Starch is the main constituent of cereals. The production of citric acid from different sources of starch (corn, potato, cassava, yam bean) has been reported (196-200). Generally, in the starch medium, the critical factors of citric acid production are the aeration efficiency of the medium and the amylase formation of the strain (196). Mourya and Jauhri (197) tested 17 strains of A. niger for their capacity to produce citric acid using starch hydrolysate as a substrate. The most efficient strain, A. niger ITCC-605, was selected for further improvement in citric acid content by mutation. In this case, the maximum citric acid concentration was 64.7 g/kg of glucose consumed. Bolach et al. (198) reported that liquefied starch (DE <25) was a better substrate for the production of citric acid than saccharified starch (DE >80). Tan, et al. (199) found that a typical medium containing 2% cassava starch yielded 5.4 g/L biomass dry weight and 74.8 g/L organic acids (citric, malic, gluconic, succinic, fumaric). Sarangbin and Watanapokasin (200) investigated the production of citric acid from yam bean starch by a protease negative mutant strain of A. niger. They found a maximum citric acid concentration of 106.0 g/L, whereas the parental strain produced 58.0 g/L from 140.0 g/L of soluble starch in semisolid culture after 5 days of fermentation. Esuoso et al. (201) reported the production of citric acid from imumu Cyperus esculentus and maize Zea mays by A. niger. Imumu, although a weed, has a tuber rich in carbohydrate. It is widely distributed and constitutes one of the world's worst weeds. Proximate analysis reveals a high percentage of starch, with pockets of sugar, protein, and lipids. It was observed that the maximum citric acid concentration (14.3 g/L) was obtained in unagitated and defatted cultures compared to agitated and undefatted cultures, respectively. Hang and Woodams (202,203) studied the production of citric acid from corncobs and corn husks by A. niger. Corncobs and corn husks are important byproducts of the sweet corn processing industry that have been either used as animal feed or are returned to the harvested field. Methanol had a significant effect on the 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 corncobs) after 72 h of incubation at 30°C in the presence of 3% methanol. The yield of citric acid was over 50% based on the amount of sugar consumed. Combined treatments of corn husks with dilute NaOH and Rapidase Pomaliq (a commercial apple juice processing enzyme preparation) significantly enhanced the yield of citric acid. Under favorable conditions (pretreated with 0.5 M NaOH, followed by 120 h fermentation at 30°C in the presence of Rapidase Pomaliq and 3% methanol), the yield of citric acid was 260.0 g/kg of dry matter of corn husks. Yuguo et al. (204) investigated citric acid production from dried sweet potato mash with its dregs by A. niger in an external loop airlift bioreactor and in a stirred tank fermentor. They found that an average of 10.6 g/L of citric acid was obtained in the external loop airlift bioreactor under an airflow rate of 1.3 vvm and liquid volume of 8.5 L as compared to 9.6 g/L of citric acid in the 10 L stirred tank fermentor at an agitation rate of 200 rpm, airflow rate at 1.0 vvm, and liquid volume of 6.5 L.
Khare et al. (205) reported the production of citric acid from okara (soy residue) in SSF using a cellulolytic A. terreus and citric acid producing A. niger. Okara or soy residue is generated as a byproduct from soy milk and tofu production. The medium supplemented with ammonium sulfate (0.1% N) when fermented by A. niger with simultaneous sac-charification using A. terreus at pH 8.3 and incubation temperature of 30°C resulted in the production of 5.1 g citric acid/100 g dry solids.
Other workers (158) produced citric acid from kumara (Ipomoea batatas), taro (Colacasia esculenta), and potato (Solanum tuberosum) by A. niger in SSF. Raw tubers were peeled, heated at 121°C for 30 min, and blended into a paste. The fermentation was carried out in 250 mL conical flasks containing 40 g of paste, inoculated with 108 spores per flask and incubated at 30°C as surface fermentation. The results showed that kumara and taro were excellent substrates for citric acid production, while potato was a poor medium, although it supported profuse fungal growth. The maximum citric acid concentrations were 69.0, 66.0, and 3.0 g/kg wet weight of the substrate from kumara, taro, and potato, respectively, after 6 days of fermentation. The optimum moisture content of the kumara for citrate production was >65% (w/w), while metal ions were shown not to be inhibitory to the process.
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