Chemical analysis and survey of mycotoxins in South African produced grains with emphasis on those produced by stenocarpella maydis in maize

Abstract

During the latter half of the previous century, new mycotoxins were discovered as secondary metabolites of fungi in major grains before harvest. The prevalence of these toxic compounds and the negative health effects caused when consumed in contaminated food and feed present a significant challenge for producers, traders, processors, and consumers worldwide. Until recently, there was a lack of information on the occurrence of well-known mycotoxins such as aflatoxins, fumonisins, and deoxynivalenol in commercially produced maize and wheat in South Africa (SA). Additionally, there was no available information on whether the toxins produced by the fungus Stenocarpella maydis in maize also occur in maize delivered after harvest for further processing. The main objective of this study was to gain insight into the prevalence of grain-related mycotoxins in South African-produced wheat and maize and to better understand the potential risks associated with toxins produced by Stenocarpella maydis infection in maize. Accurate reporting of the presence and concentration levels of specific mycotoxins is crucial in managing the risk associated with mycotoxins to guarantee the safety of food and feed products for consumers. However, developing analytical methods for the simultaneous analysis of a large number of organic compounds at µg/kg concentration levels is challenging. This is mainly due to the varying physical and chemical properties of the compounds, the availability of chemical standards for quantitative analysis, and the complex nature of the commodities (grains, fruits, nuts etc.) to be tested. In this study, extraction methods were investigated to develop a fit-for-purpose and cost-effective LC-MS/MS analytical method to analyse different mycotoxin groups simultaneously in grain samples. The developed LC-MS/MS method was able to analyse 14 different mycotoxins, including diplodiatoxin, in grains. It was successfully validated according to international method guidelines to ensure reliable results. A comprehensive survey was conducted over four wheat and maize production seasons in South Africa to determine the occurrence and concentration ranges of aflatoxins (AFB1, AFB2, AFG1, AFG2), deoxynivalenol (DON) and 15-acetyl-deoxynivalenol (15-ADON), fumonisins (FB1, FB2, FB3), ochratoxin A (OTA), T-2 toxin (T-2), HT-2 toxin (HT-2) and zearalenone (ZON) in commercially produced wheat and white and yellow maize in all the production provinces. The wheat and maize samples collected after harvest at the storage facilities were analysed using the validated LC-MS/MS method. The mycotoxin results of the first comprehensive maize survey were based on analyses of 1400 maize samples collected over 4 production seasons from 2014 to 2017. A follow-up survey was conducted to determine for the first time the prevalence of diplodiatoxin in commercially produced maize in South Africa, the primary toxin produced by Stenocarpella maydis. A low mycotoxin risk was reported in SA produced wheat. Only deoxynivalenol was present in 12.5% of the 160 wheat samples collected over 4 production seasons (2015–2018). The maximum concentration levels found ranged from 362 µg/kg to 593 µg/kg, far below the regulated maximum allowable levels in wheat. Aflatoxin B1 was seldom present in commercially produced maize in South Africa. The mycotoxins found were DON, 15-ADON, fumonisins, and ZON with at least one of these mycotoxins found in 83% of the maize in the first season, and 80%, 63% and 62% in the successive seasons. Deoxynivalenol and fumonisins were the most prevalent in maize. Sixty-nine percent of the maize samples was contaminated with DON in the first year, and 41%, 23% and 37% of the samples in the following three seasons. Occurrence in white and yellow maize only differed in the 4th season with 54% of white maize and only 19% of yellow maize contaminated with DON. Higher annual mean DON concentrations were reported in white maize in the seven different production provinces. Between 41% and 57% of the maize samples contained fumonisins, with similar prevalence in white and yellow maize in most of the production provinces. The mean FUM concentrations were well below the 4000 µg/kg regulatory value. Overall, less than 10% of the samples were contaminated with ZON. Diplodiatoxin was the major toxin found in maize inoculated with S. maydis isolates. The inclusion of diplodiatoxin in the validated LC-MS/MS analytical method was possible with the availability of pure diplodiatoxin extracted from the inoculated maize. In the maize survey, conducted from 2019 to 2022, diplodiatoxin was for the first time reported in commercial produced white and yellow maize in 5 of the production provinces. A large seasonal variation in prevalence was found with diplodiatoxin present in 13%, 20%, 1% and 51% of the maize samples in the 4 consecutive seasons. The presence of diplodiatoxin in maize delivered to food and feed processors highlighted the importance to further investigating the toxicity of diplodiatoxin. The suitability of the zebrafish embryo toxicity test method was verified with diplodiatoxin assays. This study has successfully reported on the occurrence of mycotoxins related to grains in commercially produced wheat and maize in South Africa. The concentration ranges, regional variations, and seasonal trends confirmed the importance of continuously monitoring mycotoxins in the grain value chain to ensure that the food and feed are safe for consumers.