Effects of dextran with different molecular weights on the quality of wheat sourdough breads

Accepted Manuscript Effects of dextran with different molecular weights on the quality of wheat sourdough breads Yao Zhang, Lunan Guo, Dan Xu, Dandan Li, Na Yang, Feng Chen, Zhengyu Jin, Xueming Xu PII: DOI: Reference:

S0308-8146(18)30394-7 https://doi.org/10.1016/j.foodchem.2018.02.146 FOCH 22530

To appear in:

Food Chemistry

Received Date: Revised Date: Accepted Date:

9 October 2017 6 February 2018 27 February 2018

Please cite this article as: Zhang, Y., Guo, L., Xu, D., Li, D., Yang, N., Chen, F., Jin, Z., Xu, X., Effects of dextran with different molecular weights on the quality of wheat sourdough breads, Food Chemistry (2018), doi: https:// doi.org/10.1016/j.foodchem.2018.02.146

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effects of dextran with different molecular weights on the quality of wheat sourdough breads Yao Zhang b, Lunan Guo b, Dan Xu b, Dandan Li b, Na Yang b, Feng Chen d, Zhengyu Jin a, b, Xueming Xu a, b, c, * a

State Key Laboratory of Food Science and Technology, Jiangnan University, 1800

Lihu Road, Wuxi 214122, PR China b

School of Food Science and Technology, Jiangnan University, 1800 Lihu Road,

Wuxi 214122, PR China c

National Engineering Laboratory for Cereal Fermention Technology, Jiangnan

University, 1800 Lihu Road, Wuxi 214122, PR China d

Department of Food, Nutrition and Packaging Sciences, Clemson University,

Clemson, South Carolina 29634, USA

*Corresponding author: Email address: [email protected] (X. Xu).

Abstract This research aimed at investigating the effects of different weight-average molecular weights (Mw: T10, T70, T250, T750, T2000) of dextran (α-(1→6)-linked linear backbone, α-(1→3)-linked branching) on wheat sourdough bread qualities. Texture analyzer showed that dextran contributed to a significant inhibition of bread staling, particularly dextran T2000. Pasting profiles (Rapid visco-analysis) and steady rheological properties (steady shear measurements) of samples indicated that dextran 1

T2000 suppressed the swelling and gelatinization of wheat starch granules and bound a great amount of water, thus retarding the aging process. Dextran T2000 markedly improved the elastic properties of sourdough-containing dough. Wheat sourdough breads containing dextran T10 exhibited unexpected less firm crumb structure. The improvement of bread qualities was a function of the Mw of dextran. Dextran with high Mw values has the potential for improving the quality of wheat sourdough breads, especially extending shelf life. Keywords: Dextran; Weight-average molecular weight; Bread quality; Pasting property; Rheology.

Chemical compounds studied in this article Dextran (PubChem CID: 4125253); Starch (PubChem CID: 24836924); Amylose (PubChem CID: 53477771); Amylopectin (PubChem CID: 439207);

2

1. Introduction Sourdough fermentation plays an important role in baked and steamed breads by improving aroma, texture, shelf life and nutritional properties (Clarke & Arendt, 2005). It has been attributed to the metabolites of lactic acid bacteria (LAB) during sourdough

fermentation

including

organic

acids,

volatile

compounds,

exopolysaccharides (EPS) or antifungal compounds and barteriocins. Recently, EPS produced by LAB have been paid great research attention due to increasing bread volumes, softening crumb structures and functioning as prebiotics (Tieking & Gänzle, 2005). Because of the safe nature of LAB, microbial EPS have been gradually described as alternatives for the hydrocolloids (pectin, carrageenan, arabic gum,) currently used as bread improvers (Lynch, Coffey, & Arendt, 2017). Effects of EPS on physiological properties of dough and breads are dependent on its polymer size, monosaccharide compositions, glycosidic linkages and degree of branching (Monsan et al., 2001). Homopolysaccharides (HoPS), composed of monosaccharide (mostly fructose and glucose), have been regarded as the sufficient bread improvers (Chen, Levy, & Gänzle, 2016; Katina et al., 2009). Among these HoPS, mainly α-(1→6)-linked dextran with high weight-average molecular weights (Mw), has been extensively reported to improve bread qualities, especially inhibiting the crumb-firming process. A starter culture rich in dextran with a high Mw (95.4% α-(1→6) backbone linkages, 4.6% α-(1→3) branching linkages) significantly increased the volume of fresh breads and reduced the staling rate after two weeks of storage (Lacaze, Wick, & Cappelle,

