| Whey precipitation often occurs during storage and transportation of fermented dairy products,which seriously affects the quality and market sales of dairy products.In order to improve the quality of fermented yogurt,different stabilizers,thickeners and other chemical additives are added to fermented yogurt products.Therefore,the development of natural food additives has attracted more and more attention in recent years.Lactic acid bacteria(LAB)exopolysaccharide(EPS)has the effect of enhancing the viscosity,water holding capacity and silkiness of fermented yogurt,and has natural and organic characteristics.Therefore,as an additive for yogurt products,it can enhance the natural,organic properties of yogurt products and improve the added value of its products.Kefir grains is the fermentation starter of natural dairy product—kefir,which is composed of complex microbial phase.Kefir has a significant probiotic health effect on the human body,and 25%of its dry weight is composed of EPS secreted by microorganisms.The EPS-producing strains in the kefir grains can be screened out and isolated by experiments,and can be directly used as compatible strains in the development of composite fermentation starters for yogurt products.Using the composite strain starter as a fermentation starter,through fermentation production of yogurt,the purpose of improving the quality of yogurt,enhancing the viscosity,stability,water holding capacity of yogurt and increasing the silky mouthfeel can be achieved.In this study,the main constituent bacteria in the kefir grains-LAB were isolated and identified;secondly,the EPS-producing strains were screened from the isolated strains,and the fermentation of single strain yogurt products were carried out to analyze and evaluate the quality of fermented yogurt products,on this basis,the process of fermentation production of EPS of strain was preliminarily optimized.The physicochemical properties of the EPS were characterized,and the influence of the physicochemical properties on the yogurt product were evaluated.The high-quality EPS fermentation strain optimized by the above process was directly applied to the development of compound fermentation starter of pure yogurt strains,explore the effect of its compatibility strain on the quality of multi-strain composite fermented yoghurt,so as to lay the foundation for the further development of new yoghurt products with good flavor,stable physical and chemical properties and ethnic characteristics.The research results achieved by the experiments are as follows:(1)37 strains of LAB were isolated from six kinds of kefir grain collected from Xinjiang,Tibet,Inner Mongolia and Heilongjiang.Through morphological observation and 16S r DNA sequence analysis,37 strains of LAB were identified as:11 strains of Lactobacillus kefir,12strains of Lactobacillus casei,2 strain of Lactobacillus paracasei,3 strains of Streptococcus thermophilus,3 strains Leuconostoc mesenteroides,2 strains of Leuconostoc pseudomesenteroide,1 strain of Lactobacillus hirseri,1 strain of Lactobacillus perolens,2 strain of Lactococcus lactis subsp.lactis.The results indicate that Lactobacillus kefir and Lactobacillus casei were the dominant strains among the six kinds of kefir grains in different regions.37strains of LAB were screened for their ability to produce EPS.Through the MRS-S plate coating method,according to the morphology of the colony and the viscosity of EPS,firstly,6 EPS-producing strains were identified as candidate strains,including 1 strain of Streptococcus thermophilus(HST2),3 strains of Leuconostoc mesenteroides(KLM2,NLM5,XLM1)and 2strains of Leuconostoc pseudointestinal(YLPM1,YLPM2).Secondly,a single-strain fermentation verification was performed on the EPS-producing strains,through comprehensive evaluation of acidity,water holding capacity,acetaldehyde and diacetyl mass concentration,viscosity,sensory and other aspects,an EPS-producing strain XLM1 with excellent fermentation performance was determined.