Studies On Leavening Properties Of Fruit Sourdoughs And Their Effects On Baking Quality Of Bread

This study investigated the effects of spontaneous sourdoughs started with pear and navel orange fermentations on the quality attributes of breads,focusing on texture,flavor,nutritional properties and shelf-life.The mechanism of sourdough on improving quality of bread was also studied.Lactic acid bacteria(LAB)and yeasts in two kinds of sourdough were isolated and identified.In addition,influences of selected strains used as mono-culture or co-culture on quality of bread were investigated.First,this work successfully developed that adding sourdough could improve the technological and nutritional properties of bread,and that different substrates for sourdough led to differences in the bread characteristics.Breads made from sourdough exhibited lower pH(4.66-4.87)and greater acidity(3.95-4.46 m L)than those of control bread(CB).The retention coefficients for the pear sourdough(PSD)and navel orange sourdough(OSD)doughs were 87.2% and 90.4%.They were 8.73% and 12.72% greater than those of the control dough,respectively.The use of sourdough contributed to the specific volume,with that of pear sourdough bread(PSB)increasing by 8.99% and navel orange sourdough bread(OSB)increasing by 5.56%.The hardness of PSB was lower than that of CB by 16.14%,while there was no significant difference between those of OSB and CB.At the same time,the free amino acid content in PSB and OSB was 1.6 times that of CB.In addition,the two kinds of sourdough fermentation increased the content of ketones,acids,esters and aldehydes in the bread.Second,the yeasts and LAB isolates from the PS and OS were identified using 26 S rRNA gene and 16 S rRNA gene sequencing.All LAB isolates were identified as 3 species: Lactobacillus brevis,Lactobacillus plantarum,and Lactobacillus rossiae and all the yeasts as Saccharomyces cerevisiae.Four strains with superior performance were obtained by rescreening,which were S.cerevisiae LY4,L.brevis QB11,L.plantarum QB15 and L.rossiae QB7.Third,the mechanism of sourdough on improving quality of bread was discussed.The microstructure of the dough showed that the starch-protein structure of the PSB and OSB doughs was more compact than that of CB dough.Amylase and phytase were present in the PS and OS.L.brevis QB11 and L.plantarum QB15 could produce amylase and phytase.The sourdough fermentation slowed the loss of moisture of the bread and inhibited the retrogradation of amylopectin during storage.The amylopectin retrogradation enthalpy of PSB was 76.01% of that of CB,and OSB was 80.66%.Fourth,the effect of LAB and yeast strains used as mono-culture on bread quality was analyzed.The results showed that the effect of S.cerevisiae LY4 on bread mainly reflected in the increase of bread volume and of the content of flavor compounds,such as alcohols,esters and aldehydes.Lactobacillus fermentation significantly increased the acidity of bread.L.brevis QB11 and L.plantarum QB15 had an adverse effect on the specific volume of bread and on the hardness of the bread crumb.Compared to CB,their specific volume decreased by 15.87% and 13.23%,respectively.L.rossiae QB7 and L.plantarum QB15 significantly increased the free amino acid content of bread.The content of total FAA of their breads were 16.59% and 23.37% more than that of CB,respectively.L.brevis QB11 had a positive effect on improving the content of flavor compounds,such as alcohols,esters,aldehydes and ketones.The contents of esters and ketones compounds of its bread were 2.64 and 5.77 times those of CB,respectively.LAB had a positive effect on the reduction of moisture loss and the inhibition of amylopectin retrogradation.Fifth,the influence of different combinations of four strains on bread quality was explored.When S.cerevisiae LY4: L.rossiae QB7: L.brevis QB11: L.plantarum QB15 were inoculated at a ratio of 3:100:100:100 to prepare sourdough,quality of bread was the best.The specific volume of its bread increased by 5.20% compared to CB.Hardness of its bread reduced by half.And sensory quality was favorable.The content of total free amino acids increased by 50.75%.The content of GABA increased by 56.47%.And the contents of valine,methionine,tryptophan,phenylalanine,leucine and lysine all increased.Particularly,the content of tryptophan was increased by 92.22%.The retrogradation enthalpy of amylopectin was 83.18% of that of CB.Thus,the MC13 could be recommended for replacing spontaneous sourdough for improving the quality of bread.