3

2007). The addition of 20% W. cibaria MG1 wheat sourdough containing about 5.1 g/kg dextran of Mw up to 3 × 109 g/mol exhibited greater improvements on crumb softness of the fresh breads and the stored breads, compared to the incorporation of 10% reuteran (5.8 g/kg, 107 g/mol) enriched L. reuteri VIP sourdough (Sandra et al., 2012). Softer crumb after storage was also observed on sorghum breads consisted of W. cibaria 10M sourdough containing 8 g/kg dextran with a Mw value of 7.2 × 108 g/mol (Galle et al. 2012). The positive influence of dextran with high Mw on fresh bread volumes may be attributed to combinations with gluten network, improving dough stability and gas retention (Wolter, Hager, Zannini, Czerny, & Arendt, 2014). In addition, the hindrance on the bread staling is possibly due to interactions between high Mw dextran and starch-gluten or starch-starch associations (Gray & Bemiller, 2003). However, it seems to be deficient in clarifying whether dextran with high Mw plays a decisive role in improving the quality of breads. Literatures reported that polysaccharides of low Mw or particular sizes controlled the rate of bread firming. Oligosaccharides with low degrees of polymerization (DP: 3-5) effectively inhibited the long-term retrogradation of amylopectin (Duran, Leon, Barber, & Benedito, 2001). Spring dextrin with the lower average degree of polymerization (14.7 and 12.5) also contributed to the lower recrystallization of wheat and corn starch gels (Xu et al., 2013). The main purpose of this study was to elucidate effects of dextran with different Mw on sourdough bread qualities. Changes in wheat flour pasting properties and sourdough-containing dough rheology caused by the addition of different dextran

4

were measured for giving possible explanations to differences in bread quality characteristics. This research could promote better understanding of the influence of dextran varying with their Mw values on the quality of breads and the eventual utilization of dextran in the baking industry. 2. Materials and methods 2.1 Ingredients Wheat flour (Yihai Kerry, China) containing 72% starch, 12.8% protein and 11.2% moisture, dried yeast (Angel, China), salt (Huai, China), sugar (Taikoo, China) and shortening (Nanqiao, China) were purchased from the local market. Dextran T2000 (Mw range: 1500000-2800000 g/mol) was obtained from Sigma Co., Ltd (Shanghai, China). Dextran T10, T70, T250 and T750 (Mw: 9608, 64636, 254903, 750000 g/mol) were purchased from Aladdin Co., Ltd (Shanghai, China). 2.2 Sourdough preparation Leuconostoc mesenteroides ATCC 8239T was obtained from China General Microbiological Culture Collection Center (CGMCC, China) and routinely propagated in De Man, Rogosa and Sharpe (MRS) medium at 30 ºC. Strain powders stored in the refrigerator at 4 ºC were used to prepare starter cultures. For preparation of the strain powder, single colony was picked from MRS agar plates and subcultured twice in MRS broth. The precipitated cells washed twice with demineralized water were dissolved in the solvent composed of 2% (w/v) glycerin and 10% (w/v) skim milk powders, and then lyophilized. Sourdoughs were prepared by mixing wheat flour, demineralized water and the strain powder to gain a dough yield of 225 [100 × (wheat

5

flour + distilled water) /wheat flour] and an initial strain content at 107 CFU/g, and fermented at 30 ºC for 24 h. 2.3 Bread production Dextran-containing sourdough breads consisted of 100% wheat flour, 20% sourdough, 5% shortening, 1.4% dried yeast and 0.6% table salt (based on flour, BF). Taking a balance between the experimental viability and limited weights of dextran into consideration, various dextran samples were added at 0.1% (BF) due to preliminary bread baking trials. The control breads were prepared without dextran. Modified water addition levels were all 60% (BF) for avoiding dough of excessive viscosity, 6.7% lower than the value based on the farinograph method 54-21 (AACC, 2000b). To keep a constant flour/water ratio, flour and water in sourdoughs replaced equal amount of flour and water in the non-sourdough dough formula. Bread ingredients were blended with a kneader (SINMAG, China) at a low speed (18 rpm) to form dough and then developed at a high speed (40 rpm). The fully developed dough was scaled to 50 g, shaped, and placed in the proof box for 90 min at 38 ºC, 80% humidity. Breads were baked for 15 min at 190 ºC top and 210 ºC bottom heat in the oven and were then cooled for 2 h at room temperature prior to subsequent analyses. Bread loaves were stored in sealed plastic bags at 4 ºC for staling evaluation. Three batch replicas were prepared for each bread type. 2.4 Bread characteristics Baking loss was measured by weight determination of dough before and bread after baking. The specific volume (mL/g) of three loaves from each type of breads

6

was determined by applying the rapeseed displacement method (AACC, 2000a). The moisture of bread crumb on the baking day (day 0) and day 1, 3, 5, 7 of storage at 4 ºC were determined by the oven method at 105 ºC for 24 h. For textural crumb evaluation of three slices from three different breads per baking batch on different days of storage, texture profile analysis (TPA) was performed using a TA-XT Plus texture analyzer (Stable Micro Systems, Surrey, UK) compressing the center area of each slice to 40% of its initial height with a 25 mm aluminium cylindrical probe. The staling rate was calculated as increases in hardness within 7 days of storage (staling rate = [hardness (day 7 – day 0) / days of storage]). The crumb structure was analyzed from the center area of 9 cm2 of each slice from three bread loaves per batch in terms of cell density (ratio number of cells / 9 cm2), cell average area (ratio pore area / number of cells) and porosity (ratio pore area / 9 cm2) using the Image J 1.41 software (National Institutes of Health, USA). 2.5 Differential scanning calorimetry A SIINT instrument (X-DSC 7000 model, Japan) was used to investigate effects of dextran with different Mw values on the retrogradation enthalpy of sourdough breads after storage. Freeze-dried crumb was used, which has been frequently applied in DSC tests for investigations of amylopectin retrogradation (Bosmans, Lagrain, Ooms, Fierens, & Delcour, 2013; Gujral, Singh, & Rosell, 2008). Central crumb from three different breads per batch after the storage of 1 day and 7 days was vacuum lyophilized, milled and passed through a 0.15 mm sieve. Approximate 3 mg powders were weighed into aluminum DSC pans, mixed with 6 μL deionized water and

