Trypticase soy broth, a nutritious medium supplemented with glycerol is used as a maintenance medium for the preservation of bacterial culture.

Enzymatic digests of protein substrates act as protective colloids

Bacterial stock cultures frozen and stored at –20 °C or below will remain viable for several months, and some may remain viable for years.

Glycerol is a cryoprotective agent that provides intracellular and extracellular protection against freezing.