Effect of wheat bran insoluble dietary fiber with different particle size on the texture properties,protein secondary structure,and microstructureof noodles

Jian Zhang,Mengqin Li,Chaoran Li,Yanqi Liu

College of Food Science and Technology,Henan Agricultural University,Zhengzhou 450002,China

Keywords:

Wheat bran

Insoluble dietary fiber

Water distribution

Microstructure

Noodle quality

A B S T R A C T

This study was conducted to explore how the insoluble dietary fiber(IDF)of wheat bran with different particle size affects the texture properties,water distribution,protein secondary structure and microstructure of noodles.The results suggested that IDF addition increased the cooking loss and decreased the sensory evaluation because of the damage on dough structure,while as the IDF particle size decreased,the sensory score increased from 78.8 to 82.3 and cooking loss decreased from 8.65%to 7.65%,which could be attributed to that small particle-sized IDF limited the damage on protein network structure,decreased the T22 and t-structure,and increased the β1-structure.Moreover,IDF particle size had a significant correlation with protein secondary structures,texture properties and evaluation score of noodles.In conclusion,adding appropriate particle size would be an effective way of enhancing the nutritional and textural properties of noodles.

1.Introduction

There was an interest in food product development which not only had good taste but also functional ingredients.Wheat bran,as the by-products of food processing industry,which was a rich source of dietary,functional and fibers,the content of the total dietary fiber(TDF)in wheat bran was approximate 20%-30%,but nearly 90%of TDF was insoluble dietary fiber(IDF),which had higher water retention capacity and water swelling capacity.IDF could enhance satiety,increase fecal volume,and shorten defecation time,thereby decreased the risk of obesity,constipation,and colon cancer[1-3].

Despite its various health benefits,wheat bran IDF was not favored by the consumer due to its rough texture and bad flavor.Therefore,many efforts had been made to modify IDF, including decreasing their particle size and improving the soluble dietary fiber(SDF)content,which might result in certain changes in its structure,porosity,surface area,and functional properties,etc.Compared to native bran,adding fermented bran improved the characteristics of the dough and the quality of steamed bread[4].With the development of new technologies,some researches revealed modified DF through complex enzyme method,ultrafine comminution,high hydrostatic pressure,or hydrostatic pressure-enzyme treatment improved the content of SDF,the cholesterol adsorption and water retention capacity because of the loose microstructure[5,6].Moreover,the superheated steamstabilized wheat bran showed comparable antioxidant activities to that of native wheat bran[7].

However,dietary fiber addition influenced the dough microstructure and decreased the quality of texture and taste,which limited the consumption of dietary fiber[4].The findings of Liu et al.[8]showed that adding native wheat bran decreased the maximum tensile resistance and increased the hardness and chewiness of steamed bread.Moreover,DF incorporation affected dough characteristics and cookie quality due to change the thermal properties and water availability,and the affected extent largely depended on the type of fiber and the level of substitution[9].Therefore,most studies tried to improve the effects of DF by changing the particle size.These studies had found that changing the particle size of bran could make up for the negative effects on bread quality,the reason might be due to the increasing water holding capacity,better hydration properties,and a higher adsorption capacity,the other reason could be attributed to the increased particle surface and liberation of reactive components due to cell breakage[10].Meanwhile,other studies demonstrated that superfine grinding had not significantly improved the water holding capacity of carrot IDF[11],markedly decreased the water holding capacity and swelling capacity of citrus pomace IDF[12].These variations could be explained by several reasons,such as variation in the definition of bran,natural variation in composition and physical properties,variation in food procedures and in compensations made.However,there was scare information on the influence of wheat bran IDF with different particle size on noodle quality and its mechanism.

Therefore,in this study,not only the effect of the wheat bran IDF with different particle size on the texture and sensory qualities of noodle was determined,but also the effects on water distribution,protein secondary structure and microstructure were evaluated to understand the mechanism which caused the effects of bran fractions with different particle size on noodle quality.

2.Materials and methods

2.1.Materials

Wheat bran and wheat flour were obtained from Jinyuan Flour Mill Co.,Ltd.(Zhengzhou,China).Wheat bran was micronized by using HM-701B nano-ball-milling(Beijing Tianyuan round-Mechanical Technology Co.,Ltd.,Beijing,China).

