Study On Low-lactose Milk. Manufacture Using Endocellular β-Galactosidase From K. Lactis

Nutrient-rich milk and dairy products, easy digestion and absorption, known as "whiteblood ", is the best natural food. China dairy industry has developed rapidly in recent years,but there is still a huge gap in average level compared to the world. And lactose intolerance isone of the most important reason for restricting the development of China dairy industry.Using β-galactosidase hydrolysis of milk can generate glucose, galactose and the "bifidusfactor" galactooligosaccharide, which Not only solve the problem of lactose intolerance, butalso increase the nutritional value of milk. The main research of this thesis is production oflow-lactose milk with β-Galactosidase catalyzed from K.lactis. This dissertation focuses onthe optimization of fermentation conditions for K.lactis and preparation of β-Galactosidasecrude enzyme solution, detection of galactooligosaccharide, low-lactose milk productionprocess optimization and quality control of low-lactose milk.This paper deals with the cultural condition by orthogonal experiments for production ofβ-D-galactosidase using yeast Kluyveromyces lactis and its properties. The results showedthat the optimum temperature was28℃, pH value was5.7, inoculation size was3%, lactoseaddition was2%, liquid volume was40mL, rotation speed was80r/min and fermentationtime was22h. Under these conditions, the specific activity would reach24.54U/mg. Theoptimum temperature, pH value and substrate concentration for β-D-galactosidase was50℃,6.6and0.6mol/L respectively. The β-D-galactosidase was found to have high stability at pH6.2-7.0under40℃. The enzyme was activated by metal ions of Mn²⁺, Mg²⁺and Zn²⁺, butstrongly inhibited by the metal ions of Cu²⁺.On the basis of single factor experiment, the best hydrolysis technique parameter wereacquired by response analysis in which the galactooligosaccharide synthesis rate was themain indicator. The results showed that when the reaction temperature was45.3℃, enzymeconcentration was2.5mL and reaction time was69.7min, the maximumgalactooligosaccharide yield was6.15%. The lactulose content in low-lactose milk was158.17mg/L determined by lactulose kit. By measuring the specific activity of lipase and protease in β-D-galactosidase crudeenzyme solution, the degradation rate of fat and protein in low-lactose milk was controlled.The results showed that when the temperature was45℃, the specific activity of protease andlipase were5.41U/mg and22.06U/mg respectively. The results indicated that production oflow lactose milk using β-D-galactosidase from K.lactic could cause excessive degradation ofprotein and fat in low-lactose milk, and it could also enhance the milk flavor. This canprovide theoretical data for industrial production of low lactose milk.