African Journal of Biotechnology
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African Journal of Biotechnology Vol. 2 (10), pp. 369-373, October 2003 ISSN 1684-5315 © 2003 Academic Journals
Full
Length Research Paper
Mycology and spoilage of retail cashew nuts
L.
O. Adebajo1* and S. A. Diyaolu2 1Department
of Biological Sciences, Olabisi Onabanjo University, Ago- Iwoye, Nigeria. lawadebajo@yahoo.com 2Biology
DepartmentTai Solarin College of Education, Ijebu Ode, Nigeria. *Correspondence
author; E-mail: lawadebajo@yahoo.com. Accepted
18 September 2003
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Thirty-two samples of retail cashew nuts from Lagos,
Nigeria were examined on two media. The pH values (5.1-6.3) of all the
samples were conducive for fungal growth and mycotoxin production.
Moisture content levels ranged between 4.1 and 6.8%. Fifteen samples had
moisture contents up to or above 5.8%, the highest level estimated to be
‘safe’ for the storage of the nuts. Fourteen fungal species, mostly
toxigenic and belonging to 5 genera were isolated. Seven species were from
genus Aspergillus, 3 from Penicillium, 2 from Rhizopus
and one each from Mucor and Syncephalastrum. The most
predominant isolates were: A. niger, A. restrictus, A. flavus,
A. fumigatus and Aspergillus sp. The mean and range of total
fungal counts (CFU/g) in samples were: 3,368 (180 to 16, 300). At
acceptable fungal levels of 103 and 104/g, only 14
and 28 samples, respectively, were deemed fit for human consumption. All
the species recovered induced detectable loss in weights of the milled
nuts, though to varying extents and would be expected to cause
considerable spoilage of the nuts. Key words: Cashew nut, Anacardium occidentale, fungal
count, mycology, Aspergillus sp., Penicillium sp., spoilage.
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The cashew plant, Anacardium occidentale L, is a
small to medium sized tree belonging to the family Anacardiaceae (Cobley
and Steele, 1976). The fruit is a kidney-shaped
achene about 3c m long with a hard grey-green pericarp. The seeds are
the source of cashew nuts and they are normally removed from the
pericarp after the fruits are roasted, a process which burns off shell
oil and cooks the seeds. Worldwide,
cashew nuts are an esteemed and highly priced food delicacy because of
their pleasant taste and flavour. The post-harvest processing,
packaging and marketing have been commercialized and modern technology
and regulations adopted in major producing countries like India and
Tanzania. In Nigeria however, despite the cultivation of cashew in
plantations and the establishment of cashew-processing factories (Esuruoso,
1974), peasant processing and packaging
methods are still commonly adopted. The latter predisposes the nuts to
mould contamination especially during hawking of the product, which
usually are packaged in hand-knotted thin polyethylene bags. There are
no labels to indicate vital information such as the name and address
of producer, nutritional contents, recommendations for storage and the
best-before-date for human consumption. However, mycotoxicoses are
becoming increasingly implicated in human and animal pathology (Bacha
et al., 1988). The situation may be worsened by
consumers’ reluctance to discard fairly mouldy food samples such as
cashew nuts due to the irresistible taste and flavour. This paper is a report on the mycology of retail processed cashew nut samples from the Lagos metropolis, Lagos State, Nigeria. The roles of isolates in the degradation, and thus spoilage of the nuts, were also determined.
