Screening Of High Flavor-producing Lactic Acid Bacteria And Their Mechanism For Flavor Production

Diacetyl,acetaldehyde,and acetoin were important flavor compounds in fermented milk.Usually,these contents was added into fermented milk by external sources,to increase improving the flavor of fermented milk.With the pursuit of natural health,screening strains with flavor-enhancing abilities and studying their aroma production mechanism has becoming as a hotspot in fermented milk research.Therefore,the synergistic effect of the three compounds in fermented milk were detected,then lactic acid bacteria with flavor-enhancing abilities from traditional fermented dairy products were screened.Gene expression level in the mixed microbial system was compared.The main findings are as follows:(1)The threshold and optimal concentration of three key compounds in fermented milk of diacetyl,acetaldehyde,and acetoin in the fermented milk matrix were determined.It was concluded that the thresholds of diacetyl,acetaldehyde,and acetoin in the fermented milk matrix were 5.34±0.91 mg/L,15.39±1.07 mg/L,and 28.98±0.86 mg/L,and the three were the optimal concentration ranges are 6.65?9.12,25.87?35.45 and 37.34?49.86 mg/L,respectively.Using the Feller-model curve and ?-? plot method to study the synergy of aroma between the three compounds,synergies between three compounds were found.At the same time,the synergistic effect was the strongest when the ratio of diacetyl,acetaldehyde and acetoin was 4:16:32.Through the verification of electronic nose,concluded that synergistic effect was existed between the three compounds.(2)Lactic acid bacteria with flavor-enhancing abilities was isolated from traditional fermented dairy products.Results show that the strains SIT-17.B and SIT-20.S had higher production capacity of diacetyl,and also have good acetaldehyde and acetoin production capacity.SIT-17.B was Lactobacillus dellbrueckll subsp.Bulgaricus,SIT-20.S was Streptococcus thermophilus.The output of diacetyl,acetaldehyde and acetoin increased rapidly in fermention time,and the output of the three tended to be stable during the storage period.The whole genomes of SIT-20.S and SIT-17.B were sequenced,and the key genes involved in flavor formation were determined.It was found that the key enzymes between SIT-17.B and SIT-20.S could be Complementary relationship.(3)Different ratio of SIT-20.S coupled with SIT-17.B were prepared as starter.It was found when the strain ratio was 1:1,the production of diacetyl reached the highest,which was 53.90±3.95mg/L,the highest yield of acetaldehyde was 15.01±1.14 mg/L,and when the strain ratio was 10:1,the highest yield of acetoin was 91.93±5.23 mg/L.The fermented milk of strain ratio of 1:1 and 10:1,and SIT-20.S and SIT-17.B single bacteria fermentation were selected as samples.Transcriptome sequencing of gene expression levels.It was found that the upregulation of 6-phosphate fructose kinase between complex fermentation and separate SIT-17.B fermentation samples greatly increased the production of pyruvate(a key intermediate product in the production of diacetyl,acetaldehyde and acetoin),thus promote the production of three compounds.Between mixed fermentation and separate SIT-20.S fermentation samples,threonine synthase in SIT-17.B was up-regulated,which greatly increased the synthesis of threonine,while alcohol dehydrogenase in SIT-20.S was down-regulated,which makes it difficult for acetaldehyde converted to ethanol,thereby ensuring the production of acetaldehyde.Considering the aroma synergy research results and strains mixed fermentation results,it was found that the content ratio of acetaldehyde and acetoin in the compounding ratio of 1:1 is consistent with the synergy ratio of acetaldehyde and acetoin,and also in line with the optimum concentration range in yogurt matrix,so,flavor enhancing was favorable at this strain ratio.