Mycotoxins in sorghum bicolor and pennisetum glaucum collected from the Oshana Region of Northern Namibia

Abstract

This project formed part of a research collaboration between the Cape Peninsula University of Technology and the University of Namibia (Windhoek, Namibia). The study involved mycological and multiple mycotoxin surveillance of smallholder farms, and processed sorghum (Sorghum bicolor) and pearl millet (Pennisetum glaucum) products sold at open markets in the Oshana region of northern Namibia. Despite strict regulations worldwide, mycotoxin levels in grains are not regulated in Namibia. Smallholder farming communities in northern Namibia are heavily reliant on sorghum and pearl millet as staple food. The Oshakati smallholder farmers service both the Oshakati and Ondangwa open markets. Sorghum and pearl millet samples were collected from smallholder farmers’ households in Oshakati and from randomly selected vending stalls from the Oshakati and Ondangwa open markets. The occurrence of mycotoxigenic fungi on sorghum and pearl millet was determined using conventional mycological methods as well as validated molecular techniques utilizing species-specific primers and quantitative Real-time PCR (qPCR). The concentrations of multiple mycotoxins [aflatoxin B1 (AFB1), fumonisin B1 (FB1), fumonisin B2 (FB2), fumonisin B3 (FB3), ochratoxin A, deoxynivalenol, zearalenone (ZEA), nivalenol and moniliformin] were determined with a validated liquid chromatography with tandem mass spectrometry analytical method. Morphological analyses indicated higher fungal contamination levels in the raw (whole grain) and processed sorghum as compared to pearl millet (P<0.05). No Aspergillus spp. were detected in the raw sorghum samples. The contamination frequencies of Fusarium spp. in raw sorghum and pearl millet were both 80%. Fusarium and Aspergillus spp. co-occurred in 30% of raw pearl millet samples. In processed samples, contamination with Fusarium and Aspergillus spp. was detected in 74% and 100% samples, respectively. The infection levels of Aspergillus spp. in processed products were higher as compared to Fusarium spp. (P<0.05). No Fusarium verticillioides, F. graminearum and F. proliferatum were detected with qPCR in raw sorghum, while F. verticillioides (0.08 - 0.1 pg/µl) and F. subglutinans (0.03 pg/µl) were detected in 80% of raw pearl millet samples. Fusarium verticillioides, F. graminearum and F. proliferatum were, however, detected in 95% sorghum malt samples (0.0059 - 16.69 pg/µl). Fusarium spp. were detected in 92% pearl millet malt and bran samples (0.07 - 29.05 pg/µl). No mycotoxins were detected in raw sorghum, which corresponds with the fungal qPCR results. No mycotoxins were detected in raw pearl millet, which could be ascribed to the low levels of mycotoxigenic fungi detected with qPCR. The results indicated that contamination with mycotoxins occurred during handling and/or processing. 57% of malt and bran samples contained mycotoxins which are regulated by the European Union (EU). 20 and 54% of processed sorghum and pearl millet samples, respectively, contained AFB1 (3 - 14 µg/kg). 17% of all bran and malts contained AFB1 at levels above the regulatory maximum level of 5 µg/kg set by the European Commission (EC). 9% of bran and malt samples exceeded the fumonisin regulatory level (200 µg/kg) set by the EC for infants and young children. 4% of processed samples contained ZEA at levels far above the recommended EC level of 100 µg/kg. Cooccurrence of FB1, FB2, FB3 and ZEA was detected in both sorghum and pearl millet malts. A strong correlation (R = 0.8 - 0.83) existed between levels of F. verticillioides and F. proliferatum determined with qPCR and FB1, FB2, and FB3 concentrations in samples. The study provided important information on the degree of fungal and multiple mycotoxin contamination of raw and processed sorghum and pearl millet in the Oshana region of northern Namibia. It confirmed co-contamination of staple grains and therefore possible chronic co-exposure of communities to multiple mycotoxins regulated by the EU. The results will contribute to determining the levels of dietary exposure of these farming communities to multiple mycotoxins. It will make a valuable contribution to the development of technological or practical methods to reduce mycotoxin levels in these staple grains. By monitoring the malting process, the sources of contamination and critical control points could be identified and managed. Mycotoxin awareness campaigns and sustainable education could further contribute to reducing mycotoxin contamination. Ultimately, the project could contribute to food safety and security in northern Namibia.