Thesis Abstract

Abstract
Rice husk is an abundant agricultural byproduct that has the potential to be valorized for the production of high-value products such as citric acid. In this study, the production of citric acid from rice husk using Aspergillus niger via solid-state fermentation was investigated. Aspergillus niger is a well-known citric acid producer and solid-state fermentation offers several advantages for the production of citric acid from lignocellulosic materials like rice husk. The process involves the breakdown of the complex lignocellulosic structure of rice husk into simpler sugars which can be utilized by Aspergillus niger for citric acid production. Various process parameters such as moisture content, pH, temperature, and incubation time were optimized to enhance citric acid production. The results showed that citric acid production was significantly influenced by these parameters, with the highest citric acid yield obtained under optimized conditions. Furthermore, the use of different pretreatment methods for rice husk such as acid or alkali treatment was studied to improve the accessibility of the substrate for enzymatic hydrolysis and citric acid production. The results indicated that pretreatment of rice husk had a positive impact on citric acid production due to increased sugar availability. Additionally, the effects of nitrogen sources and supplementation on citric acid production were also evaluated. Nitrogen is an essential nutrient for microbial growth and citric acid production, and its availability can significantly affect the yield of citric acid. Various nitrogen sources such as urea, ammonium sulfate, and yeast extract were tested to determine their impact on citric acid production. The results showed that nitrogen supplementation positively influenced citric acid production, with yeast extract being the most effective nitrogen source. Overall, this study demonstrates the feasibility of valorizing rice husk for citric acid production using Aspergillus niger via solid-state fermentation. The optimization of process parameters, pretreatment methods, and nitrogen supplementation can enhance citric acid production from rice husk, thereby providing a sustainable and eco-friendly approach for the production of this valuable organic acid.

Thesis Overview

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