1.7.2
Nucleotides
Commercial interest in nucleotide fermentations developed due to the discovery of A. Kuninaka in Japan that certain purine ribonucleoside 5 ′ - monophosphates, namely guanylic acid (GMP), inosinic acid (IMP), and xanthylic acid (XMP) were enhancers of flavor for foods, beverages, and seasonings [80, 81] . Interestingly, AMP had no such activity. The intensity of flavor enhancement by these compounds surpassed that of MSG by several orders of magnitude. Combination of a flavor nucleotide with MSG had a synergistic effect on flavor enhancement. Originally, the nucleotides were made by enzymatic hydrolysis of yeast RNA but this was an expensive process. Auxotrophic mutants of the glutamic acid producer, C.
glutamicum , were then found to produce IMP and XMP [82, 83] . By further mutation
of the IMP producer, a strain was obtained which could produce a mixture of the two most potent nucleotides, GMP and IMP [84] . Direct fermentation became the method of choice for the industry [85] , although some companies produced the nucleosides (inosine and guanosine) by fermentation and then enzymatically (via phosphokinase catalysis) converted these to the 5 ′ nucleotides. Production of 5 ′ - flavor nucleotides amounted to about 40 g/l.