Awah et al, 2011
1
FUNGAL ASSESSMENT OF SWEET YAM (Dioscorea esculenta) FLOUR
Awah, N. S.1, 2, Agu, K. C.1 and Odoemena, A. C.1
Department of Applied Microbiology and Brewing
Nnamdi Azikiwe University, P. M. B. 5025, Awka, Anambra State
E-mail: nsikakawah@yahoo.com
Mobile: +2348038728766
2
To whom correspondence should be addressed
ABSTRACT
Sweet yam (Dioscorea esculenta) tubers were processed and fermented to the traditional
West African dried yam flour. Yam slices were blanched for 10 minutes and left to
ferment at room temperature for 48 hours. The fermented yam slices were oven dried at
50oC for 24 hours. The dried yam chips were ground into flour and 1g of the sample was
serially diluted. Appropriate dilution was plated out in duplicate on Potato Dextrose Agar
(PDA) prepared with added chloramphenicol to inhibit bacterial growth. Incubation of
the culture plates was done at room temperature (35-37oC) for 4 to 5 days, which gave
rise to a fungal count of 4×10-5 cfu/ml and 3×10-5 cfu/ml on plates A and B, respectively.
A total of six isolates were recovered and characterized as species of Mucor, Rhizopus
and Aspergillus with a percentage occurrence of 33.3%, 50% and 16.6%, respectively.
The pH of the fermenting medium was reduced from 7.83 at the onset of fermentation to
4.65 at the end of fermentation.
Key words: Sweet yam, chips, fermentation, flour, fungal isolates
2
INTRODUCTION
Sweet yam, Dioscorea species which are also grouped among the lesser yams are
climbing plants with glaborous leaves and twining stems, which coil readily around a
stake (1). It is an important source of carbohydrate for many people of the sub-region
especially in the yam zone of West Africa (2). Yam is the second most important
root/tuber crop in Africa after cassava (3). Sweet yam (Dioscorea esculenta) is one of the
edible yams grown in Nigeria and some other countries along the west coast of Africa (4,
5). Probably because of its Asiatic origin (6), it is sometimes confusingly called Chinese
yam in the West Indies and West Africa in place of Dioscorea opposita that is the true
Chinese yam. Out of the World production of over 50 million tonne of yams per annum,
Nigeria alone produces 35 million tonne. Despite this, the demand for yam tubers in
Nigeria has always exceeded its supply. However, it has been that an average of over
25% of the yield is lost annually to diseases and pests (7, 8, 9).
Nigeria is the largest producer of yam in the world but there is no comprehensive account
of the diseases of the crop (10). In order to minimize losses, considerable quantities of
roots and tubers are transformed into more durable products by drying, fermentation and
comminuted in different combinations and sequences producing a variety of material
each with distinctive characteristics. In some West African countries such as Nigeria,
Benin and Ghana, yams are processed into dry yam tubers/slices and flour (11, 12). When
dry yam is milled into flour, called “elubo”, which is when stirred into boiling water
makes a thick paste known as “amala” eaten with soup by the consumers (13, 12, 14).
Hitherto, this age long traditional method is still being used for the processing of yam to
dry yam (gbodo). Preliminary survey carried out on 263 processorsof the yams in South
West Nigeria revealed that the local consumers have preference for the dry yams made by
the Baruba or Baruten people of Kwara State who incidentally are the major producers of
the traditional dry-yams (5). Though lesser yam is well adapted to the yam growing zones
of Nigeria, it is largely consumed in local farmsteads and urban areas as boiled or fried
yam, that is, without any other defined secondary or even tertiary food product from the
starchy or carbohydrate rich edible tubers (16). The relatively short (sometimes about 2
months) shelf life of the harvested tubers at shaded tropical ambient condition (17) and
the observed preference of the Nigerian yam consumers of the indigenous yam species
(especially white Guinea yam or Dioscorea rotundata) over lesser yam in the preparation
of local dishes (boiled and fried yam inclusive) seem to pose a very serious constraint to
the crop’s expanded production in the country (18). Moreover, flours made with many
white yam cultivars (and even some water yam or Dioscorea alata genotypes) serve as
secondary products for the preparation of locally cherished amala and yam fufu meals
(19). Early research work in the West Indies showed that D. esculenta flour was among
those that gave stiff dough for bread making (20, 21).