7

hermetically sealed. Pans were equilibrated overnight at 4 ºC and then heated from 20 ºC to 100 ºC at 10 ºC/min (together with an empty reference pan). Each sample was run in triplicate to determine the mean value. 2.6 X-ray diffraction The bread samples stored for 7 days at 4 ºC were taken for XRD studies to further prove the inhibition effect of dextran on the sourdough bread aging. Bread crumb after the storage were subjected to lyophilization, milling and shifting through a 0.15 mm sieve for obtaining powder samples. The XRD pattern was recorded by a Bruker D8-Advance XRD instrument (Bruker AXS Inc., Germany) with nickel-filtered Cu-Kα (wavelength 1.5405 Å), radiation under the conditions of 40 kV and 30 mA. The diffractograms were collected from 5 ° to 45 ° at 3 °/min. The degree of crystallization (%) was processed by the MDI Jade 6.0 software (Materials Data, Inc., USA). 2.7 Flow curve of aqueous solutions of dextran The flow behavior of various dextran solutions at a concentration of 2% (w/v) was determined by an AR G2 rheometer (TA instrument Inc., USA). The samples were measured using a parallel plate geometry (6 cm) at gap 0.5 mm, temperature 25 ºC and the excessive sample was wiped off. Viscosities of dextran solutions were determined as a function of shear rates over a range of 0.1 - 20 s-1 and performed in triplicates. Variations in rheological properties of the samples were described by the power law model (Eq. (1)), which was extensively used to investigate the flow behavior of liquids (Barnes, Hutton, & Walters, 1989). The power law parameters (n,

8

K, and R2) were determined.

  K n

(1)

Where τ is the shear stress (Pa), γ is the shear rate (s-1), K is the consistency index (Pa sn), and n is the flow behavior index. 2.8 Pasting properties Effects of dextran of different Mw on pasting profiles of wheat flour were analyzed in a composite system using a 4500 rapid visco-analyzer (Perten Instruments Ltd., Australia). The system was prepared as follows. For better understanding functions of different Mw dextran on the pasting characteristics of wheat flour, the incorporated concentrations were all 0.5% (BF), a highest dosage within the appropriate addition range (0.05% - 0.5%) from previous bread quality analyses. The calculated amount of dextran powders was thoroughly dissolved in 25 g distilled water with continuous magnetic stirring for 2 h, and 3.5 g wheat flour was then mixed into the system. The prepared slurry samples were equilibrated at 50 ºC for 1 min before heating at a rate of 12 ºC/min to 95 ºC, maintained at that temperature for 2.5 min, cooled to 50 ºC at a rate of 12 ºC/min and held at 50 ºC for 1.5 min. Paddle speed was 960 rpm for the beginning 10 s to disperse the sample, and then 160 rpm for the remainder of the experiment. The average values for pasting parameters were obtained for each sample between triplicate measurements. 2.9 Rheological properties Dough composed of 100% wheat flour, 20% sourdough, 60% water and 0.5% dextran (BF) were developed in a Farinograph-E farinograph (Brabender, Germany)

9

at 63 r/min, 25 ºC for 11 min. Rheological measurements were conducted using an AR G2 rheometer (TA instrument Inc., USA). A parallel plate geometry (2 cm) at gap 2 mm, temperature 25 ºC was used for the oscillatory measurements. Prepared samples were transferred to the rheometer plate and the excess part was trimmed off. Silicon oil was applied to the exposed surface of samples to prevent evaporation during experiments. Samples were rested for 5 min to allow equilibration and recovery before the detection. Storage modulus (G′), loss modulus (G") and tan δ (G"/G′) were determined at a constant shear strain (0.02%) and varying frequencies (0.1-10 Hz). The 0.02% strain was in the linear viscoelastic region obtained from the strain sweep results (data not shown). 2.10 Statistical analysis Results were reported as the mean values and standard deviations. Statistical analyses were performed using Origin 8.5 (OriginLab Inc., Northampton, MA, USA) and SPSS 20.0 statistical software programs (SPSS Inc., Chicago, USA). Tukey's test was used for significance analyses. 3. Results and discussion 3.1 Bread characteristics Characteristics of wheat sourdough breads treated by the dextran samples and the control breads (without dextran, Ctrl) were summarized in Table 1. The application of dextran did not cause significant differences on baking loss of the breads. Weakened gluten network due to the incorporation of sourdoughs could increase baking loss of breads (Clarke et al., 2005). It suggested that dextran could not reduce the adverse