2.2.Preparation of insoluble dietary fiber(IDF)

IDF was produced by wheat bran according to the method of Gunenc et al.[13]with little modification.Firstly,wheat bran was exposed to enzymatic digestions(α-amylase,alcalase protease,amyloglucosidase)to remove starch and protein, thereafter centrifugation (10,000 r/min), the residue was washed sequentially with hot water,ethanol(95%),and acetone(95%),and lastly filtered and recorded as IDF.Five different grades of IDF samples were separated by the nano-ball-milling with different particle size,labeled as A(＞125 μm),B(83-125 μm),C(63-83 μm),D(50-63 μm),and E(＜50 μm).

2.3.Preparation of dry Chinese noodles

The dry Chinese noodles were made according to the method of Zhang et al.[14].Five kinds of wheat bran were mixed well with flour at a ratio of 3.0%.Noodle making dough was prepared by mixing 200 g flour,3 g salt and 6 g IDF with 72 mL water to produce dough crumbs that were aggregated by a Laboratory Mixer (Guangzhou Weierbo Machinery Co.,Ltd.,Guangzhou,China).The stiff dough obtained was calendered through the sheeting rolls of MT12.5 laboratory noodle machine(Henglian Food Mechanical Industry Co.,Ltd.,Guangzhou,China),sheeted four times using the 3.5 mm roll gap setting, and rested in a plastic bag for 40 min at room temperature,then successively sheeted using 2.5,1.8,1.3,1.1,and 1.0 mm roll gap settings.The final dough sheet was cut to produce 3.0 mm wide and 1.0 mm-thick noodle strips.The noodles were dried in a convection oven at 42°C for 4.5 h with the humidity at 70%,and then preserved in a preservative plastic bag at room temperature.

2.4.Analysis of cooking loss of noodles

Cooking loss was determined by the method described by Han et al.[15]with slight modification.For each measurement,ten strands of each type of uncooked noodles were weighed and then boiled in 400 mL of distilled water until the white core disappeared.The cooked noodles were rinsed with 200 mL deionized water and drained on filter papers, and cooking loss was determined by measuring the weight of solid substance lost from noodle strands into the cooking water.

2.5.Analysis of textural properties of noodles

The mode of texture profile analysis(TPA)was to evaluate the texture properties using the analyzer of TA-XT2i(Stable Micro Systems,Surrey,UK)equipped with a 5 kg load cell and a probe 35 mm.The pre-test,test,post-test speeds were all 1.0 mm/s, the interval time between the first and second cycles was 2 s, and the compressed ratio was 70%.The test was done in five replications,hardness,springiness,and chewiness were calculated from the curves adopting the method described by Brady and Mayer[16].

2.6.Evaluation of the sensory quality of noodles

All noodles samples were prepared and cooked in boiling water(at a ratio of 1:10)for 4.0 min and cooled for 2.0 min in 20°C water[17],and then evaluated the sensory quality.Ranking tests were conducted at a sensory laboratory,which was adequately ventilated and maintained at a comfortable temperature and humidity.Ranking tests of freshly cooked noodles were carried out by 30 semi-trained panelists,which consisted of male and female students of 20-30 years old.Noodle evaluation was conducted according to a standard of SB/T10137-1993 [18].The evaluation details were shown in Supplementary Table 1.The experiment was conducted at room temperature and relative humidity of 50%-60%.

2.7.Free thiol groups and disulfide bonds of the dry noodles

Both the free sulfhydryl group(SH)and disulfide bond(SS)contents were determined with Ellman's Reagent according to the method of Ellman[19].

2.8.Protein secondary structures of the dry noodles

Protein secondary structures in the noodles were determined using the Spectrum GX Fourier Transform Infrared Spectrometer (FT-IR) (Perkin Elmer, Massachusetts, the United States).The resolution was set to be 4.0 cm−1,and the wavelength range was between 4,000 and 400 cm−1.The FT-IR spectrum deconvolution was performed with Omnic and Peakfit software.The secondary derivative fitting exhibited 6-7 sub-peaks with＞0.96 residuals,and the protein secondary structure feature peaks were recognized,and each peak's percentage was calculated.

2.9.Microstructure of the dry noodles

The microstructures of the noodles were examined by the S-3400NII Scanning Electron Microscope (SEM) (Hitachi Inc., Tokyo, Japan).The dry noodle samples were fractured by folding and followed by mounting on specimen stubs with fractured side facing up.The mounted noodle samples were sputtered with a layer of gold,and magnifications of ×3,000 were used.