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Samples A total of 32 samples (each comprising 10 randomly
picked consumer packs per retailer) were obtained from different
locations in the Lagos metropolis. The physical conditions of the
packaging material and of the nuts were carefully noted. On the day each
sample was collected, the 10 composite packs were opened and the nuts
were aseptically transferred into a big thick sterile polyethylene bag
after which the content was thoroughly mixed together. The moisture
content, pH analysis and mycological studies were also undertaken on the
day of collection. Twelve youths with ages between 15 and 26 years were
asked to chew without swallowing the nuts from each sample and report on
the taste and crispness of the food. Moisture
content and pH analyses Approximately 75 g portion of each sample was crushed
in a mortar and the moisture contents of 5 g bits were determined after
drying at 1030C for 4.5 h. The pH values of 1:2 (w/v)
suspensions of the pulverized material in distilled water were obtained. Evaluation
of visible mould incidence Two hundred nuts were randomly obtained from a sample
and carefully observed with the naked eye for any evidence of mould
growth. The cotyledons (2 per nut) were then separated by hand and the
state of the inner surfaces was also noted. The incidence of visible
mould contamination was expressed as a percentage of the 200 nuts
examined. Mould
incidence after surface disinfections Forty cotyledons from the 200 nuts examined above were
randomly picked and surface sterilised with a 2% aqueous solution of
sodium hypochlorite for 2 min. This was followed by rinsing with three
washes of sterile distilled water before four cotyledons were plated
together equispaced from each on the two media used. Two of the
cotyledons had their inner surfaces turned up and the remaining two had
their outer surfaces turned up. Two media (Pitt and Hocking, 1985),
of low water activity (aw) were used. Malt extract agar with
40% (w/v) sucrose (MA40) and malt extract yeast extract 50% glucose agar
(MY50G). Before pouring plates, chlortetracycline and chloramphenicol
were added each at a concentration of 50 mg/L of medium. Plates were
incubated upright at 280C for 5 to 8 days during which the
number of cotyledons that yielded colonies was recorded. Also, the
colonies were noted, enumerated and subcultured for identification.
Fungal incidence was expressed as a percentage of the 40 cotyledons
plated. Mould
incidence in non- disinfected samples Another set of 200 nuts was randomly obtained from the
sample with the aid of sterile forceps, which were also used to split
each of the nuts into two cotyledons. Without any surface sterilisation,
40 randomly picked cotyledons were plated and determination made as
described above.
Estimation
of total fungal counts Randomly selected non-disinfected nuts (100) were
pulverised and used for the study. The initial dilution was prepared by
adding 10 g of sample to 90 ml diluent (40% (w/v) sucrose) and
homogenised by stomaching for 2 min. Decimal dilutions (1 + 9 ml) were
prepared down to 10-4, and spread plated in triplicate, 0.1
ml per plate. MA40 that gave higher counts in a pilot study was the only
medium used. Plates were incubated upright at 280C for 5 to 8
days. Utilsation
of cashew nuts by the isolated fungi
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The
moisture contents (%) recorded for the 32 samples ranged between 4.1 and
6.8 with a mean value of 5.4 (Table 1). More than half of the samples,
that is 17 or 53% had moisture contents below 5.8%, which in a preliminary
study conducted by the authors, was found to be approximately equivalent
to 70% relative humidity (at the prevailing ambient temperature), which is
generally considered the maximum level for safe storage (Pixton, 1982;
Henderson, 1985).
However, 15 or 47% of the samples had moisture contents up to or
above the limit (5.8%) safe for storage and consequently, were predisposed
to fungal growth and mycotoxin contamination during storage.
Most stored agricultural products, including cashew nuts, being
colloids, are hygroscopic i.e. they will absorb moisture from or give it
up to the surrounding atmosphere until they are in equilibrium with it (Pixton,
1967). In the present study, the ease with
which the thin container was perforated by the edges of the nuts suggests
a continuity in the environment inside and outside many of the retail
packs. This leakage is expected to affect the overall quality and spoilage
of the product. Indeed, research reports (Adebajo, 1992;
Bankole et al., 1999; Desrosier and Desrosier, 1977;
Frazier and Westhoff, 1978) on the influence of
environmental conditions especially relative humidity and temperature on
nuts and allied food products are well documented.
Table 1. Moisture content (MC), pH, visible mould incidence (VMI) and total fungal count (CFU/g) in retail processed cashew nuts.
The presentations in Table 1 show that the mean pH and
the range (in parenthesis) recorded for the samples were: 5.7 (5.1 to
6.3). Thus, the nut is a low acid food product and without exception, all
the samples had pH values conducive to microbial growth and activities
including the elaboration of toxic secondary metabolites (Frazier
and Westhoff, 1978; Smith and Moss, 1985). The visible mould incidence (%) estimates ranged
between 0 and 3 with a mean of 0.4. Visible mould growth was recorded in
only 8 or 25% of the 32 samples investigated (Table 1). When present,
growth was usually in traces and this made the detection difficult.