10

effect of weakened gluten structure on baking loss, regardless of their Mw values. Specific volume, related to gas holding capacity of gluten network, significantly affects the quality of breads (Maleki, Hoseney, & Mattern, 1980). Among these samples,

the

control group had

higher

specific

volumes

(p<0.05) than

dextran-containing breads, suggesting that dextran adversely affected gas holding capacity. The literature reported that higher water-soluble pentosan levels resulted in lower baking absorption levels and mixing tolerance of dough, thus decreasing bread volumes (Roels, Cleemput, Vandewalle, Nys, & Delcour, 1993). Dextran T10, T750, T2000 contributed to higher loaf volumes in comparison to dextran T70 and T250, which could originate from differences in the way dextran with various Mw interacted with starch-starch or starch-gluten associations in the dough system. It was noted that significant variance in hardness and moisture contents of fresh breads (0 day) was not obtained, indicating less influence of dextran on the amylose retrogradation and moisture migration during cooling. Starch hydrolysis products and gluten protein depolymerization catalyzed by α-amylases and proteases with improved activities under acidic conditions could be involved in inhibition of staling (Defloor & Delcour, 1999). Therefore, effects of dextran on crumb firmness and moisture levels of fresh breads were possibly covered up by acidification. With the exception of dextran T70, the staling rate of sourdough breads containing dextran T10, T250, T750 and T2000 decreased by 12.14%, 2.96%, 21.58% and 26.34% respectively, relative to the control group (Table 1). This suggested that dextran T10, T750 and T2000 significantly retarded the staling process of sourdough

11

breads, possibly due to the inhibition effect on amylopectin retrogradation (Ribotta, Cuffini, León, & Añón, 2004). The breads composed of dextran T70 staled at a faster rate, relative to the control breads, that was in agreements with a significant increase in the moisture loss rate. These results suggested that the anti-firming potential of dextran depended on their Mw, which could be attributed to different effects on amylopectin retrogradation, starch-gluten interactions or moisture transfer. The current research revealed that effects of dextran on sourdough bread qualities varied with their Mw values. Furthermore, dextran T10, T750 and T2000 were shown to markedly hinder the firming process of sourdough breads and dextran T2000 could be a promising improver used to prolong shelf life of cereal products. 3.2 Crumb structure Digital image analysis was used to characterize crumb structure of the 9 cm2 central areas of breads in terms of cell density, cell average area, and porosity (Table S1). Although marked differences in porosity of the samples were not induced by the application of dextran, a denser structure was indicated by the significant higher cell density and lower cell average area. Similarly, a coarser, more open structure was revealed by the lower number of cells and increased cell volumes (Wolter, Hager, Zannini, Czerny, & Arendt, 2014). This result coincided with lower specific volumes of the breads treated by dextran, possibly owing to the interference of dextran during the aggregation of gluten protein molecules or immobilization of an amount of water necessary for the complete gluten hydration (Roels, Cleemput, Vandewalle, Nys, & Delcour, 1993).

12

3.3 Thermal analysis The retrogradation enthalpy of breads with different treatment is presented (Fig. 1). Amylopectin retrogradation predominantly contributed to the more energy required for melting a large amount of amylopectin crystallites formed during the staling process (Chen, Ren, Zhang, Tong, & Rashed, 2015). Furthermore, Recrystallized amylopectin played a significant role in the crumb-firming performance of aged breads, decreasing consumer acceptance of these products (Cauvain S. P., 2015). The retrogradation enthalpy of all samples increased with storage periods, suggesting more amylopectin crystallites formed during the storage. After stored for 7 days, sourdough breads containing dextran T2000 had the lowest retrogradation enthalpy followed by dextran T750 and dextran T10, and the value of breads with dextran T70 was 0.22 J/g higher than that of the control. These results were consistent with changes in staling rates of the samples determined by crumb firmness analysis. It suggested that the highest Mw dextran (dextran T2000) possessed the potential for inhibiting the aging of stored breads, through decreasing the amount of formed amylopectin crystallites. The retarding effect of dextran with high Mw (8.3 × 107 g/mol, 98% α-(1→6) linkages) on the bread staling was also pronounced (Chen, Levy, & Gänzle, 2016). However, the anti-firming performance of dextran with low Mw (dextran T10) should not be ignored in order to better understand the molecular mechanism of dextran on bread staling. Low Mw solutes fitting well in the water structure retarded chain reordering of starch molecules. On the other hand, the solutes inhibited the formation of hydrogen bonds between starch granules and gluten fibrils

13

(Biliaderis & Prokopowich, 1994). 3.4 XRD pattern analysis XRD analysis was applied to further demonstrate the influence of dextran on the long-term retrogradation of amylopectin. XRD patterns and the quantitatively calculated crystallinity of bread samples are illustrated in Fig. 2. After 7 days of storage, a highlighted peak at 2θ close to 17.5° was observed in all samples, revealing a typical B-type XRD pattern formed during the bread staling (Tao, Zhang, Wu, Jin, & Xu, 2016). The formation of B-type crystallization during storage represented that the molecular aggregation state was changed in wheat starch. The peak at 20° was expected to be a well-formed V-type XRD pattern, which consisted of amylose, fatty acids and phospholipids (Lalush, Bar, Zakaria, Eichler, & Shimoni, 2005). The relatively crystallinity decreased from 23.17% to 16.78%, proving the retarding effect of some dextran (dextran T10, T750 and T2000) on amylopectin retrogradation. A work reported that less water being available for the crystal lattice formed during amylopectin crystallization in stored breads contributed to a softer crumb structure (Gray & Bemiller, 2003). It implied that these certain dextran could decrease the moisture mobility of stored bakery products, thus hindering amylopectin recrystallization. In addition, the way dextran interacted with starch-starch or starch-gluten associations could account for the anti-firming performance. 3.5 Steady shear rheological properties of dextran solutions The viscosity as a function of shear rates for various dextran solutions has been shown (Fig. S1). As summarized in Table 2, the data from flow sweep tests was fitted

