Isolation and identification of a protease-producing strain from alkekengi tofu

The commercially available alkekengi tofu was used as samples. First,the appropriate dilution times of the sample solution were screened.Pure culture was carried out on potato glucose plates and MRS plates. Then the pure culture strains were inoculated on skim milk medium.Taking transparent zone as index,casein was used as substrate to determine enzyme activity to screen and isolate the strain that producing protease. Then,naked eye observation,optical microscopic observation,scanning electron microscopy,physiological and biochemical reaction,ITS sequencing method were used to identify the bacteria. The results showed that the appropriate diluted concentration for 25 g sample was 10^-5.In the skim milk medium,under the condition of temperature of 28 ℃,aerobic training of 72 h,the strain produced a transparent circle and the enzyme activity was 9.6 U/mL. Macroscopic observation showed colony was ivory,villous,with concentric circle radiation,microscopic section spores single or in chain,sacnning electron microscopy observation showed the width of mycelium were 4~5.5 μm,and spore side were mostly(3.3~5.7) μm×(7.0~17.1) μm.According to the physiological and biochemical reaction results,refer to the Handbook of fungal identification,the strain belonged to Geotrichum candidum.The results of ITS sequencing showed homology of the strain on the potato glucose plates with Geotrichum candidum up to 100%.According to the growth curve,0~8 h was in the delay period,8~40 h was the logarithmic period,40~68 h was the stable period and after 68 h,the strain was in the decline period.