Appreciable lesser yam flour inclusion in the wheat based could give acceptable loaves
for Nigerian consumers (18). This would not only improve the value and importance of
sweet yam in the rural farmstead but could also curb its post-harvest losses during
storage, since it does not keep for a long time. Yams are consumed in substantial
3
quantities as a fresh vegetable but a large proportion is processed mainly at the level into
dried products (12). The need for processing is due largely to extreme perishability and
storage losses as a result of decay and rotting (22).
This study was carried out to isolate, purify and characterize the various fungal species
associated with the fermented sweet yam chips processed into flour.
4
MATERIALS AND METHODS
Preparation of Raw Materials
Sweet yam tubers (Dioscorea esculenta) were obtained from a local market in Uyo,
Akwa Ibom State. The yam tubers were thoroughly washed with clean water to remove
adhering soil and other undesirable materials and to reduce microbial growth on the final
product. The yam tubers were peeled, washed, cut into uniform size and mixed in the
ratio of 1:2 with water. They were blanched for 10 minutes. Thereafter, the mixture was
allowed to cool and ferment for two days at room temperature. The pH of the medium
was monitored during the fermentation period. After fermentation, the water used for the
fermentation was decanted and the yam slices dried in a hot air oven at 50 oC for 24 hours.
The dried chips were then ground with an electric blender and the flour obtained stored in
a sterile polythene bag.
MEDIA PREPARATION
The medium used was potato Dextrose agar (PDA) prepared by dissolving 20g of D(+)
glucose and 15g of agar-agar in a potato infusion from 200g of Irish potatoes. The
solution was blanched for 10minutes to enhance the dissolution of the solutes in the
solvent. Chloramphenicol (50mg/ml ethanol) was added to the solution to inhibit any
bacterial growth. The medium was then sterilized by autoclaving at 121 oC for 15minutes
at 15p.s.i and allowed to cool before pouring carefully into sterile Petri dishes.
PROCESSING OF YAM FLOUR
One gram of the yam flour was used to prepare a tenfold serial dilution. The diluent used
for the dilution contained 0.5g of sodium chloride and 1g of bacteriological peptone
dissolved in 100ml of distilled water and sterilized by autoclaving at 121oC for
15minutes. Thereafter, 0.1ml of the 10-5 dilution was plated on potato dextrose agar
(PDA). The plates were then incubated at room temperature for 72hours.
ISOLATION AND IDENTIFICATION OF MICROORGANISMS
The fungal colonies which developed on the plates were counted and purified by subculturing onto potato dextrose agar (PDA) using the point technique. The subculture was
kept at room temperature until the pure cultures became visible. Isolated colonies from
these plates were then transferred on to potato dextrose agar (PDA) slants and kept in the
refrigerator at 4oC. The isolated colonies were identified macroscopically by observing
the pigmentation, texture and elevation and also identified microscopically with the use
of lactophenol blue to view features such as hyphae and spores. The results were
compared with the specimens in the Atlas (23, 24).
5
RESULTS AND DISCUSSION
The result of pH readings during the fermentation of sweet yam slices are presented in
Table 1. There was a progressive reduction in the pH value from 7.83 at the onset of
fermentation to 4.65 at the end of the 48 th hour fermentation period. The reduction in pH
of the fermentation medium can be attributed to the actions of microorganisms involved
in medium. Microorganisms, during fermentation process produce acids and other
metabolites through its various metabolic activities thereby reducing the pH of the
fermentation medium.
The results of the fungal counts from the fermented sweet yam flour are depicted in Table
2. The duplicate plates A and B recorded a total fungal count of 4.0 x 10 -5 cfu/ml and 3.0
x 10-5 cfu/ml, respectively giving an average fungal count of the two plates as 3.5 0 x 10 -5
cfu/ml. The low fungal count obtained in this study can be attributed to the attributed to
the process of subjecting the fermented sweet yam chips to drying (25). This process of
subjecting any food substance to heat treatment is a general or normal way of sterilizing
food substances thereby reducing their microbial loads (26).
