Isolation And Identification Of Lactic Acid Bacteria From Dzo Yoghourt In Gannan Pasturing Area Of China And Its Role In Fermentation

Gannan pastoral area is located in the Gansu Province of southwest China,which is also seated in the northeast edge of Qinghai-Tibet plateau and the loess plateau in the western excessive area.It has vast grassland,which is one of "Five Major Pastoral Areas"of China.It is 2960 meters above sea level.Tu,Hui,Tibetan and other ethnic minority gather in this pastoral area,the unique ethnic culture gave birth to the ethnic characteristics of fermented dairy products.Gannan pastoral area has a wealth of Yak,Dzo resources,Dzo is the hybrid of Yak and ox,which keeps the dual advantage of Yak's resistance to cold weather and ox's high mammary volume.Local minority have been using the traditional methods to make yogurt from generation to generation,the made-up yogurt is solidified with golden surface,and the section is white and smooth,the smell is good.Yogurt can maintain the original hardness and consistency,the surface is covered with golden cream,the taste is pure with rich flavor,the quality of texture is good.After long time storage can still ensure the quality.All these demonstrate that there are excellent resources of lactic acid bacteria in Dzo yogurt,which this study is based on.The main research contents and results are shown as follows:1.The physicochemical characteristics of Dzo yogurt and molecular analysis of microbial community in Dzo yoghurt by PCR-DGGE.Firstly,the yoghurt was analyzed for different microbial indicators(lactic acid bacteria,yeasts and moulds,coliform bacteria).The results showed that number of LAB and yeast were about 107CFU/mL and 103 CFU/mL(only in Gannan area);The coliform and moulds were not detected in all Dzo yogurts.The protein,fat and ash contents of Dzo yogurt were 4.39%-5.00%,2.03%-4.32%and 0.46%-0.78%.The microbial communities structures in traditional Dzo yoghurt were analysed by using the PCR-DGGE fingerprint technique.The results showed that bacteria community similarity were 40%-75%among Dzo yoghurt samples from different regions,the diversity of Dzo yoghurt from Gannan area is more abundant than Hezuo area.The Shannon's diversity is 1.7568-2.1803,it is demonstrate that the diversity of GN-3 is most abundant,and the HZ-3 is adverse.By sequencing V3 region of 16s rDNA,bacteria community were composed of Lactobacillus fermentum,Lactobacillus delbrueckii subsp.bulgaricus,Streptococcus thermophilus,Lactobacillus delbrueckii,Uncultured Bacillus,Uncultured bacteria and Uncultured Lactobacillus.2.Isolation and identification of lactic acid bacteria.Firstly,96 LAB were isolated from Dzo yogurt,seen through microscope,It includes 53 strains ofcocci and 43 strains of bacillus.By analysis of 16S rRNA gene sequence,the 4 strains which selected by the fermentation test were identified as Enterococcus faecium,Streptococcus thermophilus,Lactobacillus delbrueckii ssp bulgaricus and Lactobacillus fermentum.3.The fermentation performance of LAB which isolated from Dzo yogurt.The curd time of 4 kinds of lactic acid bacteria is not the same.The PQ-4 spend the longest time curding.PG-3 need the shortest time and about 4.5 h to reach,PQ-8 and PG-30 can curd with 6.5 h and 7 h respectively.Through the fermentation experiment found that PQ-4 achieve the highest pH when it curds.When the PQ-8 curd,storage under 4°C after 1 day or 7days,there was no significant difference of pH,which can significantly reduce the acidification.The pH value of PG-13 is minimum when it curds,and at the end of the storage had reached its lowest pH.The color of yogurt made by PG-13 changed acutely after production and during storage.CFU values of PQ-4 after fermentation were higher than others,and viable counts of PG-13 were lowest when it curds and after 14 days of storage.The yogurt fermented by PG-13 resulted in higher value of G' and G" when it curds,storage under 4°C after 1 day,the value of G' and G" of PQ-8 was higher and higher,show the highest value of G' and G" at the end of storage.4.The study of the solidify yoghurt with good flavor.Combining PQ-8(isolated from Dzo yogurt)and MB2-1(isolated from Sailimu yogurt)to ferment the yogurt.PQ-8 and MB2-1 were combined(1:1,1:1,2:1 ratio)to produce yogurt.Through combination,the curd time was shortened,and the viable counts were promoted compared with single bacteria.Combination 2:1 show the highest WHC and pH,and the highest value of G' and G" during the process.The results of sensory evaluation reveal that the highest flavor score own to combination 1:2 and 1:1.By means of combining LAB,the counts of diacetyl content had obvious improvement,almost close to the PQ-8 single fermentation,far higher than that of MB2-1 single fermentation.Above all,two kinds of bacteria grew well together,they has obvious synergy effect:PQ-8 continues to produce flavoring materials,and show good rheological properties,and high protease activity of MB2-1 can shorten the time of curd and make yogurt production time to suit the requirements of industrial fermentation.