Wheat Starch Deterioration During Frozen Storage: Mechanism And Effect On The Quality Of Dough

Frozen dough technology has been used as a typical way for the industrialization of baking.Loss of yeast viability and rupture of the gluten network induced by frozen pressure were generally believed to be the predominant factors for the deterioration of frozen dough.Although starch was the main constituent of wheat flour,no literatures were published about the starch deterioration in frozen dough and its effect on the breadmaking quality.This paper started with the deterioration mechanism of wheat starch during frozen storage,further explored the freezing-induced effects of wheat starch on the quality of reconstituted dough,and finally aimed to decrease the frozen deteriorations of starch granules through hydrocolloids.These fingdings could provide effectiveness for the quality modulation of frozen dough.The main research contents and conclusions were summarized as follows:Freezing rate affected the physicochemical properties of wheat starch.Wheat starches dispersed in one and a half times of volumes of deionized water were frozen at 0.18?0.37 and 1.54 oC/min until the core temperature arrived at-20 oC.The changes of physicochemical properties of starches were considered after the storage of 8 weeks.Results indicated that lower freezing rate resulted in higher content of damaged starch and lower contents of protein,lipid and amylose in starch.It caused an increase in relative crystallinity,gelatinization parameters and pasting characteristics.Meanwhile,higher water absorption was observed as the addition of starches frozen at slow freezing rate,which prolonged the dough development time.When it came to slow freezing rate(0.18?/min),damaged starch(DS)content was increased from 1.36 to 2.49%,while the protein,lipid,and amylose contents in starch were decreased from 0.41 to 0.20%,0.55% to 0.24% and 28.9% to 26.2%,respectively.The slow freezing rate had no significant effect on the crystal structure of starches,showing an A-type pattern,but the relative crystallinity(RC)value was increased from 31.4% to 34.6%,as analyzed by X-ray diffraction(XRD).Differential scanning calorimetry(DSC)indicated that the onset temperature(To)and enthalpy(?H)were increased from 56.55 to 58.15 oC and 9.54 to 11.83 J/g,respectively when the starches were subjected to slow freezing rate.Meanwhile,the viscosities of peak(PV),breakdown(BV),and setback(SV)were increased from 2352 to 2952 cP,560 to 771 cP,and 1053 to 1356 cP,respectively.Then,the native and freezing-treated starches were individually blended with gluten at a ratio of 86/14(starch/gluten).The specific volume of bread containing slow freezing-treated starches was decreased from 3.26 to 1.96 cm3/g,while the crumb hardness increased from 1.18 to 2.71 N.The starch properties were deteriorated by multiple freezing/thawing treatment.The changes in starch properties were investigated after freezing and thawing wheat starch suspension for 3,7,and 10 cycles.Multiple freezing/thawing treatments caused an increase in damaged starch and leaching materials,as well as the gelatinization and pasting parameters.These alterations facilitated an irreversible decrease in bread volume and an increase in crumb hardness.The granular surface of starch with 10 cycles of freezing and thawing treatment became coarser than that of native starch,as accompanied by higher DS value of 4.50%.At the same time,the protein,lipid,and amylose contents were decreased to 0.10%,0.06%,and 24.5%,respectively,while the RC was increased 35.7%.The values of To and ?H were also increased to 59.32 oC and 13.62 J/g,exhibiting a more ordered molecular structure of the starch granules.During heating process,PV was significantly increased to 2645 cP due to the 10 cycles of freezing treatment.However,high shearing treatment by a rapid visco-analyzer broken down the starch granules,resulting in that more amylose was leached and BV and SV were increased to 620 cP and 1238 cP,respectively.The bread containing freezing-treated starch(10 cycles)was in lower specific volume(2.49 cm3/g)compared with that of native starch,and had a harder texture structure(6.78 N).The smaller starch granules were,the more frozen deteriorations they had due to their high sensitivity to freezing process.The physicochemical properties of wheat starch with different particle size(A-type and B-type)were analyzed during 3 cycles of freezing/thawing treatment.The freezing treatment caused apparent damage,more loss of chemical components,and an increased crystallinity of B-type granules but not for A-type granules,increasing the gelatinization and pasting parameters.The bread quality was further deteriorated by the addition of freezing-treated B-type granules that failed to support the bread structure.A cracked structure was observed on freezing-treated B-type granules with an increased damaged starch content of by 1.71%,while that of freezing-treated A-type granules was increased by 0.95%.Meanwhile,freezing treatment also decreased the protein and lipid contents of B-type granules by 0.31% and 0.28%,respectively.This decrease led to an absence of peak at 2?