Recent advances in quality deterioration and improvement of starch in frozen dough

Wenjuan Feng,Sen Ma,Xiaoxi Wang

College of Food Science and Technology,Henan University of Technology,Zhengzhou 450001,China

ABSTRACT Starch with different particle sizes has different compositions and different physical and chemical properties.The amylopectin content and protein content and lipid content of small granule B starch are higher than that of A-type starch, which can form spiral complex to limit the expansion of starch.Under the same treatment conditions,B-type starch is more vulnerable to damage,sensitive to freezing storage,even breaks in freezing storage,and has higher gelatinization temperature than other starch during freezing.Amylose contained in A-type has high content,high crystallinity,high degree of ordering after frozen storage and insensitivity to frozen storage,which is beneficial to improve the stability of pores and texture of frozen dough.With the extension of freezing time,the content of amylose decreased,while the content of various amylopectin increased and the conformation of polysaccharide changed.Starch deterioration is different under different frozen storage conditions.The deterioration mechanism of frozen dough can be evaluated by variety of starch characteristics during freezing process,which is of great guiding significance to the improvement of frozen dough technology.

Keywords: Frozen dough Starch structure Poor quality Improved dough technology

1.Introduction

Frozen dough technology was developed in the 1950s,it refers to the use of refrigeration technology to produce finished or semi-finished dough, which can be refrigerated for a certain period of time and subsequently processed using thawing and other follow-up processing processes to obtain the finished baked product [1].In the past few years,the frozen dough technology has been widely used in the baking industry.High-quality frozen dough is an obvious choice to accelerate the industrialization of cereal products[2].The products made from the finished frozen dough often experience a decrease in volume,and have a rough internal structure in the bread core, and a short shelf life.These characteristics have seriously hindered the development of frozen dough technology[3].Starch is the major component of flour,accounting for about 75%of the weight of flour[4].The water absorption expansion rate of starch determines the water holding capacity of flour and affects the softness of the finished products[5],which is highly correlated with its final quality[6].In 1989,Toyokawa et al.[7]suggested that a good quality dough should have an appropriate ratio of amylose/amylopectin.Shi[8]proposed that high-quality dumpling flour should have lower amylose content and amylose/amylopectin ratio.The increase of the amylose content will prolong the optimal cooking time and reduce the water absorption rate and cooking loss rate of dumpling skin,and therefore it is determined that the amylose content in high-quality dumpling wrapper should be less than 20%,while the amylose/amylopectin ratio should be less than 0.4.In the processing of flour products,the role of starch is mainly reflected in:(i)diluting gluten to the desired consistency,(ii)producing sugar by the action of amylase;(iii)providing a surface suitable for gluten binding;(iv)partially gelatinizing to soften the structure and further stretch the gas cell membrane during heating; and(v)promoting gluten hydrolysis by gelatinization of starch,which hardens the membrane, solidifies the appearance structure of flour products,and forms the external texture characteristics of flour products[9].

During the frozen storage of the dough,the formation of ice crystals puts pressure on starch granules, making the surface of the granules rough,imperfect and leading to the release of leaching solutions of amylose, protein, lipids,etc.Also, the freezing process weakens the double helix structure of amylose,accelerates the degradation of starch,which in turn affects the overall performance of the frozen product[10,11].The dough made by mixing starch and gluten, obtained from different wheat varieties, in different proportions has different rheological performance[12].Another study used tailored wheat flour as raw material to prepare starch and gluten,with 100%flour as blank control group,and other groups containing a certain proportion of added crude protein and starch with a flour content of 50%as experimental groups to study the changes in their internal structures after a number of freeze-thaw cycles[13].The study found that increasing the ratio of gluten to starch content can reduce,to a certain extent,the degree of deterioration of non-fermented dough during the freezethaw process.Currently,however,as the main ingredient of dough,the research on wheat starch in the frozen dough is limited,and there is a lack of in-depth research and regular summary on the specific deterioration mechanism of starch during the freezing process.This article summarized the current domestic and foreign research results on this topic to provide some guidance for wheat starch researchers.

