| Lactic acid bacteria(LAB)were used in sourdough fermentation technology,and they could produce extracellular polysaccharides as a natural hydrocolloid to improve bread quality.Some studies have found that high molecular weight(HMw)dextran(Mw>7.6×10~6)could improve the specific volume of bread and prolong the shelf life.However,these studies were all based on the separated EPS,and few studies were carried out on the role of LAB-fermented sourdough with HMw dextran.In this paper,the Leuconostoc mesenteroides ATCC 8293 was selected for fermentation to obtain high-yield HMw dextran sourdough(EPSD),and its fermentation conditions and the addition amounts in bread were optimized.In the whole process of bread making,including sourdough fermentation,mixing,proofing and baking,the effect of EPSD on the characteristics of wheat starch and gluten protein was analyzed to clarify the improvement mechanism of bread quality.The main research contents were as follows:1.The fermentation conditions of EPSD were optimized by single-factor and response surface test analysis as follows:fermentation time was 24 h,fermentation temperature was 20°C and 15%sucrose addition.The content of in-situ enriched HMw dextran in EPSD reached15.16±0.56 g/kg.Compared to before optimization,the growth level of this strain was increased to 2×10~9 CFU/g and the p H could be reduced to 3.8～3.7 due to Le.mesenteroides ATCC 8293promoted the synthesis of organic acids to form an acidic environment.2.The effects of EPSD addition on the basic properties of bread,such as specific volume,baking loss,moisture content,texture,color and internal texture were explored.The results showed that the 25%EPSD addition of bread(EPSB)had the best quality:compared with the control,the specific volume of fresh bread increased from 4.82 cm~3/g to 5.15 cm~3/g,the baking loss decreased from 11.84%to 11.16%,the moisture content of bread increased from 43.07%to 44.03%,and the hardness of crumb decreased to 311.33 g.In the bread texture,the cell average area increased from 0.24 mm~2 to 0.28 mm~2,and the porosity decreased from 18.28%to 16.92%,making the bread loose.On the bread color variation,the L*of crust decreased from60.86 to 57.28,the a*and b*increased by 14.26%and 7.47%respectively;the L*value of crumb decreased from 62.7 to 59.53,and b*decreased by 7.07%,but it was within the acceptable range.After storage for 7 days,the hardness of crumb decreased from 731.14 g to437.95 g,and the recovery increased from 15.36%to 18.25%.The retrograded enthalpy values decreased from 3.51 J/g to 2.82 J/g,and the relative crystallinity decreased from 14.02%to12.04%.The addition of EPSD in bread increased specific volume,reduced baking losses,inhibited moisture migration,loosens the texture,and maintains the softness and elasticity of bread during storage,slowing down the staling process.3.Differential scanning calorimetry(DSC),X-ray diffraction(XRD)and rapid viscosity analysis(RVA)were used to measure the retrogradation degree,amylopectin crystallinity and gelatinization characteristics in bread,and the EPSD improving bread quality and wheat starch characteristics was explored.The results showed that,compared with the three control breads,the retrogradation enthalpy of EPSD decreased from 3.36 J/g to 2.89 J/g after 7 days of storage,a maximum decrease of 16.3%,and the relative crystallinity decreased from 15.20%to 13.53%.In addition,the addition of EPSD had no significant effects on gelatinization temperature at all stages.At sourdough stage,the peak viscosity,breakdown and setback viscosity of sourdough decreased from 562 c P,229 c P and 395 c P to 345 c P,140 c P and 249 c P.At mixing stages,the peak viscosity and setback viscosity of sourdough decreased from 772 c P,303 c P and 501 c P to 681 c P,272 c P and 465 c P.Under weak acid or acidic conditions,HMw dextran ensured the integrity of starch granules,reduced the ordered arrangement of starch granules,and then interfered with the interaction between amylose,thus inhibiting the short-term retrogradation of amylose.4.The molecular weight distribution,protein polymerization degree,free sulfhydryl content and secondary structure of protein were analyzed by high performance size exclusion chromatography(SE-HPLC),sodium dodecyl sulfate-polyacrylamide gel electrophoresis(SDS-PAGE),Fourier transform infrared spectroscopy(FTIR),revealing the correlation between the improvement of bread quality by EPSD and the change of protein characteristics.The results showed that,compared with the three controls,the highest content of SDS-DTT soluble protein(Mw≈16 k Da～92 k Da)was found in EPSD,and the content of glutenin macropolymer(GMP)was significantly increased to 62.25%,with a maximum increase of19.14%.The content of SDS-soluble high-Mw(HMW)proteins and SDS-soluble low-Mw(LMW)proteins decreased significantly to 9.3%and 28.45%,with a maximum decrease of4.03%and 15.11%,which was beneficial to the polymerization of gluten protein by HMw dextran under acidic conditions.At proofing stage,the GMP content in EPSB was significantly reduced to 45.94%,with a maximum decrease of 8.67%.LMW proteins increased significantly to 43.19%,with a maximum increase of 7.7%.In addition,the content of antiparallelβ-sheets increased significantly,which indicated that the existence of HMw dextran promoted the depolymerization of GMP and enhanced the extensibility of dough under weak acid conditions. |
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