To restrict microbial growth during enzyme hydrolysis; an Erlenmeyer flask (114) in connection with the vessel (112) for preparing ethanol by mixing 20 g/l of pre-treated rice hulls from hydrolysis with 200 ml of citrate buffer (pH 5.0 ± 0.2, 50 mM), followed by sterilization for 15 minutes at 15 psi (121°C) with rice hulls soaked in citrate buffer and supplemented with cellulase at a substrate to enzyme ratio of 1:5 (20 g pretreated substrate: 100 ml crude cellulase), inoculating immobilized yeast cells at various concentrations, fermented and centrifuged to analyze the ethanol concentration.