Advances in Microbiology, 2013, 3, 333-342
http://dx.doi.org/10.4236/aim.2013.34047 Published Online August 2013 (http://www.scirp.org/journal/aim)
Occurrence of Aspergillus Species and Aflatoxin
Contamination in Raw and Roasted Peanuts from
Formal and Informal Markets in Eldoret
and Kericho Towns, Kenya
Helene Nyirahakizimana1, Lizzy Mwamburi1, Johnstone Wakhisi2,
Charity Kawira Mutegi3, Maria Elisa Christie4, John Maina Wagacha5,6*
1Department of Biological Sciences, University of Eldoret, Eldoret, Kenya
2School of Medicine, Moi University, Eldoret, Kenya
3International Institute of Tropical Agriculture (IITA), Nairobi, Kenya
4Office of International Research, Education and Development, Virginia Technology, Blacksburg, USA
5International Crops Research Institute for the Semi Arid Tropics (ICRISAT), Nairobi, Kenya
6School of Biological Sciences, University of Nairobi, Nairobi, Kenya
Received May 11, 2013; revised June 14, 2013; accepted July 14 2013
Copyright © 2013 Helene Nyirahakizimana et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The population and diversity of fungal species and levels of aflatoxin contamination were investigated in 228 marketed
peanut samples; 140 from formal and 88 from informal markets, in Kericho and Eldoret towns of Kenya. Ground pea-
nut samples were cultured on Modified Dichloran Rose Bengal (MDRB) agar while aflatoxin level was quantified
based on indirect competitive ELISA. Correlation between the incidence of major aflatoxin-producing fungal species
and aflatoxin levels was also established. Fungal species commonly isolated from the peanut samples included Asper-
gillus flavus L strain, A. flavus S strain, A. parasiticus, A. tamarii, A. caelatus, A. alliaceus (all of Aspergillus section
Flavi) and A. niger. Fungi isolated in low frequency included Fusarium spp., Penicillium spp., Mucor spp. and Rhi-
zopus spp. Aflatoxin levels in peanut products ranged from 0 to 2345 µg/kg in raw peanuts, 0 to 382 µg/kg in roasted
coated peanuts, and 0 to 201 µg/kg in roasted de-coated peanuts. Overall, levels of total aflatoxin were higher in sam-
ples from informal (mean = 97.1 µg/kg) than formal (mean = 55.5 µg/kg) market outlets. There was a positive and sig-
nificant correlation (R2 = 0.63; p ≤ 0.05) between aflatoxin levels and the major aflatoxin producing fungi in raw pea-
nuts from formal markets in Eldoret town. Additionally, total aflatoxin in raw peanut samples from informal markets in
Kericho was positively and significantly correlated (R2 = 0.81; p ≤ 0.05) to the population of A. flavus (L and S strains).
In roasted coated peanuts sampled from formal market outlets in Eldoret, aflatoxin levels correlated positively and sig-
nificantly (R2 = 0.37; p ≤ 0.05) with A. flavus S strain. There is need to create awareness among peanut traders and con-
sumers on proper handling of peanuts and health risks associated with consumption of unsafe peanut products.
Keywords: Aflatoxin; Aspergillus Section Flavi; Peanuts; Peanut Market Outlets
Peanut (Arachis hypogeae L) is one of the main crops
grown in Kenya , primarily for local consumption but
also export mainly through the World Food Programme
in Kenya . In 2010, FAO statistics indicated produc-
tion of 99,072 metric tons of peanuts with shell in Kenya,
harvested from 19,291 hectares . Peanut is rich in pro-
tein (26% to 39%), fat (47% to 59%), carbohydrates
(11%), Na (42.0 mg/100 g), K (705.11 mg/100 g), Mg
(3.98 mg/100 g), Ca (2.28 mg/100 g), Fe (6.97 mg/100 g),
Zn (3.2 mg/100 g) and P (10.55 mg/100 g) [4,5], as well
as vitamins E [6,7] and B . Due to its high nutritional
value, it has several uses such as in therapeutic food ,
confectionery , and as an animal feed .