2.The main composition of starch and related properties

2.1.The composition and nature of wheat starch

Wheat starch constitutes 70%-75%of the flour and is composed of three distinct types of granules, namely Atype,B-type and C-type.The A-type granules are lenticular in shape with a diameter greater than 16 μm, the B-type granules are spherical with a smaller diameter of 5-16 μm and the C-type granules are spherical with the smallest diameter of less than 5 μm[14].The difference between Atype and B-type granules is due to the difference in synthesis time during wheat grouting[15].The granule size of wheat starch will affect the quality of the dough.Bread made with reconstituted flour containing 100% small wheat starch granules was significantly softer than that made with reconstituted flour containing 100% large wheat starch granules[16].There are three possible reasons for that difference:(i)small wheat starch granules would exude less amylose during baking and as a result there will be less amylose between starch and denatured gluten thus leading to cross-linking reduction of the starch-gluten network; (ii)small granular wheat starch contains more lipids than the large granules, which may decrease firmness; and (iii)higher swelling power of small wheat starch granules may increase the retention of moisture in the bread crumb[16-19].A-type granules have higher amylose content and crystallinity,and since A-type starch(contains large amount of A-type granules)gelatinizes easily,it can improve the volume and texture quality of the bread[20].According to the tensile test, A-type starch and B-type starch (starch with large amount of B-type granules)have different effects on the tensile properties of the dough.The small particles of B-type starch(6.5-9.5 μm)improve the extensibility of the dough and reduce the extension resistance of the dough.When the diameter of starch particles is more than 27.5 μm,the tensile properties of the dough can be significantly improved [21].Accordingly, wheat starches consisting of different size particles have different effects on the properties of dough,which may be considered as a way to control the deterioration in the quality of frozen dough.Wheat starch can be divided into amylopectin and amylose-rich starch types according to its chain type, in which amylose has a linear structure,and amylopectin has a complex branched(tree)structure.Starch surface binding lipids are mainly triglycerides,free fatty acids,etc.,and the lipids in starch granules are mainly free fatty acids,monoacyl lipids, lysophospholipids, etc.[22,23].Wheat starch is also divided into hard starch and soft starch according to whether the source is hard or soft wheat.The fat content of hard starch is much lower than that of soft starch[24].Hard starch has slow water absorption rate, high water absorption capacity and long gelatinization time.In contrast,soft starch has fast water absorption rate and low water absorption capacity[25].

2.2.The crystal type and properties of wheat starch

Wheat starch has a semi-crystalline structure, with a crystalline content between 15%and 45%,and depending on its architecture and hydration state it exists in four polymorphic forms with distinctive crystalline patterns,namely the A-type, B-type, C-type and V-type.X-ray diffraction(XRD)analysis and molecular simulation technology have confirmed that the double helix in the polymorph with A-type crystalline structure is closely arranged with water molecules to form a monoclinic system,and each double helix in the crystal cell contains four water molecules[26].Instead,the arrangement between the double helices in the polymorph with B-type crystalline structure is relatively loose,and form a hexagonal crystal,and each hexagonal arrangement of six double helices in the crystal cell contains 36 water molecules,with the water molecules located in the cavity surrounded by the six double helices[27](Fig.1).The crystallinity of high-amylose wheat starch is about 9%,while that of waxy wheat starch is about 40%.Generally speaking,the higher the content of amylose,the lower the crystallinity.The moisture content of starch granules and the ambient temperature will also affect the crystallization of starch granules.For instance, a moisture content of 10%-50%does not affect the proportion of double helix structure in starch granules,but the moisture content from 1%to 3%will lead to a significant reduction in the proportion of double helices.The changes in ambient temperature will also affect the crystallinity of starch granules,since as a semi-crystalline polymer,the glass-rubber transition temperature of starch is affected by the water content.Thus,the higher the water content,the lower the glass transition temperature.This property has guiding significance for the improvement of frozen dough technology.When starch gelatinization is completed, the “Maltese cross”pattern in starch granules will disappear.XRD analysis can reveal that the crystallization region disappears at this time.The temperature rate that the crystallization region disappears depend on the moisture content and the type of starch.With the decrease of the moisture content,the disappearance temperature of the crystallization region increases.When the moisture content is less than 50%,the temperature for the complete disappearance of the crystallization region can reach 100°C[9].

