Changes In Yeast Stability And Its Effect On Dough Quality During Frozen Storage

Dough freezing is an innovative technology for the industrialization of bakery products.The decrease of yeast activity during frozen storage is considered to be one of the most vital factors.Previous researchers studied the yeast freezing in yeast suspension system,which could not reflect the changes of yeast properties in real frozen dough and their effect on dough quality.In addition,few studies focused on the effect of changes in yeast properties on bread texture,flavor,color and bread aging.The vacancy of the knowledge remains the issue of reliability to elucidate the detailed deterioration mechanism of frozen dough.Therefore,this study was mainly to investigate the changes in yeast activity and metabolites which related to the stability of yeast in frozen dough and their effects on quality of dough and bread,in order to reveal the mechanism of yeast-induced dough deterioration.This could promote the theoretical guidance and technical support for the application of frozen dough and the improvement of frozen dough quality.The main research contents and conclusions were summarized as follows:(1)Yeast in dough was more susceptible to freezing than that in suspension,and the degree of the freezing damage was closely related to yeast properties.Using yeast suspension as control,the properties of sugar tolerant-instant dry yeast(ST-IDY)and instant dry yeast(IDY)in dough were investigated during frozen storage.Scanning electron microscopy(SEM)and confocal laser scanning microscopy(CLSM)were used to observe the morphological structure and physiological state of yeast cells.It was found that freezing process induced the deformation of yeast cells,and wrinkles and holes on cell surface.IDY cell structure was more severe damaged than that of ST-IDY.After frozen storage,the integrity of yeast cell membranes was broken.After 90 days of frozen storage,compared with yeast in suspension,the survival rate and gas production capacity of ST-IDY in dough were reduced by 56.3%and 64.4%,respectively,while IDY was decreased by 69.8%and 66.5%.The qualitative and quantitative analyses of yeast leachates released from lysed yeast cells by high performance liquid chromatography(HPLC)showed that,after 90 days of frozen storage,the contents of GSH,trehalose,free amino acids and reducing sugars in ST-IDY suspensions were increased.The GSH content in dough was significantly increased,while the trehalose content was decreased.Compared with yeast suspension,GSH content in ST-IDY containing dough increased by 81.0%(p<0.05).(2)Changes in the properties of yeast during dough frozen storage caused the decrease of pH value,and promoted the deterioration of bread flavor and texture.After 90 days of frozen storage,the pH value of yeast-containing dough(YD)decreased from 5.67 to 5.37,while the pH value of dough without yeast(NYD)decreased from 6.07 to 6.00.After 90 days of frozen storage,the contents of lactic acid,acetic acid,citric acid and succinic acid in YD were markedly increased,while the content of organic acids in NYD showed no significant difference(p>0.05).Gas chromatography-mass spectrometry(GC-MS)analysis of the flavors in bread crumb showed that the total content of alcohols,aldehydes and esters which showed pleasant odors in YD dough was reduced by 76.7%after 90 days of frozen storage.The total content of substances which showed unpleased order such as acetic acid,octanoic acid,2-methylbutyric acid,hexanoic acid,and 3-octene-2-one increased by 460.74%.Taking dough made by frozen NYD dough and fresh yeast(NYD-FY)as control,the quality of YD bread was analyzed by texture analyzer,image scanning and colorimeter.Results confirmed that changes of yeast properties during frozen storage were important factors causing the decline of bread specific volume,hard bread crumb,deterioration of bread color,and acceleration of bread aging.The specific volume of YD bread was reduced by 8.7%than that in NYD-FY,and the hardness was increased by 15.1%after freezing for 90 days.During frozen storage,the color of the bread crust changed from yellowish brown to light yellow,and the bread crumb changed from light yellow to yellowish brown.Compared with NYD-FY,the aging rate of bread crumb increased by 19.9%,the water loss rate increased by 19.4%,and the retrogradation enthalpy increased by 9.68%.