Preparation of spherical pellets of dried bacteria by isodensity freezing

A density gradient was established in a cold mixture of Freon 113 and heptane by cooling the base of the container. In this freezing mixture, interfacial tension forms aqueous drops into spheres. As soon as freezing begins, the pellets of bacteria rise to the surface and, on transferring to suitable containers, are dried under high vacuum. Viability of the bacteria was retained.