THE NITROGEN AND SULFUR SOURCE

Organic sources of nitrogen in synthetic media are specific amino acids, purines, pyrimidines, and urea. Urea, depending upon the buffer capacity of the system, will raise the pH-value of the medium. Organic urea is also formed in the urea cycle reaction, starting with ammonia  

Ammonium sulfate produces acidic conditions because the ammonia is rapidly utilized and fi ee acid is then liberated. 

Industrial fermentations are generally more rapid and efficient when these materials are used, since they reduce the number of compounds which the cells would otherwise have to synthesize de novo . The availability of nitrogen as well as the concentration in the media has to be considered in each case. Proteins can only be assimilated by microorganisms that secrete extracellular proteases, which enzymatically hydrolyze the proteins to amino acids. Microorganisms without this ability require protein hydrolysates, peptones, or digests composed of free amino acids prepared by hydrolyzing proteinaceous materials with acids or enzymes. 

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