Evaluation of partial-vacuum baking for gluten-free bread: Effects on quality attributes and storage properties

Evaluation of partial-vacuum baking for gluten-free bread: Effects on quality attributes and storage properties

Journal of Cereal Science 91 (2020) 102891 Contents lists available at ScienceDirect Journal of Cereal Science journal homepage: http://www.elsevier...

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Journal of Cereal Science 91 (2020) 102891

Contents lists available at ScienceDirect

Journal of Cereal Science journal homepage: http://www.elsevier.com/locate/jcs

Evaluation of partial-vacuum baking for gluten-free bread: Effects on quality attributes and storage properties Sezin Tuta S¸ims¸ek Çankırı Karatekin University, Faculty of Engineering, Department of Food Engineering, 18100, Çankırı, Turkiye



Keywords: Baking Partial-vacuum Gluten-free bread Storage Starch

The purpose of this study was to evaluate the impact of partial-vacuum baking on the quality and storage properties of gluten-free bread (GFB). Conventional (180� C-30 min at atmospheric pressure) and partial-vacuum (180� C-15 min at atmospheric pressure and at 180� C-15 min at 60 kPa vacuum pressure) methods were con­ ducted to bake GFB. Quality attributes (specific volume, colour, texture, total water loss) were assessed, DSC and SEM analyses were carried out to understand the effect on the bread’s microstructure when using vacuum during baking. No significant differences (p > 0.05) were observed in the hardness and specific volume of the partialvacuum baked GFB; however, changes in the total water loss and in the total colour change were statistically significant (p < 0.05). The DSC, SEM and XRD results showed that more crystalline structure and different starch crystal types formed after partial-vacuum baking. Storage properties were also investigated over a 3-day period. Partial-vacuum baking significantly affected the total water loss and the texture parameters (p < 0.05) during storage. Partial-vacuum baked samples were softer and had a tendency to become stale more slowly than the control. The findings indicate that the partial-vacuum baking method increases the shelf life of gluten-free products by modifying the microstructure of the bread.

1. Introduction Bread is an essential foodstuff that is highly important in meeting the nutritional needs of people around the world, depending on economic and cultural habits. The dough quality for bread production depends on the quality and amount of protein. Gluten is the protein in bread that comprises approximately 80–85% of the total protein of wheat. The primary role of gluten is its production of the desirable viscoelastic structure of wheat bread. The poor quality and sensorial properties of gluten-free bread compared to wheat bread are mainly due to lack of this structure. Gluten-free bread without improvers usually displays a hard and crumbly texture, low volume, poor crust colour, taste, and aroma, and a short shelf life due to a rapid staling rate (Aguilar et al., 2015). Rapid staling and the short shelf life of gluten-free bread are relevant problems for consumers and producers. Lack of a gluten network causes faster moisture loss in gluten-free products, which increases the staling rate. Staling begins after the bread is removed from the oven, with mi­ crobial deterioration being slower than staling. The staling process is caused by starch retrogradation and water loss in bread products, resulting in hardening of the bread texture. Staling occurs as a result of various reactions that have not yet been fully elucidated; however, they

include recrystallisation of amylopectin, rearrangement of the water distribution in the bread crumb and changes in the amorphous phase € (Ozkoc et al., 2009). Slowing the staling rate and extending the shelf life are economic and safety issues to be overcome by the bakery industry. Microwave-assisted baking (Sumnu et al., 2010), high-pressure baking, ultrasound aeration, extrusion, hydrothermal treatments (Naqash et al., 2017), and low-pressure (vacuum) baking (Rondeau-­ Mouro et al., 2019) are novel technologies that have been applied to improve the quality and storage properties of gluten-free bread. Vacuum technologies have recently been applied to food processes for various purposes such as cooling, cooking and frying. Studies on low-pressure €kmen, 2014; baking have also recently gained attention (Mogol and Go �lu et al., 2015; Yıldız et al., 2017; Rondeau-Mouro et al., 2019). Palazog Increasing the shelf life can be achieved by vacuum-combined baking. Ruttarattanamongkol et al. (2011) reported that the use of a vacuum in conventional baking resulted in a softer bread product than conven­ tional methods during yeast-free bread production using supercritical fluid extrusion of dough. Rondeu-Mouro et al. (2019) applied partial-vacuum baking to gluten-free bread at 20 kPa and used nuclear magnetic resonance (NMR) to investigate the effect on the water dis­ tribution and state of macromolecules. In a previous limited study, the