2.10.Statistical analysis

The experimental data obtained in this study were expressed as the mean value±SD of at least three replicated determination results.Oneway analysis of variance was used to analyze the data,and significant differences among the obtained mean values were compared by Duncan's test using SPSS13.0(SPSS Inc.,Chicago,USA).Differences were considered significant at P ＜0.05.

3.Results and discussion

3.1.Effect of IDF particle size on the cooking loss of noodle

Some soluble starch and protein in noodle were immersed in water,thus making the water turbid,cloudy and thick during cooking.The cooking loss of noodles was an important parameter for evaluating their overall quality[20].Several studies reported that fiber incorporation in pasta increased cooking loss by influencing a uniform diffusion in cooking water[21]and by weakening gluten network,which retained the amylose during cooking[22].Our results also showed that cooking loss was significantly increased by IDF addition(from 6.57%to 8.65%)(Table 1).While the finer particle size reduced the risingtendency(from 8.65%to 7.65%)(Table 1),the reason could be explained by that some active groups would be exposed and combined with the gluten protein in the noodles,thus limited the water mobility and improved the formation of stronger protein networks surrounding starch particles,thereby preventing soluble components from entering the cooking water.

3.2.Effect of IDF particle size on the noodle sensory evaluation

Noodle sensory attributes such as color,flavor,palatability,toughness,stickiness,smoothness,and taste were highly associated with the structural and textural properties of cooked noodle.Generally,the optimal characteristics for noodles would be a chewy noodle with a smooth texture.Noodles that were too stiff or too sticky would obviously be less popular.Compared with the control(83.1),IDF-added noodles showed the lower scores(from 78.8 to 82.3)on the sensory properties,might be related to the higher hardness and lower springiness(Table 1).While,the sample with small particle size(D,E)had higher score that that of large particle sized sample(A,B,C).The results were also consistent with the noodle microstructure as examined by SEM,and textural properties,which indicated that IDF disrupted noodle structure and reduced springiness values.

3.3.Effect of IDF particle size on the noodle texture analysis

Texture profiles were widely used to represent the texture,taste,and feel of food.TPA could simulate the chewing action of teeth and was a quantitative evaluation method for the textural properties of foods,on the basis of the analysis of a chewing curve.The particle size of IDF significantly influenced the hardness and chewiness of noodles(Table 1).The hardness of noodles,which was an indicator of its resistance to the destructive force during processing and application,decreased significantly(P ＜0.05)to 2186.2 g with the C(63-83 μm)and then remained almost unchanged.Similar to hardness,chewiness decreased almost in the same pattern with the decrement of particle size(data not shown).

These results were in line with the textural properties of the DF added tofu[23]and milk protein concentrate powder added bars [24].It was well known that the protein structure had significant influence on the wheat noodle texture,the reduced hardness and chewiness of lager particle size(A,B,and C)maybe due to anionic groups on the surface of IDF,which reduced protein-protein interaction,thereby damaging the gluten network[25].However,the continuous decrease of particle size would increase the amorphous area in IDF,and the active groups such as pentosan would be exposed and combine with the gluten protein in the noodles through the active bonds of phenolic acid,thereby gradually enhancing the gluten of the noodles.The other reason was possibly due to that particle size of the IDF aected protein chain association during making of noodles,which was confirmed by the results that insoluble large particles other than protein acted protein chain association during gelation of tofu[25].The results were consistent with the changes in water distribution due to changes in microstructure of the IDF added noodles(Fig.1).

The springiness was also affected by IDF addition.Compared to the control,springiness decreased by the IDF addition with all particle size,while springiness of cooked noodles containing IDF with finer particle size was significantly higher (P ＜0.05) than that of coarse sample.The higher springiness was caused by the addition of finer IDF which could reduce the water mobility, in turn minimize the damage of fiber on gluten network.

Table 1 Effect of IDF particle size on the noodle texture analysis and sensory evaluation.