The microscopic and macroscopic characteristics of the fungi isolated from fermented
sweet yam are depicted in Table 3 while the percentage occurrences of the isolates are
presented in Table 4. A total of six isolates were obtained and purified as species of
Mucor, Aspergillus and Rhizopus with relative occurrence of 33.3%, 16.6% and 50%,
respectively. These fungi may have been present in the atmosphere in the form of spores
as reported by (25). These fungi were not different from the earlier reports of mycoflora
of sweet yam (25). However in this study, yeast species such as Candida, Geotrichium
and Pichia were not detected. Perhaps due to the available nutrients and prevailing
environmental conditions that determines the nature and density of the colonizers. Fungi
isolates of dried yam flour isolated were identified based on their microscopic and
macroscopic characteristics and a further comparison with the specimens in the atlas (23,
24).
The source of fungi obtained from this study may probably have come from the washing
and steeping water and also improper handling (27). These fungi are also found to be
associated with the spoilage of yam chips and flour. The Mucor species have been
implicated in soft rot of fresh tuber (10). The Aspergillus species are also known to be
involved in mycotoxin (afflatoxin) production but the presence of the toxin is influenced
by the nature of the substrate, climatic conditions, processing factors and presence of
competing fungi. The Rhizopus species which was predominant have been found to be
associated with post-harvest rot (28).
6
TABLE 1: pH Levels of the Fermentation Medium of Sweet Yam
Fermentation Period (h)
0
24
48
pH Level
7.83
6.45
4.65
TABLE 2: Total Fungal Count Observed After the First Culture Process
Plate
Count
Total Count (cfu/ml)
A
4
4×10-5
B
3
3×10-5
Average
3
3.5×10-5
Table 3: Fungal Isolates from Sweet Yam Flour
Isolates
A
Suspected fungal species
Mucor
B
Mucor
C
Aspergillus
D
Rhizopus
E
Rhizopus
F
Rhizopus
7
TABLE 4: Frequency distribution of the isolates from the yam flour
Isolate
Number
Percentage (%)
Mucor species
2
33.3
Aspergillus species
1
16.6
Rhizopus species
3
50
Total
6
100
8
CONCLUSION
This work has that fungi are present in processed yam flour and that they gain access
through various ways. The presence of these organisms is undesirable and they are
involved in the storage rot of yams and some species are involved in the production of
mycotoxins. Hence proper and hygienic processing of yam flour is of utmost importance
to make the sweet yam flour an acceptable food, food ingredient/input for the production
of other food products. Also the yam flour should be properly checked for the presence of
mycotoxins.
9
REFERENCES
1. Udensi, E. A., Oselebe, H. O. and Iweala, O. O. (2008). The investigation of
chemical composition and functional properties of water yam (Dioscorea alata):
Effect of varietal differences. Pakistan Journal of Science, 7: 342-344
2. Akissoe, N. H., Hounhouigan, J. D., Bricas, N., Vernier, P., Nago, M. C.
and Olurunda, O. A. (2001). Physical, chemical and sensory evaluation of
evaluation of dried yam (Dioscorea rotundata) tubers, flour and amala
flour derived product. Tropical Science, 41: 151-156
3. Food and Agriculture Organization (FAO) (1997).Food Production
Yearbook, Volume 50
4. Akoroda, M. O. and Hahn, S. K. (1995). Yams in Nigeria: Status and trends.
African Journal of Root and Tuber Crops, 1(1): 38-41
5. Raemaekers, R. H. (2001). Crop production in tropical Africa, DGIC,
Brussels, Belgium
6. Hahn, S. K. (1995). Yams: Dioscorea spp. (Dioscoreaceae), In’ Evolution
of crop plants, Smart, J. and Simmonds, N. W. (eds), Longman
Scientific and Technical, London,pp 112-120
7. Arene, O. B. (1987). Advances integrated control of economic diseases of
cassava in Nigeria. In: Integrated Pest Management for Tropical Roots
and Tubers Crops, Hahn, S. K. and Cavenes, F. E. (eds), pp. 167-175
8. Ezeh, N.O. (1998). Economics of production and post-harvest Technology, In:
Advances in research, Orkwor. G. C., Asiedu, R. and Ekanayake, I. J. (eds),
IITA and NRCRI, Ibadan, Nigeria, pp 187-214
9. Food and Agriculture Organization (FAO) (1998).Storage and Processing of
roots and tubers in the tropics.
