Fungal morphology and metabolite production in submerged mycelial processes.

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

There is a great worldwide demand for citric acid consumption due to its low toxicity when compared with other acidulants used mainly in the pharmaceutical and food industries. Other applications of citric acid can be found in detergents and cleaning products, cosmetics and toiletries, and other. Global production has now reached 1.4 million tonnes and there is annual growth of 3.5–4.0 % in demand/consumption of citric acid. As a result of the adverse market conditions, only big producers have survived. Any increase in citric acid productivity would be of potential interest and hence there is an obvious need to consider all possible ways in which this might be achieved. The production by submerged fermentation is still dominating. However, solid-state processes can create new possibilities for producers. Many by-products and residues of the agro-industry can be used in the production of citric acid. A cost reduction in citric acid production can be achieved by using less expensive substrates. The use of agro-industrial residues as support in solid-state fermentation is economically important and minimizes environmental problems. Other perspectives for citric acid production sector are the improvement of citric acid producing strains, which have been carried out by mutagenesis and selection.

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New perspectives for citric acid production and application

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.

Under otherwise identical fermentation conditions, the sugar source has been shown to have a marked effect on citric acid production by Aspergillus niger. Sucrose was the most favourable source, followed by glucose and fructose and then lactose. No citric acid was produced from galactose. Strong relationships were observed between citric acid production and the activities of certain enzymes in myccelial cell-free extracts prepared from fermentation samples. When sucrose, glucose, or fructose was the sugar source pyruvate carboxylase activity was high, but 2-oxoglutarate dehydrogenase activity was not detected. When galactose was the sugar source pyruvate carboxylase activity was low, but 2-oxoglutarate dehydrogenase activity was high. It is suggested that whereas glucose and fructose repress 2-oxoglutarate dehydrogenase, thereby causing accumulation of citric acid, galactose does not. The activity of aconitase showed a direct relationship to the citric acid production rate. Thus, the activity was highest when sucrose was the sugar source, and lowest when galactose was the source. It is suggested that when large amounts of citric acid are lost from the cell the activity of aconitase increases as a response to the diminished intracellular supply of its substrate.

The effect of the sugar source on citric acid production by Aspergillus niger

The effect of methanol on the ability of several strains of Aspergillus to produce citric acid from galactose has been investigated. In the absence of methanol, very little production (less than 1 g/l) was observed. In the presence of methanol (final concentration 1% v/v), however, citric acid production and yeilds were increased considerably. Strong relationships were observed between citric acid production and the activities of the enzymes 2-oxoglutarate dehydrogenase and pyruvate carboxylase in cell-free extracts. During citric acid production, in the presence of methanol, the activity of 2-oxoglutarate dehydrogenase was low and that of pyruvate carboxylase high. In the absence of methanol, where little citric acid was produced, the reverse was true. It is suggested that the presence of methanol may increase the permeability of the cell to citrate, and the cell responds to the diminished intracellular level by increasing production via repression of 2-oxoglutarate dehydrogenase.

The effect of methanol on citric acid production from galactose by Aspergillus niger

The use of lactic casein whey permeate as a substrate for citric acid production by fermentation has been investigated. Using a mutant strain of Aspergillus niger IMI 41874 in fermenter culture, a citric acid concentration of 8.3 g/l, representing a yield of 19% (w/w) based on lactose utilized, has been observed. Supplementation of the permeate with lactose (final concentration 140 g/l) increased the production to 14.8 g/l (yield 23%). The natural pH of the permeate (pH 4.5) was the most suitable initial pH for the process, and pH control during the fermentation was unnecessary. The addition of methanol (final concentration 3% v/v) to the fermentation increased the citric acid production to 25 g/l (yield 33%, based on lactose utilized). 13 references.

Production of citric acid from whey permeate by fermentation using Aspergillus niger

Previous work in this laboratory has demonstrated that although Aspergillus niger can readily utilize galactose, no citric acid is produced from this carbon source (Hossain et al 1984) Experiments were now conducted where galactose was added at various concentrations to synthetic growth medium containing glucose as carbon source, so that the effect of galactose on citric acid production from glucose could be observed The results showed that the presence of galactose or a product of galactose metabolism caused inhibition of citric acid production, and also reduced the rate of glucose utilization Enzyme analyses using mycelial cell-free extracts indicated that galactose interfered with the glucose-repression of the key enzyme 2-oxoglutarate dehydrogenase

M. Hossain

Papers

The effect of the sugar source on citric acid production by Aspergillus niger

The effect of methanol on citric acid production from galactose by Aspergillus niger

Production of citric acid from whey permeate by fermentation using Aspergillus niger

Galactose inhibition of citric acid production from glucose by Aspergillus niger