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PROJECT TOPIC: VALORIZATION OF RICE HUSK FOR CITRIC ACID PRODUCTION USING ASPERGILLUS NIGER BY SOLID STATE FERMENTATION.

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CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Chemistry and Occurrence of Citric Acid

Citric acid or 2-hydroxypropane 1, 2, 3-tricarboxylic acid is an alpha-hydroxyl acid with a three carbon skeleton, which has three carboxylic acid groups (COOH), and one hydroxyl group (Max, et al., 2010), with molecular formula of C6H8O7 and molar mass of 192.12 g/mol., it’s also known as p-hydroxyl tricarboxylic acid is a weak organic acid occurring in high concentrations in citrus fruits (Anastassiadis and Rehm, 2006). It is ubiquitous in nature as it serves as an intermediate in citric acid cycle, where by carbohydrates are oxidized to CO2. The widespread presence of citric acid in animal and plant kingdom is an assurance of its non- toxic nature and it has been used as an acidulant in manufacture of soft drinks, jams and confectioneries (Anastassiadis and Rehm, 2006). Citric acid is found as colorless translucent crystals, odorless, with strongly acid taste. The solid has density of 1.66 g/mL, melting point of 153°C and boiling point of 175°C. It is highly soluble in water to give an acidic, sour tasting solution (Pratiti, 2013). Citric acid is found in large quantities in citrus fruits with lime having the highest concentration of the acid (Pratiti, 2013). In addition to fruits, citric acid is found in all animal species. The citric acid cycle is vital in the oxidation of sugars and acetate to CO2 and water, releasing energy for physiological functions (Pratiti, 2013). The chemical structure of citric acid is presented in Figure 2.1


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