10. Amusa, N. A. and Baiyewu, R. A. (1999). Storage and market disease of yam
tubers in south western Nigeria, Ogun. Journal of Agricultural Resource
(Nigeria), 11: 211-225
11. Bricas, N., Vernier, P., Ategbo, E., Hounhouigan, J. Mitchekpe, E., Nkpenu, K.
and Orkwor, G. (1997). Le development La Filiere cossettes d'Igname en
Afrique de L'Quest. Cashiers de la recherche' Development, 44(2): 100-114
12. Achi, O. K. and Akubor, P. I. (2000). Microbiological characterization
of yam fermentation for yam flour (elubo) production. World Journal
of Microbiology and Biotechnology. 16(1): 3-7
10
13. Akingbala, J. O., Oguntimein, T. B., and Sobanda, A. O. (1995).
Physico-chemical properties and acceptability of yam flour substituted with
Soy flour. Plant Food for Human Nutrition, 48: 73-80
14. Akissoe, N. H., Hounhouigan, J. D., Bricas, N., Vernier, P., Nago, M. C. and
Olurunda, O. A. (2001). Physical, Chemical and Sensory Evaluation of
dried yam (Dioscorea rotundata) Tubers Flour and Amala- A
Flour Derived Product. Tropical Science, 41: 151-156
15. Babajide, J. M., Oyewole, O. B. and Obadina, O. A (2005). An assessment of
the microbiological safety of dry yam (gbodo) processed in southwestern Nigeria, African Journal of Biotechnology, 5(2): 157-161
16. Degras, L. (1993). The yam, a Tropical Root Crop, Macmillan Press, London.
Pp 1-404
17. Opara, L.U. (1999). Post-Harvest Operation: Chapter XXIV Yams.
18. Ukpabi, U. J. (2010). Fermented breadmaking potential of lesser yam
(Dioscorea esculenta) flour in Nigeria. Australian Journal of Crop Science,
4(2): 68-73
19. Ukpabi, U. J. and Omodamiro, R. M. (2008). Assessment of hybrid
white yam (Dioscorea
rotundata) genotypes for the preparation of Amala.
Nigerian Food Journal, 26(1): 111-118
20. Martin, F. W. (1974). Tropical yams and their potential. Part 1. Dioscorea esculenta.
Agricultural Handbook No. 457, USDA, Washington D.C. pp 18
21. Martin, F. W. and Ruberte, R. (1975). Flours made from edible yams
(Dioscorea spp.) as substitute for wheat flour. Journal of Agricultural for
University of Peutro Rico, 59(4): 255-263
22. Coursey, D. G. (1967). Yam storage 1. A review of storage practices and information
on storages losses. Journal of Stored Product Research, 2: 227-244
23. Bulmer, G. S. (1978). Medical Mycology, Airborne fungi, UpJohn Company,
Michigan, pp78-91
24. Frey, D., Oldfield R.J. and Bridger, R.C. (1979). A color atlas of pathogenic
fungi,Wolfs, Medical Publisher. pp 83-93
25. Okigbo, R.N and Nwakammah, P.T., (2005). Biodegradation of white yam
(Dioscorea rotundata Poir) and water yam (Dioscorea alata L.) slices dried
under different conditions; KMITL Science Technology Journal, 5:3
11
26. Rozis, J.F. (1997). Drying Food Stuffs. Backhugs Publisher Leiden. pp 311.
27. Babajide, J. M., Oyewole, O. B., Henshaw, F.O., Babajide, S. O. and
Olasantan,
A. O. (2006). Effect of local preservatives on quality of
traditional dry
yam slices “Gbodo” and its products. World Journal of
Agricultural Sciences,
2(3): 267-273
28. Okigbo, R.N. (2002). Mycoflora of tuber surface of white yam (Dioscorea rotundata)
and post-harvest control of pathogens with Bacillus subtilis, Mycopathologia,
156: 81-85.