E-mail address: [email protected]. https://doi.org/10.1016/j.jcs.2019.102891 Received 28 August 2019; Received in revised form 27 November 2019; Accepted 28 November 2019 Available online 29 November 2019 0733-5210/© 2019 Elsevier Ltd. All rights reserved.

S. Tuta S¸ims¸ek

Journal of Cereal Science 91 (2020) 102891

effect of a vacuum on the quality and storage properties of gluten-free bread was evaluated. However, no studies have evaluated all of the quality and storage properties together. For this reason, conventional and partial-vacuum baking methods were compared by investigating the quality characteristics (texture, total water loss, total colour change and specific volume) and changes in the structure of samples as analysed by SEM and DSC. The storage properties were evaluated, and the effect of storage on the physicochemical properties (moisture content, texture and total water loss) of the bread samples was investigated. XRD and SEM analyses were conducted to understand changes in the starch structure (presence of starch granules and starch crystal type) of gluten-free bread during storage period.

initial bread dough (mdough) and the bread 1 h after baking (mgfc) (Eq. (1)) (Ahrne et al., 2007). � � mdough mgfc � 100 (1) WLð%Þ ¼ mdough 2.4.2. Total colour change The crust colour of the gluten-free bread samples was determined using chromometer (Model CR400, Konica Minolta, Japan). Colour pa­ rameters (L* ; a* and b* of the bread crust) were measured at five randomly selected regions of the bread upper crust. The L* parameter represents the colour lightness, which ranges from 0 (black) to 100 (white). The a* parameter can vary between 60 (green) and þ60 (red); the b* parameter can vary between 60 (blue) and þ60 (yellow) €kmen and S¸enyuva, 2006). The total colour change was calculated (Go with Eq. (2) by applying the average values of the L* ; a* and b* param­ eters (Ahrne et al., 2007): qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi �2 �2 2 ΔE ¼ L*0 L* þ ða*0 a* Þ þ b*0 b* [2]

2. Materials and methods 2.1. Material Rice flour (Ege, gluten-free white rice flour, Turkey), hydroxypropyl methyl cellulose (HPMC, Sigma-Aldrich, Japan), gluten-free instant yeast (Dr. Oetker, Turkey), sunflower oil (Komili, Turkey), salt, sugar and distilled water were used for gluten-free bread production.

where L*0 ; a*0 and b*0 correspond the values of the reference white plate,

which were L*0 ¼ 94.84, a*0 ¼ 1.34, and b*0 ¼ -0.74. The white cali­ bration was accomplished with a white calibration plate (standard illuminant C, Y: 85.8, x: 0.3164, y: 0.3233) prior to the colour measurements.

2.2. Gluten-free bread formulation The dough formulation was adjusted through preliminary experi­ mentation with the fermentation duration, atmospheric baking tem­ perature and time. Baking parameters were selected that minimised the firmness and total water loss values of the gluten-free bread and maxi­ mised the total colour change and specific volume. According to the preliminary experiments, the final gluten-free bread formulation was 100 g rice flour, 0.5 g HPMC, 1 g instant yeast, 1.8 g salt, 8.0 g sugar, 6.0 g oil and 110 mL of distilled water mixed at 25 � C. The dry ingredients were mixed first and then the other ingredients were added. HPMC and instant yeast were dissolved in water before adding to the mixture; the rest of the water was then added in. The ingredients were mixed using a stand mixer (Model A480, Korkmaz, Turkey) at speeds of 85, 110 and 140 rpm for 2, 1 and 1 min, respectively. The pH value of the gluten-free dough was 5.74 � 0.06 at 25 � C, and the specific weight was 1.176 � 0.011 g/mL. The dough was weighed (80 � 0.05 g) in aluminium baking pans with dimensions of 88 � 38 mm and proofed in an oven (Model MST-55, TEKLAB, Turkey) at 35 � C for 45 min.