14

well with the power law equation with high determination coefficient (R2: 0.9991 0.9999). All samples displayed Newtonian behavior because their viscosity did not significantly change as shear rates increased and flow behavior index values (n) approached one (0.980 - 0.996). The viscosity of solutions of commercial dextran (from Pharmacia Fine Chemicals, Sweden), with Mw up to 2 × 106 g/mol and concentrations up to 30% (w/v), was independent of the applied shear stress (Nomura, Koda, & Hattori, 1990) A study reported that solutions of dextran originating from Leuconostoc mesenteroides B-512, of Mw ranging from 7 × 104 to 5 × 105 g/mol and concentrations up to 25% (w/v), presented Newtonian fluids (Carrasco, Chornet, Overend, & Costa, 1989). The flexible conformation, a linear backbone of consecutive α-(1→6)-linkages and less α-(1→3)-branching, induced Newtonian flow performance. The dextran T70 solution showed a slight non-Newtonian shear-thinning behavior, reflected by a decreased n value (0.885) compared to the control (water solutions, Ctrl). This indicated an increase in the degree of branching (Tirtaatmadja, Dunstan, & Boger, 2001). Dextran solutions all had higher K values than the control, increasing with their Mw. These results accounted for differences in bread characteristics, especially staling rates and moisture loss rates. Dextran T2000 could hold a larger amount of water, available for plasticization, resulting in less amylopectin recrystallization and softer texture of stale breads (Guarda, Rosell, Benedito, & Galotto, 2004). Nevertheless, inhibitions effects of dextran T10 on crumb firming of sourdough breads were less affected by its water-binding capacity, likely attributable to interactions with starch molecules and gluten fibrils.

15

3.6 Pasting properties of wheat flour The influence of dextran on pasting profiles of wheat flour including peak viscosity, breakdown and setback values are displayed in Fig. 3. Changes in pasting properties of wheat flour slurry were mainly related to various behaviors of starch granules in the composite system. An apparent decrease in peak viscosity (p<0.05) was observed on the slurry consisting of dextran T2000, relative to the control (without dextran, Ctrl). Similar results were reported when the Mw of purified food-grade fenugreek was equal to or larger than 1.8 × 106 g/mol, which was relevant to the decrease in the amount of amylose leaching out starch granules during gelatinization (Funami et al., 2008). It was assumed that dextran T2000 would suppress the swelling and gelatinization of starch granules through wrapping around the surface (Chen, Tong, Ren, & Zhu, 2014; Nawab, Alam, Haq, & Hasnain, 2016), therefore contributing to less amylose leaching into the heated slurry. In addition, since dextran T2000 had high water-binding capacity drawn from the previous steady shear rheology experiments, less moisture would be available for the swelling and gelatinization process of wheat starch. This was consistent with the speculation that the anti-retrogradation mechanism of xanthan on rice starch might partially derived from the thickening effect and the water-holding capacity (Tang, Hong, Gu, Zhang, & Cai, 2013). The breakdown viscosity was the difference between peak viscosity and trough viscosity, representing the degree of granule disruption during heating. The formation of amylose junction zones was responsible for a setback viscosity due to the rapid

16

aggregation of leached amylose molecules, indicating degrees of the short-term retrogradation of starch. Dextran T2000 led to a decrease in breakdown and setback viscosities in comparison to the control. Dextran T2000 could inhibit the short-term retrogradation of wheat starch, therefore possessing the potential for anti-firming of sourdough breads (Chen, Ren, Zhang, Tong, & Rashed, 2015). Taking the retarded staling of sourdough breads into consideration, the inhibition influence of dextran with high Mw on the short-term retrogradatin may be conspicuous by acidifying the composite system. 3.7 Dynamic rheology properties Wheat dough was a viscoelastic material formed by mixing wheat flour with water.

