MICROBIOLOGY Textbook - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.4. INCOMPLETE OXIDATIONS

9.4.2. Production of Organic Acids by Fungi

Molds possess all the Enzymes of The Glycolytic Pathway for carbohydrate degradation. Metabolic bottlenecks lead to the accumulation of intermediates, which are excreted into the medium either directly or after minor modifications. Metabolic deviations frequently occur under conditions of substrate excess.

Lactic acid is produced by zygomycetes such as Rhizopus nigricans and R. oryzae. However, unlike Homofermentative lactic acid Bacteria, Fungi never produce lactic acid as the sole product. Along with lactic acid, they synthesize small amounts of fumaric, succinic, malic, formic, and acetic acids.

Gluconic acid is produced by numerous Aspergillus and Penicillium species. It is a product of glucose oxidation catalyzed by glucose oxidase, an enzyme secreted by fungi into the nutrient medium:

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Oxalic acid is synthesized by the fungus Aspergillus niger in an alkaline environment (pH 7.0) via the Hydrolysis of oxaloacetic acid:

Citric acid. Previously, natural citric acid was extracted from citrus fruits. Today, it is manufactured via microbial synthesis using specific strains of the fungus Aspergillus niger. Traditionally, the producer Organism was cultivated via surface culture, where the fungus grew On the surface of a nutrient medium poured into large flat trays. A more advanced method is submerged Fermentation in closed, aerated, and agitated bioreactors.

Citric acid is secreted in large quantities into the medium by Aspergillus niger at initial pH values ranging from 2.5 to 3.5. As the pH rises, gluconic acid appears first, followed by oxalic acid (Fig. 27).

Fig. 27. Pathways of organic Acid production by fungi

Citric acid is widely used in the food, medical, pharmaceutical, and coatings industries.