2.4.3. Specific volume The rapeseed displacement method according to AACC method 10–05.01 was used to define the volume of the bread samples. The specific volume of the gluten-free bread samples were calculated by the ratio of bread volume to bread weight (Turabi et al., 2008). 2.4.4. Texture profile analysis (TPA) Texture profile analyses were performed using a texture analyser (Model TA-XT2i, Stable Micro Systems, UK, 30 kg load cell) equipped with 36 mm radius cylindrical probe (P/36R). Bread pieces (2.5 cm � 4 cm x 4 cm) were taken from the middle of bread crumb (Brady and Mayer, 1985). The test speed was set 1 mm/s, and the probe compressed the sample to a depth of 10 mm. The compression was repeated 5 s after the first compression. The hardness, cohesiveness, springiness, chewi­ ness and adhesiveness of the gluten-free bread were assessed using two bite force-time curves. To calculate these parameters, Texture Analyzer software (Exponent, TEE 32 V 6,1,4,0, 2013) was used with the standard TPA macro. Hardness was defined as the maximum peak force during the first compression cycle. Adhesiveness was the negative force area obtained after the first compression and represents the work required to overcome the attractive forces between the surface of a food and another surface. Cohesiveness was defined as the ratio of the positive force area during the second compression to that during the first compression. Springiness was the height that the food recovered during the time that passed between the end of the first bite and the start of the second bite (the ratio of time difference during the second bite to the first bite). Chewiness was calculated as hardness x cohesiveness x springiness and represents the energy required to masticate solid food. Resilience was the ratio of the area during the first withdrawal over the area of the first penetration (Ruttarattanamongkol et al., 2011).

2.3. Baking Atmospheric baking was carried out at full atmospheric pressure (Pabs ¼ 101.3 kPa) in a vacuum oven (Model OV-11, Jeiotech, North Korea) at 180 � C for 30 min without applying the vacuum pump. Partialvacuum baking was performed in the same vacuum oven with the use of the vacuum pump (Model MVP6, WooSung vacuum pump, Korea). The first period of baking was carried out at atmospheric pressure and 180 � C for 15 min after which time baking was continued at 60 kPa vacuum pressure at 180 � C for 15 min. The atmospheric bake time was selected according to visual observation of the crust formation, which is impor­ tant to prevent the bread collapsing. Lowering the vacuum pressure lead to decreasing the boiling point of water; hence, the vacuum pressure was determined so as not to be lower than the gelatinization temperature of starch. To achieve adequate starch gelatinization and completely cook the crumb, a vacuum pressure of 60 kPa (Pabs ¼ 41.3 kPa, Twater boiling � point ¼ 77 C) was applied. The baking experiments and quality analyses were carried out in two runs, with two replicates in each run. Quality properties were analysed after cooling for 1 h.

2.5. Scanning Electron Microscopy (SEM) SEM analyses were performed on the gluten-free dough and bread samples. Images were taken with a scanning electron microscope (Model 1430 VP Zeiss, LEO, England) after being coated with a special adhesive, the surfaces of the bread pieces were coated with gold. First, the dough and bread samples were frozen at 80 � C in the freezer for approxi­ mately one week and then freeze-dried (Labconco Corporation, Kansas

2.4. Quality attributes 2.4.1. Total water loss Total water loss during baking was determined by weighting the 2

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Journal of Cereal Science 91 (2020) 102891

City, MO, USA). The freeze-dried samples were milled by a grinder (Model SCM-2934, Sinbo, Turkey). After both baking methods, SEM � images of the milled dough and bread samples were obtained (Lap�cíkova et al., 2019). The SEM images were obtained at 1000x, and 3000x magnifications for the dough, control and partial-vacuum baked (PVB) bread samples. In the staling analyses, gluten-free bread (fresh and stored samples) was cut into smooth blocks (approximately 1.5 cm in size) from the middle part of the bread. The blocks were frozen at 80 � C and then freeze-dried. SEM images at 60x magnification were used to determine the effect of storage on the bread samples. The SEM images were taken two times.

