Microencapsulation of rhizobacteria by spray-drying: formulation and survival studies

This research deals with the microencapsulation of the bacteria Pseudomonas fluorescens-putida, using the spray-drying technique. These bacteria act as a plant growth stimulator and the microparticles are subsequently included in the seed coating or pelleting material. The objective was to maintain living encapsulated bacteria for a minimum of 5-6 months. Three polymers were tested: a methacrylic copolymer from the Eudragit range, an ethylcellulose and a modified starch. A silica additive to the spraying food was also examined. The granulometric distribution and the morphology of the microparticles were studied and the residual moisture was measured after each bacteria survival control test. The best results were obtained with the methacrylic copolymer Eudragit, particularly for microparticles collected in the cone of the spray-drying chamber. The mean diameter of the microspheres was 44.6 microns, 85% of these particles having a size between 6.2 and 84.4 microns. The bacterial survival time of a particular strain incorporated in these microparticles, strain M3.1, was one year. A relationship was found between the bacterial survival and the microspheres' residual moisture, the best survival of the stored bacteria being observed when the residual moisture was around 25%.