A major challenge in peanut production is fungal and
aflatoxin contamination. Aflatoxins are a group of my-
cotoxins that adversely affect food safety, mainly of
grains and peanuts, as well as trade and human and ani-
opyright © 2013 SciRes. AiM
H. NYIRAHAKIZIMANA ET AL.
mal health. Aflatoxins are the most toxic and carcino-
genic compounds among the known mycotoxins  and
are mainly produced by Aspergillus flavus and A. para-
siticus [11-13]. There are four major aflatoxin types: B1,
B2, G1 and G2 so designated based on their blue and yel-
low-green fluorescence . Aspergillus flavus produces
aflatoxin B1 and B2 while A. parasiticus produces B1, B2,
G1 and G2 . Other aflatoxin producing species in-
clude A. nom ius which produces B and G aflatoxins ,
A. pseudotamarii, A. bombycis and A. ochraceoroseus
Peanuts and maize are the main sources of human ex-
posure to aflatoxin due to their high level of consumption
e.g. 13.3 million tons of peanuts were consumed in Ken-
ya in 2001-2003 with a projected consumption of 16.32
million tons in 2030 . Reference  found Eurotium
repens, A. parasiticus, and A. flavus to be the most potent
aflatoxigenic species with average levels of aflatoxin
above 100 µg/kg in peanuts from markets within Nairobi.
High prevalence of A. flavus L strain (> 77%) and A. fla-
vus S stain (> 65%) has been reported in peanuts from
Busia and Homa bay counties in Kenya . Marketing
of peanuts in Kenya is generally through informal market
outlets [1,20], where peanuts are not properly protected
from environmental influence and are not properly pac-
kaged; making them susceptible to fungal contamina-
tion. According to [20,21], peanuts at market level in
Kenya are more contaminated with aflatoxin than those
stored by farmers.
Recent reports indicate that aflatoxin is common in
peanuts and grains in different parts of Kenya [1,2,20],
posing a serious health challenge. In order to minimize
consequences of aflatoxin on food security, trade, health
and meet national and international mycotoxin regulatory
standards, there is need to monitor fungal species and
mycotoxin contamination periodically. The objective of
this study was to investigate the incidence and diversity
of aflatoxin producing fungal species and aflatoxin levels
in marketed raw and roasted peanuts in Eldoret and Keri-
cho towns in Kenya. The correlation between the popula-
tion of major aflatoxin producing fungi and total afla-
toxin levels in peanuts marketed in different outlets was
2. Materials and Methods
2.1. Study Area
The study was conducted in Eldoret and Kericho towns
within the Rift Valley region in Kenya. Eldoret town
(0˚31′54″N, 35˚15′58″E) is located in Western Kenya at
2100 - 2700 m above sea level, 300 km North West of
Nairobi on the trans-African highway and 65 km North
of the equator. It has a cold and wet climate with an ave-
rage temperature of 27˚C and 1124 mm mean annual
rainfall. Kericho town (0˚22′0″S, 35˚16′59″E) lies within
the highlands west of the Great Rift Valley in Kenya,
adjacent to Kenya’s biggest water catchment area, the
Mau forest. It is located in the south west of the country
at 2096 m above sea level, 263 km North West of Nairobi.
The climate in Kericho is characterized by cool tempera-
tures ranging from 16˚C to 20˚C, and high rainfall aver-
aging between 1400 mm and 2000 mm per annum.
2.2. Market Survey and Collection of Peanut
Market survey was conducted in June 2011 and collec-
tion of peanut samples took place from June 2011 to
January 2012. Two hundred and twenty eight (228) pea-
nut samples of 0.5 kg each were collected from formal
and informal markets from Eldoret (118) and Kericho
(110) towns. Seventy four peanut samples (raw-15,
roasted-36, roasted de-coated-23) and 66 samples (raw-
15, roasted-30, roasted de-coated-21) were collected
from formal markets in Eldoret and Kericho towns, re-
spectively. Additionally, 44 samples (raw-24; roasted-20),
and another 44 (raw-24; roasted-15, roasted de-coated-5)
were collected from informal markets in Eldoret and
Kericho towns, respectively.
Within each town, stratified systematic sampling plan
was followed in acquiring samples of peanut products on
sale. Formal markets referred to stockists, shops and su-
permarkets while informal markets included hawkers and
open markets. Half a kilogram sample of raw shelled,
roasted and roasted de-coated peanuts was collected from
each vendor operating formal and informal market out-
lets. The peanut samples collected from informal markets
were packaged and sealed in a sterile polythene bag. All
samples were then transported to the laboratory where
they were stored in a cold room at 8˚C until laboratory
Information on the source and handling of peanuts on
sale was gathered through direct observation and inter-
view using a semi-structured questionnaire that captured
the type of peanut product traded, nature of market outlet,
packaging material used, source of the peanut products,
mode of transport to the market, storage structures and
conditions, whether or not peanuts were sorted before
selling, the sorting criteria used, and the time interval
between buying and selling (data not shown).
