Isolation,Identification And Products Characterization Of Strain EW-220 Producing Thermostable β-galactosidase

220 strains of bacteria were isolated from soil in Tibet and 23 were isolated as the thermostable β-galactosidase producer. Among them a bacterial strain producing a large amount of thermostable β-galactosidase was selected. By testing morphologic and physiological-biochemical characteristics, the strain was identified as Bacillus sp.. We call it EW-220.The cells of the strain are Gram-positive and the shape of single cell is pole. The strain has endospore shape of which is oval and the sporangium is swelling. EW-220 can grow in the temperature range from 45 ℃ to 70 ℃ and its salt-resisting is about 10%. It can grow in the pH range from 5 to 8 and its optimum pH is 5.5-7.0. It can make use of D-glucose, D-xylose, D-asabinose, D-mannose to produce acid. It can decompose amylum and glutin. It can deoxidize nitrate. And, the M.R and V.P tests are all negative. It can grow in the culture medium with sodium azide.Studying on the effects of carbon sources and other influence factors optimized the β-galactosidase fermentation condition for strain. The optimal medium composition for strain growth is(%) Lactose 1%, Glucose 1%, Yeast extract 1%, pepton1%, NaCl0.5%, pH 6.8.The optimal medium composition for β-galactosidase producing is (%) Lactose 1%, Glucose 1%, Yeast extract 0.5%, peptone 1%, Toween-80 1%, pH 6.8. The optimal conditions for β-galactosidase producing are inoculation for 10%, seeds age for 12h, culture medium volumes for 40mL in 250mL flask, initial pH7.0, cultivated at 50 ℃ with an aeration rate of 140 rpm for 48h. Then a maximum yield of 14U/mL was obtained.By studying on the supernatant of culture fluids, the enzymatic characteristics were concluded. The optimum temperature and pH for β-galactosidase activity was 60℃ and pH6.5.It was stable in the pH range 5.5-8.0 and had good thermal stability. The enzyme retains 100% of activity after 48h of heat treatment at 50 ℃ and half-lives were 18h at 55 ℃.It also showed through hydrolysis tests of lactose in milk that β-galactosidase was exhibited strong ability to hydrolyze lactose in milk and reached above 50 percent of hydrolysis ratio by reacting with 1 U/mL enzyme into milk holding under 55 centigrade for 2.5 hours.