Water, temperature and gas composition interactions affect growth and ochratoxin A production by isolates of Penicillium verrucosum on wheat grain

Abstract

Aims: To examine the effect of interactions between water, temperature and gas composition on growth and ochratoxin A (OTA) production by isolates of Penicillium verrucosum in vitro and in situ on grain-based media and wheat grain. Methods and Results: Three isolates of P. verrucosum were examined in relation to radial growth rate and OTA production, and to interacting conditions of water activity (a_w), temperature and gas composition on a milled wheat medium. Subsequently, detailed temporal studies were carried out on gamma irradiated wheat grain over the range 0·75–0·995 a_w, 10–25°C and air, 25 or 50% CO₂. This showed that optimum growth of P. verrucosum was at 0·98 a_w in vitro at 25°C, but at 0·95 a_w and 25°C on wheat grain. The a_w minimum for growth was about 0·80 a_w, although no OTA was produced under this condition even after 56 days. Significant inhibition of growth and OTA production occurred with 50% CO₂, and 0.90–0·995 a_w at 25°C. Conclusions: The optimum and marginal conditions for growth and OTA production on wheat grain have been identified. At least 50% CO₂ is needed to inhibit growth and OTA production by >75% in moist grain (0·90–0·995 a_w). Significance and Impact of the Study: First detailed identification of optimal and marginal interacting conditions of water/temperature and gas composition on growth and OTA production by P. verrucosum on wheat grain. This is a critical component of the postharvest management strategy for minimizing contamination by this important mycotoxin and predicting risk, based on environmental conditions, during drying and storage.