2.3. Sample Preparation
Each peanut sample was thoroughly mixed and 250 g
drawn and ground to a fine powder using a Black and
Decker blender machine (BX525-B5 Type 02, Shanghai,
China). Two replicates of 100 g each were weighed
where one replicate was used for mycological analysis
and the other for aflatoxin analysis.
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H. NYIRAHAKIZIMANA ET AL. 335
2.4. Microbial Analysis
2.4.1. Preparation of Culture Media and Peanut
Peanut samples were cultured on modified dichloran rose
bengal (MDRB) agar medium . The medium was
prepared by mixing 10 g glucose, 2.5 g peptone, 0.5 g
yeast extract, 1 g KH2PO4, 0.5 g MgSO4, 7H2O, 20 g agar,
and 25 mg rose bengal in 1 L distilled water. The pH of
the medium was adjusted to 5.6 using 0.01 M HCl. The
medium was autoclaved for 20 minutes at 121˚C and 15
psi, and cooled in a water bath at 60˚C. To inhibit the
growth of bacteria and ensuring that the medium was
semi selective for Aspergillus species, 5 ml of 4 mg/l
dichloran (in acetone), 40 mg/l streptomycin (in 5 ml
distilled water) and 1 mg/l chlorotetracycline (in 10 ml
distilled water) were added to the medium through a ster-
ile 0.25 µm syringe filter paper after cooling to 50˚C. Ap-
proximately 20 ml of the medium was dispensed in dis-
posable Petri plates, and allowed to settle for two to three
days before use.
From the 100 g ground peanut sub-sample, two sub-
samples of 2.5 g each were weighed and transferred into
falcon tubes into which 10 ml of 2% water agar solution
(2 g of agar dissolved in 100 ml sterile distilled water)
were added and mixed thoroughly. The first sub-sample
was serial diluted to 10−1 and the second to 10−2. A 0.2 ml
aliquot of the suspension from each dilution was pipetted
and spread onto MDRB plates under aseptic conditions.
There were six replicates for each sample (three for 10−1
and three for 10−2). The plates were incubated for seven
days at 30˚C after which fungal colonies were counted.
The fungal colonies were sub-cultured on clean MDRB
agar medium Petri plates for identification.
2.4.2. Identification of Fungal Species and Counting
Pure colonies on MDRB agar medium were sub-cultured
onto the Czapek yeast extract agar (CYA; 1 g K2HPO4,
10 mL Czapek concentrate, 5 g powdered yeast extract,
30 g sucrose, 15 g agar), whose pH was adjusted to 7.2
and the plates incubated at 30˚C for 7 days. Species of
Aspergillus section Flavi were identified based on cul-
tural and morphological characteristics including colony
colour, size of sclerotia, texture and conidial morphology
characteristics , and by comparison with reference
strains obtained from the International Crops Research
Institute for the Semi-Arid Tropics (ICRISAT) Plant Pa-
thology laboratory. The reference cultures were sub-cul-
tured at the same time of plating the peanut samples. Co-
lonies of other isolated fungal pathogens were identified
to genus level. The colony forming units (CFU) of each
fungal species were counted using the Gallenkamp col-
ony counter (Gallenkamp manufacturer, Frodsham, Eng-
land). Equation (1) was used to determine the population
(CFU/g peanuts) of the fungal species.
CFUg peanuts=volume plateddilutionfactor (1)
The volume plated was 0.2 ml while the dilution fac-
tors were 0.25 for the first dilution (10−1) and 0.025 for
the second dilution (10−2).
2.5. Aflatoxin Analysis
The level of total aflatoxin in each peanut sample was
determined by indirect competitive Enzyme Linked Im-
munosorbent Assay (ELISA), a method approved by the
Association of Analytical Chemists (AOAC) . A 100
g sub-sample peanut powder was well mixed and 20 g
taken, triturated in 70% methanol (70 ml absolute metha-
nol in 30 ml distilled water, v/v) containing 0.5% (w/v)
potassium chloride in blender for 2 minutes. The extracts
were transferred to a conical flask and shaken for 30
minutes at 300 rpm. The extract was filtered through
Whatman number 41 filter paper and then transferred to a
sterile container, stored in a freezer (−20˚C) until analy-
sis for total aflatoxin. The extracts were analyzed for
aflatoxin level at the Plant Pathology laboratory in IC-
2.6. Data Analyses
Data on fungal population and aflatoxin levels in peanuts
were compared based on the type of market (formal and
informal), type of peanut product (raw, roasted coated
and roasted de-coated) and towns (Eldoret and Kericho).
