation is the isolation of starch from the tubers of 
D. schimperiana and to carry out a comparative study of some of its properties with 
those of starches derived from other members of the family as done by J. C. Favier 
(Soni et al., 1985).
Materials and Methods
Dioscorea schimperiana was collected along the Great Dyke area near Banket in 
Zimbabwe, during summer (April 1993) and a voucher specimen No. 921201 is 
deposited with the National Herbarium and Botanic Garden (Zimbabwe). Fresh 
tubers of D. schimperiana (3.5 kg) were peeled, cut into small pieces and macerated, 
using a Heavy Duty Juice Extractor, to an off-white slurry (3.3kg). The tubers were 
soaked, overnight, in a methanol-water solution (1:1) into which sodium 
metabisulphite had been added to make a 1 percent solution. The suspended starch 
was sieved using a dish towel and the residue washed with distilled water until the 
solution was clear. The starch so extracted was allowed to sediment overnight and 
the supernatant was decanted off. The starch was washed with copious quantities 
of water (29L) until the decanted water was dear. The starch was then filtered off 
and air-dried to give an off-white solid (0.1kg; 3 percent) on a fresh weight basis.
Analysis of starch
The physicochemical properties of D. schimperiana starch were analysed according 
to the general protocol of Henkel Inc. Harare (Zimbabwe) (Muzimbaranda, 1993). 
Parameters such as viscosity, phosphorus and amylose content, water-binding 
capacity and solubility were determined by Sychawska (Poland). Corn starch 
(universally used as the 1 percent standard solution) (Muzimbaranda, 1993) was 
obtained from Henkel Inc. (Zimbabwe).
Physical properties o f the starch
The gelatinization of a 7.8 percent starch solution of D. schimpei iana starch occurred 
over the temperature range 74 to 84°C with complete gelatinization occurring over 
93°C. This was done using a Rheotest-2-rotary viscometer. Characteristics of 
gelation of the starches of D. schimperiana and D. dumetorum are shown in Table 1.
Table 1: Characteristics of gelation.
Parameters D. schimperiana D. dumetorum
Gelation temperature (°C) 81 79
Maximum viscosity (mPas) 207 173
Temperature at maximum velocity (°C) 93 86
Stabilisation after 20 min at 96°C (Mpas) 150 72
A comparative study of wild yam starch from Dioscorea schimperiana 81
The viscosities were taken using Contraves and Brookfield viscometers. Brabender 
and Contraves viscographs are shown in Figures 1 and 2 below. The viscosities after 
a 20 minute stabilisation at 96 C for D. schimperiana and D. dumetorum starches are 
200 mPa and 75 mPa (Nkala and Sibanda, 1994) respectively. A higher viscosity 
shows that the granules are fully swollen and a lower viscosity indicates that the 
granules are only moderately swollen. Ideally, D. schimperiana starch would be 
preferred in uses such as glues and adhesives, and in some food applications, 
because it is stable and does not easily retrograde
Figure 1: Bradender viscograph of D.schimperiana starch.
Figure 2: Contraves viscosities ofD. Schimperiana starch.
82 Sibanda, Sibanda and Sychawska •  JASSA •  Vol. 6 No. 2 • 2000
Other parameters
A summary of the results of a comparison of some of the properties of the sta relies 
of D. schimperiana and D. dumetorum is shown in Table 2 below.
Values in parentheses are those specified by the International Organisation for 
Standardization (ISO) (Nkala, 1994; ISO3947,1977; IS03188 ,1978; ISO3593, 1981).
Table 2 : Selected properties of starches from D. schimperiana  and D. dumetorum  
(comparative basis).
Parameters D. schimperiana D. dumetorum
Moisture content (%) 14.3 (14.2) 
(ISO 3947,1997)
12.35
Amylose content (%) 24.5 14.8
Lipid content (%) 0.44 (s 0.7) 
(ISO 3947, 1977)
3.96
Protein content (%) 0.40 (s0.7) 
(ISO 3947, 1977)
1.73
Phosphorus content (%) 19 28.8
Ash content (%) 0.90 (s 0.2) 
(ISO 3188, 1978)
0.16
Absorption of l2 Xmax (nm) 500 595
pH 5.52 (5-6) 
(ISO 3593, 1981)
5.25
Thus, D. schimperiana would seem to be more suitable source of starch for both domestic 
and industrial purposes than D.dumetorum.
