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.

Amylase is an enzyme that is used in various industries such as food, paper, and textile industries. Amylase can be produced by microorganisms through fermentation. Rhizopus oryzae is one of microorganisms that can produce amylase through fermentation. Tapioca solid waste has a good potential as a carbon source for microorganisms for producing amylase since it contains high carbohydrate. Tapioca solid waste produced in Toba Regency, North Sumatera, reaches 126 tonnes per day, and the use is limited as compost and forage. It is possible to use tapioca solid waste as a substrate for amylase production thus providing added value to it. Fermentation was carried out for 7 days at 35 °C. After extraction, amylase was partially purified by ammonium sulfate at 40% and 60% saturation level followed by dialysis. Amylase activity was determined spectrophotometrically using dinitrosalicylic acid (DNS) method. Partially purified amylase in 60% ammonium sulfate fraction has the highest activity of 47.6278 U/mL and the highest yield of 97.51% with purity level of 0.653 times. Optimum condition of amylase activity was at pH 6.5 and temperature of 60 °C with 30.1797 U/mL activity. The ability of amylase in hydrolyzing starch was analyzed in various time duration (3, 6, 9, and 12 h) and various ratio of enzyme against substrate (1: 2, 1:5, and 1:10). The highest hydrolysis activity of amylase was 44.3778 U/mL, obtained from the ratio 1:10 of enzyme against substrate and 12 h of hydrolysis time.