Carbon sources, cellulose, glucose

The following order of activity for methanol electro-oxidation was observed for PtRu/C electrocatalysts prepared with different carbon sources cellulose > starch > glucose. Some studies have been shown that mesoporous carbons are very active as supports of electrocatalysts for Proton Exchange Membrane Fuel Cell [3,4],... 

The series of degradation steps comprising mineralization is similar, whether the carbon source is a simple sugar (e.g., glucose), a plant polymer (e.g., cellulose), or a pollutant molecule [49,50,62 - 64,72,73]. Each degradation step in the pathway is facilitated by a specific catalyst (i. e., an enzyme) made by the degrading cell. Enzymes are found mostly within a cell (i. e., internal enzymes),... 

The induction of this operon responds to the intracellular concentration of cAMP, which is determined by the carbon source available to the cell. When cells are grown on cellobiose or cellulose that do not inhibit adenylate cyclase, cAMP is made in sufficient quantities for induction of cellulase. On the contrary, when cells are grown on glucose or other readily metabolized carbohydrates that do inhibit adenylate cyclase. 

Succinic acid is commonly produced in microbes because it exists as a part of the TCA cycle, one of the ordinary metabolic pathways for production of energy. Several groups in the world are developing this production system to produce cheaper succinic acid from renewable resources like starch, glucose, cellulose and so on. If succinic acid could be produced from cheap carbon sources and the price were competitive with the petroleum-base product, many C4 chemicals could be expected as derivatives. 1,4-Butanediol is the typical one, which has a huge market. 

The metabolism of anaerobic chytrids has not been studied in great detail, but it is known that most anaerobic chytrids studied so far produce formate, acetate, succinate, lactate and ethanol besides hydrogen and carbon dioxide when growing on cellulose, glucose or fructose as a carbon source (Julliand et al. 1998). Such a mixed acid fermentation is very similar to bacterial mixed acid fermentations that are, for example, well known for facultative anaerobic enteric bacteria, such as Escherichia coli. 

The objective of the current work was to characterize carefully the dynamics of cellulase production and metabolic activity following cellulose addition in a batch cultivation of the strain T. reesei Rut-C30. Cells were initially grown on glucose as the carbon source, and after its depletion, cellulose was added. Since it is difficult to follow the growth directly after addition of a solid substrate, on-line measurements of C02 evolution were used to follow the metabolic activity of the cells. Frequent samples were also taken to measure enzyme activity and sugar concentrations. 

Fig. 2. Reducing sugars ( ) determined by DNS method and OD (A) vs time for aerobic batch cultivation of T. reesei Rut-C30. The initial growth medium was a Mandels medium with 10 g/L of glucose as the carbon source. At t = 67 h, Solka Floe was added to a concentration of 10 g/L. (There were no measurements of OD after the addition of cellulose.)...

Major biotechnological uses of the biomass carbohydrate moiety have attracted worldwide attention. Controlled cellulose degradation by cellulases may produce materials for important multifarious applications carbohydrates that can be used in the food and beverage industries, cellulose microfibril fragments for non-caloric food additives, hyperabsorbent cellulose fibers from fragmented cellulose microfibrils which can be used in biomedical, commercial and house-hold absorbent materials. Biomass-derived glucose syrups can also be used as carbon source in industrial fermentations for the production of antibiotics, industrial enzymes, amino-acids, and bulk chemicals. 

In terms of the carbon source, glucose is a catabolite repressor, but many oligomers and polymers of glucose induce enzyme production. Lactose, cellobiose, and sophorose are all dimeric sugars that induce cellulase production. Cellulose is also an inducer. A list of the performance of several carbon sources is found in Table 5 [37]. 

In the submerged fermentation process for cellulase production, crystal cellulose and paper pulp are commonly used substrates which are expensive. The solid-state fermentation for cellulase production should use a cheap medium. Because cellulase is an inducible enzyme, the inducer must be included in the medium. Cellulose is the best inducer [45,46] and its hydrolysate (glucose) is the carbon source for microbes to grow therefore, cellulosic material is the basic component in the medium. Complex nitrogen sources are used to supply nitrogen and to regulate the pH value. Other nutrient salts, such as Mn + and Zn etc., should also be involved in the medium. 

T t has been reported that fungal cellulases are induced enzymes and that cellulose preparations induce cellulolytic activity while easily assimilable carbon sources give the best fungal growth but less production of enzyme activity (9,12, 14). For example, Horton and Keen (10) found that 7.5 X 10"3M D-glucose repressed the synthesis of cellulase to a basal level in Pyrenochaeta terrestris and suggested that cellulase synthesis was regulated by an induction-repression mechanism. 

Usually, glucose and sucrose are used as carbon sources for cellulose production, although other carbohydrates such as fructose, maltose, xylose, starch and glycerol have also been tried. The effect of initial glucose concentration on cellulose production is also important, since the formation of gluconic acid as a byproduct in the medium decreases the pH of the culture and ultimately decreases the production of cellulose. Cellulose yields at initial glucose concentrations of 6, 12, 24 and 48 g/L were studied, and the consumption of glucose was found to be 100, 100, 68 and 28 % of the initial concentration, respectively [14]. 

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