Study On Effect Of Dextran On Wheat Bread Quality And Its Mechanism

Sourdough technology has been widely used in breadmaking since the late 20^th century as it can improve the aroma,texture,shelf life and nutritional properties of final bread products and meet consumers’demands for“clean labels”.During sourdough fermentation,lactic acid bacteria（LAB）accumulates sufficient homopolysaccharides（HoPS）to satisfactorily enhance bread quality,especially crumb softness after storage.Among these HoPS,linear dextran has been regarded as a promising bread improver.However,little information was involved in the performance of dextran with various molecular weights（Mw）in enhancing bread quality,as well as the underlying mechanisms.This vacancy is disadvantageous for bakers or researchers to essentially understand their potential for increasing consumers’acceptance of bread or other cereal products.In this research,effect of dextrans varying with Mw values on characteristics of straight breads and type I sourdough breads were compared to elucidate the relationship between M_w of dextran and the quality of bread products.Differences in rheological properties of doughs and sourdough-containing doughs were then explored.Moreover,changes in behaviors of starch granules and gluten proteins were determined by the method of separation and reconstitution,revealing the underlying mechanisms for improved bread quality.Yields of the targeted dextran in sourdough and culture broth were increased when fermentations were conducted under the moderate low temperature（20℃）.The effectiveness of the sourdough enrich in dextran on inhibiting bread aging was also proved.Main research contents and conclusions were listed as follows:1.Effectiveness of exogenously added dextrans on improving bread quality,especially on delaying bread staling,was related to their M_w values and the processing methods of bread products.Results from image scanning,texture analyzer and differential scanning calorimeter（DSC）showed that addition of dextran T2000（M_w:2.0×10⁶ g/mol）under the concentration of0.1%（w/w）caused a more open crumb structure and decreased hardness of the fresh straight bread,as well as the lowered staling rate and retrogradation enthalpy after the storage of 7days at 4℃.The hardness of fresh bread,the staling rate and enthalpy of stored breads were respectively decreased by 32.2%,17.6%and 23.1%relative to the control.In comparison to the control sourdough bread,incorporation of 0.1%dextran T2000 contributed to a denser crumb structure of sourdough bread on the baking day.Dextran T10,T750 and T2000 of 0.1%respectively reduced the staling rate by 12.2%,21.5%and 26.3%and the retrogradation enthalpy by 11.7%、14.0%and 17.1%.Moreover,the relatively crystallinity separately decreased from 23.17%to 19.31%,17.03%and 16.78%.Dextran T2000 possessed the potential for improving the quality of straight breads and sourdough breads.In addition,its inhibition on the aging of sourdough breads was superior to that of straight breads.Based on baking trials of addition optimization,concentration of 0.1%is proper for improving the quality of the two types of breads.Dextran T2000 of 0.1%exhibited stronger inhibition on bread aging than other commercial hydrocolloids under the same amount,including hydroxypropyl methyl cellulose（HPMC）,carboxymethyl cellulose（CMC）and xanthan gum.Therefore,dextran T2000 is a promising bread quality improver.2.Influence of dextran on dough rheology was connected to the way by which the dough was processed.The farinograph analysis clarified that incorporation of dextran T2000with 0.8%（w/w）increased dough water absorption from 66.7%to 68.7%,and decreased dough developing time from 2.5 min to 2.2 min and dough stable time from 4.3 min to 3.4min.This suggested that high Mw dextran had an adverse effect on dough processing quality.Analysis on the flow behavior of different dextran solutions showed that viscosities of the aqueous solution of dextran T2000 were independent on shear rates,and its consistency index was higher than that of other dextran solutions.High M_w dextran thus tended to hold water molecules,decreasing its mobility.Tensile properties of straight dough were investigated using a texture analyzer.The result showed that dextran T2000 of 0.1%did not cause significant differences（p>0.05）in resistance and extensibility.A rheometer was applied to explore the dynamic rheological properties of straight dough.It was found that dextran T2000increased storage modulus（G′）and loss modulus（G′′）,as well as tanδ（G′′/G′）within the scanning frequency from 0.3 Hz to 1.0 Hz.Results from small amplitude oscillatory shear tests also showed that 0.1%dextran T2000 led to lower tanδ.It could be concluded that dextran T2000 could improve the elastic property of sourdough-containing dough,while hardly had positive effects on that of straight dough.3.Dextran obviously inhibited the short-term retrogradation of amylose and the long-term retrogradation of amylopectin.Furthermore,the weakly acidic condition highlighted the inhibition of dextran on the amylopectin recrystallization.Rapid visco-analyzer and lodine colorimetric method were combined to determine the gelatinization of starch.The data showed that dextran T2000 of 0.5%（w/w,based on starch weight）enhance the thermal stability of starch granules under the non-acidic environment and the weakly acidic condition（pH 5.5）,which contributed to less amylose leaking into the gelatinized system and less formation of amylose crystallites.On the opposite,the weakly acidic environment had detrimental effect on the thermal property of starch granules,promoting the leaking of amylose and the formation of amylose crystallites.Moisture