How are different varieties of pollen grains stored for long period of time in pollen banks?

Different varieties of pollen grains are stored for long period of time in pollen banks with the help of cryopreservation.

Pollen banks help to preserve the biodiversity by preserving valuable genetic resources. The pollen grains are stored in viable conditions. In this process, the pollen grains are first dried to bring water content below a certain level. Then it is stored in liquid nitrogen at -196 degree centigrade.