Similar observations were recorded by Adebajo (1993),
on stored tiger nuts; and it was responsible for the ease with which
mouldy low water activity food products, especially nuts are presented for
sale after adulteration and “dressing.” This practice by retailers is
possible because many consumers are casual in their observation and thus
unable to detect mould deteriorated samples. In the present study however,
mouldy nut samples were easily detected in the mouth because they were not
crispy though the pleasant aroma of the nut was still perceived and
strongly too. The latter may cause children and even adults that are
poorly discriminating in the choice of foods they consume to ingest mouldy
samples thus potentiating a public health hazard (Adebajo, 1993). The
total fungal counts/ g obtained ranged between 180 (sample 19) and 16, 300
(sample 16) while the mean count recorded for the entire 32 samples was
3,368 (Table 1). The spread plate method adopted in this study had been
found by several workers (King et al., 1986;
ICMSF, 1986), to be preferable to pour plate method
as it gave batter recoveries of fungi and lower dilution errors.
Specifications for fungal contamination in packaged
nuts and allied foods are usually related to the nature of the product and
the level of standard set by the producing company. Consequently, wide
variations exist among companies and this prompted the International
Commission on Microbiological Specifications for Foods (1986), to set
fungal tolerances for flour, cereal and packaged nut products in the range
102 to 104/g. However, for most strict companies,
acceptance standards of less than 103/g is adopted (King et
al., 1981). The results in Table 1 therefore
indicate that only 14 or 43. 8% and 28 or 87.5% of the cashew nut samples
investigated were fit for human consumption at 103 and 104/g
acceptable fungal levels, respectively. Altogether, 14 fungi belonging to 5 genera were
recovered at varying levels from the cashew nuts (Table 2). The most
predominantly encountered species in decreasing order of isolation from
the non-disinfected nuts were Aspergillus niger, A. restrictus, A.
flavus, A. fumigatus, Aspergillus, sp., Rhizopus nigrians, R.
arrhizus and M. pusillus; while A. tamarii; Penicillium
citrinum, A. ochraceus, Pencicllium sp. While P. digitatum and Syncephalastrum sp. were the less frequent isolates
and were not recovered from the surface-disinfected nuts. Some of the species, especially of Aspergillus and
Penicillium (Table 2) associated with the nuts are known to have
strains that produce toxic metabolites (Bamburg et al., 1969;
Cole and Cox, 1981). Thus, they pose a potential
hazard to consumers’ health. The
conditions generally known to influence the production of mycotoxins in
foods and allied agricultural products include presence of a toxigenic
mould, a suitable substrate for the growth of the mould, and an
environment conducive for the toxin production by the mould (Betina, 1984).
Thus, the high ambient temperatures (25 to 330C) and relative
humidities (³80%)
prevalent in southern Nigeria (Ogundero, 1987)
suggest that elaboration of mycotoxins in poorly packaged and stored
retail cashew nut samples is to be expected. However, there is limited
information as to the degree of the suitability of cashew nut as substrate
for myctoxin production, especially aflatoxins, by A. flavus (Abdel-Gawad,
1993; Manay and Shadaksharaswamy, 1987).
This dearth of information is worrisome because A. flavus was
recorded in all the nut samples analysed and ranked among the three most
frequently encountered species (Table 2). Of the known mycotoxins, the
most important from the viewpoint of direct hazard to human health are
aflatoxins (Hsieh, 1986). They pose a quadruple
threat to both human and animals as they are mutagenic carcinogenic,
teratogenic and toxic (Wogan et al., 1974; Chu, 1977;
Pitt and Hocking, 1997). In the light of this, we
have designed a series of experiments (unpublished) to determine the
suitability of cashew nuts as substrate for the elaboration of aflatoxins
by A. flavus and also to characterise the interacting effects of
temperature, moisture content, water activity and time on the production
of the toxins.
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