| Hurood is a traditional dairy product made of raw milk through natural fermentation,heating and stirring.At present,the production of traditional hurood has not been industrialized,and most of them are mainly produced in small workshops,which are easily affected by environmental microorganisms during the production process.However,there are few studies on the composition and changes of microflora in the production of traditional hurood.The isolation of lactic acid bacteria strains in traditional hurood and the elucidation of the changes in microorganisms during the production process are of great significance for the quality improvement and industrial production of hurood.In this study,by combining pure culture and non-culture sequencing methods,a total of 27 samples of raw milk,yogurt,jiaoke and hurood in the production process of traditional hurood were used for lactic acid bacteria isolation and microbial diversity research to explain the composition and changes of the microbial community during the production of hurood,the main research results are as follows:(1)A total of 319 strains of lactic acid bacteria were obtained from 27 samples by pure culture method,which were identified as 4 genus and 9species.Among them,the dominant bacteria in raw milk,yogurt,jiaoke and hurood are Lactococcus lactis,accounting for 52.4% of the total isolates,and also include Streptococcus thermophilus(15.7%),Lactiplantibacillus plantarum(11.0%),Lactobacillus helveticus(9.4%),etc.The viable count of lactic acid bacteria in the raw milk is 6～7 log CFU/m L,and the viable count in the yogurt after natural fermentation were increased to 8～9 log CFU/m L.After heating,the lactic acid bacteria cannot tolerate high temperature,grow and multiply will be inhibited,so the count of viable bacteria in the hurood were decreased to 4～5 log CFU/m L.(2)Metagenome sequencing showed that the microorganisms contained in yogurt,jiaoke and hurood were mainly lactic acid bacteria.Among them,Lactococcus lactis has the highest relative content in raw milk(27.6 %),yogurt(98.2 %),jiaoke(75.9 %)and hurood(87.3 %)samples,which is consistent with the isolation and identification of lactic acid bacteria in the pure culture method.After the raw milk was naturally fermented,Lactococcus lactis became the dominant bacteria,resulting in a single bacterial structure and a decrease in microbial diversity.The microflora of raw milk was different from that of yogurt,jiaoke and hurood,while the microflora of yogurt,jiaoke and hurood was similar.(3)The microflora changes of raw milk,yogurt,jiaoke and hurood in each process stage showed that the microflora in raw milk was relatively complex,such as 5 bacteria: Meiothermus silvanus,Thermus scotoductus,Escherichia coli,Acinetobacter guillouiae and Acinetobacter johnsonii were all detected to have a certain relative content in raw milk.After natural fermentation,the relative content decreased significantly(P<0.05);The relative contents of Enhydrobacter aerosaccus,Macrococcus caseolyticus,Acinetobacter johnsonii and Klebsiella oxytoca in jiaoke were significantly higher than those in yogurt and hurood(P<0.05).After correlation analysis among all dominant strains,it is found that Lactococcus lactis may inhibit the growth and reproduction of bacteria from the environment,such as Acinetobacter guillouiae and Acinetobacter johnsonii.(4)The metabolic pathway research of microbial gene family shows that there are 13 metabolic pathways with significant differences among different types of samples.Among them,the metabolic pathway of6-hydroxymethyldihydropterin diphosphate biosynthesis is significantly different between raw milk and yogurt,jiaoke,hurood(P < 0.05),and is more concentrated in the samples after natural fermentation,It indicates that this pathway may be related to the natural fermentation in the process of traditional hurood.Among them,9 metabolic pathways are more concentrated in jiaoke,which may be related to microorganisms from the environment.Lactobacillus helveticus may be related to the metabolic pathway of(5Z)-dodec-5-enoate biosynthesis and galactose degradation I.In summary,this study analyzed the composition and changes of the microflora in the production process of hurood,and obtained valuable lactic acid bacteria resources,which laid a theoretical foundation for the process improvement of traditional hurood. |
