Beef extract and peptanes, or hydrolyzed protein, are the basic ingredients of nutrient broth.
These materials supply a variety of carbon sources, nitrogen compounds in the form of amino acids, and a mixture of cofactors such as vitamins.
The addition of agar, a complex carbohydrate extracted from seaweed, results in a solid medium.
Agar is an ideal solidifying agent for microbiological media because of its melting properties and because it has no nutritive value for the vast majority of bacteria.
Solid agar melts at 194-212°F (90-100°C); liquid agar solidifies at about 103°F (42°C).