Citric acid is widely used in the food and beverage industry, pharmaceutical industry and elsewhere. World-wide demand for this compound is in excess of 200,000.metric tons a year. Historically, and even today, the chief processes for the production of citric acid are microbial fermentation processes. A large number of organisms including fungi, yeast and bacteria have been examined as microbial agents to produce citric acid.
Aspergillus niger employed since the early part of this century is still apparently the organism of choice.
The basic methodology for citric acid production is throughly reviewed in "Prescott & Dunn's Industrial Microbiology" 4th Ed., 1982, Reed, edit., AVI Pub. Co., Inc., Westport, Conn., pps. 709-747 (hereby incorporated by reference).
Three principal methods of citric acid production by microbes are known: solid state culture, continuous culture and multi-stage fermentation processes. Most new technology is apparently based on submerged fermentation processes. The usual carbon sources are sucrose or molasses.
Where solid state fermentation has been employed the fermentation medium has been impregnated in porous solid materials such as sugarcane bagasse, potato or beet pulp, pineapple pulp and the like and then inoculated.
Grape pomace is the residue left from juice extraction and constitutes about 16% of the original fruit (Rice, Am. J. Enol. Vitic., 27:21-26). In 1984, approximately 4.9 million metric tons of grapes were produced in the United States. Grape pomace is rich in carbohydrates, but its nitrogen and phosphorus contents are low (Rice, supra). At present, most pomace is dumped despite the increasing disposal problem posed and efforts made at by-product utilization (Famuyima and Ough, 1982, Am. J. Enol. Vitic., 33:44-46).