3. Results and discussion 3.1. Quality properties of gluten-free bread The total water loss, specific volume, colour and texture results of the control and PVB gluten-free bread are presented in Table 1. The mois­ ture content of the control was higher than that of PVB sample, but the differences were not statistically significant (p > 0.05). The total water loss of the PVB gluten-free bread samples was higher than that of the control, and this difference was statistically significant (p < 0.05). The water evaporation rate increases in the vacuum processes due to a reduction in the boiling point of water. This is the main disadvantage of vacuum baking, as discussed by Wang and Sun (2001). Alterations in the baking step did not significantly affect the specific volume of the gluten-free bread (p > 0.05). Air bubbles, which form due to the aeration of dough, are the primary cause of the structure (texture and pore production) of bread and cake. The production of gas (CO2, ethanol) in the dough causes the development of volume and pore structure. This starts with kneading and continues with the fermentation step; gas bubbles that form during these stages expand during baking (Elgeti et al., 2015). The nonsignificant changes seen in the specific volume of the bread samples produced by the two baking methods may have been due to bread expansion during the atmospheric baking step of the partial-vacuum baking process, because gas bubbles expand mostly during the early stages of baking, as discussed in Breadariol et al. (2019). Compared with the control, the total colour change decreased significantly in the PVB samples (p < 0.05). Among the colour param­ eters (L*, a*, b*), only the differences of b* were not statistically sig­ nificant (p > 0.05). Lower total colour change and a* values indicate that lighter bread was obtained when using partial-vacuum baking compared to the conventional method. Having a lighter colour than wheat bread is one of the drawbacks of gluten-free bread, and consumer preferences desires darker colour formation. Colour forms during baking via the Maillard reaction. Increases in baking temperature and time lead to an increase in the a* value of bread crust due to increased surface €kmen and temperature, which accelerates the Maillard reaction (Go S¸enyuva, 2006). Lighter colour formation was in agreement with the �lu et al. (2015), Mogol and Go €kmen, 2014 after studies of Palazog partial-vacuum baking of biscuits. During vacuum baking, the surface temperature of bread is decreased; therefore, colour formation is limited �lu et al., 2015). Obtaining a darker crust colour may be (Palazog accomplished by adjusting the baking temperature-time combination and/or the dough formulation to include darker gluten-free ingredients. People’s perception of food during mastication and swallowing is

2.6. Differential scanning calorimetry (DSC) DSC analysis was carried out with a Mettler-Toledo DSC1 700 (USA). Freeze-dried dough and bread samples were milled to sizes of between 250 and 350 μm. The samples were weighed (3 � 1 mg) into aluminium pans, and water was added at a sample: water ratio of 1:3. The aluminium pans were hermetically sealed and kept in a refrigerator for 24 h. For the DSC scans, the samples were heated from 20 � C to 150 � C at a heating rate of 10 � C/min (Demirkesen et al., 2014). An empty pan was used as a reference. The accuracy of the DSC analysis was �0.0001� C. The analyses were carried out in triplicate. 2.7. Storage properties Staling analyses were carried out in a cooled incubator (Model Thermostable IR-150, Daihan Scientific, Korea) at 22 � 1 � C for 3 days (moulding occurred after 3 days). During this time, the samples were covered with a stretch film and kept in polyethylene bags (Demirkesen et al., 2014). The moisture content, total water loss and texture of the bread samples were analysed 1 (Day 0), 24, 48, and 72 (Day 3) hours after baking. Changes in hardness and cohesiveness of the bread samples were assessed by TPA analysis, as described in Section 2.4.4. After each storage period, the samples were frozen at 80 � C and freeze-dried for approximately 24 h. X-ray diffraction (XRD) and SEM analysis were conducted for the freeze-dried samples on Day 0 and Day 3. SEM ana­ lyses were carried out as mentioned in Section 2.5. Three runs were conducted for the staling analyses. 2.7.1. Moisture content After grinding, approximately 2 g of the bread samples were weighed and dried at 105 � C in an oven until a constant weight was achieved (Model UN 160 Plus, Memmert, Germany). The moisture content of the samples was calculated from the initial and final weight values (AOAC, 2000).

