Study On Microbial Community Flora And Their Correlation With Flavor In Milk Fan

Milk fan is a traditional fermented dairy production in Yunnan,China.It is popular by local consumers due to its rich nutrition and typical flavor.However,the traditional production process restricts the industrial production process of milk fan.This study focused on the contribution of microorganisms in milk fan to the formation of characteristic flavors.The combination of high-throughput sequencing and flavor analysis were used to analyze the variation of microorganisms and volatile compounds in the milk fan samples according to the region and the production process,and then the contribution of the microorganisms to the formation of aroma compounds was explored.On this basis,microorganisms with greater contribution to aroma compounds were isolated and identified,and freeze-dried bacteria powder milk fan starter for the purpose of aroma enhancement were prepared.It provides theoretical reference and data support for the industrial production and quality control of milk fan.The main results and conclusions of this study are as follows:(1)High-throughput sequencing technology was used to comprehensively analyze the composition of microorganisms in milk fan samples from different regions.The differences in the structure and composition of microbial communities and dominant genus of microorganisms in milk fan samples from different regions were found.The dominant bacterial genus mainly were lactic acid bacterias including Lactococcus and Lactobacillus,and the fungal genus mainly were yeasts such as Rhodotorula,Torulaspora and Candida.The content of microorganisms in milk fan samples varies from different regions.Lactococcus was predominant in samples K,Y,X and D,while in samples J and W,its relative abundance was lower than that of Lactobacillus.Rhodotorula had the highest relative abundance in samples J,X and D,but in samples K and W,the dominant genus was Candida.The analysis of volatile compounds also found the influence of the geographical factors on the compounds.As the characteristic aroma compounds of milk fan,the concentrations of butyric acid and caproic acid were highest in all samples,while the concentration of 2-pentanone,ethyl acetate,2-nonanone,ethyl caproate,ethyl caprylate,ethyl butyrate and propionic acid were greatly depended on the regions.Correlation analysis demonstrated that Lactobacillus,Rhodotorula,Lodderomyces and Debaryomyces were potential aroma-contributing genus in milk fan.(2)High-throughput sequencing technology was used to track the changes in the microbial community during the production of milk fan.The results showed an alternating trend of microbial relative abundance.The relative abundance of Lactobacillus and Issatchenkia decreased at first and then increased during the fermentation of sour juice,while increased at drying process of the milk fan,and became the dominant microorganisms.It showed same variation trend in the abundance of microbial communities,the metabolic function of the bacterial community,and the amino acid transport and metabolism function during the milk fan production.It was increased at first and then decreased with the fermentation of sour juice,and increased during the drying process of the milk fan.The main metabolic functions in bacterial communities was amino acid transport and metabolism which was related to the generation of various aldehydes.Lactobacillus and Rhodotorula were again identified as core aroma-contributing microorganisms by correlation analysis.(3)Ten Lactobacillus and seven Rhodotorula were isolated from milk fan and sour juice,in which Lactobacillus plantarum-4 and Rhodotorula gracilis-3 were the best aroma-producing microorganisms.These two strains were added to ferment sour juice to make milk fans,which verified their aroma-producing ability.Compared with milk fan produced by traditional sour juice,the addition of Lactobacillus plantarum could increase the concentration of esters,such as ethyl acetate,isoamyl acetate,ethyl caproate and ethyl caprylate,in milk fan,while the addition of Rhodotorula glucoides could increase the concentration of acetic acid,butyric acid,caproic acid,capric acid,ethanol and pentanol.The aroma-producing dominant bacteria and the acid-producing dominant bacteria were combined in 5 ratios to make milk fan starter to prepare milk fan.The results of sensory evaluation showed that the milk fan prepared by a ratio of 1:1 has the best sensory characteristics and excellent aroma intensity and acceptance.Although the milk fan made by this ratio didn’t have the highest aroma intensity,its aroma acceptance is the best,indicating that it is the best combination proportion.The artificial milk fan starter was prepared with this combination proportion,which was subsequently made into freeze-dried bacterial powder.Response surface optimization experiments proved that the best formula of the artificial milk fan lyophilized protective agent was: skimmed milk powder 20.09%,polyethylene glycol 4.42%,sodium glutamate 1.46%,Maltodextrin is 21.87%.The lyophilized protective agent prepared with this formula can effectively improve the survival ability of microorganisms in the freeze-drying process,and the maximum lyophilized survival factor is 0.952.By verifying the acid-producing and aroma-producing capacity of the artificial starter,the results showed that it not only shorten the time used for acid-water fermentation,but also effectively enhance the overall aroma of the milk fan.