sequential optimization strategy

Susana (2005) worked that Solid state fermentation (SSF) has become a very attractive alternative to submerged fermentation (SmF) for specific applications due to the recent improvements in reactor designs. This paper reviews the application of SSF to the production of several metabolites relevant for the food processing industry, centred on flavors, enzymes (α-amylase, fructosyl transferase, lipase, pectinase), organic acids (lactic acid, citric acid) and xanthan gum. In addition, different types of biorreactor for SSF processes have been described.

Ali and Haq (2005) investigated deals with the promotry effect of different additives and metallic micro minerals on citric acid production by Aspergillus niger MNNG-115 using different carbohydrate materials. For this, sugar cane bagasse was fortified with sucrose salt medium. Ethanol and coconut oil at 3.0% (v/w) level increased citric acid productivity. Fluoroacetate at a concentration of 1.0 mg/ml bagasse enhanced the yield of citric acid significantly. However, the addition of ethanol and fluoroacetate after 6 h of growth gave the maximum conversion of available sugar to citric acid. In another study, influence of some metallic micro-minerals viz. copper sulphate, molybdenum sulphate, zinc sulphate and cobalt sulphate on microbial synthesis of citric acid using molasses medium was also carried out. It was found that copper sulphate and molybdenum sulphate remarkably enhanced the production of citric acid while zinc sulphate was not so effective. However, cobalt sulphate was the least effective for microbial biosynthesis of citric acid under the same experimental conditions. In case of CuSO4, the strain of Aspergillus niger MNNG-115 showed enhanced citric productivity with experimental (9.80%) over the control (7.54%). In addition, the specific productivity of the culture at 30 ppm CuSO4 (Q(p) = 0.012a g/g cells/h) was several folds higher than other all other concentrations. All kinetic parameters including yield coefficients and volumetric rates revealed the hyper productivity of citric acid by CuSO4 using blackstrap molasses as the basal carbon source.

Haq et al. (2005) investigated is concerned with the optimization of nitrogen for enhanced citric acid productivity by a 2-deoxy D-glucose resistant culture of Aspergillus niger NGd-280 in a 15 l stirred tank bioreactor. Nutrients, especially nitrogen source have a marked influence on citrate productivity because it is an essential constituent of basal cell proteins. Citric acid has been known to be produced when the nitrogen source was the limiting factor. Ammonium nitrate was employed as a nitrogen source in the present study and batch culture experiments were carried out under various concentrations of ammonium nitrate. Specific growth rate was decreased and the biosynthesis of citric acid was delayed at higher concentrations of ammonium nitrate. Specific citric acid production rate was the highest when intracellular ammonium ion concentration was between 2.0 and 3.0 mmol g(-1) cells.

Xie and West (2006) determined which citric acid-producing strain of Aspergillus niger utilized wet corn distillers grains most effectively to produce citric acid. Citric acid and biomass production by the fungal strains were analysed on the untreated grains or autoclaved grains using an enzyme assay and a gravimetric method respectively. Fungal citric acid production on the grains was found to occur on the untreated or autoclaved grains. The highest citric acid level on the grains was produced by A. niger ATCC 9142. The autoclaved grains supported less citric acid production by the majority of strains screened. Biomass production by the fungal strains on the untreated or autoclaved grains was quite similar. The highest citric acid yields for A. niger ATCC 9142, ATCC 10577, ATCC 11414, ATCC 12846 and ATCC 26550 were found on the untreated grains. Treatment of the grains had little effect on citric acid yields based on reducing sugars consumed by A. niger ATCC  9029 and ATCC 201122. It is feasible for citric acid-producing strains of A. niger to excrete citric acid on wet corn distillers grains whether the grains are treated or untreated. The most effective citric acid-producing strain of A. niger  was ATCC 9142. The study shows that the ethanol processing co-product wet corn distillers grains could be utilized as a substrate for the commercial production of citric acid by A. niger without treatment of the grains.

Kim et al (2006) investigated that citric acid is an effective remediation agent for soils contaminated by heavy metals. The large-scale field use of this organic acid requires the development of efficient production techniques using low cost substrates such as sugar rich wastes. To develop such techniques, the objective was to simultaneously optimize the initial glucose, nitrogen (N), phosphorus (P) and NaCl levels of a nutrient solution used to wet peat moss (PM) simulating a sugar rich waste on which Aspergillus niger NRRL 567 was grown to produce citric acid. Seventeen different combinations of nutrients were tested to grow A. Niger at 30 °C for 48 and 72 h, and to measure the resulting citric acid production. With the central composite design method (CCD), the results were used to produce a second order equation defining citric acid production as a function of initial glucose, N, P and NaCl levels. Initial glucose and N levels were found to have a significant positive and negative effect on citric acid production, while P and NaCl were found to have a limited positive and an insignificant effect, respectively. A peak citric acid production of 82 g kg−1 dry peat moss (DPM) was reached after 72 h with the following optimized nutrient solution, in terms of g kg−1 DPM: 967.9 glucose, 15.4 (NH4)2SO4, 43.9 KH2PO4 and 4.0 NaCl. This represented a 50% increase in production, over a shorter fermentation period, compared to a basal nutrient solution optimize solely for initial glucose level.

Lofty et al. (2007) sequential optimization strategy, based on statistical designs, was employed to enhance the production of citric acid in submerged culture. For screening of fermentation medium composition significantly influencing citric acid production, the two-level Plackett-Burman design was used. Under our experimental conditions, beet molasses and corn steep liquor were found to be the major factors of the acid production. A near optimum medium formulation was obtained using this method with increased citric acid yield by five-folds. Response surface methodology (RSM) was adopted to acquire the best process conditions. In this respect, the three-level Box-Behnken design was applied. A polynomial model was created to correlate the relationship between the three variables (beet molasses, corn steep liquor and inoculum concentration) and citric acid yield. Estimated optimum composition for the production of citric acid is as follows pretreated beet molasses, 240.1g/l; corn steep liquor, 10.5g/l; and spores concentration, 10(8) spores/ml. The optimum citric acid yield was 87.81% which is 14 times than the basal medium. The five level central composite design was used for outlining the optimum values of the fermentation factors initial pH, aeration rate and temperature on citric acid production. Estimated optimum values for the production of citric acid are as follows initial pH 4.0; aeration rate, 6500ml/min and fermentation temperature, 31.50C.