Citric Acid Fermentation

The production of organic acids covers two aspects: first, the metabolic pathways involved in the biosynthesis, and, second, the industrial process strategy adopted The review seeks to show the underlying biochemical similarities in the biosynthesis of organic acids and the resulting similarities in the commercial processes Two groups of acids are defined, those with a "long" biosynthetic path from glucose, involving much of the glycolytic pathway and the tricarboxylic acid cycle, and those acids with a "short pathway", essentially a biotransformation of glucose The regulation of the pathways and the future developments in metabolic control theory and genetic manipulations relating to them are considered The organisms used industrially are also limited, Aspergillus sp and Candida yeasts; again the underlying metabolic similarities lead to similar strategies for all the acids discussed

The production of organic acids.

Fungal production of citric acid.

Citric acid is a commodity chemical produced and consumed throughout The World. It is used mainly in the food and beverage industry, primarily as an acidulant. Although it is one of the oldest industrial fermentations, its World production is still in rapid increasing. Global production of citric acid in 2007 was over 1.6 million tones. Biochemistry of citric acid fermentation, various microbial strains, as well as various substrates, technological processes and product recovery are presented. World production and economics aspects of this strategically product of bulk biotechnology are discussed.

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Citric acid production.

Facility in the targeted manipulation of the genetic and metabolic composition of organisms, combined with unprecedented computational power, is forging a niche for a new subspecialty of biotechnology called metabolic engineering. First published in 2002, this book introduces researchers and advanced students in biology and engineering to methods of optimizing biochemical systems of biotechnological relevance. It examines the development of strategies for manipulating metabolic pathways, demonstrates the need for effective systems models, and discusses their design and analysis, while placing special emphasis on optimization. The authors propose power-law models and methods of biochemical systems theory toward these ends. All concepts are derived from first principles, and the text is richly illustrated with numerous graphs and examples throughout. Special features include: nontechnical and technical introductions to models of biochemical systems; a review of basic methods of model design and analysis; concepts of optimization; and detailed case studies.

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Pathway Analysis and Optimization in Metabolic Engineering

Determinations of the momentary levels of various intermediates related to the activity of the tricarboxylic acid cycle have been made during citric acid production in high-accumulating (manganese deficient) and lowaccumulating (manganese supplemented) mycelia of Aspergillus niger. During the growth period the levels of almost all TCA cycle acids, with the exception of 2-oxo-acids, were unusually high; during the induction phase of citrate accumulation malate, fumarate, and isocitrate decreased, whereas pyruvate, oxalacetate, and citrate increased. The presence of succinate could not be demonstrated. The interrelations of the momentary concentrations of the intermediates mainly demonstrate a lack in activity of 2-oxoglutarate dehydrogenase, representing a block in the TCA cycle concomitant with a strongly operating glycolysis as a prerequisite for citrate accumulation. Inhibition studies with crude enzyme preparations suggest that an inhibition of malate dehydrogenase by citrate and also inhibition of isocitrate dehydrogenase by citrate and 2-oxoglutarate occur during the production phase as additional factors.

M. Röhr

Papers

The role of the tricarboxylic acid cycle in citric acid accumulation by Aspergillus niger