distributions of different starch gel systems after the 7-day storage at 4℃ were measured by a low field nuclear magnetic instrument（LF-NMR）.Owing to the treatment by dextran T2000 of 0.5%and the weakly acidic environment,T₂₂ was respectively increased from 3.453 ms to 3.802 ms and 3.813 ms,and A₂₂ was decreased from 61.442 au to 55.107 au and 55.279 au.This result indicated that dextran T2000 and the weakly acidic environment both increased the mobility of water molecules and decreased the water content within the amylopectin recrystallization.Moreover,their synergetic retardation on the reordering of amylopectin was revealed by the highest T₂₂ value（4.008 ms）.Analyses on the long-term retrogradation of gelatinized starch and starch-gluten systems after the storage of 7 days at 4℃ by DSC showed that 0.5%dextran T2000,weakly acidic conditions and their combination caused the retrogradation enthalpy of the starch system decreased by 4.0%,7.1%and 5.8%and that of the starch-gluten system lowered by 9.0%,8.7%and 14.3%.It implied that the presence of gluten proteins highlighted the inhibited effect of the combined dextran T2000 and weakly acidic conditions on amylopectin retrogradation.4.Dextran had beneficial impacts on the secondary conformation,thermal stability and viscoelasticity of the weakly acidified gluten network.Electrophoresis（SDS-PAGE）and size exclusion chromatography（SE-HPLC）were applied to analyze the distribution of gluten proteins.The results showed that dextran T2000 of 0.5%（w/w,based on gluten protein weight）and weakly acidic conditions（pH 5.5）both decreased the band intensity in the high Mw area（Mw>175 kDa）.Furthermore,the amount of glutenin macropolymer（GMP）was decreased from（62.43±0.16）%to（57.70±0.90）%when the gluten network was exposed to the weakly acidified environment.Contents of soluble high M_w（HMW）and low M_w（LMW）protein molecules were corresponding increased.The analysis on free thiol（SH）contents revealed that 0.5%dextran T2000 led to the accumulation of free SH through disturbing the polymerization of gluten proteins.However,dextran T2000 did not obviously change the amount of free SH of the weakly acidified gluten structure,suggesting that acids tended to weaken the adverse influence of high Mw dextran on gluten polymerization.Contents of secondary structures of gluten network were measured by Fourier transform infrared spectroscopy（FTIR）,including antiparallelβ-sheet,α-helix,β-turn andβ-sheet.In comparison to the control,0.5%dextran T2000 and the acidic environment both resulted in decreased amounts of antiparallelβ-sheet and increased amounts ofβ-turn,representing lower contents of"train"（meaning compact structures）and higher contents of"loop"（meaning loose structures）.This revealed that gluten networks had lower coherence and resistance to extension.Nevertheless,dextran T2000 increased the compact structure and lowered the loose structure of the weakly acidified gluten dough,indicating an improvement on their properties of coherence and resistance to extension.DSC was used to analyze the thermal characteristics of different gluten structures.Addition of 0.5%dextran T2000 resulted in lower denaturation temperature（T_p）,and the weakly acidified environment led to the reduction of enthalpy（ΔH）.The high M_w dextran obviously increased T_p andΔH of the acidified gluten network,suggested its improved stability.The analysis on phase angle（δ）of gluten dough showed that dextran T2000 of 0.5%had no significant influence onδvalues of the gluten structure and the weak acidic condition caused the increasedδ.However,lowerδof weakly acidified gluten network was obtained when the high M_w dextran was combined,elucidating the improved elasticity.There was a positive correlation between the secondary structures of gluten dough and its thermodynamic and rheological properties.5.In contrast to the conventional fermentation temperature（30℃）,moderate low-temperature（20℃）is beneficial for Leuconostoc mesenteroides ATCC 8293 to increase production of high M_w dextran,regardless of the sourdough or culture broth fermentation.When sourdough without addition of substrate sucrose was fermented at 30℃ and 20℃ for24 h,their final cell counts and pH values were comparable,while slightly higher amounts of colonies and lower pH values were observed when sourdough was co-fermented with 15%and 30%sucrose（w/w,based on flour weight）.Cold temperature（10℃）incubation led to the lower cell number and the higher pH of sourdough without or with sucrose after the 24 h fermentation.When the culture broth was fermented under 37℃,30℃ and 20℃,the strain grew fast after 2 h and grew stable after 14 h.The lag duration was respectively extended to24 h and 36 h when the strain was incubated at 10℃ and 6℃.Furthermore,the optical density（OD600）of the stable phase under 10℃ and 6℃ was also lower than that of the stable period at 30℃.These results suggested that the cold environment delayed the growth of the strain,while the mild cold condition did not affect the strain growth.Results from SE-HPLC and ion exchange chromatography（SE-HPLC）showed that the yield of EPS in sourdough fermented with 15%sucrose（w/w）for 24 h under 20℃ was the highest,8.7%higher than that of the same sourdough fermented under 30℃.The type and amounts of panose-series oligosaccharides were also apparently increased.Evaluation of bread quality showed that the sourdough fermented with 15%sucrose at 20℃ contributed to a lower staling rate compared with the control,decreased by 26.2%.The yield of EPS in the media composed of 15 g/L sucrose reached the highest when the LAB was grown to the stationary phase under 20℃,increased by 75.0%relative to the same media incubated at 20℃.