El-Samragy et al. (1996) investigated the effect of pH value, methanol and salt concentration on the production of citric acid from cheese whey by two strains of Aspergillus niger. Lactose concentration, utilized lactose, citric acid concentration, conversion coefficient of lactose to citric acid and mycelial dry weight were measured during the fermentation process. The maximum citric acid concentration (1.06 and 0.82 g/l), were obtained at pH 3.5 after 9 days of fermentation for Aspergillus niger 1 and 2, respectively. The presence of 4% (v/v) methanol in the fermentation medium increased the amount of citric acid produced by Aspergillus niger 1 and 2 by 23% and 18% respectively. Both strains showed a high ability to utilize lactose for the production of citric acid when grown in the presence of 10% (w/v) salt, the conversion coefficient of lactose to citric acid was 28.24% for Aspergillus niger 1 and 25.60% for Aspergillus niger when the fermentation medium had a 10% (w/v level of salt. The cumulative effect of fermentation medium pH (3.5), methanol concentration (4% v/v) and salt concentration (10%, w/v) during the fermentation process of whey did not enhance the production of citric acid by Aspergillus niger strain, 1 while it did increases the production of citric acid by Aspergillus niger strain 2 by about 4-fold.
Vergano et al. (1996) studied the type of sporulation medium and time of incubation. They found that these had an effect on spore viability and citric acid production by mycelia grown from Aspergillus niger spores. They found that viability increased with time of incubation, but higher production of citric acid was achieved with spores incubated for less than 7 days.
Anonymous (1998) stated that Corn cobs could serve as a substrate for citric acid production by Aspergillus niger of the four cultures examined, Aspergillus niger was found to produce the highest amount of citric acid (250 g/kg dry matter of corn cobs) after 72 h of growth at 30oC in the presence of 3% methanol. They yield of citric acid was over 50% based on the amount of sugar consumed.
Roukas (1998) investigated the production of citric acid from carob pod extraction by Aspergillus niger in surface fermentation. A maximum citric acid production (4.07 g/l) was achieved at pH of 6.5 and temperature of 30oC. Other kinetic parameters, namely, citric acid yield, biomass yield, specific biomass production rate, and fermentation efficiency were maximum at pH 6.5, temperature 30oC and an initial sugar concentration of 100 g/l. The external addition of methanol into the carob pod extract at a concentration upto 4% (v/v) improved the production of citric acid.
Hang and Woodams (1998) that Corncobs could serve as a substrate for citric acid production by Aspergillus niger. Methanol had a significant effect on fungal production of citric acid from corncobs. Of the four cultures examined, A. Niger NRRL 2001 was found to produce the highest amount of citric acid (250 g/kg dry matter of corncobs) after 72 h of growth at 30°C in the presence of 3% methanol. The yield of citric acid was over 50% based on the amount of sugar consumed.
Karaffa et al. (2001) fungi, in particular Aspergilli, are well known for their potential to overproduce a variety of organic acids. These microorganisms have an intrinsic ability to accumulate these substances and it is generally believed that this provides the fungi with an ecological advantage, since they grow rather well at pH 3 to 5, while some species even tolerate pH values as low as 1.5. Organic acid production can be stimulated and in a number of cases conditions have been found that result in almost quantitative conversion of carbon substrate into acid. This is exploited in large-scale production of a number of organic acids like citric-, gluconic- and itaconic acid. Both in production volume as well as in knowledge available, citrate is by far the major organic acid. Citric acid (2-hydroxy-propane-1,2,3-tricarboxylic acid) is a true bulk product with an estimated global production of over 900 thousand tons in the year 2000. Till the beginning of the 20th century, it was exclusively extracted from lemons. Since the global market was dominated by an Italian cartel, other means of production were sought. Chemical synthesis was possible, but not suitable due to expensive raw materials and a complicated process with low yield. The discovery of citrate accumulation by Aspergillus niger led to a rapid development of a fermentation process, which only a decade later accounted for a large part of the global production. The application of citric acid is based on three of its properties: (1) acidity and buffer capacity, (2) taste and flavour, and (3) chelation of metal ions. Because of its three acid groups with pKa values of 3.1, 4.7 and 6.4, citrate is able to produce a very low pH in solution, but is also useful as a buffer over a broad range of pH values (2 to 7). Citric acid has a pleasant acid taste which leaves little aftertaste. It sometimes enhances flavour, but is also able to mask sweetness, such as the aspartame taste in diet beverages. Chelation of metal ions is a very important property that has led to applications such as antioxidant and preservative. Moreover, it is a "natural" substance and fully biodegradable.
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