The diversity of fungal species contaminating peanut
products sampled from the two market types and towns
was compared based on Simpson diversity index (D)
values. Equation (2) was used to compute D values. Low
index value indicates that a few species dominated over
Where S = number of species; ;
i= 1, 2,10
CFUg peanuts for species i
Aflatoxin level was not normally distributed and did
not have constant variance implying that the assumptions
for parametric t-test did not hold (Shapiro-Wilk test for
Normality, p < 0.001 and Bartlett’s test for homogeneity
of variances, p < 0.001). Therefore, in comparing any
two groups of the variables, the non-parametric Mann-
Whitney U (Wilcoxon rank-sum) statistical test was used
to analyze the data. In comparing more than two groups,
data were analyzed using analysis of variance (ANOVA)
Copyright © 2013 SciRes. AiM
H. NYIRAHAKIZIMANA ET AL.
Copyright © 2013 SciRes. AiM
under unbalanced design (GenStat version 14). The
means were separated using Fisher’s protected least sig-
nificant difference (LSD) at 5% level of significance.
SPSS version 16 statistical software was used to conduct
3.1. Fungal Species Identified from Various
Seven Aspergillus species—A. flavus L strain, A. flavus S
strain, A. parasiticus, A. tamarii, A. caelatus, A. alliaceus
(members of Aspergillus section Flavi) and A. niger—
were isolated from 69% of the peanut samples (Table 1).
Out of 228 total peanut samples analyzed, 28% were
infected with one Aspergillus species or strain; while
41% were contaminated with more than one Aspergillus
species. Other fungal genera isolated from the peanut
samples included Mucor, Rhizopus and Fusarium. Table
1 shows the mean population (CFU/g peanuts) of fungal
species isolated from various peanut products from dif-
ferent market outlets. The incidence of major fungal
pathogens was as follows in decreasing order: A. flavus L
strain (mean = 574 CFU/g), A. tamarii (mean = 109) and
A. flavus S strain (mean = 97). Aspergillus niger, A.
parasiticus, A. alliaceus and A. caelatus were isolated in
low frequency with averages of 39, 18, 4 and 3 CFU/g
Generally, the incidence of fungal pathogens in peanut
samples from informal markets was significantly higher
(p ≤ 0.05) than from formal markets. For example, the
incidence of fungal pathogens in raw peanuts from in-
formal market outlets was significantly higher (p ≤ 0.05)
than from formal market outlets, both in Eldoret and
Kericho towns. However, the incidence of fungal patho-
gens was significantly higher (p ≤ 0.05) in roasted coated
peanuts from formal markets than in samples from in-
formal markets. There was also variability in infection
levels of peanuts sampled from Eldoret and Kericho
towns. The incidence of fungal pathogens in raw and
Table 1. The population (CFU/g peanuts) of fungal species in different peanut products sampled from formal and informal
market outlets in Eldoret and Kericho towns.
