Studies On The Microbial Community Structure Of Raisin And Apple Spontaneous Fermentation Sourdough And The Baking Quality Of Sourdough Bread

This study was used spontaneous fermentation sourdough made from raisin and apple for bread production. The microbial community structure was analyzed, main lactic acid bacteria and yeast were isolated and identified. The spontaneous fermentation sourdough was use to replace ordinary commercial yeast and use optimise production of spontaneous fermentation sourdough bread. The advantage of spontaneous fermentation sourdough in bread baking properties, nutritional properties, flavor characteristics and storage properties were investigated. Also the improvement effect of spontaneous fermentation sourdough on bread specific volume, texture, free amino acids, volatile flavor, moisture migration, hardening and staling during storage were studied.The p H and TTA of two kinds of spontaneous fermentation sourdough made from raisin and apple were measured with p H meter, respectively. The number of lactic acid bacteria and yeast in them were counted. The results showed that the average p H of the two kinds of spontaneous fermentation sourdough were 5.08 and 3.88, with corresponding TTA values of 7.70 m L and 12.32 m L, respectively. The mean value of lactic acid bacteria was 6.67 log CFU/g and 7.92 log CFU/g, while for yeast 4.93 log CFU/g and 5.84 log CFU/g, respectively. The ratio of the lactic acid bacteria to yeast was close to 100: 1, hence steady state.The lactic acid bacteria and yeast in the spontaneous fermentation sourdough were isolated and identified by 16 S r DNA and 26 S r DNA D1/D2 amplification and sequencing method, respectively. The microbial community structure was also analyzed. The results showed that the lactic acid bacteria in raisin spontaneous fermentation sourdough were Lactobacillus sake and little amount of Lactobacillus plantrum and Pediococcus pentosaceus, while in apple spontaneous fermentation sourdough were Lactobacillus plantrum. For yeast, one strain of Saccharomyces cerevisiae was found in the two kinds of spontaneous fermentation sourdough.The amounts of spontaneous fermentation sourdough added and proofing time were varied to produce bread, and the baking properties of bread were determined. The results showed that combination of these two process parameters produced different effects on specific volume, hardness and sensory characteristics of bread. When spontaneous fermentation sourdough was added at a fixed amount, at the shortest proofing time, the specific volume increased, but with the increase in the time, the specific volume become stable and unchanged, while the hardness of bread decreased, then increased. When proofing time was fixed, at the lowest spontaneous fermentation sourdough percentage, the specific volume increased, but with the increase in the spontaneous fermentation sourdough percentage, the specific volume decreased, while the hardness of bread decreased, then increased. The maximum specific volume of both raisin and apple spontaneous fermentation sourdough bread occurred at 30% spontaneous fermentation sourdough addition with proofing time of 6 h and 8 h, respectively. The minimum hardness occurred at 30% spontaneous fermentation sourdough addition with proofing time of 6 h. The samples with the best sensory evaluation appeared at 30% spontaneous fermentation sourdough addition with proofing time of 6 h. From response surface, we can learn the optimum combination of spontaneous fermentation sourdough added and proofing time of raisin spontaneous fermentation sourdough bread was 33.39%-6.27 h, with corresponding specific volume and hardness of 7.26 m L/g and 159.07 g, respectively. While the optimum combination of spontaneous fermentation sourdough added and proofing time of apple spontaneous fermentation sourdough bread was 34.08%- 6.41 h, with corresponding specific volume and hardness of 7.57 m L/g and 169.01 g, respectively.Using HPLC technology, the free amino acid content of bread was measured. The results revealed that the raisin spontaneous fermentation sourdough increased essential amino acid content by seven kinds, while the apple spontaneous fermentation sourdough increased by six kinds. The proportion of essential amino acids in the total amino acids increased from 14.15% to 35.02% and 28.79%, respectively. Therefore, spontaneous fermentation sourdough have positive effect on nutritional properties of bread.The result of GC-MS analysis of volatile flavor compounds in bread showed that spontaneous fermentation sourdough enhanced the flavor of the bread. The type of volatile flavor compounds increased and the total flavor compounds were increased by 53.27% and 49.83%, respectively. After fermentation by two kinds of spontaneous fermentation sourdough, new flavor compounds such as ethyl, 3-methyl butanal, azelaic acid ethyl ester, limonene, Phenylacetaldehyde, 2-heptene were detected. However, different flavor compounds characteristic were noted between raisin and apple spontaneous fermentation sourdough bread, this maybe due to the different microbial community structure. In raisin spontaneous fermentation sourdough bread, styrene, 2-Furanmethanol, 1-octene, enanthate were unique flavor compounds, while in apple spontaneous fermentation sourdough bread, butyl acetate, heptanoate, decanoic acid were unique flavor compounds.Using Texture Analyzer and DSC, the change of texture, water content and amylopectin retrogradation enthalpy of bread during storage was determined. The results showed that the texture parameters of spontaneous fermentation sourdough bread significantly deteriorated at a slower rate than ordinary yeast bread. The moisture migration rate, the amylopectin retrogradation enthalpy and staling during storage followed similar trend.