Discussion
The characteristics of gelation indicate that starch from D. schimperiana completely 
gelatinizes at a temperature above 93'C as compared to a temperature of 86°C for 
D. dumetorum. This is an indication that the associative forces in D. schimperiana 
starch are stronger than those in D. dumetorum starch, thus requiring prolonged 
heating for the disruption of the starch matrix. These results show that the starch 
from D. schimperiana has a higher gel strength than that of D. dumetorum. The 
protein content was determined by the Kjeldahl method and that of the D. 
schimperiana starch was found to lie within the internationally accepted range (s0.7 
percent) whereas that of D. dumetomm was 1.73 percent. The advantage of low 
protein content is that it reduces the retrogradability of the starch.
A comparative study of wild yam starch from Dioscorea schimperiana 83
D. schimperiana starch has a lipid content of 0.44 percent (s 0.7 percent) whereas 
that of D. ditmetorum is 3.96 percent. A low lipid content is ideal for commercial 
purposes (ISO 3947, 1977) in the sense that it prevents the starch from turning 
rancid on storage.
The ash content of D. schimperiana starch was found to be 0.9 percent, a value 
which is far higher than that of D. dumetonim (0.16 percent) and even higher than 
the internationally accepted value of 0.2 percent. This could have been due to 
incomplete combustion and also to the conversion of various metals into their 
oxides.
The amylose content of D. sciiimpei iana starch is 24.5 percent whereas that of D. 
dumetonim is 14.8 percent. The difference is also confirmed by the differences in 
iodine absorption. Inspite of its other favourable properties, a high amylose content 
in D. schimperiana starch might promote retrogradation, particularly on long 
storage.
Grain size was analysed using the screen Deck Grab sample method at Henkel 
Chemicals Inc. Results are tabulated in Table 3 (a) below. A microscope analysis 
was also taken [Table 3(b)] and photomicrographs of the starch granules are also 
shown as Figure 3.
Table 3(a): Screen analysis for grain size distribution.
Sieve Micron percentMass
+70 +212 0.86
+100 + 150 0.22
+120 + 125 0.25
+ 170 +90 0.62
+200 +75 0.42
+240 + 63 0.90
Receiver 240 97
Table 3(b): Grain size analysis by a microscope.
The inference from these results is that the starch granules are made up of very 
small grains and 97 percent of them are below 63 urn. When passed through the 
150mm sieve; 0.22 percent of the sample was obtained. It u as observed that most
Micron percentMass
>31 m
21.9 -  30.9 mm
13.9 -  21.8 mm 
<13.9 mm
16.49
4.26
19.15
60.10
84 Sibanda, Sibanda and Sychawska •  JASSA •  Vol. 6 No. 2  •  2000
Figure 3: Photomicrograph of starch granule (with scale).
Figure 4: Solubilization pattern of D. schimperiana star.
A comparative study of wild yam starch from Dioscorea schimperiana 85
of the matter was fibrous implying that the amount of starch retained could lie 
within the International requirements of 0.1 percent (ISO 1666, 1973) retainment.
Solubility and water-binding capacity
The solubility and water-binding capacity ofD. schimperiana starch are shown in 
Figures. 3 and 4 below. The water-binding capacity of the starch was found to be 
88 percent. This is an average value which is attributed to the strong association of 
starch polymers in the granules. A high water binding capacity (above 100 percent) 
indicates a loose association of the polymers.
Figure 5: Water-binding capacity ofD. schimperiana starch.
The solubility of the starch was quite low (1.4 percent to 15.5 percent). It was 
observed from the curve that solubility value increased with temperature. Solubility 
is a measure of the extent of the association of hydrogen bonding within the 
molecules and the compactness of the granules.
Conclusion
From the results discussed above, it is evident that the properties ofD. schimperiana 
and D. dumetorum starches do not vary widely although most of the parameters of 
D. schimperiana starch lie within the interna tionally accepted values. The difference cou Id 
be due to the variation in seasonal changes to which the tubers ofD. schimperiana 
and D. dumetorum were subjected (Nkala, 1992). Structure and composition vary 
according to the size of the granules and we would therefore expect more pronounced 
properties in the starch found in the wet seasonfD. schimperiana) than that found 
in the dry season (D. dumetorum) (Nkala, 1992).
86 Sibanda, Sibanda and Sychawska • JASSA • Vol. 6 No. 2 •  2000
ACKNOWLEDGEMENTS
The authors would like to acknowledge technical help that one of us (Ms SS) 
received from Henkel Inc. (Harare), specifically from Mr C. Muzimbaranda, and 
the Research Board of the University of Zimbabwe for financial assistance, Grant 
No. 3YYH10/3511. Help is also being gratefully acknowledged from Mr R.B. 
Drummond for the botanical identification of D. schimperimm.
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A comparative study of wild yam starch from Dioscorea schimperiana

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A comparative study of wild yam starch from Dioscorea schimperiana

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