Citric Acid is Ubiquitous in Nature

Citric acid is ubiquitous in nature. Citric acid is solid at room temperature, melts at 153ºC and decomposes at higher temperatures into other products (Rajoka et al. 1998). It is non-toxic and easily oxidized in the human body. Because of its high solubility, palatability and low toxicity, it can be used in food, biochemical and pharmaceutical industries. These uses have placed greater stress on increased citric acid production and search for more efficient fermentation process. The worldwide demand of citric acid is about 6.0 x 105 tons per year and it is bound to increase day by day (Ali et al. 2001).

The production of citric acid by Aspergillus niger is one of the most commercially utilized examples of fungal overflow metabolism. Many microorganisms such as fungi and bacteria can produce citric acid. The various fungi, which have been found to accumulate citric acid in their culture media, include strains of Aspergillus niger, A. awamori, Penicillium restrictum, Trichoderma viride, Mucor piriformis and Yarrowia lipolytica (Arzumanov et al. 2000). But Aspergillus Niger remained the organism of choice for the production of citric acid. (Mattey and Allan, 1990; Ali et al. 2001). Molasses is a desirable raw material for citric acid fermentation because of its availability and relatively low price.

Incubation temperature plays an important role in the production of citric acid. Temperature between 25-30ºC is usually employed for culturing of Aspergillus niger but temperature above 35ºC is inhibitory to citric acid formation because of the increased the production of by-product acids and also inhibition of culture development. Sanjay and Sharma (1994) reported that citric acid production by Aspergillus Niger is sensitive to the initial pH of the fermentation medium.

Solid-state fermentation has long been applied to the food industry. SSFs is a process carried out with microbes growing on nutrient impregnated solid substrate with little or no free water. Solid state fermentation (SSF) can be directly carried out with low-cost biomaterials like corn Stover, corncobs, banana stalk, wheat bran etc. abundant and available in Pakistan with minimal or no pretreatment, and thus is relatively simple, uses less energy than submerged fermentation (SmF), and can provide unique microenvironments conductive to microbial growth and metabolic activities. The present project will be designed to use corncobs as carrier substrate for SSF of molasses based medium by Aspergillus niger. 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