Table 1 Weight loss, moisture content, specific volume, colour and TPA results of control and PVB gluten-free bread.

2.7.2. X-ray diffraction (XRD) X-Ray diffraction analysis was performed using CuMo radiation on an X-ray diffractometer (Model D8 Advance, Bruker AXS, USA) at 40 kV and 30 mA. Freeze-dried samples were milled to a size of 250–350 μm. The milled bread samples were compressed to a thickness of 1–2 mm and a diameter of 13 mm. Scanning interval of a diffraction angle (2Ɵ) was conducted between 10� and 40� , and the scanning rate was 4� /min. The € analyses were carried out in duplicate (Ozkoç et al., 2009).




p (p ¼ 0.95)

Weight loss (%)

12.07 � 0.14

13.85 � 0.46


Moisture content (%)

50.36 � 0.10

49.19 � 0.86


Specific volume (mL/g)

1.44 � 0.05

1.53 � 0.13


71.44 � 0.61 1.11 � 0.03 15.87 � 0.11 28.70 � 0.55

74.76 � 0.64 0.83 � 0.01 16.19 � 0.16 26.28 � 0.39

0.004 0.001 0.090 0.004

20.12 � 2.15 0.94 � 0.24 0.78 � 0.01 0.63 � 0.01 9.83 � 1.11 0.33 � 0.00

17.82 � 1.80 10.71 � 1.57 0.52 � 0.02 0.41 � 0.02 3.78 � 0.43 0.17 � 0.02

0.156 0.010 0.010 0.010 0.020 0.000

Colour L* a* b* ΔE

2.8. Statistical analysis

TPA Hardness (N) Adhesiveness (g.sec) Springiness Cohesiveness Chewiness Resilience

Paired-t-tests (p ¼ 0.95, Microsoft Excel, 2007) were performed to measure differences between quality properties (colour, texture, specific volume and total water loss) of the control and partial-vacuum baked (PVB) bread samples. Differences between the control and PVB samples during storage were examined by calculating the statistical significance between the 24 h, 48 h and 72 h stored control and PVB counterparts.

* p-value lower than 0.05 means statistical differences were significant. 3

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Journal of Cereal Science 91 (2020) 102891

tested through texture profile analysis. The effect of the vacuum on the textural properties was perceived as giving rise to a more adhesive and less stiff, chewable, cohesive and flexible nature compared to the atmospheric-baked counterparts. No statistical difference was found between the hardness values of the control and PVB bread. Changes in the other textural parameters (adhesiveness, springiness, cohesiveness, chewiness and resilience) of the gluten-free bread were statistically significant (p < 0.05). Springiness and resilience give information about the elasticity of food. More adhesiveness indicates less elasticity in the PVB bread samples. A previous study showed lower values for springi­ ness, chewiness and cohesiveness of rice bread produced by PVB, indi­ cating that it was more brittle than the control (Kadan et al., 2001). There is an inverse relationship between ambient pressure and the volume of gas bubbles. This might have caused the stretching of gas bubbles in the PVB breads; hence, fewer internal bonds formed in the bread matrix.

