Water activity and temperature effects on mycotoxin production by Alternaria alternata on a synthetic tomato medium

Alternaria spp. have been reported to be the most frequent fungal species invading tomatoes. Certain species, in particular the most common one, A. alternata, are capable of producing several mycotoxins in infected plants and in agricultural commodities. Alternariol (AOH), alternariol monomethyl ether (AME), and tenuazonic acid (TA) are some of the main Alternaria mycotoxins that can be found as contaminants of food. The objective of this study was to determine the effect of water activity (a(w), 0.904, 0.922, 0.954, and 0.982) and temperature (6, 15, 21 and 35 degrees C) on mycotoxin production on a synthetic tomato medium of a cocktail inoculum of five strains of A. alternata isolated from tomato fruits affected by Blackmould. The optimum AOH production occurred at 0.954 a(w) after 28days of incubation at 21 degrees C. A temperature of 21 degrees C was the most favourable for AOH synthesis at all a(w) levels. The maximum concentration of AME was determined at 0.954 a(w) and 35 degrees C. The optimum conditions for TA accumulation were 0.982 a(w) and 21 degrees C. At the 0.904 a(w) no growth or germination was registered at 6 degrees C and 15 degrees C over the whole incubation period. At 21 degrees C and 35 degrees C growth occurred slowly but none of the toxins were detected at this a(w) level. In general, high a(w) levels were favourable for mycotoxin production. None of the other toxins was detected at quantifiable levels at 6 degrees C after the whole incubation period. A storage temperature of 6 degrees C or below could be considered as safe for tomato fruits and high moisture tomato products (a(w)>0.95), in relation with Alternaria toxins. The results obtained here could be extrapolated to evaluate the risk of spoilage in tomato fruits and tomato products caused by this pathogen.