Raw cashew nut quality as function of contamination by mycotoxins and other secondary metabolites of Aspergillus spp. and farmer practices

Cashew nut is a cash crop in tropical regions of Africa including Benin, providing substantial revenues to small-scale farmers and contributing significantly to the national gross domestic product. In Beninese context, about 95% of raw cashew nuts are exported. Therefore, the production of raw cashew nuts of good quality is mandatory to enter the international market. Moreover, health issues related to the consumption of nuts contaminated with mycotoxins are of high concern for human being. The objective of this research was to investigate the occurrence of mycotoxins and other secondary metabolites of Aspergillus sp. on raw cashew nuts and to evaluate farmers’ practices that affect the quality of raw nuts.

In surface disinfected samples 90.2% of raw kernels were contaminated by strains of A. section Nigri in the northern guinea (NG) zone compared to 87.2% in the southern guinea (SS) zone. The level of contamination of raw cashew kernels by strains of A. section Flavi was 6.7% in the NG zone whereas it was 4.6% in the SS zone. When non disinfected kernels were plated, A. section Nigri was predominant in both NG and SS zones, with percentages of 89.7% and 93.4%, respectively. Ten and 15.7% of raw cashew kernels were contaminated by strains of A. section Flavi in NG and SS zones, respectively. A. flavus, A. tamari, A. costaricaensis, A. minisclerotigenes and A. nomius were identified as strains in A. section Flavi. The average water content and the cashew nut count were respectively 8.6% and 172 nuts/kg in the NG zone and 8.7% and 174 nuts/kg in the SS zone.

The occurrence of aflatoxins, analysed by ultra-high performance liquid chromatography tandem mass spectrometry, showed that none of the analysed samples was positive for natural occurrence of aflatoxins with a detection limit of 0.05 - 0.2 µg/kg. All the above results showed that, at present, cashew nuts from Benin were in the range of good quality for export.

Seven species of black aspergilli were isolated based on morphological and chemical identification namely A. tubingensis (44% of total strains), A. niger (with a chemical profile similar to A. welwitschiae) (32%), A. brasiliensis (10%), A. carbonarius (8.7%), A. luchuensis (synonym to A. kawachi or A. acidus) (2.7%), A. aculeatus (2%) and A. aculeatinus (0.7%). Forty five different metabolites and their isomers were identified and classified in 2 groups. Group A of known mycotoxins: fumonisin B2, fumonisin B4, ochartoxin B, ochratoxin A and secalonic acid A, secalonic acid D and secalonic acid F. Group B included other secondary metabolites like nigragillin, pyranonigrin A, nigerazine A, nigerazine B, nigerapyrone E, tensyuic acid A, tensyuic acid F, pyranonigrin B, pyranonigrin C, pyranonigrin D, fonsecin, tensidol A, pyrophen, atromentin, tensyuic acid B, funalenone, rubrofusarin, orlandin, asperazine, tensyuic acid C, tensyuic acid D, nigerasperone A, tensidol B (pestalamide A), fonsecin B, malformin A2, tubingensin A, tubingensin B, malformin C, kotanin, nominine, antafumicin A, antafumicin B, aurasperone C, aurasperone F, aurasperone E, aurasperone B, flavasperone and nafuredin. DNA analysis resulted in a phylogenetic trees based on combined sequences data of β-tubulin and calmodulin of 27 strains of A. section Nigri that clustered in 5 clades. Also, the chemical profile clustering showed also 5 groups confirming the species specific metabolites production.

Sorting and grading resulted in two grades. First (good) grade (Grade 1) consisted of nuts with less than 50% of their shell surface affected by fungi and/or insect injuries, whereas second (bad) grade (Grade 2) were nuts that had 50% or more of their shell surface covered by fungi and/or insect symptoms. Nut counts in Grade 1 were from 151 to 174 nuts/kg and from 142 to 182 nuts/kg in NG and SS, respectively. In Grade 2, nut counts were from 168 to 202 nuts/kg in NG and from 171 to 197 nuts/kg in SS. These results indicated that sorting and grading resulted in Grade 1 of better quality with high nut count, and low fungal contamination.

Interviews revealed the dominance of men (95%) in cashew nut cultivation indicating that it as a gendered activity. Cashew nut farming is labour intensive with 55% of farmers in the range of 35 to 55 years old. Unfortunately, most cashew farmers were less educated with up to 76% not exceeding primary educational level, making the educational level of farmers to be the main constrain for the production of cashew nut of good quality. This resulted of 80% of cashew farmers to select their seedlings from uncertified sources and 59% to never ask or received advice from extension services about cashew farming practices. Cashew nut farmers reported that cashew revenue contributed significantly to the household income for 44% and 54% of farmers in the NG and the SS zone, respectively. Other constrains for good cashew nut production were lack of sorting, drying and farm management. The farmer’s education level, limited access to extension services, lack of sorting, drying and farm management are major parameters that affect the quality of raw cashew nuts. Improvement of these parameters can positively affect raw nut quality and therefore contribute to better revenue from raw cashew production.

Together with good farming and post-harvest management practices, the implementation of specific extension service for cashew growers and an incentive for sorting and grading as low cost technology, will impact the quality parameters of raw cashew nuts intended for export from Benin.