(3)During frozen storage,the changes of yeast properties promoted the depolymerization of gluten protein,but the degree of its promoting ability was weaker than the damage of gluten protein by freezing itself.Taking NYD and fresh dough made by yeast leachates extracted by frozen yeast suspension(YLD)as control,the protein molecular weight distribution,free sulfhydryl content,protein secondary structure,freezable water content and dough rheological properties of YD dough were measured using size exclusion-high performance liquid chromatography(SE-HPLC),visible light spectrophotometer,Fourier infrared spectrometer(FTIR),differential scanning calorimeter(DSC)and rheometer during frozen storage.The results showed that changes in yeast properties promoted the depolymerization of macromolecular gluten polymers(GMP)by the breakage of intermolecular disulfide bond.After 90 days of frozen storage,the relative content of SDS soluble protein in YD,NYD and YLD dough increased by 5%,4%and 3%,respectively,and the content of SDS-DTT soluble protein decreased by 17%,12%and 8%,respectively,while the free sulfhydryl content increased by 46.3%,35.0%,and 25.8%,respectively.The degree of damage to gluten protein induced by freezing itself was greater than the effect of yeast properties on gluten.Fourier infrared spectroscopy analysis shows that the changes of yeast properties were closely related to the partial conversion of the protein secondary structure from ?-helix and ?-turn structure to a ?-sheet structure.Freezable water content and rheological properties of dough indicated that the changes in yeast properties could weaken the binding ability of gluten protein and water,leading to protein dehydration and reduction of dough viscoelasticity.After 90 days of frozen storage,the freezable water content of YD,NYD and YLD dough increased by 47.7%,33.2%and 21.3%,respectively.The viscoelasticity of YD was significantly weaker than that of NYD.A reduction was also observed in YLD viscoelasticity.(4)The content of GSH released by lysed yeast cells during frozen storage was positively correlated with the deterioration of frozen dough and bread quality.Based on the GSH contents in yeast dough and yeast suspension system,the bread quality,protein molecular weight distribution,free sulfhydryl group,moisture state and rheological properties of dough were investigated by adding different content of GSH.Baking loss of bread showed a linear increasing trend with the increase of GSH content(R2=0.95,p<0.05),while the specific volume of bread showed a significant linear negative correlation with the addition of GSH(R2=0.92,p<0.05).GSH promoted the crumb microstructure of bread to be rough and coarse.SE-HPLC results showed that the content of SDS soluble protein was linearly positively correlated with the increase of GSH content(R2=0.96,p<0.05),while the relative content of SDS-DTT soluble protein showed a significant linear decline with the increase of GSH content(R2=0.98,p<0.01).With the addition of 0.04%GSH,the relative content of SDS soluble protein was increased by 66%,and the relative content of SDS-DTT soluble protein decreased by 52%.Using low-field nuclear magnetic resonance(LF-NMR)and DSC to estimate the moisture state and distribution in dough.It was found that GSH weakened the hydration capacity of gluten protein.The water molecules in gluten structure changed from fixed water to bound water.The results of rheological properties of dough showed that GSH reduced the stability and viscoelasticity of the dough by inducing gluten protein depolymerization and water migration,and showed a negative impact on gas retention capacity of dough.Bread storage experiments showed that GSH accelerated starch recrystallization and promoted bread aging.(5)The promotion of bread aging induced by GSH was highly related to the depolymerization of gluten protein,acceleration of the separation of starch and protein in dough and long-term retrogradation of starch.The gelatinization properties of wheat flour(WF)and wheat starch(WS)were measured using rapid viscosity analyzer(RVA).The results indicated that 0.1%GSH reduced the peak viscosity of WF by 19.3%and the retrogradation value by 19.6%.The peak viscosity of WS showed no significant difference(p>0.05),and the retrogradation value decreased by 6.8%.The results of the relative molecular weight distribution of WF protein suggested that the depolymerization behavior of WF protein caused by GSH weakened the strength of the gel network.The depolymerized protein was intercalated between gelatinized starch chains and inhibited the short-term retrogradation of amylose.The results of DSC,rheometer and LF-NMR showed that the depolymerization of GSH on WF protein alleviated the hindrance effect of protein to starch granules,promoting the combination of water and starch,making starch easier to gelatinization.It promoted the recrystallization of amylopectin molecular in WF gel during the long-term retrogradation.GSH weakens the water holding capacity and strength of WF gel.After storage at 4? for 7 days,GSH significantly increased the retrogradation enthalpy(?Hret)of WF,and slightly decreased the ?Hret of WS.GSH reduced the water mobility of WF gel and accelerated the retrogradation of gel.