3.Changes in starch property in the process of freezing

3.1.Effect of frozen storage on the basic composition of wheat starch

The main indexes used to study the effect of frozen storage on the basic components of wheat include the content of protein,fat,amylose,and damaged starch.Some of the factors considered include freezing rate, frozen storage time and freeze-thaw cycle.With the increase of the freezing rate,the loss of basic components of wheat flour decreased[28].Meat products are usually treated by quick-freezing instead of slow-freezing as the small ice crystals formed by quick-freezing cause little damage to tissues,and there is less juice loss after thawing and little influence on flavor.However,wheat starch in frozen storage may also be a reason.Slow-freezing causes the increase of ice nuclei resulting in mechanical damage to starch,and more fat,protein,and amylose are extracted after thawing[29].The binding protein,amylose and fat contents of starch granules all decrease with the number of freeze-thaw cycles,while the damaged starch content increases due to the dissolution of amylose in amorphous regions [30].With the extension of the freezing time, the amylopectin content of each type of starch granule increases, while the amylose content decreases.The decrease in the ratio of amylose to amylopectin further accelerates the conformational change of polysaccharide chains.However, the damaged starch content of B-type granules is higher than that of A-type granules.This is due to the higher content of amylopectin in Btype granules than that in A-type granules,as a result,it is easier to degrade during the freezing process, which confirms that of B-type granules are more sensitive to freezing than A-type granules [11].

3.2.Effects of frozen storage on the microstructure of wheat starch

Fig.1.Diagram for packing configuration of double helixes and water molecules in starch granules.

Yang et al.[31]studied the physical and chemical properties of whole wheat starch and its A-type and B-type granules obtained from frozen dough at 0,4 and 8 weeks of storage.The scanning electron microscopy(SEM)images of whole wheat starch(Fig.2)revealed that,whether frozen or not,the morphology of A-type and B-type granules is similar to that of natural starch.Although A-type granules are lenticular in shape,and B-type granules are spherical in shape,they all showed different degrees of damage during the freezing process.For instance, the surface of A-type granules is slightly rough after freezing,whereas the morphology of Btype granules is greatly changed,such as incomplete granules and irregular edges,indicating that the freezing process led to the fracture of some B-type granules[32,33].Not only the freezing temperature and time will affect the starch structure of frozen dough,but also some factors affecting the water activity will cause the change of the starch microstructure.Fig.3 is a scanning electron microscope picture of freeze-thaw cycles for 3 times,7 times and 10 times.The surface of starch without freeze-thaw treatment is very smooth,and some grooves appear in the treated starch.These changes became more evident after multiple exposures to ice crystals during freezing.It could be ascribed to the effect of freezing pressure on the granules.Starch granules were compressed and crumbled due to the phase transformation.The internal or external ice crystals of granules occupied more space than an equal amount of water, generating a squeezing force.Freezing treatment resulted in the deformation, disruption, or coalescence of wheat starch granules.When the structure of starch changes,the system composed of starch and gluten is also destroyed[34].

