Effects Of Protein Cross-linking On The Qualities Of Buckwheat Chinese Steamed Bread And The Mechanism

Buckwheat Chinese steamed bread?CSB?contains rich nutrition,which meets the requirements of consumers for healthy staple food.However,due to the lack of gluten protein in buckwheat,it is not conducive to the formation of gluten network structure,which affects the quality of buckwheat CSB.In this thesis,the ascorbic acid?Vc?,alkali?sodium bicarbonate,sodium carbonate?,transglutaminase?TG?and glucose oxidase?GOD?were applied as additives into buckwheat CSB in order to improve the quality of buckwheat CSB and then its mechanism was discussed.Firstly,the effects of different buckwheat additions on the quality of CSB were investigated.The results indicated that the rheological properties of yeast dough decreased gradually along with the increased buckwheat flour.The leakage time was accelerated as well.The specific volume and the pore size of the CSB became smaller.Meanwhile,the increased hardness,adhesiveness,chewiness and decreased springiness,resilience lead to a poor texture quality of CSB,especially for the addition over 30%.Secondly,effects of Vc on the fermentation characteristics of buckwheat dough,quality characteristics of buckwheat CSB,and protein cross-linking were studied.The results showed that adding Vc in a certain weight could increase the maximum swelling height,gas release volume and gas retention ratio of the dough.The specific volume of buckwheat CSB was increased with Vc,as well as the springness,chewiness,resilience and hardness.With the raise of Vc,the SDS polyacrylamide gel electrophoresis?SDS-PAGE?exhibited that the intensities of the relatively low molecular weight protein bands and the top bands of the concentrated gel decreased.The size exclusion chromatogram?SE-HPLC?showed that for the non-reducing condition,the SDS extractable protein?SDSEP?and the peak area of protein were both decreased,even the peak area was obviously smaller compared to the one under reducing condition.It was found from laser confocal microscopy?CLSM?that the addition of Vc made the structure of protein network in the buckwheat CSB become continuous and compact,and can fully combine the starch filled therein.Meanwhile,the effects of NaHCO₃ and Na₂CO₃ on the quality characteristics of buckwheat steamed bread were researched.The results indicated that the addition of NaHCO₃and Na₂CO₃ could improve the specific volume and height-diameter ratio of the CSB,increasing the gumminess,hardness and resilience.The best sensory score was during 0.6%NaHCO₃,0.2%Na₂CO₃.At the same time,higher peak and final viscosity and decreased breakdown value of starch were shown with the increase of alkali level.The microstructure proved the more continuous and compact protein network structure could be fully embed with the gelatinized starch.Furthermore,an obvious increase in the intensity of protein bands with higher molecular weight,while intensity of lower molecular protein bands was decreased.The SE-HPLC under non-reducing conditions showed the significant?p<0.05?decrease in the area of protein peak and the SDSEPs.The SDSEPs under reducing condition were close to 100%when the addition of NaHCO₃ and Na₂CO₃ were below 0.2%and 0.1%respectively,indicating that the lower concentration of alkali could promote the disulfide bond of protein in the buckwheat CSB.In combination,the continuous increase in the addition obviously decreased SDSEPs under reducing conditions,which illustrated that the higher concentration of alkali can induce non-disulfide cross-linking of protein.In addition,the changes of free thiol content,decrease of cystine content,and production of dehydroalanine and lanthionine have verified that the high addition of alkali can induce the dehydroalanine-derived cross-linking of protein in buckwheat CSB.Fourthly,the effects of adding transglutaminase?TG?on the fermentation characteristics of buckwheat dough and the quality characteristics of buckwheat CSB.The increased addition of TG can improve fermentation characteristic,enhance the specific volume,hardness and chewiness of CSB.The band width of the relatively low molecular weight protein in SDS-PAGE was narrowed,and the band at the top of the separation gel got deepened with the decreased intensities.The decrease of protein peak area and SDSEP under reducing conditions indicated that TGase could induce the cross-linking of protein in buckwheat CSB.In addition,the effects of glucose oxidase?GOD?on the buckwheat CSB were studied.With the addition of GOD,the maximum expansion height,time to start leakage,and the retention ratio of gas were increased during the dough fermentation,while the total volume of the released gas decreased.The addition of GOD can increase the specific volume,height-diameter ratio and texture of the CSB.In SDS-PAGE,the width of the relatively low molecular weight protein bands got broad and the intensities at the top of separation gel were incereased.SE-HPLC showed that under non-reducing conditions,SDSEPs were decreased,and the peak area of SDS extractable protein was lower than that under reducing conditions.In conclusion,adding 20 mg/kg GOD during the second dough-making could get better qualities of buckwheat CSB.The link between degrees of protein cross-linking and quality characteristics of buckwheat steamed bread after adding Vc,alkali,TG and GOD was discussed.The results showed that protein cross-linking induced by additives was correlated with the specific volume,hardness and chewiness of buckwheat CSB.Alkali?NaHCO₃ or Na₂CO₃?was found to have a great influence on the degree of cross-linking of proteins,followed by TG,Vc and GOD.In buckwheat CSB,alkali could increase the specific volume significantly,while its hardness and chewiness were obviously improved by Vc and GOD.