FUNGAL ASSESSMENT OF SWEET YAM (Dioscorea esculenta) FLOUR
Awah, N. S.1, 2, Agu, K. C.1 and Odoemena, A. C.1
Department of Applied Microbiology and Brewing
Nnamdi Azikiwe University, P. M. B. 5025, Awka, Anambra State
E-mail: nsikakawah@yahoo.com
Mobile: +2348038728766
2
To whom correspondence should be addressed
ABSTRACT
Sweet yam (Dioscorea esculenta) tubers were processed and fermented to the traditional
West African dried yam flour. Yam slices were blanched for 10 minutes and left to
ferment at room temperature for 48 hours. The fermented yam slices were oven dried at
50oC for 24 hours. The dried yam chips were ground into flour and 1g of the sample was
serially diluted. Appropriate dilution was plated out in duplicate on Potato Dextrose Agar
(PDA) prepared with added chloramphenicol to inhibit bacterial growth. Incubation of
the culture plates was done at room temperature (35-37oC) for 4 to 5 days, which gave
rise to a fungal count of 4×10-5 cfu/ml and 3×10-5 cfu/ml on plates A and B, respectively.
A total of six isolates were recovered and characterized as species of Mucor, Rhizopus
and Aspergillus with a percentage occurrence of 33.3%, 50% and 16.6%, respectively.
The pH of the fermenting medium was reduced from 7.83 at the onset of fermentation to
4.65 at the end of fermentation.
Key words: Sweet yam, chips, fermentation, flour, fungal isolates
2
INTRODUCTION
Sweet yam, Dioscorea species which are also grouped among the lesser yams are
climbing plants with glaborous leaves and twining stems, which coil readily around a
stake (1). It is an important source of carbohydrate for many people of the sub-region
especially in the yam zone of West Africa (2). Yam is the second most important
root/tuber crop in Africa after cassava (3). Sweet yam (Dioscorea esculenta) is one of the
edible yams grown in Nigeria and some other countries along the west coast of Africa (4,
5). Probably because of its Asiatic origin (6), it is sometimes confusingly called Chinese
yam in the West Indies and West Africa in place of Dioscorea opposita that is the true
Chinese yam. Out of the World production of over 50 million tonne of yams per annum,
Nigeria alone produces 35 million tonne. Despite this, the demand for yam tubers in
Nigeria has always exceeded its supply. However, it has been that an average of over
25% of the yield is lost annually to diseases and pests (7, 8, 9).
Nigeria is the largest producer of yam in the world but there is no comprehensive account
of the diseases of the crop (10). In order to minimize losses, considerable quantities of
roots and tubers are transformed into more durable products by drying, fermentation and
comminuted in different combinations and sequences producing a variety of material
each with distinctive characteristics. In some West African countries such as Nigeria,
Benin and Ghana, yams are processed into dry yam tubers/slices and flour (11, 12). When
dry yam is milled into flour, called “elubo”, which is when stirred into boiling water
makes a thick paste known as “amala” eaten with soup by the consumers (13, 12, 14).
Hitherto, this age long traditional method is still being used for the processing of yam to
dry yam (gbodo). Preliminary survey carried out on 263 processorsof the yams in South
West Nigeria revealed that the local consumers have preference for the dry yams made by
the Baruba or Baruten people of Kwara State who incidentally are the major producers of
the traditional dry-yams (5). Though lesser yam is well adapted to the yam growing zones
of Nigeria, it is largely consumed in local farmsteads and urban areas as boiled or fried
yam, that is, without any other defined secondary or even tertiary food product from the
starchy or carbohydrate rich edible tubers (16). The relatively short (sometimes about 2
months) shelf life of the harvested tubers at shaded tropical ambient condition (17) and
the observed preference of the Nigerian yam consumers of the indigenous yam species
(especially white Guinea yam or Dioscorea rotundata) over lesser yam in the preparation
of local dishes (boiled and fried yam inclusive) seem to pose a very serious constraint to
the crop’s expanded production in the country (18). Moreover, flours made with many
white yam cultivars (and even some water yam or Dioscorea alata genotypes) serve as
secondary products for the preparation of locally cherished amala and yam fufu meals
(19). Early research work in the West Indies showed that D. esculenta flour was among
those that gave stiff dough for bread making (20, 21).