3.4.Effect of IDF particle size on the noodle's T2 relaxation time

In food,most of the protons were from water.When a water molecule tightly interacted with the substrate,mobility became limited,the relaxation was accelerated,and the T2time was shorter.Whereas for free water molecules, it took a longer time to reach equilibrium, thus the T2time was longer.Therefore, measuring T2relaxation times provided insight into the water distribution and microstructure of network system.The two-exponential curve fitting revealed two water populations in the noodle samples,whose relaxation times were in good agreement with the T2relaxation times of various starch-water mixtures [26].T21ranged from 0.448 ms to 0.528 ms,T22was between 8.815 ms and 17.515 ms,which were on behalf of higher and lower mobility,respectively(Fig.1).These results agreed with that rice flour suspensions with turanose before and after gelatinization had two water populations[27].Compared to the control,addition of IDF decreased the T21in different degrees,which was possibly due to that the binding force between fiber and water was greater than that between gluten/starch and water[28].This result indicated that IDF appeared to be effective at holding more water and limiting the water movement,probably impeding the interactions between water and protein/starch,which correlated with okara dietary fiber at lower added proportion decreased the T2of tofu [23].Moreover, with the particle size decreasing,the time of T2decreased firstly(A,B,C),and then increased(D,E).The reason could be explained that the bran IDF was pulverized to grade C or finer size would damage the crystal structure and internal network or the strong mechanical shearing provided more hydrophilic groups.

3.5.Effect of IDF particle size on the SH and SS contents of the noodles

Fig.2 demonstrated the effect of different bran IDF particle size on the amount of free sulfhydryl groups(SH)and disulfide groups(SS)of the noodles.The ratio of disulfide cross-links and free thiol groups were necessary for aggregation of gluten and quality of noodles,representing the degree of tightness of protein spatial structure[29].The SS content of noodles was reduced from 1.73 to 0.65 μmol/L(P ＜0.05)when decreasing the IDF particle size in the dough formula.This result was like a previous study where the replacement of wheat flour with DF leaded to a decrease of SS content from 54.1 to 41.5 μmol/g[30]and Konjac glucomannan resulted in a decrease in the concentration of SS.While,the free SH increased from 4.38(control) to 5.36 μmol/L(B) (P ＜0.05), and then remained almost unchanged.Furthermore,the decrease of SS in the IDF groups was not balanced by an increase in free SH(C,D,E),which indicated an irreversible loss in the thiol equivalent groups.The slight decrease in SS groups was not balanced by an increase in free SH content of overcooked pasta and DF-ferulic acid added gluten protein[30,31].The possible reason might be due to that when the particle size was large(A,B),IDF could relatively easily split off the gluten network structure,and the inter-chain disulfide groups were more easily broken,thereby the SS content decreased,and the SH content increased.While,the bran IDF's crystal structure was partially translated to an amorphous structure,produced small segments,and released more active groups at the smallest particle size (＜63-83 μm),which interacted with gluten protein.

3.6.Effect of IDF particle size on protein secondary structure of the noodles

The effect of different sized IDF particles on the noodles'protein secondary structure was studied by FT-IR.The vibrations of CO,CN,and NH that were associated with protein secondary structure,and could be evaluated by FT-IR spectroscopy,which was a rapid and non-destructive technique.The amide I band (1,600-1,700 cm−1) and the amide II band(1,220-1,330 cm−1)were two major bands that were commonly used to recognize protein secondary structure[28].The proportion of peak area as β1-structure(the strongest intermolecular H-bond,1,611-1,624 cm−1),β-structure(intramolecular H-bond,1,625-1,640 cm−1),γ-structure(random coil,1,641-1,648 cm−1),α-helix(1,650-1,659 cm−1),t-structure(β-turn,1,660-1,689 cm−1)was determined(Table 2).

Fig.1.Effect of particle size on the water distribution of the noodles.

Compared to sample without the added IDF,the IDF addition increased the α-helix in different ways,which was associated with the high holding capacity(data not shown).Moreover,particle size of IDF had significant influence on the secondary structure(P ＜0.05).As the IDF particle size decreased, t-structure (from 34.07% to 24.18%) and α-structure (from 17.36%to 16.31%) decreased, while the β1-structure(from 16.66%to 21.46%), β-structure (from 15.14% to 19.34%), and r-structure (from 16.78% to 19.10%) exhibited a slight upward trend.This result was in agreement with that when there was relatively low water content in the system(35%-45%),the bran's hydrophilic groups competed for water molecules,which caused water re-distribution and gluten partial dehydration,thus the elastic t-structure collapsed and turned into the lack-of-elasticity β1-structure and r-structure[32].This result agreed with that the dietary fiber in the dough caused the gluten to become less elastic and stiffer,which was consistent with the t-β1transformation hypothesis[33].The reason could be explained by that as the IDF particle size decreased,the surface areas between the IDF and protein interactions increased and the active group was exposed (especially the ferulic acid monomers from the arabinoxylan side chain);on the other hand,the pulverization destroyed the IDF cell wall and released a few cell-active substances,such as the embedded enzyme,conjugated ferulic acid monomers,and so on.Meanwhile,IDF's active groups and the active substances inside the cell together affected the charge of the protein, the formation and stabilization of αstructure.The β1-structure formation(intermolecular β-fold)as well as its aggregation in the gluten protein network might be due to the baked bread's smaller volume that resulted from adding bran.Overall,changing the IDF particle size consequently influenced the protein secondary structure content as well as the protein interactions.However,the total interactions within protein molecules (α + β) only changed slightly, and the protein molecule still retained its internal structural integrity.