kPa) may have influenced the degrees of gelatinization and protein degradation. The lower cohesiveness value of the PVB bread compared to the control (Table 1) may have resulted from incomplete gelatiniza­ tion and limited protein degradation due to the reduced pressure. 3.3. DSC The onset temperature (To), peak temperature (Tp), end temperature (Te) and enthalpy (ΔH) values of the dough, control and PVB bread samples are shown in Table 2. The gelatinization temperature of rice starch ranges from 80 to 90 � C, and the amylose–lipid complex melting endotherm ranges from 100 to 110 � C (Prakaywatchara et al., 2018). Gelatinization enthalpy means the loss of the molecular order of starch during the gelatinization process (Cornejo and Rosell, 2015). The amylose–lipid complex enthalpy gives information about the amount of amylose–lipid complex formation, and this is related to food qualities such as textural and rheological properties (Prakaywatchara et al., 2018). Fessas Schiraldi (2000) reported that when the moisture content decreases, a significant overlap occurs between starch gelatinization and amylose–lipid decomposition; additionally, the gelatinization tempera­ ture tends to shift toward a high temperature. In the present study, peaks were overlapped for baked samples and peaks were separated for dough samples. The ΔH values showed that gelatinization and the formation of amylose-lipid complex in the bread matrix was higher for the control samples than for the PVB samples, and these values were low for the dough samples. According to SEM results (Fig. 1), PVB might have more gelatinization enthalpy, hence low amylose-lipid complex enthalpy.

3.2. SEM SEM images of the gluten-free bread dough, control, and PVB bread samples are depicted in Fig. 1. Starch granules are clearly indicated in the dough images depicted in Fig. 1-a (1000x magnification) and Fig. 1a’ (3000x magnification), and differentiated dough structure was observed in the resulting bread samples. In the control samples, swollen starch granules and gas bubbles could be seen in the bread crumb. Starch granules were distributed throughout the protein fibrils (Fig. 1-b); the stretched structures are the protein fibrils, as shown in Fig. 1-b’ (black arrow). In the PVB crumb, as demonstrated in Fig. 1-c (1000x magnifi­ cation) and Fig. 1-c’ (3000x magnification), distorted starch granules were observed along with fewer gas bubbles, and no protein fibrils were detected. In contrast to the rigid structure of the PVB samples, a ho­ mogeneous and compact structure formed in the control. In the baking process, starch granules gelatinise and protein degradation occurs. The degree of these changes is affected by the baking temperature and the amount of water (Kim et al., 2003). During vacuum baking, rapid water evaporation and a colder inner temperature compared to conventional baking due to the lower boiling point of water (Tbp,water ¼ 76.6 � C at 60

Table 2 DSC results of dough, control and PVB gluten-free bread. Samples

To (� C)

Tp (� C)

Te (� C)


94.80 � 4.11 110.83 � 1.94

103.91 � 1.29 111.63 � 2.32

111.67 � 1.35 116.54 � 1.76

ΔH (J/g) 167.93 � 96.30 119.26 � 87.75


95.94 � 0.82

105.50 � 0.62

121.24 � 1.62

821.56 � 14.03


92.43 � 2.24

106.18 � 0.52

119.64 � 0.84

687.04 � 73.71

Fig. 1. SEM images of dough (a, x1000 magnification; a’,x3000 magnification), control (b, x1000 magnification; b’,x3000 magnification) and PVB gluten-free bread crumb (c, x1000 magnification; c’,x3000 magnification) (White arrows: starch granule, black arrows: protein fibrils). 4

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These results were consistent with the TPA results (Table 1), especially regarding the lower cohesiveness value due to poor intermolecular in­ teractions in PVB bread.

hardness values of the PVB samples decreased along with cohesiveness and resilience; only the hardness values of the control samples increased, while the cohesiveness and resilience decreased over time. Although the initial hardness values of the samples were not statistically significant (Table 1), statistical differences between the hardness values during the 3-day storage for the control and PVB samples were signifi­ cant (p < 0.05). Cohesiveness provides information about the freshness of bread samples and on the internal resistance and interactions between ingredients; cohesiveness tends to decrease during storage (Roman et al., 2020; Ronda and Roos, 2011). The cohesiveness values decreased during storage for both bread samples. The values were lower at the beginning and during storage, indicating that the vacuum caused the formation of weaker internal bonds between the ingredients compared to the fully atmospheric conditions. HPMC is used in bread as improver and anti-staling agent. HPMC improves bread structure by forming a barrier against gas diffusion; hence, the total water loss decreases and the crumb moisture content increases. Exposure to high temperatures �rcenas leads to the formation of a polymeric network during baking (Ba and Rosell, 2005). During storage, increases in total water loss and de­ creases in moisture content were observed for the PVB samples compared to control (Table 3). During vacuum baking, the internal temperature of bread decreases due to the decreased boiling point of water and diminishes of internal pressure; this may have caused the poor and/or different HPMC network formation. The higher adhesiveness values of the PVB samples (although showing similar changes in mois­ ture content) could be related to the inability to fully incorporate HPMC into the bread.