Population of fungal species (CFU/g substrate)
Town Peanut product
AFL AFS AP AT AC AA AN PEN Others
Raw 43.8 114.0 1.6 8.9 11.3 1.3 250.743.6 236.0 711 79.0
Eldoret Roasted coated 0.4 0.1 0.3 1.4 1.8 0.0 0.4 0.9 78.1 83 9.2
Roasted de-coated 5.8 1.9 4.4 9.1 1.7 0.0 1.6 0.4 11.5 36 4.0
Mean 17.0 39.0 2.0 7.0 5.0 0.4 84.0 15.0 109.0 277.0 31.0
Kericho Raw 9.3 217.3 19.8 0.0 3.1 0.0 25.8 0.0 0.0 275.0 31.0
Roasted coated 1685.9 17.9 0.0 71.9 0.0 0.0 1.1 0.1 0.2 1777.0197.4
Roasted de-coated 1254.9 56.4 4.9 437.90.0 0.0 0.0 0.0 0.0 1754.0195.0
Mean 983.0 97.0 8.0 170.01.0 0.0 9.0 0.0 0.1 1269.0141.1
Grand Mean 500.0 68.0 5.0 88.0 3.0 0.2 47.0 8.0 53.0 773.0a
Raw 188.5 191.1 33.5 83.8 12.6 36.0 156.8136.5 442.8 1282.0142.4
Eldoret Roasted coated 0.2 0.0 1.7 0.0 0.0 0.0 0.0 0.2 14.0 16.0 2.0
Roasted de-coated - - - - - - - - - - -
Mean 94.0 96.0 18.0 42.0 6.0 18.0 78.0 68.0 228.0 1298.072.2
Kericho Raw 1326.9 448.5 110.0441.51.7 0.0 47.9 0.0 6.5 2383.0265.0
Roasted coated 1094.4 0.0 0.0 25.8 0.0 0.0 0.0 0.0 4.9 1125.0125.0
Roasted de-coated 1685.3 0.0 0.0 13.3 0.0 0.0 0.0 0.0 0.0 1699.0189.0
Mean 1369.0 150.0 37.0 160.01.0 0.0 16.0 0.0 4.0 5207.0193.0
Grand Mean 859.0 128.0 29.0 113.03.0 7.0 41.0 27.0 90.0 6504.0b
AFL: A. flavus L strain; AFS: A. flavus S strain; AP: A. parasiticus; AT: A. tamarii; AC: A. caelatus ; AA: A. alliaceus; AN: A. niger; PEN: Penicillium spp.
Different letters accompanying grand means indicate that they are significantly (p ≤ 0.05) different. Roasted de-coated peanuts were not on sale in informal
markets of Eldoret town.
H. NYIRAHAKIZIMANA ET AL. 337
roasted de-coated peanuts sampled from Kericho town
was significantly higher (p ≤ 0.05) than in similar sam-
ples from Eldoret town. However, there was no signifi-
cant (p ≥ 0.05) difference in the incidence of fungal
pathogens in roasted de-coated peanuts sampled from
formal and informal markets in Kericho town. Similarly,
there was no significant (p ≥ 0.05) difference in the inci-
dence of fungal pathogens between roasted coated pea-
nuts and roasted de-coated peanuts from formal and in-
formal markets in Eldoret and Kericho towns.
3.2. Diversity of Fungal Species in Different
Peanut Products and Market Types
The diversity of fungal species was generally higher in
peanut products sampled from Eldoret than Kericho town
(Figure 1). However, there was no significant (p ≥ 0.05)
difference in the diversity of fungal species among dif-
ferent peanut products.
3.3. Population of Major Aflatoxin-Producing
Fungi in Peanut Samples
The population of major aflatoxin-producing species (A.
flavus L strain, A. flavus S strain and A. para siticus) was
significantly (p ≤ 0.05) higher in peanuts from Kericho
than in Eldoret town. However, the population of these
species was not significantly different in roasted coated
and roasted de-coated peanuts from both formal and in-
formal market outlets (Figure 2).
The incidence of A. flavus (L and S strains) and A.
parasiticus was significantly lower (p ≤ 0.05) in raw
peanuts sampled from formal than informal markets in
Eldoret town (Figure 2). Aspergillus flavus L strain was
pre-dominant in roasted coated and roasted de-coated
peanuts from formal markets with an incidence of 98.9
and 94.9%, respectively (Table 2). The corresponding
incidence in raw, roasted coated and roasted de-coated
peanuts from informal markets was 68.5, 99.9 and 100%.
The incidence of A. flavus S strain was significantly
higher (82%) in raw peanuts from formal markets than
from informal markets (28%) while Aspergillus para-
siticus was isolated in low incidence in same raw peanuts
from both formal (5%) and informal (6%) markets.
3.4. Aflatoxin Levels in Different Peanut
There was variation in total aflatoxin levels between pea-
nut samples from formal and informal market outlets,
Eldoret and Kericho towns as well as among peanut
products (Table 3). Eighty one percent (185 out of 228)
of the peanut samples analyzed had detectable levels of
total aflatoxin. Aflatoxin levels in peanut products rang-
ed from 0 to 2345 µg/kg in raw peanuts, 0 to 382 µg/kg
in roasted coated peanuts, and 0 to 201 µg/kg in roasted
de-coated peanuts. Generally, raw peanuts were the most
Figure 1. Fungal species diversity in peanut products marketed in formal and informal markets in Kericho and Eldoret
towns. Bars accompanied by the same letter are not significantly (p ≥ 0.05) different.