Appreciable lesser yam flour inclusion in the wheat based could give acceptable loaves
for Nigerian consumers (18). This would not only improve the value and importance of
sweet yam in the rural farmstead but could also curb its post-harvest losses during
storage, since it does not keep for a long time. Yams are consumed in substantial
3
quantities as a fresh vegetable but a large proportion is processed mainly at the level into
dried products (12). The need for processing is due largely to extreme perishability and
storage losses as a result of decay and rotting (22).
This study was carried out to isolate, purify and characterize the various fungal species
associated with the fermented sweet yam chips processed into flour.
4
MATERIALS AND METHODS
Preparation of Raw Materials
Sweet yam tubers (Dioscorea esculenta) were obtained from a local market in Uyo,
Akwa Ibom State. The yam tubers were thoroughly washed with clean water to remove
adhering soil and other undesirable materials and to reduce microbial growth on the final
product. The yam tubers were peeled, washed, cut into uniform size and mixed in the
ratio of 1:2 with water. They were blanched for 10 minutes. Thereafter, the mixture was
allowed to cool and ferment for two days at room temperature. The pH of the medium
was monitored during the fermentation period. After fermentation, the water used for the
fermentation was decanted and the yam slices dried in a hot air oven at 50 oC for 24 hours.
The dried chips were then ground with an electric blender and the flour obtained stored in
a sterile polythene bag.
MEDIA PREPARATION
The medium used was potato Dextrose agar (PDA) prepared by dissolving 20g of D(+)
glucose and 15g of agar-agar in a potato infusion from 200g of Irish potatoes. The
solution was blanched for 10minutes to enhance the dissolution of the solutes in the
solvent. Chloramphenicol (50mg/ml ethanol) was added to the solution to inhibit any
bacterial growth. The medium was then sterilized by autoclaving at 121 oC for 15minutes
at 15p.s.i and allowed to cool before pouring carefully into sterile Petri dishes.
PROCESSING OF YAM FLOUR
One gram of the yam flour was used to prepare a tenfold serial dilution. The diluent used
for the dilution contained 0.5g of sodium chloride and 1g of bacteriological peptone
dissolved in 100ml of distilled water and sterilized by autoclaving at 121oC for
15minutes. Thereafter, 0.1ml of the 10-5 dilution was plated on potato dextrose agar
(PDA). The plates were then incubated at room temperature for 72hours.
ISOLATION AND IDENTIFICATION OF MICROORGANISMS
The fungal colonies which developed on the plates were counted and purified by subculturing onto potato dextrose agar (PDA) using the point technique. The subculture was
kept at room temperature until the pure cultures became visible. Isolated colonies from
these plates were then transferred on to potato dextrose agar (PDA) slants and kept in the
refrigerator at 4oC. The isolated colonies were identified macroscopically by observing
the pigmentation, texture and elevation and also identified microscopically with the use
of lactophenol blue to view features such as hyphae and spores. The results were
compared with the specimens in the Atlas (23, 24).
5
RESULTS AND DISCUSSION
The result of pH readings during the fermentation of sweet yam slices are presented in
Table 1. There was a progressive reduction in the pH value from 7.83 at the onset of
fermentation to 4.65 at the end of the 48 th hour fermentation period. The reduction in pH
of the fermentation medium can be attributed to the actions of microorganisms involved
in medium. Microorganisms, during fermentation process produce acids and other
metabolites through its various metabolic activities thereby reducing the pH of the
fermentation medium.
The results of the fungal counts from the fermented sweet yam flour are depicted in Table
2. The duplicate plates A and B recorded a total fungal count of 4.0 x 10 -5 cfu/ml and 3.0
x 10-5 cfu/ml, respectively giving an average fungal count of the two plates as 3.5 0 x 10 -5
cfu/ml. The low fungal count obtained in this study can be attributed to the attributed to
the process of subjecting the fermented sweet yam chips to drying (25). This process of
subjecting any food substance to heat treatment is a general or normal way of sterilizing
food substances thereby reducing their microbial loads (26).