3.4. Storage properties 3.4.1. Quality characteristics Changes in the total water loss, moisture content and TPA results (hardness, adhesiveness, springiness, cohesiveness, chewiness and resilience) during the 3-day storage are shown in Table 3. The total water loss of PVB samples was higher than that of the control at the initial stage of baking and during storage. The total water loss is related to the movement of free water in the bread. In vacuum applications, such as cooling and baking, increased water loss was observed compared to conventional methods. The change in the moisture content of the bread crumb was not statistically significant (p > 0.05) (Table 3) during the storage period. Water migration is the main reason for hardening of the bread during storage. The moisture content of the samples was decreased during storage because of moisture migration from crumb to crust (Monteau et al., 2017). Changes in the values of hardness, springiness, cohesiveness, chew­ iness and resilience during the 3-day storage of the bread samples were statistically significant (p < 0.05), but no significant change was observed for the adhesiveness values (p > 0.05). Hardness, cohesiveness and resilience are some of the most desired textural parameters of gluten-free bread (Roman et al., 2020). During the storage period, the

3.4.2. SEM SEM images (60x magnification) of the control and PVB gluten-free bread samples after 1 h and 72 h are depicted in Fig. 2. Deep and lon­ gitudinal cracks were observed PVB bread after 1 h baking (Fig. 2-b), and the existing cracks from baking were deepened after 72 h of storage (Fig. 2-b’). The control bread had a more compact and firmer structure than the PVB bread after 72 h baking. These results were in agreement with the harder texture (Table 3) seen for the control compared to the PVB counterpart after 72 h.

Table 3 Changes of total water loss, moisture content and texture during 3-day storage. Parameters


Total Water Loss (%)


Moisture Content (%)




Storage time

p* (p ¼ 0.95)