Copyright © 2013 SciRes. AiM
H. NYIRAHAKIZIMANA ET AL.
Figure 2. Colony forming units (CFU/g substrate) of Aspergillus flavus (L and S strains) and A. parasiticus in different peanut
products sampled from formal and informal markets in Eldoret and Kericho towns. Bars accompanied by the same letter(s)
are not significantly (p ≥ 0.05) different.
Table 2. Incidence (%) of major aflatoxigenic species in different peanut products sampled from formal and informal market
outlets in Eldoret and Kericho towns.
Incidence (%) of aflatoxigenic species
Market type Peanut product A. flavus L strain A. flavus S strain A. parasiticus
Formal Raw 13.10 81.60 5.30
Roasted coated 98.90 1.10 0.02
Roasted de-coated 94.90 4.40 0.70
Informal Raw 68.50 27.80 6.20
Roasted coated 99.90 0.00 0.10
Roasted de-coated 100.00 0.00 0.00
Table 3. Aflatoxin levels (µg/kg) in different peanut products sampled from formal and informal markets in Eldoret and
Formal market Informal market Grand mean
Eldoret Kericho Mean Eldoret Kericho Mean
Raw 37.8 129.0 83.4 80.1 340.2 210.2 146.8
Roasted coated 93.1 55.5 74.3 48.1 29.4 38.8 56.5
Roasted de-coated 7.9 9.6 8.8 - 42.3 42.3 19.9
Mean 46.3 64.7 55.5 64.1 137.3 97.1
Roasted de-coated peanuts were not on sale in informal markets of Eldoret town.
Copyright © 2013 SciRes. AiM
H. NYIRAHAKIZIMANA ET AL.
Copyright © 2013 SciRes. AiM
contaminated (mean = 146.8 µg/kg), while roasted de-
coated peanuts were the least contaminated (mean = 19.9
µg/kg). Similarly, raw peanuts sampled from informal
markets had higher levels of aflatoxin (mean = 210.2
µg/kg) than samples from formal market outlets (mean =
83.4 µg/kg). In contrast, roasted coated peanuts from
formal markets were more contaminated (mean = 74.3
µg/kg) than samples from informal markets (mean = 38.8
The level of total aflatoxin in roasted coated peanuts
was higher in Eldoret than Kericho town. Raw peanuts
sampled from informal markets in Kericho town had sig-
nificantly higher levels of aflatoxin (mean = 340.2 µg/kg,
with 83% contaminated samples), compared to roasted
de-coated peanuts from formal markets in Eldoret (mean
= 7.9 µg/kg, with 74% contaminated samples). Overall,
the levels of total aflatoxin were higher in informal
(mean = 97.1 µg/kg) than formal (mean = 55.5 µg/kg)
3.5. Correlation between the Population of
Major Aflatoxin Producing Species
and Aflatoxin Levels
The population of A. flavus (S and L strains) and A. pa-
rasiticus in raw peanuts had a significant positive corre-
lation (R2 = 0.69; p ≤ 0.05) with total aflatoxin level
(Figure 3). However, there was no significant (p ≥ 0.05)
correlation between the two fungal species and aflatoxin
levels in roasted peanuts. The population of A. flavus and
A. parasiticus significantly influenced the levels of afla-
toxin in peanuts sampled from formal markets in Eldoret
town (R2 = 0.63; p ≤ 0.05). On the contrary, aflatoxin
levels in roasted coated peanuts from informal markets
were not significantly correlated (R2 = 0.09; p ≥ 0.05) to
the population of A. flavus and A. parasiticus. In formal
markets, the population of A. flavus S strain significantly
positively correlated (R2 = 0.37; p ≤ 0.05) with the levels
of aflatoxin. However, the level of aflatoxin was not sig-
nificantly correlated (R2 = 0.102; p ≥ 0.05) to the popula-
tion of A. flavus and A. parasiticus in roasted de-coated
peanuts sampled from formal markets.