The microscopic and macroscopic characteristics of the fungi isolated from fermented
sweet yam are depicted in Table 3 while the percentage occurrences of the isolates are
presented in Table 4. A total of six isolates were obtained and purified as species of
Mucor, Aspergillus and Rhizopus with relative occurrence of 33.3%, 16.6% and 50%,
respectively. These fungi may have been present in the atmosphere in the form of spores
as reported by (25). These fungi were not different from the earlier reports of mycoflora
of sweet yam (25). However in this study, yeast species such as Candida, Geotrichium
and Pichia were not detected. Perhaps due to the available nutrients and prevailing
environmental conditions that determines the nature and density of the colonizers. Fungi
isolates of dried yam flour isolated were identified based on their microscopic and
macroscopic characteristics and a further comparison with the specimens in the atlas (23,
24).
The source of fungi obtained from this study may probably have come from the washing
and steeping water and also improper handling (27). These fungi are also found to be
associated with the spoilage of yam chips and flour. The Mucor species have been
implicated in soft rot of fresh tuber (10). The Aspergillus species are also known to be
involved in mycotoxin (afflatoxin) production but the presence of the toxin is influenced
by the nature of the substrate, climatic conditions, processing factors and presence of
competing fungi. The Rhizopus species which was predominant have been found to be
associated with post-harvest rot (28).
6
TABLE 1: pH Levels of the Fermentation Medium of Sweet Yam
Fermentation Period (h)
0
24
48
pH Level
7.83
6.45
4.65
TABLE 2: Total Fungal Count Observed After the First Culture Process
Plate
Count
Total Count (cfu/ml)
A
4
4×10-5
B
3
3×10-5
Average
3
3.5×10-5
Table 3: Fungal Isolates from Sweet Yam Flour
Isolates
A
Suspected fungal species
Mucor
B
Mucor
C
Aspergillus
D
Rhizopus
E
Rhizopus
F
Rhizopus
7
TABLE 4: Frequency distribution of the isolates from the yam flour
Isolate
Number
Percentage (%)
Mucor species
2
33.3
Aspergillus species
1
16.6
Rhizopus species
3
50
Total
6
100
8
CONCLUSION
This work has that fungi are present in processed yam flour and that they gain access
through various ways. The presence of these organisms is undesirable and they are
involved in the storage rot of yams and some species are involved in the production of
mycotoxins. Hence proper and hygienic processing of yam flour is of utmost importance
to make the sweet yam flour an acceptable food, food ingredient/input for the production
of other food products. Also the yam flour should be properly checked for the presence of
mycotoxins.
9
REFERENCES
1. Udensi, E. A., Oselebe, H. O. and Iweala, O. O. (2008). The investigation of
chemical composition and functional properties of water yam (Dioscorea alata):
Effect of varietal differences. Pakistan Journal of Science, 7: 342-344
2. Akissoe, N. H., Hounhouigan, J. D., Bricas, N., Vernier, P., Nago, M. C.
and Olurunda, O. A. (2001). Physical, chemical and sensory evaluation of
evaluation of dried yam (Dioscorea rotundata) tubers, flour and amala
flour derived product. Tropical Science, 41: 151-156
3. Food and Agriculture Organization (FAO) (1997).Food Production
Yearbook, Volume 50
4. Akoroda, M. O. and Hahn, S. K. (1995). Yams in Nigeria: Status and trends.
African Journal of Root and Tuber Crops, 1(1): 38-41
5. Raemaekers, R. H. (2001). Crop production in tropical Africa, DGIC,
Brussels, Belgium
6. Hahn, S. K. (1995). Yams: Dioscorea spp. (Dioscoreaceae), In’ Evolution
of crop plants, Smart, J. and Simmonds, N. W. (eds), Longman
Scientific and Technical, London,pp 112-120
7. Arene, O. B. (1987). Advances integrated control of economic diseases of
cassava in Nigeria. In: Integrated Pest Management for Tropical Roots
and Tubers Crops, Hahn, S. K. and Cavenes, F. E. (eds), pp. 167-175
8. Ezeh, N.O. (1998). Economics of production and post-harvest Technology, In:
Advances in research, Orkwor. G. C., Asiedu, R. and Ekanayake, I. J. (eds),
IITA and NRCRI, Ibadan, Nigeria, pp 187-214
9. Food and Agriculture Organization (FAO) (1998).Storage and Processing of
roots and tubers in the tropics.
