Isolation And Application Of Cassava Raw Starch Digesting Glucoamylase Producing Strains

The global oil reserve is limited, industrial alcohol as a renewable energy is becoming more important. Cassava starch fermentation for industrial alcohol has a broad prospects for development, but needs high energy input in the gelatinization and saccharification process, thus hinder the development of starch alcohol.Raw starch fermentation technology is the fundamental method to reduce energy consumption. Raw starch digesting glucoamylase(RSDG) is the important support for the raw starch fermentation. The efficiency to screen microbial strains that produce cassava raw starch glucoamylase is the key of obtained RSDG.1. A novel method for isolating raw starch digesting enzyme producing microorganisms was established, including new methods to sterilize raw starch and the method to prepare the screening plates.Raw starch sterilizing method:Raw starch was sterilized either in 4% NaC102 or 0.36% HgCl for 20 min, or in 3% H2O2 for 20 min, followed by washing in sterilizing water three times. The color and quality of the sterilized raw starch was unchanged, and this sterilization process was easy and highly efficient. The plates prepared from the sterilized raw starch were smooth and uniform and suitable in the screening of the RSDG microbes.The method for preparing the plates containing the sterilized raw starch was optimized. The best starch concentration was 1.5%, the optimal temperature of the autoclaved medium when the raw starch was added in was 75℃, and the incubation time for the plates after the microbes were plated on was 3-4 days. Under these conditions, the clearance zones around the colonies were the clearest. The clearance zones were further proved to be starch-free area by iodine staining.2. Aspergillus niger strain ITBB FC2 was isolated from decayed raw cassava root. This strain showed strong raw-starch-digrading-glucoamylase (RSDG) activity in liquid fermentation medium with 2% cassava flour. The crude enzyme had an activity of 495.04 U/mL and produced 970 mg/L of glucose in the 2% cassava flour fermentation medium when incubated at 37℃,200 r/min for 3 days, while it had an activity of 6330 U/g in solid fermentation medium(bran:water=1:1). The optimum pH was 4-5.2 mmol/L. Ca2+promoted the enzyme activity by 133.74%.3. Cassava raw starch fermentation for high concentration alcohol helps to save energy. Single factor and orthogonal experiments were performed to optimize the fermentation conditions for high concentration alcohol production from cassava raw starch. The optimized conditions were:material to water 1:2.5, enzyme dosage 315 U/g, yeast dosage 7500 cfu/mL, and fermentation time 5 d. The alcohol concentration of fermentation liquor reached 12.9% under the optimized conditions, while the content of the residual sugar was 0.38%, and the starch utilization rate was 73.86%. The main factors that influenced the alcohol production were fermentation time, raw starch digesting glucoamylase and yeast dosage.