Temperature and water activity effects on growth rate of ochratoxigenic species isolated from Argentinian human food.

ASTORECA A.; BARBERIS C.; RAMIREZ M.L.; MAGNOLI C.E.; DALCERO A.

Carlos Paz, Cordoba, Argentina

Congreso; Advances in research on toxigenic fungi and mycotoxins in South America ensuring food and feed safety in a myco-globe context.; 2006

Comision Europea, Universidad Nacional de Rio Cuarto

Ochratoxin A (OTA) is receiving worldwide increasing attention because of the hazard it means to human and animal health. It is a toxin naturally produced by Aspergillus ochraceus and some species of Aspergillus section Nigri formed by A. niger group and A. carbonarius which have been reported on a great variety of substrates from regions with warmer and tropical climate. In Argentina, species included in Aspergillus section Nigri have been frequently isolated on peanut seeds, maize kernels, dried grapes and coffee cherries. Besides, the ability of these strains to produce OTA has been demonstrated. The aim of this study was to determine the effect of water activity (aW), temperature, and their interactions on ´in situ´ mycelial growth rate of four strains of Aspergillus section Nigri isolated from peanuts and maize kernels. Peanut grains (variety Runner) and maize kernels were exposed to an appropiate gamma irradiation. Water activity was adjusted to 0.93- 0.95- 0.97 and 0.99; the amount of water necessary to produce the required aW was previously determined by a moisture absorption curve and the aW levels were confirmed by using a water activity meter. Also, control plates were prepared and measured when the experiment finished in order to detect any significant deviation of the aW. Rehydrated grains were placed in sterile 9 cm Petri dishes forming single layers of grain. Treatments containing grains of the same aW were placed in sealed closed containers containing beakers of glycerol/ water mixtures of the same aW as the plates to maintain the relative humidity equilibrium. The Petri plates were daily examined and two diameters at right angles were measured of each colony. The increase in radial growth was determined and used to calculate the growth rate (mm day-1) by linear regression, under each set of treatment conditions for each isolate. The results showed that the maximum growth rates for the four isolates of A. niger were obtained at 0.99 aW and 30 ºC and that temperature influenced the effect of aW on growth. The minimum aW for growth of the isolates of A. niger changed with temperature. At 0.91 aW, the lowest level of water stress tested, all the isolates were unable to grow at 15 ºC. The variance analysis of the effect of the factors: isolate, aW and temperature, and their two- and three- interaction way showed that all factors and all interactions were statistically significant  (p<0.001) in relation to growth rates for all strains assayed. In the present study, the knowledge of interacting environmental conditions provides very useful information for predicting the possible risk factor for fungal contamination and thus, the mycotoxin contamination since it can easily be extrapolated to natural systems as agricultural and food products.