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Citric acid consumption is escalating gradually, witnessing high annual growth rate due to more and more advanced applications coming to light. The present review discusses different aspects of fermentation and effects of various environmental parameters and deals with the potential ways to increase the yield of citric acid to meet the ever-increasing demands of this commercially important organic acid. Different techniques for the hyperproduction of citric acid are continuously being studied from the past few decades and still there is a gap, and hence, there is an obvious need to consider new pragmatic ways to achieve industrially feasible and environmentally sustainable bio-production of citric acid. The utilization of inexpensive agro-industrial wastes and their by-products through solid-state fermentation by existing and genetically engineered strains is a potential route. This review also deals with downstream processing considering the classical and advanced approaches, which also need significant improvement. In situ product recovery method which leads to improved yields and productivity can be further optimized for large-scale production and recovery of citric acid. [ABSTRACT FROM AUTHOR]

Abstract: In view of ever growing demand of citric acid, there is an urgent need to look for inexpensive and novel substrates for feasible production of citric acid. In this context, the present study was carried out to evaluate the potential of different agro-industrial wastes for hyper production of citric acid through solid-state and submerged fermentation by Aspergillus niger NRRL 567 and NRRL 2001. It was found that among all the solid substrates utilized, apple pomace with 66.0±1.9g/kg of dry substrate proved to be an excellent substrate for citric acid production by A. niger NRRL 567 at 72h of incubation. A. niger NRRL 2001 resulted in slightly lower citric acid concentration of 61.0±1.9g/kg of dry substrate at the same incubation time. APS-1 (apple pomace ultrafiltration sludge-1) gave highest citric acid production rate of 9.0±0.3g/l and 8.9±0.3g/l of substrate by A. niger NRRL 567 and NRRL 2001 by submerged fermentation, respectively. Further study with apple pomace and apple pomace ultrafiltration sludge-1 by A. niger NRRL 567 was carried out. Addition of 3% (v/w) ethanol and 4% (v/w) methanol to apple pomace gave significantly higher citric acid values of 127.9±4.3g/kg and 115.8±3.8g/kg of dry substrate by A. niger NRRL 567 by solid-state fermentation. Higher citric acid values of 18.2±0.4g/l and 13.9±0.4g/l of apple pomace ultrafiltration sludge-1 were attained after addition of 3% (v/v) ethanol and 4% (v/v) methanol, respectively by A. niger NRRL 567. Apple pomace solid waste and apple pomace ultrafiltration sludge-1 thus proved to be an excellent source for citric acid production, of the different substrates chosen. [Copyright &y& Elsevier]