Study On The Mechanism Of Lactobacillus Sanfranciscensis On Sourdough Bread Quality And Improvement Of Its Bread Flavor

Sourdough is fermented with grains,water and active microorganisms such as lactic acid bacteria and/or yeast.Studies have shown that the use of sourdough improves the texture and enhance the flavor of bread,extends the shelf life and improves the nutritional value of the product.However,the changes of components and the mechanism of improvement on the bread quality during sourdough making remains unclear.Lactobacillus sanfranciscensis is one of the predominant microorganisms in sourdough.In this study,the improvement mechanism of L.sanfranciscensis fermented sourdough on bread quality was revealed.Based on the findings,the methods to enhance bread flavor were further studied.This could provide a theoretical basis and technical support for the production control of sourdough bread industrial production in China.The main research contents and conclusions were summarized as follows:(1)The depolymerization of gluten was promoted by acidification and enzyme activities of L.sanfranciscensis during sourdough fermentation and the whole bread making process.However,the volume and texture of sourdough bread were less dependent on energy input during mixing and the gluten macropolymers(GMP).L.sanfranciscensis DSM20451~T was used as the main research object,and its glutathione reductase(GshR)mutant strain(L.sanfranciscensis?gshR),NADH oxidase(NOX)mutant strain(L.sanfranciscensis?nox)and Lactobacillus sakei LS8 without GshR were used as controls.The effects of L.sanfranciscensis on the bread quality were studied by comparing the bread volume,free SH and molecular weight changes of gluten.The results showed that the sourdough bread volume were less dependent on the content of GMP.In wheat sourdoughs fermented with L.sanfranciscensis,GshR facilitated the accumulation of free SH,but had an adverse effect on the specific volume of bread.However,excessive accumulation of free SH promoted the exchange of SH/SS,leading to the accumulation of GMP,which was less affected by NOX.(2)L.sanfranciscensis consumed sucrose to produce EPS in situ during sourdough fermentation.The EPS producted by L.sanfranciscensis improved the gluten network structure of the dough,and then improved the texture of the bread.The EPS also inhibited the recrystallization of amylopectin during bread storage,thereby retard the staling of the bread and prolonging its shelf life.Through analyzing with the gas production and gas holding capacity of the dough,scanning electron microscopy(SEM),dynamic rheology,DSC thermodynamic changes,X-ray diffraction,bread texture etc.,it is proved that EPS enhanced the gas holding capacity of the dough,enhanced cross-linking between gluten proteins,reduced the viscoelastic modulus(G'and G'')of the dough,retard the staling of bread during storage,and inhibited the recrystallization of amylopectin,thereby improving the quality of the bread.This effect was confirmed by adding an equal amount of purified exopolysaccharide to the straight dough as control.The results also indicated that EPS of L.sanfranciscensis can compensate for the effect of GMP degradation on the reduction of bread specific volume during sourdough fermentation.(3)The texture and aroma of sourdough breads fermented by L.sanfranciscensis in combination with yeast were effected by gas capacity,protein degradation and yeast esterification and the metabolism of amino acids.The effect of sourdough fermented by L.sanfranciscensis in combination with its symbiotic yeast(Kazachstania humilis FUA4001,Saccharomyces cerevisiae FUA4018 and Wickerhamonyces anomalus FUA4025)on the quality of the dough,especially the bread flavor was investigated by comparing the EPS content,gas capacity,free SH and total peroxides content,gluten molecular weight,free amino acids content and volatile compounds of bread crumb.The results showed that the addition of the three yeasts did not reduce the GMP content during sourdough fermentation when compared with the bread fermented only with L.sanfranciscensis.The bread volume and crumb softness were positively correlated with the gas capacity.The addition of yeast in the sourdough was not conducive to the accumulation of EPS,but beneficial to enrich the flavor profile of bread,and the production of these aroma substances was mainly related to the degradation of low molecular weight proteins,which leaded to an increase in pyrazines and furans in bread crumb to enhance the bread aroma profile at the same time.The difference of bread flavor depended on the esters production ability and amino acids metabolism ability of the yeast strain.(4)Two aroma-producing yeast strains isolated from Chinese liquor Daqu were used as mixed starter cultures with L.sanfranciscensis for preparing sourdough bread,and the bread aroma was improved.Two maltose-negative yeast strains(Pichia kudriavzevii EP1and Meyerozyma guilliermondii EH1)with decent tolerance to acids(pH=3)and osmotic pressure(60%glucose or 8%NaCl)were obtained from Chinese liquor Daqu by the preliminary screening of the bread quality and sensory evaluation,and rescreening by growth test with L.sanfranciscensis during sourdough fermentation.The two strains contained different degrees of protease,lipase,phytase,and xylanase activity.These two strains significantly promoted the formation of lactic acid and acetic acid.The sourdough bread had less baking loss,lower staling rate and better texture,especially with improved flavor profile.The types of alcohols,aldehydes,esters and heterocyclics in the L.sanfranciscensis fermented sourdough bread were increased from 41 species to 44 species in the P.kudriavzevii EP1 fermented bread and 46 species in the M.guilliermondii EH1fermented bread,respectively.The bread made with isolated yeasts also contained some unique and aroma active substances,such as 1-octen-3-ol,3-methylthio-propylaldehyde,1-octen-3-one,isopropyl myristate,2-methoxy-4-vinylphenol and dimethyl trisulfide.The sensory evaluation showed improved flavour complexity and increased consumer acceptance of bread when using mixed starters.(5)The low-temperature fermentation method accumulated aroma compounds and improved the bread flavor.The effect of temperature on the sourdough bread fermented by L.sanfranciscensis DSM20451~T in combination with P.kudriavzevii EP1 and M.guilliermondii EH1 was investigated based on the growth and acidity capacity of microorganisms,reducing sugars,free amino acids,organic acids and aroma compounds.The results showed that the low-temperature fermentation method accumulated aroma compounds and improved the bread flavor.Important volatiles such as 1-octen-3-ol,phenylethyl alcohol,2-methoxy-4-vinylphenol,3-methylbutanal,ethyl decanoate,2-pentylfuran,2-furanmethanol and 2-furanmethanol were accumulated with decreased temperature.