Study On Effect Of Yeast On Lactose Metabolism In Fermented Milk

Yeast plays an extremely important role as the main microorganisms in traditional fermented dairy products,not only endowed the products with special flavor,but also had effect to resist oxidation,lower cholesterol and repress the harmful germs.Developing new fermented milk with potential probiotic yeast can enhance the quality of the product and nutritional value.Saccharomyces cerevisiae and kluyveromyces lactis from the Tibet milk slag were selected as tested strains,and in this paper the effects of these two yeasts on fermented milk characteristics were studied,the changes of key enzyme and sugar content during mixed fermentation process by lactic acid bacteria and yeast were analysised.At last,the processing technology for the low lactose-fermented milk was preliminarily studied.By comparing two different fermentation ways that lactic acid bacteria alone fermentation and mixed fermentation with yeast,then the effects of different yeast on the growth of lactic acid bacteria,acidity and texture of fermented milk comparatively analyzed.The results showed that owing to increased the number of viable lactic acid bacteria in fermented milk the yeast can significantly promote the growth of lactic acid bacteria and increase the acidity of the fermented milk as well as the hardness,viscosity and consistency during fermented milk storage.Moreover,Kluyveromyces lactis had more significant effect on fermented milk’s characteristics than saccharomyces cerevisiae.Comparing the changes of key enzyme such as β-galactosidase,lactic dehydrogenase and changes of key sugar such as lactose,glucose and galactose in fermented milk during fermentation and storage can reveal lactose metabolism process.The research showedβ-galactosidase activity in fermented milk by lactic acid bacteria alone and mixed with saccharomyces cerevisiae and kluyveromyces lactis fermentation respectively reached 0.48,0.58 and 0.65U·mL-1 when the milk curd,and all decreased to 0.01 U·mL-1 after storage for two weeks.Lactic dehydrogenase activity in above different fermented milk respectively achieved to be 0.69,0.81 and 0.99 U·mL-1 when the milk curd,then decreased to 0.18,0.21 and 0.24 U·mL-1 respectively after storage for two weeks.Lactose,glucose and galactose contents in mixed fermented milk were detected by HPLC method,their recoveries were respectively achieved to be 91.30%,103.28%and 93.54%.Results showed degrees of lactose degradation were varied and thus changes in glucose and galactose content were different due to the participation of different yeasts.The descending trend of lactose significantly different,glucose and galactose contents in three different fermented milk also have changed significantly different.Main factors affecting degradation rate of lactose were studied by single factor and orthogonal experiments,which including inoculation amount of kluyveromyces lactis,fermentation temperature and fermentation time,the technical parameters were determined.The results showed that initial concentration of inoculated lactic acid bacteria was 7.0 lgCFU·mL-1,initial concentration of inoculated yeast was 5.5 lgCFU·mL-1,fermentation temperature was 32℃ fermentation time was 5.5h,under the above condition,the average lactose degradation rate was 25.45%.