24 h

48 h

72 h

12.09 � 0.22 13.98 � 0.82

11.45 � 0.01 13.95 � 0.58

12.24 � 0.58 14.20 � 0.15

13.27 � 0.60 14.45 � 0.29


50.40 � 0.09 48.83 � 0.59

49.47 � 0.04 48.21 � 0.09

49.01 � 0.34 47.79 � 0.21

47.54 � 0.07 47.10 � 0.37


17.36 � 2.87 17.20 � 2.95

30.25 � 7.43 18.46 � 1.87

33.03 � 3.13 17.30 � 1.62

36.36 � 7.18 16.45 � 2.42


2.06 � 1.58 10.11 � 1.38

2.17 � 2.12 5.56 � 3.65

0.46 � 0.47 5.41 � 4.64

0.57 � 0.50 2.45 � 1.82


0.80 � 0.03 0.56 � 0.06

0.68 � 0.04 0.52 � 0.03

0.75 � 0.02 0.54 � 0.07

0.70 � 0.02 0.45 � 0.00


0.61 � 0.01 0.40 � 0.03

0.43 � 0.04 0.35 � 0.03

0.36 � 0.06 0.31 � 0.04

0.35 � 0.08 0.30 � 0.05


8.44 � 1.33 3.85 � 0.42

10.00 � 3.44 3.39 � 0.62

8.99 � 1.45 2.95 � 0.90

9.12 � 3.43 2.16 � 0.09


0.33 � 0.00 0.17 � 0.02

0.20 � 0.02 0.14 � 0.01

0.17 � 0.03 0.13 � 0.00

0.17 � 0.05 0.12 � 0.02


3.4.3. XRD Crystalline patterns of starches were investigated by XRD analysis. Starch granules have different crystalline types according to botanical origin and include the A, B and C-type. A-type and B-type are mainly detected in cereal starches and tubers, respectively. C-type is a mix of these two types (Lopez-Rubio et al., 2008). The crystal type of native rice flour typically shows A-type patterns, with diffraction peaks at 15� , 17� , 18� and 23� (Sittipod and Shi, 2016). Another crystal type, called a V-type, forms after heat treatment through the complexion of starch and lipid. V-type crystal patterns have been observed at 20� with a 13� peak (Lopez-Rubio et al., 2008). The XRD spectrums of the dough, control and PVB samples (at 1 h and 72 h after baking) are shown in Fig. 3. The bread dough exhibited an A-type crystal structure, revealing the appearance of peaks at 15.24� , 17.6� , 23� and 26.59� , according to Lopez-Rubio et al. (2008). After conventional baking, the peak at 15� disappeared, and additional peaks were observed at 13� and 20� , indicating a V-type crystal structure. Peaks were observed at 13� , 15� , 17.91� , 19.77� and 22.99� , indicating that both A-type and V-type crystals existed in the PVB samples. The peaks of the control samples were at approximately 13� , 17� and 20� , and additional peaks formed at approximately 15� and 23� for the PVB samples. These additional peaks revealed that the PVB samples had more crystalline structure due to swelling and the degree of starch gelatinization, which differentiated the PVB samples from their conventional counterparts. These results can be related to the high ΔH values of the control samples compared to the PVB samples (Table 2). Peaks that formed 1 h after baking were the same after 72 h of storage; the only difference was observed for the control, which showed an additional peak at 22� that may be related to starch retrogradation.

TPA Results Hardness (N)

Control PVB

Adhesiveness (g.sec)





PVB Cohesiveness

Control PVB

Chewiness (N)

Control PVB


Control PVB

*p-values were calculated between 24 h, 48 h and 72 h of control and PVB samples, p-value lower than 0.05 means statistical differences were significant. 5

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Journal of Cereal Science 91 (2020) 102891

Fig. 2. SEM images of control (a, 1 h after baking; a’, 72 h after baking) and PVB (b, 1 h after baking; b’, 72 h after baking) gluten-free bread samples.

4. Conclusion The positive impacts on the quality and staling properties of glutenfree products have recently highlighted the benefits of vacuum baking. In this study, partial-vacuum baking was applied to alleviate the draw­ backs of gluten-free bread, such as undesirable quality characteristics and rapid staling. Limited studies have been conducted on how partialvacuum baking affects the storage properties of bread at the micro­ structural level. Partial-vacuum baking significantly impacted the total water loss, total colour change and TPA results (adhesiveness, springi­ ness, cohesiveness, chewiness and resilience) when assessed 1 h after baking; however, the hardness was not significantly affected. Remaining starch granules, different starch crystal types (mixed A and V crystal types) and poor chemical interactions (low ΔH) were observed for the PVB breads according to SEM, XRD and DSC. The effects of 3-day storage were significant for the total water loss and TPA results. During storage, the hardness of the PVB samples tended to decrease, while the hardness of the control almost doubled compared to its initial value. The XRD results revealed that although having similar moisture contents, the different moisture distributions due to a more crystalline structure of starch may have affected the storage behaviour of the PVB bread sam­ ples. Overall, to improve the efficiency of partial-vacuum baking, optimal process and product characteristics need to be identified for gluten-free bread.

Fig. 3. XRD spectrum of gluten-free dough, control, and PVB gluten-free bread.

Rondeau-Mouro et al. (2019) reported that although having a greater oven rise and gas fraction, no significant differences related to starch were found by NMR when comparing two baking methods (atmospheric pressure and reduced pressure). In the present study, the effect of the vacuum on the starch crystal type was evaluated, and differences be­ tween the XRD spectra provided information about how the vacuum affected the bread structure of the fresh and stored samples.

Author statement Sezin Tuta S¸ ims¸ ek: Conceptualization, Methodology, Formal 6

S. Tuta S¸ims¸ek

Journal of Cereal Science 91 (2020) 102891

analysis, Investigation, Writing-Original Draft; Writing-Review.

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