(2)First,under the same culture conditions(liquid volume 40 m L,inoculation volume3x10~6 cfu/m L,temperature 30?,cultured for 24 h),9 kinds of EPS production medium were screened for testing,among the 9 kinds of media,ammonium sulphate sucrose medium(LMM)showed the best performance and the highest yield of EPS(XLM1-EPS)was 12.04 mg/m L.Secondly,for ammonium sulphate sucrose medium LMM,the EPS fermentation process conditions were re-optimized,the optimization results were as follows:medium LMM,liquid volume 80 m L,inoculation volume 3x10~6 cfu/m L,temperature 30?,cultured for 96 h,the fermentation yield of XLM1-EPS was 25.01 mg/m L,which was 2.07 times the output of EPS in ammonium sulfate sucrose medium LMM under initial screening test conditions.The physicochemical properties of XLM1-EPS were tested and analyzed.Firstly,the XLM1-EPS was analyzed by Fourier transform infrared spectroscopy,and it was found that there were characteristic absorption peaks of polysaccharides such as hydroxyl group and pyranose ring.Secondly,gas chromatography and thermodynamic tests were performed on XLM1-EPS,the results of gas chromatography showed that XLM1-EPS consisted of glucose;thermodynamic test results showed the degradation starts at 225?,its melting point is 355.7?,and the enthalpy change value(H)required for melting is 676.5 J/g.These research results show that XLM1-EPS can withstand the high temperature in the dairy processing industry and has the potential to be applied to the thermal processing of food.Thirdly,scanning electron microscope observation revealed that XLM1-EPS has a smoother surface morphology than xanthan gum,indicating that it can be used to make food films;for the dairy industry,XLM1-EPS used as a dairy product additive,it can give dairy products a smoother mouthfeel and a more delicate tissue state.Finally,the results of emulsification property and flocculation analysis showed that XLM1-EPS has similar flocculation characteristics as xanthan gum,but is better than locust gum and guar gum,and has higher emulsifying activity compared with locust gum.It indicates that XLM1-EPS has the potential to be used as flocculant and emulsifier in food industry.The rheological test results indicate that XLM1-EPS has a higher apparent viscosity in high concentration,low and medium p H conditions,skim milk,it can effectively improve the texture of fermented dairy products with a p H between 4.0 and 4.5,which is of great significance for improving the quality of fermented dairy products.(3)The strain XLM1 with good fermentation performance and excellent characterization of the physicochemical properties of EPS was compounded with Lactobacillus kefir XL1 and Kluyveromyces marxianus FXJ1 in the laboratory,the purpose of the compounding was to develop a compound culture starter for ropy yogurt.After repeated fermentation experiments,the best compatibility ratio between strains was obtained,and the high-quality ropy yoghurt with ethnic characteristics was produced.In the compound starter of ropy yogurt,the optimal compatibility ratio between the strains is:lactic acid bacteria:yeast=50:1,in which the ratio between lactic acid bacteria is:Leuconostoc mesenteroides XLM1:Lactobacillus kefir XL1=5:1;the optimal fermentation conditions for the composite strain starter are:natural p H,3x10~6cfu/m L inoculation volume,30?,fermentation for 16 h.Under the optimal fermentation conditions mentioned above,compared with the yogurt produced by fermentation without compatibility with the EPS-producing strain XLM1,the retention of water power during the storage period(21d)was maintained at 100%,and the titration acidity was only increased by 8.24°T,the acidification capacity was weaker,which can effectively increase the shelf life of products;the number of viable bacteria was kept at 10~8 cfu/m L,and the sensory score of fermented yogurt was maintained above 85,it indicates that the quality of yogurt products is stable during storage.By comparing the rheological characteristics of the control group and the experimental group during storage,the results showed that the compound culture starter of EPS-producing strain XLM1 will significantly increase the content of EPS in fermented yogurt,which enhances the elasticity,viscosity,and cohesive ability of fermented yogurt,enables fermented yogurt to withstand a certain external force during long-distance transportation,and helps prevent the phenomenon of whey precipitation.Secondly,the compound culture starter of EPS-producing strain XLM1makes the fermented yogurt maintain a stable solid-like state during storage,and the smoothness of the yogurt on the taste is significantly improved.Therefore,the results of this study prove that the strain XLM1 has excellent application potential and economic value in the industrial production of dairy products. |
PDF Full Text Request