The possession of proteolytic properties is an important secondary characteristic of many species of bacteria (especially enterobacteria and Clostridia) and is determined by the ability of the culture to hydrolyse gelatin. In the traditional method a nutrient gelatin butt is inoculated and incubated, hydrolysis being indicated by liquefaction of the medium. Up to 14 days incubation at 37°C may be required, and the butt must be briefly refrigerated to prove liquefaction or solidity.