Simple diffusion is the primary mechanism for glucose uptake during the production phase of the Aspergillus niger citric acid process

An Experimental Study on Citric Acid Production by Aspergillus niger Using Gelidiella acerosa as a Substrate

Citric acid (CA) is the most important commercial product which is produced by using various sugar substrates in the terrestrial environment. The present study made an attempt to produce citric acid by the fungal strain Aspergillus niger from red seaweed Gelidiella acerosa is the best alternative to sugar substrate in the marine environment. In this study three types of production media were prepared including control (sucrose) by following standard fermentation conditions. The acid production was indicated by the reduction of pH levels. The control medium gave the highest yield of 80 g/l at pH 1.5 and the medium containing crude seaweed powder and other compositions gave the yield of 30 g/l at pH 3.5 whereas the medium containing crude seaweed and 10% sucrose gave the yield of 50 g/l at pH 3.0. When calculating the benefit cost ratio, crude seaweed powder and 10% sucrose yielded 50 g of citric acid at the lower cost of Rs. 35, whereas the other two media gave the yield of 80 and 30 g respectively with the cost of Rs. 77 and 28. In economic point of view, the medium containing seaweed and 10% sucrose showed more benefit with lower cost.

Characteristics of glucose uptake by glucose- and NH4-limited grown Penicillium ochrochloron at low, medium and high glucose concentration

Glucose uptake by Penicillium ochrochloron (formerly Penicillium simplicissimum) was studied from 0.01 to 400 mM glucose using chemostat culture and bioreactor batch culture. The characteristics of glucose uptake varied considerably with the conditions of growth, harvest and uptake assay. Glucose-limited grown mycelium showed one saturable transport system [K(S) below 0.01 mM; v(max) 1.1-1.2 mmol (g dry weight)(-1)h(-1)] plus a first order process (permeability P=1.2x10(-7)cm s(-1)). Ammonium-limited grown mycelium showed only one saturable transport system [K(S) 0.3-0.7 mM; v(max) 0.5-0.8 mmol (g dry weight)(-1)h(-1)]. During exponential growth at high glucose concentration (300-400 mM) a first order process was found with a P value of 5.6-9.3x10(-7)cm s(-1). After ammonium exhaustion a second first order phase showed a lower P value (6.1-9.3x10(-8)cm s(-1)). A similar change in permeability was also found after a re-evaluation of published data for Gibberella fujikuroi, Aspergillus niger, Aspergillus awamori and Saccharomycopsis lipolytica. For the first order processes simple diffusion was ruled out as a mechanism for glucose uptake. Glucose uptake by P. ochrochloron was controlled more strongly by metabolism than by transport and was not rate limiting for overflow metabolism.

Engineering primary metabolic pathways of industrial micro-organisms

Metabolic engineering is a powerful tool for the optimisation and the introduction of new cellular processes. This is mostly done by genetic engineering. Since the introduction of this multidisciplinary approach, the success stories keep accumulating. The primary metabolism of industrial micro-organisms has been studied for long time and most biochemical pathways and reaction networks have been elucidated. This large pool of biochemical information, together with data from proteomics, metabolomics and genomics underpins the strategies for design of experiments and choice of targets for manipulation by metabolic engineers. These targets are often located in the primary metabolic pathways, such as glycolysis, pentose phosphate pathway, the TCA cycle and amino acid biosynthesis and mostly at major branch points within these pathways. This paper describes approaches taken for metabolic engineering of these pathways in bacteria, yeast and filamentous fungi.

Advances in citric acid fermentation by Aspergillus niger: biochemical aspects, membrane transport and modeling

Citric acid is regarded as a metabolite of energy metabolism, of which the concentration will rise to appreciable amounts only under conditions of substantive metabolic imbalances. Citric acid fermentation conditions were established during the 1930s and 1940s, when the effects of various medium components were evaluated. The biochemical mechanism by which Aspergillus niger accumulates citric acid has continued to attract interest even though its commercial production by fermentation has been established for decades. Although extensive basic biochemical research has been carried out with A. niger, the understanding of the events relevant for citric acid accumulation is not completely understood. This review is focused on citric acid fermentation by A. niger. Emphasis is given to aspects of fermentation biochemistry, membrane transport in A. niger and modeling of the production process.

Citric acid production

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.

Changes in primary metabolism leading to citric acid overflow in Aspergillus niger

For citric acid-accumulating Aspergillus niger cells, the enhancement of anaplerotic reactions replenishing tricarboxylic acid cycle intermediates predisposes the cells to form the product. However, there is no increased citrate level in germinating spores and a complex sequence of developmental events is needed to change the metabolism in a way that leads to an increased level of tricarboxylic acid cycle intermediates in mycelia. A review of physiological events that cause such intracellular conditions, with the special emphasis on the discussion of hexose transport into the cells and regulation of primary metabolism, predominantly of glycolytic flux during the process, is presented.

Aspergillus niger citric acid accumulation: do we understand this well working black box?

This Mini-Review summarizes the current knowledge on the biochemical and physiological events leading to massive citric acid accumulation by Aspergillus niger under industrially comparable conditions, thereby particularly emphasizing the roles of glycolytic flux and its control, excretion of citric acid from the mitochondria and the cytosol, and the critical fermentation variables. The potential of novel techniques for metabolic analysis and genomic approaches in understanding this fermentation is also discussed.