10. Amusa, N. A. and Baiyewu, R. A. (1999). Storage and market disease of yam
tubers in south western Nigeria, Ogun. Journal of Agricultural Resource
(Nigeria), 11: 211-225
11. Bricas, N., Vernier, P., Ategbo, E., Hounhouigan, J. Mitchekpe, E., Nkpenu, K.
and Orkwor, G. (1997). Le development La Filiere cossettes d'Igname en
Afrique de L'Quest. Cashiers de la recherche' Development, 44(2): 100-114
12. Achi, O. K. and Akubor, P. I. (2000). Microbiological characterization
of yam fermentation for yam flour (elubo) production. World Journal
of Microbiology and Biotechnology. 16(1): 3-7
10
13. Akingbala, J. O., Oguntimein, T. B., and Sobanda, A. O. (1995).
Physico-chemical properties and acceptability of yam flour substituted with
Soy flour. Plant Food for Human Nutrition, 48: 73-80
14. Akissoe, N. H., Hounhouigan, J. D., Bricas, N., Vernier, P., Nago, M. C. and
Olurunda, O. A. (2001). Physical, Chemical and Sensory Evaluation of
dried yam (Dioscorea rotundata) Tubers Flour and Amala- A
Flour Derived Product. Tropical Science, 41: 151-156
15. Babajide, J. M., Oyewole, O. B. and Obadina, O. A (2005). An assessment of
the microbiological safety of dry yam (gbodo) processed in southwestern Nigeria, African Journal of Biotechnology, 5(2): 157-161
16. Degras, L. (1993). The yam, a Tropical Root Crop, Macmillan Press, London.
Pp 1-404
17. Opara, L.U. (1999). Post-Harvest Operation: Chapter XXIV Yams.
18. Ukpabi, U. J. (2010). Fermented breadmaking potential of lesser yam
(Dioscorea esculenta) flour in Nigeria. Australian Journal of Crop Science,
4(2): 68-73
19. Ukpabi, U. J. and Omodamiro, R. M. (2008). Assessment of hybrid
white yam (Dioscorea
rotundata) genotypes for the preparation of Amala.
Nigerian Food Journal, 26(1): 111-118
20. Martin, F. W. (1974). Tropical yams and their potential. Part 1. Dioscorea esculenta.
Agricultural Handbook No. 457, USDA, Washington D.C. pp 18
21. Martin, F. W. and Ruberte, R. (1975). Flours made from edible yams
(Dioscorea spp.) as substitute for wheat flour. Journal of Agricultural for
University of Peutro Rico, 59(4): 255-263
22. Coursey, D. G. (1967). Yam storage 1. A review of storage practices and information
on storages losses. Journal of Stored Product Research, 2: 227-244
23. Bulmer, G. S. (1978). Medical Mycology, Airborne fungi, UpJohn Company,
Michigan, pp78-91
24. Frey, D., Oldfield R.J. and Bridger, R.C. (1979). A color atlas of pathogenic
fungi,Wolfs, Medical Publisher. pp 83-93
25. Okigbo, R.N and Nwakammah, P.T., (2005). Biodegradation of white yam
(Dioscorea rotundata Poir) and water yam (Dioscorea alata L.) slices dried
under different conditions; KMITL Science Technology Journal, 5:3
11
26. Rozis, J.F. (1997). Drying Food Stuffs. Backhugs Publisher Leiden. pp 311.
27. Babajide, J. M., Oyewole, O. B., Henshaw, F.O., Babajide, S. O. and
Olasantan,
A. O. (2006). Effect of local preservatives on quality of
traditional dry
yam slices “Gbodo” and its products. World Journal of
Agricultural Sciences,
2(3): 267-273
28. Okigbo, R.N. (2002). Mycoflora of tuber surface of white yam (Dioscorea rotundata)
and post-harvest control of pathogens with Bacillus subtilis, Mycopathologia,
156: 81-85.