Precursor addition fermentation

The most successful way to achieve overproduction is to make use of mutants. Another way to overcome feedback regulation is to naake use of a kind of semi-fermentaticHi process called precursor addition fermentation this will be considered later in this chapter. 

Table 8.7 The most important process conditions for the production of L-phenylalanine by direct fermentation precursor addition, phenylpyruvic add (PPA) bloconversion, acetamidocinnamic add (ACA).

Extra cost compared to direct fermentation are mainly concerned with addition of PPA (precursor addition) or ACA (bioconversion). Table 8.9 compares estimated costs for foe PPA and ACA process based on foe data considered previously (section 8.6.1 and 8.6.2). Costs are estimated for production of 100 tonnes per year. 

The homologation reaction was first reported nearly 40 years ago (2). The catalyst precursor was Co (CO). Subsequent workers utilized cobalt catalysts but also employed iodide promoters (, 4 ), a Ru co-catalyst ( ), and trivalent phosphines ( ) to increase the yield. The reaction is carried out at 180-200 °C and 4000-8000 psig. In the better cases, the ethanol rate and selectivity are 1-6 M/hr and 50-80 %. Unsatisfactory conversion, selectivity, and the required high operating pressure have prevented commercialization of the current homologation technology. Additionally, fermentation routes to ethanol have now... 

Certain factors and product precursors are occasionally added to various fermentation media to iacrease product formation rates, the amount of product formed, or the type of product formed. Examples iaclude the addition of cobalt salts ia the vitamin fermentation, and phenylacetic acid and phenoxyacetic acid for the penicillin G (hen ylpenicillin) and penicillin V (phenoxymethylpenicillin) fermentations, respectively. Biotin is often added to the citric acid fermentation to enhance productivity and the addition of P-ionone vastly iacreases beta-carotene fermentation yields. Also, iaducers play an important role ia some enzyme production fermentations, and specific metaboHc inhibitors often block certain enzymatic steps that result in product accumulation. 

From fermentation solutions of Penicillium notatum Westling or Penicillium chrysogenum Thom by addition of phenylacetic acid as precursor. 

Fermentation. Dalbaheptides are produced by submerged fermentation and this topic has been reviewed. In a few cases addition of a biosynthetic precursor of the core aglycones such as tyrosine and p-hydroxyphenylglycine increased the yields. 

In addition to heterocycles in grape juice, there are also many acyclic precursors that are converted by fermentation, aging, and other modifications into heterocycles for example, hydroxyl carboxylic acids, polyphenols, and amino acids. 

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