Effects

of

the

different

dextran

on

rheological

properties

of

sourdough-containing dough were evaluated by frequency sweep tests, which were essential to predict the processability and quality of final products (Yang & Song, 2011). Variations of storage modulus (G′), loss modulus (G") and tan δ (G"/G′) as a function of frequency for the dough samples are shown in Fig. 4. G′ and G" of all samples increased as frequency increased and G′ were always higher than G", meaning that the dough had a solid, elastic-like behavior. Dough consisting of dextran exhibited higher G′ and G" than that of the control samples (Ctrl). The dough prepared with dextran T2000 had the highest G′ and G", implying the highest stiff level of the dough. This may be attributable to the greater water-binding capacity of dextran with higher Mw. Therefore, the moisture was not enough for the complete hydration of dough and the storage modulus (G′) and loss modulus (G") were

17

increased (Navickis, Anderson, Bagley, & Jasberg, 2010). As known, tan δ is an important indicator of the relative contribution of the viscous and elastic components to the viscoelastic properties of dough (Karim, Norziah, & Seow, 2000). Tan δ values of dough with dextran T2000 were lower than that of other samples (Fig. 4), suggesting an increasing contribution of the elastic properties. The molecular interactions between amylose and polysaccharides promoted the formation of tri-dimension gel structure and caused the increase in elasticity (Bemiller, 2011). This implied that interactions between dextran T2000 and starch-gluten associations in dough accounted for improvements in elastic properties, then improving sourdough bread qualities. 4. Conclusions This work clarified that positive effects of dextran on the quality of wheat sourdough breads varied with their Mw values. Dextran T2000 effectively retarded the staling of wheat sourdough breads. The lower retrogradation process could be attributed to the decreased swelling and gelatinization with lower leached amylose and less moisture being available for amylopectin recrystallization. Therefore, dextran with high Mw could be a promising staling inhibitor to increase consumer acceptance of stored bakery products. On the other hand, dextran T10 presented the marked anti-firming effect on breads, which should receive increasing attention. Dextran with low Mw disturbing chain reordering of starch molecules and hydrogen bond formation of bread components could lead to the above-mentioned differences. Acknowledgements

18

This study was financially supported by the National Natural Science Foundation of China (No. 31471584).

19

References AACC. (2000a). Approved Methods of the American Association of Cereal Chemists. (10th ed.). St. Paul, MN: AACC, international, (Methods 10-05). AACC. (2000b). Approved Methods of the American Association of Cereal Chemists. (10th ed.). St. Paul, MN: AACC, international, (Methods, 54-21). Barnes, H. A., Hutton, J. F., & Walters, K. (1989). An introduction to rheology. (1st ed.). Amsterdam: Elsevier Science, (Charpter 7). Bemiller, J. N. (2011). Pasting, paste, and gel properties of starch-hydrocolloid combinations. Carbohydrate Polymers, 86(2), 386-423. Biliaderis, C. G., & Prokopowich, D. J. (1994). Effect of polyhydroxy compounds on structure formation in waxy maize starch gels: a calorimetric study. Carbohydrate Polymers, 23(3), 193-202. Bosmans, G. M., Lagrain, B., Ooms, N., Fierens, E., & Delcour, J. A. (2013). Biopolymer interactions, water dynamics, and bread crumb firming. Journal of Agricultural & Food Chemistry, 61(19), 4646-4654. Carrasco, F., Chornet, E., Overend, R. P., & Costa, J. (1989). A generalized correlation for the viscosity of dextrans in aqueous solutions as a function of temperature, concentration, and molecular weight at low shear rates. Journal of Applied Polymer Science, 37(8), 2087-2098. Cauvain, S. P. (2015). Technology of Breadmaking. (3rd ed.). Switzerland: Springer (Chapter 10). Chen, L., Ren, F., Zhang, Z., Tong, Q., & Rashed, M. M. (2015). Effect of pullulan on

20

the short-term and long-term retrogradation of rice starch. Carbohydrate Polymers, 115, 415-421. Chen, L., Tong, Q., Ren, F., & Zhu, G. (2014). Pasting and rheological properties of rice starch as affected by pullulan. International Journal of Biological Macromolecules, 66(5), 325-331. Chen, X. Y., Levy, C., & Gänzle, M. G. (2016). Structure-function relationships of bacterial and enzymatically produced reuterans and dextran in sourdough bread baking application. International Journal of Food Microbiology, 239, 95-102. Clarke, C. I., & Arendt, E. K. (2005). A review of the application of sourdough technology to wheat breads. Advances in Food and Nutrition Research, 49(5), 137-161. Clarke, C. I., Schober, T. J., Dockery, P., Arendt, E. K., Cauvain, S. P., Salmon, S. S., & Young, L. S. (2004). Wheat sourdough fermentation: effects of time and acidification on fundamental rheological properties. Cereal Chemistry, 81(3) 409-417. Defloor, I., & Delcour, J. A. (1999). Impact of maltodextrins and antistaling enzymes on the differential scanning calorimetry staling endotherm of baked bread doughs. Journal of Agricultural and Food Chemistry, 47(2), 737-741. Duran, E., Leon, A., Barber, B., & Benedito, d. B. C. (2001). Effect of low molecular weight dextrins on gelatinization and retrogradation of starch. European Food Research and Technology, 212(2), 203-207.

