Freeze-drying of a mixture of bacterium and yeast for application in postharvest control of pathogenic fungi

The use of formulations based on microorganisms for the control of phytopathogenic fungi increases continuously, due to the risk of pesticide residues in food and the negative impact on the environment. Herein we present the obtaining by lyophilization of a dry mixed formulation containing two biological control agents, the bacterium Kosakonia radicincitans and the yeast Cryptococcus laurentii for the biocontrol of Botrytis cinerea and Penicillium expansum on stored apple fruit. For the formulation is used a complex cryoprotectant, that consists of 10% skimmed nonfat milk, 0.5% yeast extract, and 1% glucose. Two pre-freezing temperatures and different concentrations of the cryoprotectant are tested. Survival and antagonistic capacity of the formulation are compared with the mix of the bacterium and the yeast separately lyophilized. Results show higher antagonism of the mixture against B. cinerea and P. expansum than each microorganism separately. Concerning survival, it decreases significantly for both microorganisms in comparison when they are lyophilized individually. The bacterium seems the most affected by joint lyophilization. In the lyophilized mixture, the viability of the microorganisms is 38.8% for K. radicincitans and 44.7% C. laurentii. While for microorganisms that are lyophilized individually, the viability is 92.5% for the bacterium and 81.9% for the yeast. The change in concentration of complex cryoprotectant improves survival. K. radicincitans reaches the highest survival value (53.9%), but the survival of C. laurentii is not altered. The antagonistic activity of the lyophilized mixture also improves. The incidence of disease for P. expansum decreases to 26% and for B. cinerea to 16%. The formulation developed could have practical advantages for its commercialization and application since the two biological control agents are present in a single product.