Citric acid is an important commercial product with a global production reaching 736,000 tons per year (Quimica and Derivados, 1997).
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).
Almost the entire quantity of citric acid is produced by fermentation, mainly through submerged fermentation of starch or sucrose based media, using the filamentous fungus Aspergillus niger. The food industry is the largest consumer of citric acid, using almost 70% of the total productions followed by about 12% by the pharmaceutical industry and 18% for other applications, (Shah et al., 1993).
There is an annual growth of 3.5 – 4.0% in demand/consumption rate of citric acid. In Brazil, the entire demand for citric acid is met by imports. This necessitates development of indigenous biotechnological processes with economic feasibility.
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. Karaffa et al. (2001)
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.
industrial developments in the citric acid production techniques are closely guarded by confidentiality agreements. Factors which affect Aspergillus growth and citric acid yields are many and may include, substrate and nitrogen concentrations, initial pH, dissolved oxygen and cation (especially Fe+2, Mn+2 and Cu+2) levels of the medium (Ali et al. 2001). Considerable amounts of citric acid are required by large-scale industrial processes. The demand is steadily increasing and the industry is seeking cheap, economic and newer process technology.
Morphological studies have been considered very important in fungal fermentation. Morphological parameters and the type of mycelia present (free mycelia without any branches, branched mycelia and branched mycelia with conidiophore) were measured to correlate citric acid production with the morphology of Aspergillus niger. We observed that morphological parameters and the type of mycelia present varied with substrate concentration. They also depended on the type of substrate (molasses and glucose) used.( Pazouk & Panda 2002)
The techniques of ultraviolet irradiations, or N-methyl, N-nitro-N-nitroso-guanidine (MNNG) induced mutagenesis are useful to improve the yield of citric acid by A. niger. Mutagenesis can minimize the formation of side products and increase citric acid accumulation. The complexing reagents can bind metal ions and enhance citric acid production by Aspergillus niger. Citric acid is produced mainly by Liquid state fermentation (LSF)/submerged fungal fermentation of sucrose or molasses media using Aspergillus nigher (Kapoor et al., 1982; Roher et al., 1983).
A number of carbon sources may be used for citric acid fermentation. For commercial reasons, the use of molasses, sucrose or glucose syrups are favoured. The use of molasses in particular is desirable because of its low cost availability. Considerable research effort has been expended in developing the citric acid production protocol (Alvarez-Vazquez et al. 2000
).In the recent years, however, there has been increasing interest in the use of SSF process as an alternative to submerged ferementation (Kumar et al., 2003, Prado et al., 2004).
Solid State Fermntation (SSF) of molasses medium using cellulosic waste as carriers has the potential to increase the microbial growth and efficiency for citric acid production (Kumar et al., 2003, Prado et al., 2004).
Sugarcane bagasse, banana stalk, corn stover and wheat bran are abundantly available in Pakistan. These materials can be utilized as cost effective carriers/support materials for SSF of molasses.
Citric acid fermentation is one of the rare examples of industrial fermentation technology where academic discoveries have worked in tandem with industrial know-how, in spite of an apparent lack of collaboration, to give rise to an efficient fermentation process. The current world market estimates suggest that upwards of 4.0 x 105 tonnes citric acid per year may be produced (Kristiansen et al. 1999).
Citric acid production has always been a subject of interest for many workers e.g. Chaudhry et al., 1978; Eikmeier and Rehmn, 1984; Hang and woodmans, 1987; Lee et al., 1989; Gutierrez Rojas et al., 1996; Lu et al., 1997 and Pintado et al ; 1998. Different agro industries residues, such as apple pomace, coffee husk, wheat straw, pineapple waste, cassava bagasse, banana, sugar beet cosset, kiwi fruit peel etc. have been investigated with SSF techniques for their potential to be used as substrates.
When applied to appropriate mass balances, it is possible to predict the utilization of substrates and the yield of individual products. Fermentation media for citric acid biosynthesis should consist of substrates necessary for the growth of microorganism, primarily the carbon, nitrogen and phosphorus sources. Moreover, water and air can be included as fermentation substrates (Singh et al. 1998; Haq et al. 2001).
The objective of this project is to develop SSF process for enhanced citric acid production on industrial scale to meet the increasing demand of local industry. The study of factors that limit the production and accumulation of citric acid in SSF and metabolic engineering to minimize their effect will provide cutting-edge work in the utilization of molasses using a distinctive approach that combines elements of applied biology, process engineering, and evaluation of resource, environmental and economic efficacy.
Development of indigenous biotechnology will provide a route to simultaneously create new employment opportunities for skilled and unskilled labour, and decrease dependence on imported citric acid, an increase in yield and decrease in fermentation recovery costs could change the situation as we can save a huge amount of foreign exchange by commercializing the production process for this most versatile industrial chemical.
No comments:
Post a Comment