Studies On Screening Of Lactase-producing Strains, Optimum Of Liquid Fermentation And Application In Low-lactose Milk

It has been estimated that over 70ï¼…of world's inhabitants suffer from eitherlactose maldigestion or intolerance. The enzymeβ-D-galactoside galactohydrolase(β-galactosidase, E.C. 3.2.1.23, trivially lactase) hydrolyzes lactose, the milk sugar,into two moieties_glucose and galactose. The development of lactose hydrolyzedproducts presents one of the possible approaches to diminish the lactose maldigestionproblem.By using X-gal test and ONPG test, theβ-galactosidase activity quality andquantity were detected respectively, in order to screen the microorganism from soiland milk which produces large quantity ofβ-galactosidase. There are twenty-onestrains that appeared blue colour on the agar plate containing X-gal. And there arefive strains produced high quality and large quantity ofβ-galactosidase. And theβ-galactosidase activity of yeast Y6 is the best of these five strains.The fermentation culture medium of lactase-producing yeast Kluyveromyceslactis was optimized by orthogonal design and single-factor experiments. Theoptimal culture medium for fermentation of lactase-producing yeast Kluyveromyceslactis was shown as follows (ï¼…, W/V): lactose 8, glucose 0.5, yeast extract 0.7, urea0.15, KH₂PO₄ 1.5, MgSO₄ 0.1, MnCl₂ 0.01, pH 6.6～6.8. The optimum fermentationconditions on producing lactase were as follows: culture time was 18～20h,inoculation quantity was 2.5ï¼…(V/V), the volume of fermentation culture mediumwas 30mL/250mL. And the average productivity of lactase was 1.930 ONPG unitsper milliliter.The properties of the enzyme of lactase isolated from Kluyveromyces Iactis wasstudied using single-factor experiments. The optimal pH value and temperature of thelactase were found to be in the range of 6.4～6.6 and at 40～45℃, respectively. Theoptimal thermal stability exhibited at temperature of 40～45℃and in pH range of6.0～8.4. The enzyme was significantly activated by metal ions of Mn²⁺, Mg²⁺ andNa⁺, but inhibited by the metal ions of Zn²⁺ and Cu²⁺. K_m value of the lactase wasmeasured at 3.348mmol/L and 16. 873mmol/L when ONPG and lactose as substrate respectively,When milk was treated with lactase isolated from Kluyveromyces lactis at 40℃for 5 hours, with dosage of 10ï¼…, the degree of hydrolysis for lactose can reach 70ï¼….