Contamination of fermented foods in Nigeria with fungi
Introduction
Fermented foods are one of the major food groups in Africa. The process of fermentation involves the breakdown of organic compounds into acids or alcohol through enzymatic action of microorganisms particularly yeasts and bacteria under anaerobic conditions (Chilton, Burton, & Reid, 2015). Fermentation has been found to enhance the nutritional, health promoting, organoleptic and preservative properties of food (Oyewole & Isah, 2012). Fermentation plays a significant role in developing economies as a technology that increases income sources, food availability, food diversity and reduces post-harvest losses. In Africa, fermented foods are mostly produced traditionally which brings about variation in the substrate used, processing conditions, packaging materials, handling and storage practices (Babajide, Oyewole, & Obadina, 2006). Examples of such foods include iru, gari, amasi, banku, ogi, injera, mahewu and meriss. Iru from fermented African locust bean (Parkia biglobosa), ugba from African oil bean seed (Pentaclethra macrophylla) and ogiri from melon (Colocynthis citullus) are important condiments used to flavor soups and stews in West Africa particularly in Nigeria. Enujiugha, Akanbi, and Adeniran (2008) delineated that ugba is consumed by more than 40 million people in Africa and asides from being a condiment, ugba also served as a snack (Olotu, Enujiugha, & Obadina, 2014). Ugba, iru and ogiri are also examples of vegetable proteins that are used as meat substitutes in diets and are processed with methods of wild solid-state fermentation, which gives rise to extensive hydrolysis of its carbohydrate and protein constituents (Achi, 2005). In addition to their protein content, they are rich in fat, carbohydrates and are good sources of vitamins and minerals. Their typical processing method involves dehulling, boiling, draining, fermentation, salting, drying (optional) and packaging. Maize and sorghum grains are converted to ogi and ogi baba by steeping in water (±3 days), washing, wet milling and sieving. The products are the resultant sediments from this process. Ogi and ogi baba are products of lactic acid fermentation used mostly for infant feeding and often supplemented with proteinaceous foods because of their high carbohydrate content (Oloyede, Ocheme, Chinma, & Agbejule, 2015).
The microbial quality of these foods is influenced by intrinsic and environmental factors such as hygiene during processing, composition of substrates etc. The fermentation of melon, African oil bean seed and locust bean into ogiri, ugba and iru, respectively is predominantly by Bacillus species (Olasupo, Okorie, & Oguntoyinbo, 2016). Other bacteria species that can be isolated during the fermentation process are Staphylococcus, Escherichia, Micrococcus, Leuconostoc, Pseudomonas and Corynebacterium (Achi, 2005). However, fungal species have not been associated with the fermentation of ugba, iru and ogiri. Lactobacillus species such as L. plantarum and yeasts have been reported to be the fermenting organisms for ogi production (Okeke et al., 2015). Irrespective of the benefits derived from fermented foods, there are concerns about their safety because of the continuous and unpredictable pre- and post-processing contamination by pathogenic microorganisms some of which can be toxigenic fungi that produce harmful secondary metabolites including mycotoxins.
Some fungal species belonging to the Penicillium, Mucor, Geotrichum, and Rhizopus genera have been used in the fermentation of cheese and milks (William & Dennis, 2011, chap. 4), while others produce undesirable toxins and their presence in food has been attributed to their sporulating ability, which makes them easily contaminate the environment (Frisvad & Samson, 2007) and the food products therein. Hence, the assumption among consumers that fermented foods - especially those processed traditionally - are safe is a dangerous one as such foods could be potential sources of mycotoxin exposure and accompanied health complications. There has also been evidence of multiple mycotoxins in different food categories including fermented foods. Chilaka, De Boevre, Atanda, and De Saeger (2016) reported the co-occurrence of Fusarium mycotoxins such as fumonisins, nivalenol, HT-2 and deoxynivalenol-3-glucoside in fermented maize from Nigeria. Colak et al. (2012) detected Aflatoxin B1 within the range of 0.2 and 13.2 μg/kg in Tarhana, a Turkish fermented cereal. Nevertheless, most microbiological studies on African fermented foods have been based on the isolation and characterisation of microorganisms that enhance the fermentation processes and only studies with limited scope have been conducted to establish the incidence of pathogenic organisms like fungi (Olasupo et al., 2016). Even though some studies have reported the presence of fungi in some fermented foods consumed in Nigeria, there is little adequate information on the spectrum of microorganisms associated with these foods since most of these organisms were identified to genus level and only a few to species level. It is worthy to note that the safety of fermented food is largely influenced by some quality parameters such as moisture content, pH and Total Titratable Acidity (TTA) and these needs to be considered during safety studies. This research was therefore aimed at assessing the fungal diversity in some Nigerian fermented foods in order to gain insight into their safety.
Section snippets
Sampling
Cluster sampling was used to obtain fermented foods that included fermented: maize gruel (ogi), locust beans (iru), sorghum meal (ogi baba), dried locust beans (dried iru), African oil bean seed (ugba) and melon (ogiri) that are indigenous to Nigeria. A total of 108 samples (18 each) with an equivalent weight of 30 ± 5 g were purchased from some selected fermented food sellers in South West, Nigeria between Feburary and March 2015. Upon their collection, samples were placed into sterile
Results
The mean pH of the samples ranged from 3.60 to 8.08 with iru having the highest value and ogi baba having the lowest. The mean TTA of all the samples were significantly different from each other except for ugba, ogiri and ogi baba (Table 1). The mean moisture content of the samples ranged from 12 to 56% with dried iru and iru having, respectively, the lowest and highest moisture contents. Total fungal load of ugba, iru and ogiri were 9.4 × 103, 1.05 × 105 and 7.9 × 105 cfu/g, respectively. As
Discussion
The acidic pH values of the ogi (3.83) and ogi baba (3.82) analysed were expected since maize and sorghum are known to undergo lactic acid fermentation during processing into these products. The pH values of ogiri and iru were higher than the values reported by Olawuni et al. (2013) and this may be due to differences in fermentation conditions. Ogi had the highest TTA value but that did not significantly differ from that of ogi baba, the high TTA could be due to the accumulation of some organic
Conclusion
The fermented foods screened for fungal contamination indicated a wide range of species belonging to the genera Aspergillus, Penicillium, Fusarium, Saccharomyces, Mucor, Talaromyces, Rhizopus, Cladosporium, etc. some of which could be potentially toxigenic. Further to that, the heavy fungal load present in the analysed foods also indicates that these food products can be potential sources of mycotoxin exposure among humans in Nigeria. However, this can only be ascertained if the same diets are
Conflict of interest
The authors have no conflict of interest to declare.
Acknowledgment
The authors wish to appreciate the Organisation for Women in Science in the Developing World (OWSD), Italy through the Swedish International Development Cooperation Agency (SIDA), Centre of Excellence for Food Security and Safety (COE), South Africa, University of Johannesburg, South Africa via the Global Excellence and Stature (GES) Scholarship and L’OREAL UNESCO (For Women in Science) for funding this study.
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