3.3.Effects of frozen storage on the crystal structure of wheat starch

The effect of frozen storage on the wheat starch crystal structure can be analyzed by Fourier transform infrared (FTIR) spectroscopy and X-ray Diffraction(XRD).Starch has unique FTIR absorption peaks at 1,045 and 1,022 cm-1, representing the structure of the crystalline and amorphous regions of starch,respectively.The A 1,045/1,022 ratio can be used to indicate the degree of crystallization of starch granules in the short-range, which increases during slow freezing and decreases with the increase of the freezing rate [31].With the acceleration of the freezing rate, the increase of the relative crystallinity of starch gradually decreases.This is due to the loss of amylose, protein and starch ester which leads to the rearrangement of double-helical structures in the crystal region, thereby changing the proportion of crystalline region [34].After the freezing treatment, no new chemical groups and basic structural changes were found in all starch samples.The spectral intensity of the B-type starch granules showed an upward trend during storage.The content of short-range ordered crystal structure (double helix content) in starch was quantitatively determined by measuring the absorbance ratio of 1,047 cm-1/1,022 cm-1(R1047/1022) and corresponded with the hydrophilicity enhancement of starch [35].It was found that A-type granules had higher R1047/1022than B-type granules, indicating that A-type granules had more ordered crystal structure.The R1047/1022of B-type granules was found to clearly decrease with the extension of the freezing time [36,37].The crystallinity of damaged starch was found to significantly increase in the late frozen storage (FS) and continuous freeze-thaw(FT) cycles [38,39].

3.4.Effects of frozen storage on the thermodynamic properties of wheat starch

The gelatinization temperature indicates the quality of the starch crystal structure,and gelatinization enthalpy reflects the energy required to break the double-helical structure of starch [40].The gelatinization temperature and gelatinization enthalpy of starch showed an increasing trend with the slowing down of the freezing rate,which destroys the double-helical structure and increases the order of the starch structure[27].As a result,starch is difficult to gelatinize and the gelatinization temperature decreases.Decrease of the gelatinization temperature can make food gelatinize more fully for food.However,ordered starch requires more energy to unlock and melt the double-helical structure.It has been shown that, compared with A-type granules and whole wheat starch,B-type granules have a wider gelatinization temperature range,with lower starting temperature(T0)and peak temperature(Tp).All starch samples showed a wide temperature range after freezing.When the ΔH increases slightly,the thermal properties of whole wheat starch in the frozen dough are closer to those of Atype granules.Therefore,the effect of freezing on the thermodynamic properties of wheat starch can be attributed to the increase of the stability,order and crystallization of the structure of starch[31].The change of the starch structure will affect the degree of interaction between starch and protein, leading to an increase in the time required by starch and gluten to form a standard network structure,resulting in the reduction of the dough stability time and increase of the formation time.As a result,the prepared frozen dough bread has a small volume and high hardness.

3.5.Effects of frozen storage on the properties of wheat starch paste

Fig.2.Scanning electron microscope of the starch granules in frozen dough[31].

Raw starch is insoluble in cold water and can only produce a starch suspension.During the heating process,starch will irreversibly absorb a large amount of water, starch granules will expand, microcrystalline bundles will disintegrate, and amylose will dissolve to form a paste.The temperature at which the viscosity of the starch slurry starts to rise is the pasting temperature.At the same time, the viscosity of the starch slurry will continue to increase.Under the action of a shear force, the starch structure will crack, and the viscosity of the starch paste will decrease.The peak viscosity and low viscosity are usually used to express the maximum viscosity and the minimum viscosity in this process.The difference between the two is the disintegration viscosity.When the temperature falls,some starch molecules recombine to form precipitates or gels,and the viscosity increases again.The setback viscosity is used to measure this change [9].The aging pattern of starch gel is shown in Fig.4.B-type granules have highfat content and will form helical complexes to limit the expansion of starch.The hydrolysis of starch in the granular state can be increased by de-fatting starch.Therefore, the gelatinization temperature of B-type granules during the freezing process is higher than that of starches with other types of granules [41,42].Freezing leads to the loss of compactness in the grain boundary and amorphous regions of B-type granules, resulting in a weak structure and inability to resist shearing forces after heating [39].The peak, trough, breakdown and final viscosity of Atype granules slightly increase, which may be related to the little change in their amylose content.The Rapid Visco Analyser(RVA)parameters showed that the freezing time had no significant effect on the properties of wheat starch samples, which was attributed to its large proportion of A-type granules [43].The gelatinization characteristics of wheat flour are closely related to the quality of the frozen dough steamed bread.Peak viscosity is significantly negatively correlated with the texture profile analysis (TPA) hardness of the frozen dough steamed bread;trough viscosity is significantly positively correlated with resilience;decay value is significantly positively correlated with TPA hardness, and extremely correlated with a total sensory score; appearance condition and softness are significantly negatively correlated.The gelatinization temperature has a significant positive correlation with the TPA elasticity and adhesiveness.It also has a quadratic nonlinear negative correlation with the whiteness of the frozen dough steamed bread crumb.The collapse value almost has a significant negative correlation with the specific volume of the frozen dough steamed bread [44].

