Effects Of Freezing Methods On Physicochemical Properties And In Vitro Digestibility Of Starch In Dough

As the main component of dough,wheat starch has different properties after the different freezing treatments,which plays an important role in the quality of frozen dough.In this study,cryogenic refrigerator freezing method(RF),spiral tunnel freezing method(SF),and liquid nitrogen spray freezing method(LF)were selected to freeze unfermented dough from the perspectives of common household refrigerator freezing,traditional industrial production freezing,and emerging freezing methods.The effects of these three freezing methods on the physicochemical properties and in vitro digestibility of A-starch,B-starch and the whole starch in dough were studied.The data obtained clarified the relationship between starch and frozen dough quality under different freezing methods.The effects of freezing methods on the in vitro digestibility of starch were also explored to provide certain data support for scientific and reasonable evaluation of the digestibility of frozen unfermented flour products.The information obtained in this paper could have important guiding significance for the production of frozen unfermented flour products.The main conclusions were as follows.1.The effects of three freezing methods on the quality of unfermented dough were different.Freezing rates of LF,SF and RF through the maximum ice crystal formation zone were 1.33?/min,0.31 ?/min and 0.08?/min,respectively.With the decrease of the freezing rate,the water loss rate of dough gradually increased,the content of bound water(T21)and semi-bound water(T22)gradually decreased,the hardness of dough gradually increased,and the viscoelasticity of rheological properties gradually decreased.This might be because the slower the freezing rate,the larger the ice crystals formed and the greater the influence on the dough structure,which weakened the binding capacity of macromolecular substances(starch and protein)to water.The slower the freezing rate,the larger the residual holes after the ice crystals escaped,indicating that the microstructure of the dough was destroyed more obviously.Therefore,the dough quality of LF treatment was closer to that of fresh dough,followed by that of SF treatment,and the dough quality of RF treatment was the worst.This indicated that the emerging freezing method with liquid nitrogen was superior to the traditional industrial production freezing method,and the flour products treated by the household common freezing method were of poor quality.2.The basic components of the whole starch,A-starch and B-starch in unfermented dough were affected by three freezing methods,and the changes of basic components were closely related to the dough quality.The contents of lipid,protein and amylose in the whole starch,A-starch and B-starch were significantly decreased by the three freezing methods,and the content of damaged starch was significantly increased(p<0.05).The effects of LF treatment were the least,followed by SF treatment,and the effects of RF treatment were the greatest.Compared with the blank group,the B-starch in the dough after RF treatment had the greatest effects on the contents of lipid,protein and amylose,which decreased by 17.52%,12.75%and 16.83%,respectively.The Astarch in the dough after LF treatment had the least effects,which decreased by 3.42%,2.34%and 3.50%,respectively.The B-starch treated by RF had the greatest effect on the content of damaged starch,which increased by 50%,while the A-starch treated by LF had the least effect,which increased by 8.30%.Therefore,the same freezing method had the greatest influences on the parameters of B-starch,the second on the whole starch,and the least on A-starch,indicating that the sensitivity of freezing method was as follows:B-starch>the whole starch>A-starch,which was directly related to the size of starch particle.3.The structure of the whole starch,A-starch and B-starch was also affected by the three freezing methods,and the starch structure determined the properties of starch,which was closely related to the dough quality.Three freezing methods destroyed the granular structure of starch and reduced the granule size of starch in dough,which was mainly related to the loss of basic components in starch.With the decrease of the freezing rate,the more serious the damage of the granule structure of the starch was,the more the granule size of the starch decreased.After RF treatment,the surface structure of the whole starch and A-starch granules was broken,and the morphology of B-starch was significantly irregular.D50 values of the whole starch,A-starch and Bstarch decreased by 10.52%,11.11%and 37.62%,respectively.In addition,the shortrange order degree and relative crystallinity of starch were increased by the three freezing methods.With the decrease of freezing rate,R1047/1022 values and relative crystallinity of starch increased more,while three freezing methods did not produce new functional groups of starch,nor changed the crystal structure of starch.Moreover,R1047/1022 value(0.733)and relative crystallinity(36.38%)of A-starch in blank group were higher than those of B-starch(0.715,25.94%),indicating that the molecular structure of A-starch was more stable than that of B-starch.Comparing the variation range of each index under the same freezing method,it was found that the changes of surface microstructure,granule size,short-range order degree and relative crystallinity of B-starch were more obvious than those of A-starch,which also indicated that the structural characteristics of small particle B-starch in dough were more easily affected by freezing treatment.4.Three freezing methods also affected the physicochemical properties of the whole starch,A-starch and B-starch in dough,and the changes of physicochemical properties of starch were very important to the dough quality.LF,SF and RF treatments improved the swelling power,gelatinization parameters and thermodynamic properties of the whole starch,A-starch and B-starch to varying degrees.RF with the slowest freezing rate had the greatest influences on the swelling power of the whole starch,A-starch and B-starch,which increased to 10.03 g/g,12.37 g/g and 9.17 g/g.PV,BV,SV and AH of the whole starch after RF treatment increased to 2296.7 cP,601.0 cP,446.3 cP,6.63 J/g;those of the A-starch increased to 3000.8 cP,392.6 cP,761.2 cP and 7.17 J/g;and those of the B-starch increased to 2021.0 cP,336.0 cP,615.5 cP and 6.42 J/g.The effects of LF with faster freezing rate were the least,which were closer to those of the blank group,while the effects of SF treatment were between LF and RF.In addition,the gelatinization enthalpy and gelatinization parameters of A-starch in blank group were higher than those of B-starch,indicating that the gelatinization energy of A-starch was higher,which was consistent with the results of FT-IR and XRD.Comparing the changes of physicochemical properties parameters with different starch particle sizes under the same freezing method,the changes of each index of B-starch were the most obvious,while there was no significant difference between A-starch and whole starch,indicating that the physicochemical properties of B-starch were also the most sensitive to freezing method.These were related to the particle sizes of starch and the content of lipid and protein in starch.5.In vitro digestibility of A-starch,B-starch and the whole starch in the dough treated by three freezing methods were determined by simulating the in vitro digestion system.In the process of digestion,with the increase of time,the GI values of starch in dough under different treatments increased rapidly at first,then slowly,and tended to balance.The three freezing methods could improve the digestibility of A-starch,B-starch and the whole starch.After treatments,the contents of RDS increased significantly and the contents of RS decreased significantly.Moreover,freezing treatments could promote the conversion of part of RS to SDS,and promote the conversion of part of SDS to RDS.RF with the slowest freezing rate had the most obvious effect on starch digestibility,followed by SF treatment,and LF treatment had the least effect.After the same freezing treatment,the effect of B-starch was greater than that of the whole starch and A-starch,which was mainly related to the size of starch granules,the integrity of granules and the content of binding protein and lipid.This research clarified the relationship between the freezing methods and the in vitro digestibility of starch,and provided certain data support for scientific and reasonable evaluation of the digestibility of frozen flour products.