Indigo the Natural Color

Nearly all indigo is produced from N-phenylglycine (see Fig. 8.18) via fusion with potassium and sodium hydroxide, followed by treatment with sodamide [103]. The melt containing the dialkalimetal salt of indoxyl is subsequently dissolved in water, and indigo is formed by aerobic oxidation. Filtration and wash- 

The indigo production was 50% lower than expected on the basis of the Trp productivity of the parent strain [107]. This latter problem was traced to inactivation of DAHP synthase by exposure to indoxyl or its oxidation products it was also found that PEP exerted a protecting effect on DAHP synthase. The production of indigo was improved to approx. 18 g L-1 (6.2 g L 1 d 1) by increasing the gene dosage for DAHP synthase and mutations that increased the bioavailability of PEP [107]. 

A problem connected with the microbial production of indigo is the formation of a small amount of isatin, which could be partly suppressed by adjusting the 02 concentration [108]. Isatin reacts with indoxyl to give indirubin, which gives the finished denim an undesirable red cast. The introduction of an isatin-de-grading enzyme (isatin hydrolase from P. putida) satisfactorily reduced the level of indirubin [107]. 

Subsequently, indigo was successfully produced via fermentation on the 300000 L scale at a cost that was comparable with the price of chemical indigo [109]. Commercialization proved elusive, however, presumably because chemical indigo is marketed with a substantial profit margin. We note that it is common experience that the total costs of a new process, to compete, must be equal to (or lower than) the production cost of the existing process. In this particular case, it would seem that the STY of the fermentative process is too low to compete with a chemical procedure that encompasses only three steps from the basic chemicals aniline and acetic acid. 

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