Interaction of water activity and temperature on growth, gene expression, and aflatoxin B 1 production in Aspergillus flavus on Indian senna (Cassia angustifolia Vahl.).

Senna (Cassia angustifolia Vahl.) is a medicinal crop with laxative properties, and it has significant demand in the global pharmaceutical market. Senna pods are highly susceptible to aflatoxin contamination, and the successful export of pods is hindered due to the regulatory limits of importing countries. The senna pod water activity (a _w ) from harvest to storage is the key factor determining AFB ₁ accumulation. The temperature conditions from field to warehouse also interact with pod a _w , which influences fungal growth and AFB ₁ production. The determination of an ideal combination of a _w and temperature led to the assessment of the critical control point for AFB ₁ synthesis in senna. Hence, this study aimed to evaluate the influence of a _w (0.99, 0.96, 0.93, 0.90, and 0.87 a _w ) and temperature (20, 28, and 37 °C) on fungal growth, gene expression (aflR and aflS), and AFB ₁ production by A. flavus in senna agar medium. The fungus showed the longest lag time (7.7 days) at 20 °C with 0.87 a _w . We observed that 0.96 a _w (P < 0.01) was optimum for the diametric growth rate at 28 and 37 °C. However, the peak expression of regulatory genes (aflR and aflS) and the maximum AFB ₁ production were observed only at 28 °C (0.96 a _w ). The highest growth rate occurred at 37 °C, which did not favor the expression of genes and AFB ₁ production. However, at 28 °C, it positively correlated with gene expression and AFB ₁ production. The suppressed expression of regulatory genes and a trace amount of aflatoxin B ₁ were found at 20 °C with all the tested a _w . In our experiments, the low a _w (0.87 and 0.90 a _w ) suppressed the fungal growth, gene expression, and AFB ₁ production of A. flavus at all of the tested temperatures (20, 28, and 37 °C). The rapid drying of senna pods with a low water activity (≤0.87 a _w ) and storage at low temperature (20 °C) are ideal conditions to avoid AFB ₁ and ensure the quality of produce for export.
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