Citric acid is an organic acid and is highly used as a commercial product. The production of citric acid in Indonesia has not met its demand, a cheap and easy method of obtaining raw materials to boost its production is required. Submerged fermentation using Aspergillus niger is a prospective method of citric acid production. Microorganisms need substrate as a carbon source to produce citric acid. Rice husks have the potential to be used as substrate to produce citric acid because they contain high cellulose (28.6-43.3%). This research aims to obtain the optimum fermentation conditions of producing citric acid. Variations in the study were methanol concentration (1-3% v/v), sucrose concentration (0-10% w/v), substrate concentration (15-30% w/v), and agitation speed (100-400 rpm). The variations were determined by a statistical approach of Response Surface Methodology. During the fermentation process, pH and citric acid concentration were measured. The pH and citric acid concentration data was analyzed using Minitab. The optimal conditions of producing citric acid using rice husks were obtained. They were substrate concentration 22.42% w/v, sucrose concentration 10% w/v, methanol concentration 3% v/v, and agitation speed 400 rpm. These optimal conditions resulted in a citric acid concentration of 19.12 g/L in pH 2.34 in 5-day fermentation.
In today’s industry, about 80% to 90% of citric acid (C6H8O7) production is carried out using submerged fermentation method and Aspergillus niger as citric acid producing microorganism. The use of fermentation method provides advantages, including the utilization of agricultural waste as substrate. Rice husks can be used as substrate in citric acid fermentation because they contain high cellulose and hemicellulose. However, agricultural waste also contains lignin which can interfere with the hydrolysis of cellulose and hemicellulose into reducing sugars, therefore optimisation of pre-treatment process is needed to separate lignin in rice husk. In addition to the optimisation of pre-treatment process, optimisation of fermentation parameter variations to obtain maximum citric acid yield is necessary. Therefore, the aim of this narrative review is to desribe various pre-treatment methods that can be done to remove lignin from rice husks, and to provide information on the benefits of using the Central Composite Design method in the optimisation of various fermentation parameters.
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