Studies On Dough Fermented By Aroma-producing Yeast And Aroma Characteristics Of Bread

Aroma-producing yeasts have been widely used in wine, vinegar, fruit wine, rice wine, soy sauce and other traditional fermented food, owing to its unique flavor characteristics, but less in bread. After the long-time exploration, Mellanchun liquor distilled grains and Chengde wine starter were chosed as experimental samples. This study was aimed to isolate, screen and identify the yeast starins which can produce aroma components. Finally the doughs fermented by different aroma-producing yeast were used to make bread. The main results of study were as followed:A total of 6 yeast strains with producing aroma were selected from 2 samples through flavor smelling and ester titration. From 26 S rDNA sequence analysis results, 2 strains were identificated as Wickerhamomyces anomalus, 3 strains were Pichia fabianii and 1 strain was Pichia kudriavzevii. Comparing flavor characteristics and intensity of fermentation broth, 1 strain of W. anomalus and another strain of P. kudriavzevii were chosen repectively for follow-up application analysis. W. anomalus fermented broth had strong ester and bouquet aroma, while P. kudriavzevii obviously showed bouquet, ester and fruit aroma.The growth characteristic of yeast strains were investigated, the optimal temperature was 28~30°C with optimal pH 5~6. Glucose and sucrose both were the best carbon source for them. The two yeasts grew into stationary phase after 18 h both. Results of Duchenne tube and CO2 weightlessness showed that compared with dry yeast, two aroma-producing yeasts started leaven slowly with low fermentation power. W. anomalus fermentation ability is slightly higher than P. kudriavzevii. Aroma-producing yeasts should be applied with dry yeast to bread-making.The aroma-producing property was researched. From GC- MS analysis, 6 main aroma components of fermented broth and dough were included ethanol, isoamyl alcohol, phenethyl alcohol, ethanol, ethyl acetate, isoamyl acetate and phenylethyl acetate. After characterization of flavor intensity in fermented dough by GC-O, results indicated that 6 main alcohol and ester aroma substances in fermented dough had floral and fruity characteristics with aroma activity. The aroma intensity of ethyl acetate was 4.7, which was the highest value in WAD, while isoamyl acetate showed the highest aroma intensity 5 in PKD. These two esters devoted largest contribution to flavor of their fermented dough, respectively.Content of ethyl acetate in WAD and isoamyl acetate in PKD were selected as index of fermented dough respectively. It was determined that the effect of 4 single factors including DY, shaking speed, supplement carbon source(sugar) addition and fermentation time on aroma content in dough. Then response surface experiment of four factors-three levels were conducted.The optimal fermentation conditions for two aroma-producing yeast were DY = 420, shaking speed 160 r/min, the added amount of sugar 7.3(g / 100 g dough) and fermentation time 24 h.Making bread WAB and PKB with adding WAD and PKD, gas-producing ability of bread dough was increased. Compared with dry yeast bread(DYB), WAB and PKB had larger specific volume with hardness decreasing, and WAB is superior to PKB on specific volume and hardness. Sensory evaluation showed that except color, WAB and PKB achieved higher score in appearance, taste, flavor and overall acception, especially in flavor. From the description, WAB bread core had light pleasant fresh and fruity aroma, while PKB showed attractive sweetness and fruitness flavor. Revealed by GC-MS analysis of volatile flavor substances in bread, adding fermented dough improved the varieties and content of flavor substances in different degrees, and PKB had greater increase. Flavor compounds which increased most significantly were alcohol, isoamyl alcohol, phenylethyl alcohol and ethyl acetate, especially isoamyl acetate and phenylethyl acetate which DYB lacked. These substances also showed different aroma intensity in GC-O analysis, which were aroma active compounds and contributed to bread flavor. Compared with WAB, PKB showed better flavor advantage.