Preparation And Flavor Study Of Selenium-enriched Lactic Acid Bacteria Fermented Hovenia Dulcis Nutrient Solution

In this experiment,the microorganisms on the surface of Hovenia dulcis were isolated,purified and identified.A series of screenings were carried out on the identified lactic acid bacteria,and the fermentation strain was finally screened.The growth characteristics and selenium-enriched fermentation characteristics of the fermentation strain was studied and its safety was evaluated.The Se-enriched lactic acid bacteria fermented Hovenia dulcis nutrient solution was prepared by fermentation strains and its flavor was studied.The main conclusions are as follows:（1）Four kinds of lactic acid bacteria were isolated and identified from the surface of Hovenia dulcis,a total of 39 strains,including Lactobacillus dextrin,Leuconostoc enterococcus,Enterococcus faecium and Bacillus coagulans.The acid-producing ability,selenium-tolerant ability,acid-tolerant and bile-salt-tolerant ability and selenium-enriched rate were used as the indicators to screen the identified lactic acid bacteria.Finally,the strain No.12 was identified as the fermentation strain.The fermentation strain could reduce the p H of Hovenia dulcis solution（1.23+0.05）and could tolerate sodium selenite solution of 10ug/m L;its concentration of viable bacteria after treatment with p H 2.5 acid and 0.3%bile salt for 3 hours was more than 1×10⁶ CFU/m L;its optimum growth temperature was 35°C and it entered the logarithmic growth phase after 3 hours of culture,entered the stable phase after 15 hours of culture,and its culture time should be controlled at 15-18 h.（2）In the safety evaluation study,the fermentation strain was sensitive to five different antibiotics;no hemolytic activity;nitrate reductase activity and amino acid decarboxylase activity were negative.（3）In the study of selenium-enriched fermentation characteristics,the influence of four factors on the selenium enrichment/OD value of fermentation strain was as follows:the initial mass concentration of Na₂Se O₃>incubation time>incubation temperature>inoculation amount.The optimum selenium-enriched fermentation conditions of the fermentation strain were as follows:inoculation amount 2%（v/v）,initial mass concentration of Na₂Se O₃ 8 ug/m L,incubation temperature 35°C,incubation time 18 h.（4）In the study on the preparation of Se-enriched lactobacillus fermented Hovenia dulcis nutrient solution,the influence of four factors on the fuzzy comprehensive score of Se-enriched lactobacillus fermented Hovenia dulcis nutrient solution was as follows:inoculation amount=material-liquid ratio>incubation time>incubation temperature.The optimum technological conditions for the preparation of Se-enriched lactobacillus fermented Hovenia dulcis nutrient solution were as follows:inoculation amount 4%（v/v）,material-liquid ratio 1:2,incubation temperature 35°C and incubation time 30 h.（5）In the study of the flavor of Se-enriched lactic acid bacteria fermented Hovenia dulcis nutrient solution,the content of aroma substances after fermentation of the nutrient solution increased compared with that before fermentation.Nitrogen oxides and sulfides are the characteristic aroma substances of the nutrient solution.The formation of taste in the fermentation process of the nutrient solution can be divided into three stages.The difference of the nutrient solution taste in different fermentation time may be mainly caused by different acid taste.