=20o,a characteristic of amylose-lipid complex,thus increasing RC by 3.12%.Moreover,freezing treatment led to an increase of To and ?H by 2.25 oC and 2.39 J/g for B-type granules,respectively.It was followed with a marked increase in PV,BV,and SV by 794 cP,766 cP,and 522 cP,respectively,than did the A-type granules.The specific volume decreased with the substitution of B-type granules to 2.75 cm3/g,and a further decrease was visible in breads of freezing-treated B-type granules(1.29 cm3/g).In contrast,the crumb firmness of freezing-treated B-type granules was increased from 2.33 to 3.38 N.Loss of starch-associated proteins made starch granules sensitive to freezing treatment,resulting in more deterioration during freezing.1% sodium dodecyl sulfate solution was used to remove starch-associated proteins without causing any apparent damages on granular structure.Loss of starch-associated proteins led to higher water absorption for starch,resulting in significant damage on starch structure after freezing treatment,such as increased damaged starch,more leaching soluble proteins,higher gelatinization temperatures,and pasting viscosities,thus resultingin deteriorated bread quality.After removing surface proteins,freezing treatment made starch granules featured rough surface and hollowed structure,resulting in an increase of DS by 3.97%.This led to higher contents of high-molecular weight protein F1(60 kDa<Mw),low-molecular weight protein F2(16 kDa<Mw<60 kDa),and F3(Mw<16 kDa)in the supernatants of starch than frozen native starch.When the starch granules were gradually subjected to proteins removal and freezing treatments,the contents of F1,F2 and F3 increased to 0.77%,3.99% and 1.83%,respectively.Combined with protein removal and freezing treatments also significantly increased both To and ?H to 59.30 oC and 10.70 J/g,respectively,with an increase in values of PV,BV,and SV to 4775 cP,3969 cP,and 3838 cP,respectively.Bread prepared with protein-removal starch had a lower specific volume(2.61 cm3/g),while the combined protein removal and freezing treatments made a further decrease by 55.56%.The negative effect of combined protein removal and freezing treatments were also observed at the values of crumb firmness,which was increased by more than 2 times.Loss of starch-associated proteins and a certain increase in damaged starch led to a poor quality for both dough and bread.Wheat starch-gluten reconstituted dough was prepared and the ratio of starch/gluten was 86/14.The changes in the quality of reconstituted dough and bread were investigated by substitution of native wheat starch with freezing-treated starch,protein-removal starch,and damaged starch,respectively.The substitutions of these starches not only increased the water absorption of reconstituted flour,but also prolonged the dough development time.At the certain water content,the addition of these starches led to an unstable protein network structure that is manifested by a decrease in the amount of ?-helix and ?-sheet connected to the increased amount of ?-turn and random coil.This change resulted in an increase in both elastic modulus(G?)and viscous modulus(G?),and a decrease in Tan?.During heating process,the starch granule failed to get completely gelatinized,indicating a higher gelatinization temperature and lower enthalpy values.After storage of 7 days at 4 oC,the reconstituted bread got retrograded.The retrogradation enthalpies(?Hr)of freezing-treated starch,protein-removal starch and damaged starch were much higher than that of native wheat starch.Similar changes were observed in the firmness of crumb.A farinograph study showed that the addition of freezing-treated starch,protein-removal starch and damaged starch increased water absorption from 52.73 to 60.36,67.50 and 66.75%,respectively.Meanwhile,the dough development time was also increased from 6.86 to 8.17,15.19 and 9.67 min,respectively.Fourier transform infrared spectroscopy indicated a significant decrease in both ?-helix and ?-sheet when native wheat starch was exchanged for the freezing-treated starch.The amounts of ?-helix and ?-sheet were decreased from 36.5 to 24.6% and from 23.3 to 11.5%,respectively.Meanwhile,The amounts of ?-turn and random coil were increased from 22.3 to 33.5% and from 17.8 to 30.4%,respectively.Similar alterations were obtained in the groups of protein-removal starch and damaged starch.According to the analysis by DSC,the To value of dough reconstituted with freezing-treated starch,protein-removal starch and damaged starch were increased from 53.67 to 55.08,54.48 and 55.57 oC,respectively.In contrast,?H was decreased from 1.67 to 0.65,0.57 and 0.89 J/g,respectively.After storage of 7 days,?Hr was increased to 2.17 J/g for the freezing-treated starch,3.96 J/g for protein-removal starch and 3.84 J/g for damaged starch,respectively.At the same time,the crumb firmness was increased to 15.20 N,13.53 N and 12.78 N,respectively.By adding certain additives could improve the freezing-induced deteriorations of wheat starch.The effects of additives to the physicochemical properties of freezing-treated starch were investigated by adding 0.2% of guar gum,carboxymethyl cellulose sodium(CMC-Na),and monostearin individually.As the results showed,freezing-induced damage structure and leaching materials can be reduced when guar gum was used,maintaining the starch properties,improving bread volume,and softening the crumb texture.Analyzing the soluble content of the supernatant from wheat starch after freezing treatment,it was found that the soluble content was decreased from 0.71 to 0.39% when 0.2% of guar gum was added.A slight change was obtained on the contents of protein,damaged starch,and relative crystallinity.Similar changes were also observed in gelatinization and pasting parameters.In addition,the specific volume of bread containing freezing-treated starch was increased from 2.51 to 4.20 cm3/g and the crumb firmness was decreased from 4.37 to 3.50 N when guar gum was used.