Fig.2.The content of SH and SS in the dry noodles with different particle size of IDF

3.7.Effect of IDF particle size on the noodle microstructure

The noodle microstructure played an important role in understanding the internal structure of granules.The scanning electron microscopy(SEM)experiments were performed in order to understand the effect of IDF particles size on the noodle microstructure (Fig.3).High quality cooked noodle might have a continuous protein network and tangle structure.A typical structure of wheat noodle was observed in the control,the starch granules were almost wrapped in a gluten network, but some of the starch was exposed to the surface of the network.Moreover, some starch granules were deformed by the rollers;however,the shapes were still apparent as completely granular.Overall,the noodle interior could be described as loose concrete,with protein acting as the mud and starch as the pebbles.Wheat bran IDF mixed in the noodle's concrete structure looked like layered rocks.IDF with larger particle size addition decreased the extent of starch exposure and destroyed protein network(Fig.3A and 3B),which was consistent with that IDF decreased air pockets and damaged gluten structure[34].However,with the particle size decreased,a loose protein network formed was also observed(Fig.3D and 3E).Our results were confirmed by DF increased elasticity of starch gels by immobilizing water[35],and the addition of inulin to durum wheat caused an increase in pore size and thickness of matrix walls[36].Therefore,the addition of appropriate particle size IDF was considered applicable for the development of protein network in common wheat flour noodle.

3.8.The correlation analysis between IDF particle size,protein structure and noodle quality

The correlations between bran IDF particle size,protein structures,and noodle quality were shown in Table 3.The particle size of wheat bran had a significant relation with texture properties,cooking loss,and evaluation score of noodles.Moreover,the cooking loss of noodles was markedly related to the protein secondary structure(P ＜0.05).Furthermore,the particle size of fiber had extremely positive correlations with SS content as well as protein t-structure(P ＜0.05),while it revealed an extremely notably negative correlation with protein β1-structure, γ-structure, α + βstructure(P ＜0.05).The reason could be possible to that with IDF particle size decreased,the specific surface area increased,and there were more exposed active groups that could react with gluten proteins,thus the SS content and the α-structure decreased, and the β1-structure increased.The addition of bran IDF changed both the SS content and the protein secondarystructures in the noodles.The SH content was negatively correlated with the t-structure and positively correlated with β1- and t-structure.It was also found that the correlation coefficient between the SH and SS contents was negative because the active OH groups could probably remove free SH groups in the bran IDF[37].

Table 2 Protein secondary structure of the noodles with different sized IDF particles.%

4.Conclusions

The effects of the particle size of wheat bran IDF had observable influence on the cooking loss,texture properties,water distribution,protein secondary structure and microstructure of noodle.As the IDF particle size decreased,the hardness and cooking loss of noodles decreased dramatically,the reason could be related to that bran IDF with finer particle size would damage the crystal structure and internal network or the strong mechanical shearing provided more hydrophilic groups,thereby decreasing the T22,SS content and t-structure,increasing the content of β1-structure and improving the microstructure.Moreover,IDF particle size had a significant correlation with protein secondary structures and noodle qualities.Altogether, these results of the current study offer opportunities for the noodle industry to use wheat bran IDF as an ingredient for enhancing the content dietary fiber of noodles.The influence of nano-sized IDF on dough structure and noodle qualities needed to be further researched.

Conflicts of interest

The authors declare that they do not have any conflict of interest.

Fig.3.The SEM of the noodles with different size of IDF(×3,000).

Table 3 The correlation between IDF particle size and protein structures.

Acknowledgments

The research was funded by the Key Scientific and Technological Research Projects of Henan Province(Grant No.162102210108).The authors also deeply appreciated the people who had given valuable suggestions for our research and this manuscript.