Diversity and fate of fungal metabolites during the preparation ofoshikundu, a Namibian traditional fermented beverage

Abstract

Sorghum and pearl millet, ingredients for the popular Namibian traditional fermented beverage oshikundu, are prone to fungal infection, raising concerns for consumer health from mycotoxin exposure. This study aimed at determining the diversity of fungal metabolites in street-vended ingredients and their transfer rates into oshikundu. A total of 105 samples (40 sorghum malt, 40 pearl millet, 25 oshikundu) were analysed for 700 fungal, bacterial and plant metabolites, using liquid chromatography-tandem mass spectrometry. Of 98 quantified metabolites, 84 were fungal, some being mycotoxins. Aspergillus metabolites were most prevalent (50%, n=42), including aflatoxins, aflatoxin precursors, cyclopiazonic acid and 3-nitropropionic acid from Aspergillus flavus; helvolic acid, gliotoxin and fumiquinazolines from Aspergillus fumigatus and cytochalasin E, patulin and tryptoquivalines from Aspergillus clavatus. High levels of up to 2,280 μg/kg for cyclopiazonic acid and 11,900 μg/kg for 3-nitropropionic acid were quantified in sorghum malts. Other metabolites included fumonisins, curvularin, alternariol and dihydroergosine produced by Fusarium, Penicillium, Alternaria and Claviceps genera, respectively. European Union legislated mycotoxins occurred in cereals at a prevalence range of 3-75%, while none were quantifiable in oshikundu. Aflatoxin B1 was quantified in pearl millet meals (13%) and sorghum malts (50%), with 15% sorghum malts having levels above the European Union regulatory limit of 5 μg/kg. Fumonisin B1 was quantified in pearl millet meals (50%) and sorghum malts (75%) at maximum levels of 3,060 μg/kg and 123 μg/kg respectively, and levels in 5% pearl millet meals were above the European Union regulatory limit of 2,000 μg/kg. Zearalenone and ochratoxin A were quantified in the cereals at levels below European Union regulatory limits. For most metabolites quantifiable in oshikundu, transfer rates from cereals to oshikundu were above 50%, necessitating the use of good quality ingredients for preparing oshikundu and assessment of consumer exposure to mycotoxins.

Variation of Fungal Metabolites in Sorghum Malts Used to Prepare Namibian Traditional Fermented Beverages Omalodu and Otombo

“…Due to the absence of such limits in Namibia, the limits fixed by the EU [26] were used as the basis for discussion in the present study. The same mycotoxins have been recently reported in sorghum malts from Namibia using the same analytic method by [20], with the exception of patulin found in the present study and fumonisin B 4 quantified only in the previous study. Overall, 98% and 76% of the omalodu and otombo samples were contaminated with at least one of the EU regulated toxins, respectively.…”

“…These results suggest that the different malt preparation methods may not have a significant effect on levels of aflatoxins. In the previous study [20], AFB 1 was quantified with higher prevalence rate (50%) and level (average: 4.5 ± 5.5 µg/kg) in the sorghum malt flour samples compared with omalodu malts (44% prevalence; average: 2.87 ± 2.93 µg/kg).…”

“…Only 4% of the 102 metabolites were of non-fungal origin. The occurrence of fungal metabolites, including mycotoxins in sorghum malts intended for the beverage oshikundu, quantified using the same analytical technique and method, was previously reported in Namibia [20], with a total of 98 metabolites quantified. Other reports on the occurrence and quantities of fungal metabolites in sorghum malts and grains from Southern Africa have been documented in South Africa [17], Malawi [21], Zimbabwe [18], and Botswana [19].…”

“…Many reports on the occurrence and quantities of fungal metabolites in sorghum malts and related products from countries neighboring Namibia have been documented in South Africa [17], Zimbabwe [18], and Botswana [19]. A previous study in Namibia, which determined the diversity of fungal metabolites in sorghum malt samples used for oshikundu beverage production and their transfer rates into the beverage, reported that, although EU-legislated mycotoxins were not quantifiable in the beverage, transfer rates into the beverage were above 50% for most of the other fungal metabolites [20]. Due to the diversity of malting and processing methods for different beverages, the types and quantities of fungal metabolites in the specific malts may vary.…”

Mycotoxin Contamination of Beverages Obtained from Tropical Crops

“…Unusually high levels of 2.28 × 10 3 and 1.19 × 10 4 µg kg −1 for cyclopiazonic acid and 3-nitropropionic acid were found in sorghum malts, respectively. As with other beverages, fermented beverages such as oshikundu may harbor essential levels of mycotoxin contamination highly related to the quality of the raw ingredients [92].…”