3.6.Effect of frozen storage on the cooking quality of wheat starch

With the increase of damaged starch content,the quality of noodles will deteriorate.The dry matter loss rate and protein loss rate are highly positively correlated with the value of damaged starch.The dry matter loss rate increased sharply when the starch loss value exceeded 66.99 Farrand.There is a phenomenon of mixing soup[45].The crystallinity,particle size and gel aging properties of wheat starch also affect its baking properties.These physical and chemical properties are related to the changes in basic starch components, the ratio of amylose to amylopectin and the amount of damaged starch mentioned earlier[46].

Fig.3.SEM photographs of native and frozen-treated wheat starch granules:(A)native wheat starch(NWS),(B)three freezing/thawing-treated starch(3 FTS),(C)seven freezing/thawing-treated starch(7 FTS),and(D)ten freezing/thawing-treated starch(10 FTS).

4.Modification of starch property on the improvement of dough performance

4.1.Modified starch

Fig.4.Model of starch gelatinization and retrogradation.

Modified starch is a kind of starch derivative obtained by introducing new functional groups on the original starch molecules or changing the molecular size of starch components and the properties of starch particles through physical, chemical and biological methods [47].Commonly modified starches include esterified starch,carboxymethyl starch,pregelatinized starch,acetylated starch,etc.For frozen food,cross-linking and stabilization(esterification and etherification) are two important starch denaturation methods [48].Cross-linking generally involves adding a cross-linking agent to react with alkaline starch slurry at 30-50°C to reach the required reaction degree,and then neutralizing, filtering, washing with water and drying.The starch granules produced can resist excessive cooking and other changes in processing conditions,and are resistant to acid,heat,and shear.Lin et al.[49]proposed the effects of four different kinds of modified starches (potato hydroxypropyl starch, potato acetate starch, tapioca hydroxypropyl starch,and tapioca acetate starch)on the thermodynamic properties of frozen dough (glass transition temperature,ice crystal melting characteristics and freezing water content)by differential scanning calorimetry(DSC)analysis.Their study showed that adding 5% potato hydroxypropyl starch,tapioca hydroxypropyl starch,and tapioca acetate starch could significantly reduce the melting range of ice crystals in frozen dough,and the size of ice crystals formed in the dough was more homogeneous.The content of freezing water decreased with the increase of potato acetate starch and potato hydroxypropyl starch [50,51].Cassava starch has great differences in physicochemical properties after denaturation by different methods.Among them, hydroxypropyl distarch phosphate has the best freeze-thaw stability, paste viscosity is stable, and pregelatinized starch has the worst freeze-thaw stability.As shown in Fig.5,without adding modified starch,the gluten network of frozen dough was mechanically damaged due to strong ice crystals injury,there was incomplete expansion of the bottom of the bread center,a specific volume of bread significantly decreased,and the color of bread became darker.However,when modified starch was added,the above phenomenon did not occur in the three groups of bread.In particular,the cores of bread added with hydroxypropyl distarch phosphate group were uniform and still within the sensory acceptance range of consumers[52].

Converted starch is modified by acid or enzyme.After starch was treated with β-amylase(BA),trans-glycosylase(TG)and pullulanase(PUL),the ratio of short-chain and degree of polymerization (DP) 6-12 chain increased by 21.03%.Compared with the original wheat starch,the relative crystallinity of the enzyme-modified chain decreased by 6.62%,but an orderly and stable structure was formed.In terms of gelatinization viscosity, the BA →TG →PUL treatment increases the peak viscosity.The final viscosity has a very good effect on thickening pudding,jam,etc.[53].