There was a highly significant correlation (R2 = 0.807;
p ≤ 0.05) between aflatoxin level and the population of A.
flavus (L and S strain) in raw peanuts sampled from in-
formal markets in Kericho town. However, aflatoxin
level in raw peanuts sampled from formal markets in
Kericho was only significantly correlated (R2 = 0.48; p ≤
0.05) to the population of A. flavus S strain. For roasted
coated and de-coated peanuts from both formal and in-
formal market outlets, aflatoxin level was not signifi-
cantly (p ≥ 0.05) correlated to the population of A. flavus
and A. parasiticus.
This study investigated the occurrence of Aspergillus
species and aflatoxin contamination in raw and roasted
peanuts from formal and informal markets in Eldoret and
Kericho towns in Kenya.
Six Aspergillus species—A. flavus L and S strains, A.
parasiticus, A. tamarii, A. caelatus, A. alliaceus, A. ni-
Figure 3. Scatter plot of the population [CFU/g peanuts] of Aspergillus flavus (L and S strains) and A. parasiticus against
aflatoxin level in raw peanuts. Aflatoxin level = 31.50 (p ≥ 0.05) + 0.032038 CFU (p ≤ 0.05; R2 = 0.69).
H. NYIRAHAKIZIMANA ET AL.
ger—were isolated from marketed raw and roasted pea-
nuts in Eldoret and Kericho towns. Sixty-seven percent
of the peanut samples analyzed were contaminated with
the major aflatoxin producing fungi (A. flavus L strain, A.
flavus S strain and A. parasiticus) with A. flavus L strain
(> 98%) being the pre-dominant pathogen followed by A.
flavus S strain. The occurrence of these fungi especially
A. flavus S strain implies a high risk of aflatoxin con-
tamination of peanuts marketed in Eldoret and Kericho
towns. The incidence of the three fungi in peanuts con-
curs with the findings of a recent study in western Kenya
. Similar to findings of the current study, two mor-
photypes of A. flavus, the S and L strains, have been iso-
lated in other studies on peanuts in Kenya [9,19]. The
role of A. tamarii, A. alliaceus and A. caelatus as com-
mon pathogens of peanuts in Kenya has also been docu-
mented . Generally, in both towns, peanuts from in-
formal markets had higher fungal species diversity in raw
peanuts an observation which concurs with the findings
reported in .
The type of market outlet influenced the incidence of
pathogenic fungi in peanuts. This could be attributed to
handling practices including superior packaging, sorting
and storage conditions that were characteristic in formal
markets. In contrast, raw peanuts sold in informal mar-
kets were generally not packaged or sorted and were
stored in stalls exposed to weather fluctuations. In addi-
tion, some peanuts were sold in open air systems sub-
jecting them to weather changes and abrupt rainfall
which could promote fungal proliferation.
The incidence of aflatoxin producing fungi was sig-
nificantly higher in peanuts sampled from markets in
Kericho than in Eldoret town. This could be attributed to
the fact that raw peanuts sampled from informal markets
in Eldoret were sold under covered structures whereas in
Kericho, open air markets were more common. Peanut
roasting and de-coating reduces fungal population in
and/or on kernels . Indeed, during roasting process,
peanuts are exposed to dry heat at high temperatures 
that kill or reduce the population of fungi. However, the
incidence of aflatoxin producing fungi was not signify-
cantly lower in roasted peanuts sampled from Kericho
town. This could be attributed to handling practices
which could result in re-contamination of roasted ker-
Aspergillus flavus and A. parasiticus are fungal species
that have an affinity for nuts and oilseeds, and are the
main producers of aflatoxin which are the most toxic and
carcinogenic compounds among the known mycotoxins
. Aspergillus flavus produces AFB1, AFB2 in addition
to cyclopiazonic acid  which targets the liver, kidney
and gastrointestinal tract in animals, while A. parasiticus
produces AFB1, AFB2, AFG1 and AFG2 . Aspergillus
tamarii produces AFB1, AFB2 and cyclopiazonic acid 
while A. alliaceus produces ochratoxin . Ochratoxin
is nephrotoxic, hepatotoxic, immunotoxic and possibly
neurotoxic . Fusarium spp. produce trichothecenes,
fumonisins, and zearalenone among other mycotoxins
while Penicillium sp p. produce ochratoxin A and patulin
. Although the above toxins were not the subject of
investigation in this study, the presence of fungal species
known to produce them implies a greater health risk to
consumers of peanut products. In addition, this observa-
tion reveals the need for management strategies that tar-
get the control of both aflatoxin-producing fungi and
pathogens that produce other types of mycotoxins.