21

Funami, T., Kataoka, Y., Noda, S., Hiroe, M., Ishihara, S., Asai, I., Takahashi, R., Inouchi, N., & Nishinari, K. (2008). Functions of fenugreek gum with various molecular weights on the gelatinization and retrogradation behaviors of corn starch-2:

Characterizations

of

starch

and

investigations

of

corn

starch/fenugreek gum composite system at a relatively low starch concent. Food Hydrocolloids, 22(5), 777-787. Galle, S., Schwab, C., Dal, B. F., Coffey, A., Gänzle, M. G., & Arendt, E. K. (2012). Influence of in-situ synthesized exopolysaccharides on the quality of gluten-free sorghum sourdough bread. International Journal of Food Microbiology, 155(3), 105-112. Gray, J. A., & Bemiller, J. N. (2003). Bread Staling: Molecular Basis and Control. Comprehensive Reviews in Food Science and Food Safety, 2(1), 1-21. Guarda, A., Rosell, C. M., Benedito, C., & Galotto, M. J. (2004). Different hydrocolloids as bread improvers and antistaling agents. Food Hydrocolloids, 18(2), 241-247. Gujral, H. S., Singh, G. S., & Rosell, C. M. (2008). Extending shelf life of chapatti by partial baking and frozen storage. Journal of Food Engineering, 89(4), 466-471. Karim, A. A., Norziah, M. H., & Seow, C. C. (2000). Methods for the study of starch retrogradation. Food Chemistry, 71(1), 9-36. Katina, K., Maina, N. H., Juvonen, R., Flander, L., Johansson, L., Virkki, L., Tenkanen, M., Laitila, A., Gänzle, M., & Vogel, R. (2009). In situ production

22

and analysis of Weissella confusa dextran in wheat sourdough. Food Microbiology, 26(7), 734-743. Lacaze, G., Wick, M., & Cappelle, S. (2007). Emerging fermentation technologies: development of novel sourdoughs. Food Microbiology, 24(2), 155-160. Lalush, I., Bar, H., Zakaria, I., Eichler, S., & Shimoni, E. (2005). Utilization of amylose-lipid complexes as molecular nanocapsules for conjugated linoleic Acid. Biomacromolecules, 6(1), 121-130. Lynch, K. M., Coffey, A., & Arendt, E. K. (2017). Exopolysaccharide producing lactic acid bacteria: Their techno-functional role and potential application in gluten-free bread products. Food Research International, In Press, Corrected Proof. Maleki, M., Hoseney, R. C., & Mattern, P. J. (1980). Effects of loaf volume, moisture content, and protein quality on the softness and staling rate of bread. Cereal Chemistry, 57(2), 138-140. Monsan, P., Bozonnet, S., Albenne, C., Joucla, G., Willemot, R. M., & Remaudsimeon, M. (2001). Homopolysaccharides from lactic acid bacteria. International Dairy Journal, 11(9), 675-685. Navickis, L. L., Anderson, R. A., Bagley, E. B., & Jasberg, B. K. (2010). Viscoelastic properties of wheat flour doughs: variation of dynamic moduli with water and protein content. Journal of Texture Studies, 13(2), 249-264. Nawab, A., Alam, F., Haq, M. A., & Hasnain, A. (2016). Effect of guar and xanthan gums on functional properties of mango (Mangifera indica) kernel starch.

23

International Journal of Biological Macromolecules, 93(Pt A), 630-635. Nomura, H., Koda, S., & Hattori, F. (1990). Viscosity of aqueous solutions of polysaccharides and their carboxylate derivatives. Journal of Applied Polymer Science, 41(11-12), 2959-2969. Ribotta, P. D., Cuffini, S., León, A. E., & Añón, M. C. (2004). The staling of bread: an X-ray diffraction study. European Food Research and Technology, 218(3), 219-223. Roels, S. P., Cleemput, G., Vandewalle, X., Nys, M., & Delcour, J. A. (1993). Bread volume potential of variable-quality flours with constant protein level as determined by factors governing mixing time and baking absorption levels. Cereal Chemistry, 70(3), 318-323. Sandra, G., Schwab, C., Bello, F. D., Coffey, A., Gänzle, M., & Arendt, E. (2012). Comparison of the impact of dextran and reuteran on the quality of wheat sourdough bread. Journal of Cereal Science, 56(3), 531-537. Tang, M., Hong, Y., Gu, Z., Zhang, Y., & Cai, X. (2013). The effect of xanthan on short and long-term retrogradation of rice starch. Starch-Stärke, 65(7-8), 702-708. Tao, H., Zhang, B., Wu, F., Jin, Z., & Xu, X. (2016). Effect of multiple freezing/thawing-modified wheat starch on dough properties and bread quality using a reconstitution system. Journal of Cereal Science, 69, 132-137. Tieking, M., & Gänzle, M. G. (2005). Exopolysaccharides from cereal-associated lactobacilli. Trends in Food Science and Technology, 16(1), 79-84.

24

Tirtaatmadja, V., Dunstan, D. E., & Boger, D. V. (2001). Rheology of dextran solutions. Journal of Non-Newtonian Fluid Mechanics, 97(2-3), 295-301. Wolter, A., Hager, A. S., Zannini, E., Czerny, M., & Arendt, E. K. (2014). Influence of dextran-producing Weissella cibaria on baking properties and sensory profile of gluten-free and wheat breads. International Journal of Food Microbiology, 172(172C), 83-91. Xu, J., Fan, X., Ning, Y., Wang, P., Jin, Z., Lv, H., Xu, B., & Xu, X. (2013). Effect of spring dextrin on retrogradation of wheat and corn starch gels. Food Hydrocolloids, 33(2), 361-367. Yang, Y., & Song, Y. (2011). Rheological behaviors of doughs reconstituted from wheat gluten and starch. Journal of Food Science and Technology, 48(4), 489-493.