4.2.Additives that interact with starch

Most of the starch improvers strengthen the threedimensional mesh structure of gluten by interacting with gluten protein,or forming a complex or promoting gluten protein absorption to form more resilient gluten,thereby inhibiting the melting of starch.The improvement associated with starch is divided into the following four types.

(i) Hydrophilic colloid can be adsorbed on the surface of starch particles,thus reducing the dissolution of soluble amylose after the starch gel is expanded and broken, weakening the recrystallization of starch and ultimately slowing down the aging rate of starch[54].The addition of guar gum,sodium carboxymethylcellulose and monoglyceride can significantly reduce the structural damage and protein dissolution of freeze-thawed wheat starch,and maintain the crystalline properties,gelatinization properties and thermodynamic properties of starch granules.Additionally,it improves the specific volume of dough bread and softens the texture of bread[28].

(ii) The interaction between an emulsifier and starch can delay the retrogradation process of starch,slow down the water migration between gluten and starch, and delay the water absorption aging of starch, thereby delaying the aging of bread [55].However, Teng et al.[56] found that adding lecithin could significantly improve the specific volume of fresh bread,but had no noticeable effect on the specific volume of frozen dough baked bread.

Fig.5.The cross-section graphs of steamed bread by frozen dough[52].

(iii) The α-amylase can provide the energy source needed for yeast growth and reproduction during dough fermentation,and can ultimately hydrolyze the damaged starch in flour into maltose and glucose,ensuring the continuous fermentation of the dough.In the process of dough mixing,the air is mixed into the dough system, and glucose oxidase can play an oxidation role to convert glucose into gluconic acid and generate strong oxidant hydrogen peroxide to oxidize sulfhydryl(SH)groups in gluten molecules into disulfide bonds(SS) to strengthen the gluten network and enhance the dough structural strength.Hydrogen peroxide oxidation in the dough can generate free radicals,which can also promote water solubility in the dough system.The peroxidation of sugar forms a gel, which is crosslinked with the gluten network to strengthen the three-dimensional network structure of dough[57].

(iv) The addition of dietary fiber reduces the relative content and concentration of starch in the gelatinization system,which leads to a decrease of the gelatinization parameters.The water absorption capacity of dietary fiber is much higher than that of starch,and protein in wheat flour reduces the transportation of available water in the gelatinization system and further hinders the water absorption and gelatinization of starch particles.At the same time, the starch/water ratio in the gelatinization system will be increased to different degrees[58,59].The dilution effect of dietary fiber on the starch concentration in the gelatinization system may be greater than the increase of the starch/water ratio caused by its water absorption capacity[60].

5.Conclusions and prospects

In summary, frozen storage affects the basic composition, microstructure, crystal structure, gelatinization properties and thermodynamic properties of wheat starch, and further affects the cooking properties and baking properties of its flour products.Modification of raw materials is one of the most important methods of food processing.Cross-linking and stabilization of starch endow modified starch with new properties and have added significance for the development of the food industry.It is not enough to address the technical problems of frozen dough only by compounding additives.All additions should consider cost, safety and effectiveness issues.Therefore, accurate addition and control can be realized by researching the deterioration of basic components.Future research work should focus on the role of the freezing system, composed of basic components and ice crystals, in maintaining the steady-state.Specifically, the functional relationship between ice crystal growth and freezing conditions, as well as the quantitative assessment indices the deterioration of basic components could be established through separation and recombination of basic components and analysis the microstructure of the frozen dough.In addition,it is very necessary to thoroughly research the mechanochemical effects on the dough during frozen storage.

Conflicts of interest

The authors declare that there are no conflicts of interest.

Acknowledgments

This work was supported by the Key scientific and technological project of Henan Province(No.202102110143),Talent Projects from Henan University of Technology(No.2018RCJH08) and the Joint Foundations from the NSFC-Henan Province(No.U1604235).