This study also investigated the levels of total afla-
toxin in different peanut samples. Aflatoxin levels ranged
from 0 to 684.8 μg/kg and 0 to 2344.8 μg/kg in samples
from formal and informal markets, respectively. These
results are consistent with the findings in  where afla-
toxin levels ranging from 0 to 2687.6 μg/kg and 0 to
1838.3 μg/kg were reported in peanuts sampled from
Busia and Homa bay regions in Western Kenya. High
incidence of aflatoxin in raw peanuts (83% of raw peanut
samples had levels of aflatoxin averaging 340.2 µg/kg)
corroborated findings from Botswana , where con-
tamination incidence of 78% of raw samples and afla-
toxin concentration ranging from 12 to 329 μg/kg were
reported. Previous studies [25,32] have shown that pea-
nut roasting and de-coating processes reduce the risk of
aflatoxin production. This study revealed that there was
higher risk of exposure to aflatoxin through raw than
roasted peanuts. Roasting kills aflatoxin producing fungi
thereby reducing the risk of aflatoxin contamination.
Aflatoxin contamination of peanuts should be a public
health concern not only in Eldoret and Kericho towns but
also in other parts of Kenya as well as other tropical
countries. Reference  reported a mean content of 40
μg/kg of aflatoxin B1 in over 85% of peanut oil samples
from Senegal, while  reported high aflatoxin content
of 25 to 600 μg/kg in Sudanese peanuts. Different levels
of aflatoxin were also reported in peanuts collected from
processors, stockers, farmers and traders in Benin ,
while  reported total aflatoxin level of 56 μg/kg in
unprocessed peanuts in Brazil. Roasted de-coated pea-
nuts had high percentage (74%) of contaminated samples
whereas the concentration of aflatoxin was relatively low
with an average of 7.9 µg/kg. Previous studies have
shown that exposure of humans to high levels of afla-
toxin leads to acute aflatoxicosis and that long-period of
exposure to aflatoxin, even in low concentration, may
lead to liver cancer, stunted growth in children and to
immune system disorders through chronic aflatoxicosis
There was a strong positive correlation between the
population of aflatoxin-producing fungi and total afla-
toxin levels detected in raw peanuts. However, total afla-
Copyright © 2013 SciRes. AiM
H. NYIRAHAKIZIMANA ET AL. 341
toxin levels in roasted peanuts from formal and informal
markets were not significantly correlated to the popula-
tion of aflatoxigenic fungal species. These findings con-
cur with those in [9,25,32] who have reported that roast-
ing and de-coating processes reduce fungal population.
The population of A. flavus S strain was found to sig-
nificantly influence aflatoxin production. This concurs
with the findings by  who reported that the incidence
and population of A. flavus S strain significantly and
positively correlated with the levels of total aflatoxin in
peanuts. The presence of A. flavus S strain implies a ma-
jor health problem to consumers of peanuts because it
has been reported to produce greater amount of aflatoxin
especially aflatoxin B1  which is also classified as
class 1 carcinogen . Aspergillus flavus S strain pro-
duces greater quantities of aflatoxin than A. flavus L
strains . Reference  reported A. flavus S strain to
be the primary cause of contamination events in North
America and Africa.
In conclusion, the incidence of aflatoxin producing
fungi in different peanut products analyzed was high (up
to 76%), and the levels of aflatoxin differed in peanuts
sampled from formal and informal markets. The highest
population of aflatoxin-producing fungi was recorded in
raw peanuts sampled from informal market outlets. The
high incidence of aflatoxin producing fungi in peanuts
and peanut products implies poor quality of peanuts
marketed in Eldoret and Kericho towns, and cones-
quently, a high risk of aflatoxin contamination and health
risk to consumers of peanut products. There is therefore
need to improve quality standards of peanuts marketed in
the two towns. The significantly higher aflatoxin con-
tamination of raw peanuts compared to roasted de-coated
peanuts implies that processing—combining roasting and
de-coating—potentially reduces the incidence of afla-
toxin-producing fungi and aflatoxin production in pea-
nuts. There is need for raising awareness among peanut
traders and consumers on proper handling of peanuts,
and the health risks associated with consumption of afla-
toxin contaminated products.
This research was funded in part by the Peanut Collabo-
rative Research Support Program (Peanut CRSP) funded
by USAID under cooperative agreement USAID ECG-
A-00-07-00001-00. The authors express their gratitude to
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