25

Figure Captions Fig. 1. The retrogradation enthalpy of wheat sourdough breads containing different Mw dextran and control breads over seven days of storage at 4 °C Fig. 2. X-ray diffraction patterns of wheat sourdough breads compared with different Mw dextran and control breads after storage at 4 °C for 7 days. Fig. 3. Pasting curves (a) and variations of peak, setback and breakdown values (b) of wheat flour containing different Mw dextran. Fig. 4. Effects of different Mw dextran on storage modulus (G′), loss modulus (G") and tan δ of sourdough-containing dough. Fig. S1. Viscosities as a function of shear rate of dextran with various Mw values at 2% concentration in aqueous solutions and control solutions (water solutions)

26

Fig. 1

Ctrl SD-T10 SD-T70 SD-T250 SD-T750 SD-T2000

Enthalpy change (J/g)

5.0

4.5

a a

b

4.0 a

3.5

b

b

a a

a

a

a

3.0

2.5 1

b

Time (day)

27

7

Fig. 2

Ctrl

Intensity

23.17%

SD-T10

19.31%

SD-T70

24.23%

SD-T250

21.92%

SD-T750

17.03%

SD-T2000

16.78%

5

10

15

20

25

2θ (°)

28

30

35

40

Fig. 3

3500

100 Temperature curve

Viscosity (cp)

3000

80

2500 60

2000 1500

Ctrl T10 T70 T250 T750 T2000

1000 500

40

20

0

0 0

2

4

6

8

10

12

Time (min)

3500

Peak viscosity Breakdown viscosity Setback viscosity

(b)

3000

a

Viscosity (cP)

a

ab

bc

2500

c

c

2000 1500 1000

a

ab

ab

ab

ab

b

a

a

a

a

a

a

Ctrl

T10

T70

T250

T750

T2000

500 0

29

Temperature (°C)

(a)

Fig. 4

Ctrl SD-T10 SD-T70 SD-T250 SD-T750 SD-T2000

4

G' (Pa)

2x10

10

4

0.1

1

Frequency (Hz)

30

10

1.5x10

Ctrl SD-T10 SD-T70 SD-T250 SD-T750 SD-T2000

4

G" (Pa)

10

4

3

5x10

0.1

1

Frequency (Hz)

31

10

0.7 Ctrl SD-T10 SD-T70 SD-T250 SD-T750 SD-T2000

Tan δ

0.6

0.5

0.4 0.1

1

Frequency (Hz)

32

10

Table 1 Characteristics of wheat sourdough breads treated by different M w dextran and the control (without dextran, Ctrl). Ctrl Baking loss (%) Specific volume Moisture (%), 0 day Moisture loss (%/day, 7 days)

17.33±0.52

SD-T10 a

SD-T70

16.75±0.01

a

17.33±0.44

SD-T250 a

16.41±0.51

SD-T750 a

17.13±0.68

SD-T2000 a

17.03±0.15a

Hardness (g), 0 day

5.84±0.33a 38.73±0.32a 0.77±0.06b 64.82±7.14a

5.47±0.12ab 39.88±0.50a 0.77±0.09b 60.57±1.71a

5.20±0.21b 40.10±0.32a 1.17±0.04a 65.76±2.22a

4.59±0.25c 40.02±0.67a 0.83±0.07b 71.60±5.74a

5.41±0.20ab 38.84±0.55a 0.74±0.01b 64.37±5.85a

5.44±0.08ab 39.17±0.77a 0.75±0.06b 69.03±3.97a

Staling rate (g/day, 7 days)

41.84±2.38ab

36.72±1.45bc

43.84±1.24a

40.60±1.90ab

32.81±1.26c

30.82±2.59c

Mean ± standard deviation values in the same row for each sample followed by different letters are significantly different (p<0.05) by Tukey’s test.

33

Table 2 Power law parameters of different Mw dextran at 2% concentration in aqueous solutions and water solutions (Ctrl)a. Sample

K×10-3 (Pa sn)

n (-)

R2

Ctrl T10 T70 T250 T750 T2000

0.862±0.026e 1.044±0.029d 1.909±0.022c 1.822±0.008c 2.146±0.078b 2.936±0.019a

0.981±0.003ab 0.980±0.006b 0.885±0.005c 0.986±0.003ab 0.994±0.003ab 0.996±0.001a

0.9999 0.9999 0.9991 0.9999 0.9996 0.9999

K, consistency index; n, flow behavior index; R2, determination coefficient. a All tests were performed in triplicates. Means within a column with different letters are significantly different (p<0.05) by Tukey’s test.

34

•

Effects of dextran on wheat sourdough bread qualities were a function of their Mw.

•

Dextran T2000 presented the most significant inhibition effects on the aging process.

•

Dextran T2000 suppressed gelatinizaiton of starch and bound a large amount of water.

•

Dextran T2000 improved the elastic properties of sourdough-containing dough

•

Dextran T10 contributed to a substantial decrease in staling rates.

35

36