Research On Fermentation Characteristics And Technology Of Tibetan Kefir Freeze-Drying Culture Starter

Tibetan Kefir?TK?,also known as Tibetan Kefir Grain,is a type of culture starter originating from Qinghai-Tibet Plateau.It contains various kinds of microorganisms,such as lactic acid bacteria,acetic acid bacteria,yeast,etc.As a traditional fermenting starter,TK can proceed lactic acid,acetic acid and alcohol fermentation simultaneously,and thus obtains yogurt containing lactic acid,CO₂,acetaldehyde,acetoin,and trace amounts of alcohol.In this research,bacterial strains with excellent fermentation performance were isolated from TK,for ease of identifying its category and studying its probiotic properties,and a sort of Directed Vat Set was produced based on Vacuum Freezing&Drying Technology.The optimum parameters of fermentation were confirmed through the response surface experiment,and applied on three different kinds of milk pretreated in distinct ways.The physicochemical properties,texture characteristics and flavors of the fermentation products were studied.The main conclusions of this research are as follows:First,after separating 28 Gram-positive bacterial strains from TK,physiological and biochemical identification and 16S rDNA sequence homology analysis was conducted on them,through which a lactobacillus casei G2 was screened out.It is characterized by faster acid-producing ability,stronger acid resistance?pH4.0⁶.0?,higher bile salt tolerance?concentration of bile salt?0.4%?,and stronger heat resistance?the optimum temperature of growth is 40?C?.In addition,it has certain inhibitory effect on the common colon bacillus.This research demonstrates that,the lactobacillus casei separated from TK possesses favorable probiotic properties and fermentation characteristics,and thus qualifies for an ideal starter of fermentation.Second,the TK freeze-drying culture starter and its fermentation technology were studied.By mixing lactobacillus casei with streptococcus thermophiles and applying Vacuum Freezing&Drying Technology,Directed Vat Set was produced successfully.Then,through the single factor analysis and response surface experiment,better parameters of technological process and condition were found:strain ratio=1.27:1,inoculation amount=3.3%,time of fermentation under 43.8?C=8.8 hours.After that,the acidity,lactobacillus colony counts and texture characteristics of fermented milk with different fermentation parameters were tested separately.It was proved that the fermented milk produced under the optimum technological conditions is characterized by white color,solid clot,uniform texture,smooth surface,rich flavor and delicate taste,without any precipitation and whey separation.Third,different kinds of milk pretreated in distinct ways were fermented by TK freeze-drying culture starter,and the physical properties and flavors of the milk were studied.This research compared three kinds of fermented milk,including pasteurized milk,ultra-high temperature instantaneous sterilization milk?UHT?,and membrane filtration milk?FM?,and analyzed them in various aspects:physicochemical properties,viscosity and rheological characteristics,TPA analysis,microstructure observation,free amino acid content,and transformation of volatile flavor substance.Each kind of fermentation product showed its own fermentation characteristics correspondingly,and thus provided useful reference for further study of fermentation application.