Comparison of three commonly used drying technologies with respect to activity and longevity of aerial conidia of Beauveria brongniartii and Metarhizium anisopliae

We compared three commonly used drying technologies for preservation and formulation of 14- to 20-day-old aerial conidia of Beauveria brongniartii and Metarhizium anisopliae. Conidia were dispersed in an aqueous binary mixture composed of skim milk (SM) and polyvinylpyrrolidone (PVP K90) and subsequently subjected to heat stress tests, lyophilization, and spray- and fluid-bed drying. The initial heat stress tests revealed that M. anisopliae is slightly more heat resistant in the suspending media than B. brongniartii, tolerating exposure to 50 °C for 2 min without loss in viability. In comparing the drying technologies with respect to germination rates, lyophilization was the most favorable for B. brongniartii and most destructive for M. anisopliae. Viability levels of 68 and 4% resulted after freeze-drying. The influence of different inlet/outlet temperature adjustments during spray- and fluid-bed drying were examined according to their influence on conidial viability. Both drying methods resulted in severe damage to the fungal material already at an inlet/outlet temperature regime of 60/40 ± 2 °C, reflected by low and prolonged germination rates.