Occurrence and variation of Fusarium free and masked mycotoxins in maize from agriculture regions of South Africa
In the past years, it has become very clear that in mycotoxin-contaminated foodstuffs, many structurally related compounds generated from plant metabolism or during food processing can coexist with the native mycotoxins. The presence of mycotoxins in cereal grain is a very important food safety issue with the occurrence of masked mycotoxins extensively investigated in recent years. This study investigated the occurrence and variation of different Fusarium fungal species and their mycotoxins (free and masked) in maize grains from different maize producing regions of South Africa. The risk of exposure associated with consumption of Fusarium mycotoxin contaminated maize grains was also conducted and a relationship between the maize producing regions, the maize type and the occurrence of different Fusarium fungi as well as their mycotoxins was established. A total of 123 maize samples harvested during the 2015/2016 season were obtained from randomly selected silo sites in the two (western and eastern) agriculture regions of South Africa. Fungal contamination of samples was investigated using conventional (macroscopic and microscopic) and molecular methods for species identification. Mycological analyses revealed that the maize samples were contaminated with different Fusarium species. Most of the samples were contaminated with at least one fungal species, while co-contamination with different Fusarium spp. occurred in a majority of the samples. Seven Fusarium species found to contaminate the maize in both the western and eastern regions were Fusarium verticiloides, Fusarium oxysporum, Fusarium subglitans, Fusarium proliferatum, Fusarium napiforme, Fusarium fujikuroi and Fusarium graminearum with total incidence rate of 96 %, 84 %, 66 %, 83 %, 25 %, 24 % and 34 %, respectively. Fusarium verticiloides was the predominant Fusarium species irrespective of the agricultural regions. Screening of the Fusarium isolates for the presence of Fum13, Tri 6 and Zea13 genes, which underlie Fusarium mycotoxins production showed that the isolates have the biosynthetic genes. The outcome of mycotoxin analysis showed that maize types were contaminated with a mixture of both free and masked mycotoxins across the maize producing regions of South Africa. Generally, all the maize samples analysed were contaminated with an average of 5 and up to 24 out of 42 mycotoxins, including 1 to 3 masked forms at the same time. Data obtained (Table 4.3) highlights the relevance of fumonisin B₁, B₂, B₃, B₄ and A₁ vorstufe in the samples with 98 %, 91 %, 80 %, 82 % and 54 % of 123 samples contaminated with maximum contamination levels 8907.7, 3383.3, 990.4, 1014.4 and 51.5 μg/kg, respectively. Deoxynivalenol occurred in 50 % of the samples with mean concentration of 152 μg/kg (max 1380 μg/kg). Thirty-three percent of the samples were contaminated with zearalenone at a mean concentration of 13.6 μg/kg (max 145.6 μg/kg). Occurrences of HT-2 and T-2 in the samples were at very low levels at 0.8 % each and at maximum concentration of 40.2 μg/kg and 148.0 μg/kg, respectively, while nivalenol occurred in 11 % of samples at mean concentration of 14.2 μg/kg. Of the masked mycotoxins, DON-3-glucoside occurred at a high incidence rate of 53 % than the others. Among emerging toxins, moniliformin, fusarinolic acid and beauvericin showed high occurrences at 98 %, 98 % and 83 %, respectively. High incidences of these toxins in maize, which serves as a staple food in South Africa is an important cause for concern as not much has been done about the occurrence of these mycotoxins in food in South Africa and neglecting them increases the risk of exposure to humans and animals. Also, all the 42 Fusarium toxins and metabolites investigated in the maize samples across the agricultural regions (AR) were detected and quantified except for the emerging toxin, enniantin B₂ which was only detected in 2 % of the samples from the western region. Of the major mycotoxins, HT-2 was not detected at all in the eastern region but was quantified only in 2 % of the maize from the western region. Of the fumonisin Bs, fumonisin B₁ (FB₁) occurred at more frequently than FB₂, FB₃ and FB₄. Fumonisin B₁ was the main contaminating mycotoxin, occurring at mean concentration of 752.46±1469 μg/kg from the warm western region and of 439.88±514 μg/kg in the cold eastern region with only 3 % (2 samples) not contaminated. Fumonisin B₂ was the second most occurring contaminant at mean concentration levels of 290.08±188 and 151.16±513 μg/kg from the western and eastern regions, respectively. Of the masked mycotoxins detected in the samples included DON-3-glucoside, occurred at a high incidence rate of 53% than the others. Although there was no significant difference in their distribution across the agriculture regions, there seems to be no data on the occurrence of some of these masked mycotoxins in South Africa. Hence, this is the first report on zearalenone-sulphate and HT-2-glucoside on South African maize. Exposure assessment for adults calculated through maize intake for deoxynivalenol (DON), fumonisin B₁ and B₂ across the AR showed that probable daily intake (PDI) for DON was within the maximum limit of 2000 μg/kg across the ARs. The PDI for the sum of fumonisin B₁ and fumonisin B₂ in the WR was above the maximum limit of 4000 μg/kg as stated in South African regulation while that in the ER is within the maximum limit. This suggests high exposure of the population to these mycotoxins especially in the WR and which calls for public health concern. In addition, the higher incidence of the fumonisins and most of the Fusarium free and masked mycotoxins in the WR may be explained by the higher susceptibility of the maize samples to mycotoxin producing Fusarium species. However, significant differences in contamination pattern were observed between the agricultural regions. Therefore, this study has shown that there is higher risk of Fusarium mycotoxin exposure, especially fumonisin Bs with consumption of maize grown in the western than with the eastern agriculture regions of South Africa. White maize samples from the western region (WR) had significantly higher mean levels of fumonisins. It also showed that there is no significant difference in the occurrence of the masked toxins across the agriculture regions. Although toxicological data are still limited, the occurrence or presence of masked mycotoxins will add substantially to the overall mycotoxin load and toxicity. This invariably will increase the toxic health effects by these masked mycotoxins, which may be either direct or indirect through hydrolysis, or released from the matrix during digestion into the free mycotoxins (De Boevre et al., 2015). Generally, the high prevalence and at high levels (for some) of these Fusarium mycotoxins on maize may have serious health implications on the consumers since maize constitute a major dietary staple in South Africa. There is therefore, a need to carry out periodic surveys and awareness campaigns in the higher-risk regions (WR) to educate farmers as well as other agricultural stakeholders on the benefits of good agricultural practices (GAP) in relation to reducing mycotoxin exposure. However, most of the Fusarium mycotoxin research in South Africa has mainly focused on the free mycotoxins, but the novelty of this study is that very limited data are available so far, on the impact of climatic differences on these fungi and their mycotoxins (free and masked), in the different agriculture regions of South Africa. Futhermore, literature is also minimal of information on the risk assessment of maize consumers in South Africa to contaminated maize grains.