Moisture Distribution during Conventional or Electrical Resistance

Jun 20, 2014 - Heating of the dough by means of an ERO is based on the principles of Joule's first law and Ohm's law. This study compared the changes ...
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Moisture Distribution during Conventional or Electrical Resistance Oven Baking of Bread Dough and Subsequent Storage Liesbeth J. Derde,* Sara V. Gomand, Christophe M. Courtin, and Jan A. Delcour Laboratory of Food Chemistry and Biochemistry and Leuven Food Science and Nutrition Research Centre (LFoRCe), KU Leuven, Kasteelpark Arenberg 20, B-3001 Leuven, Belgium ABSTRACT: Electrical resistance oven (ERO) baking processes bread dough with little temperature gradient in the baking dough. Heating of the dough by means of an ERO is based on the principles of Joule’s first law and Ohm’s law. This study compared the changes in moisture distribution and physical changes in starch of breads conventionally baked or using an ERO. The moisture contents in fresh ERO breads are generally lower than those in conventional breads. During storage of conventionally baked breads, water migrates from the crumb to the crust and moisture contents decrease throughout the bread crumb. Evidently, less moisture redistribution occurs in ERO breads. Also, the protons of ERO bread constituents were less mobile than their counterparts in conventional bread. Starch retrogradation occurs to similar extents in conventional and ERO bread. As a result, the changes in proton mobility cannot be attributed to differences in levels of retrograded starch and seem to be primarily determined by the overall lower moisture content. KEYWORDS: electrical resistance oven baking, conventional baking, moisture migration, physical changes in starch

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bread.9 The temperature gradient in (partially baked) dough throughout ERO baking is not completely uniform, and it has been suggested that heat insulation would increase the uniformity.8 Baker and Mize7 evaluated the temperature rise, oven spring, and pressure generated by alcohol and CO2 during ERO baking. They concluded that CO2 production and alcohol evaporation during baking lower the rate of temperature rise in dough. Furthermore, the rate of oven rise and the pressure in dough changed most markedly during starch granule swelling.7 Also, Hoseney and collaborators studied gas retention of dough during ERO baking4,5 and the effect of shortening and some surfactants on loaf volume.1,6 Finally, Martin et al.9 used the ERO to study the mechanism of bread firming. ERO bread firmed rapidly in the first 24 h, but firmness did not increase during further storage. To investigate whether ERO bread is a suitable model system for conventional bread, bread baked with an ERO was compared with conventional bread. The work included monitoring temperature changes at different places of the dough/bread during baking. Furthermore, the physical changes in starch and the moisture distribution in fresh and stored bread were studied.

INTRODUCTION Baking is one of the most critical steps in the breadmaking process. Proofing dough to the same size does not ensure that equal bread loaf volumes are obtained after baking.1 Traditionally, bread is baked in an oven in which dough is heated progressively from the outside toward the center. Dough is a poor heat conductor. During baking, a temperature gradient exists from the outside to the center of the bread. This makes the study of the changes in dough/bread during heating difficult because the extent to which temperature-triggered reactions such as protein polymerization, starch gelatinization, and pasting occur in the baking process depends on the temperature−time profile experienced by a particular sample in a baking tin. To address this problem, Baker2 designed an electrical resistance oven (ERO), in which, in principle, dough is heated uniformly. In such an oven, dough placed between two electrode plates serves as an electrical resistance and heat is generated by an electric field. As a result, it is uniformly heated and the resultant bread has no crust. The heating rate can be adjusted by controlling the distance between the electrodes, the applied voltage, and the dough area in contact with the electrodes.1,3 To date, despite earlier work,1,2,4−7 no systematic study has been performed on differences between products from conventional and ERO baking. During conventional bread baking, water vapor formed in the warmer parts condenses in the cooler center of the dough. In contrast, in ERO baking, water vapor in the center of the baking dough condenses on the colder outer surfaces in contact with the environment. Therefore, the water distribution is different from that in conventionally baked bread as noted earlier by He and Hoseney8 and Martin et al.9 Moreover, ERO breads have lower crumb moisture contents than conventionally baked breads, whereas the exterior regions of an ERO bread have a higher moisture content than the crust of a conventional © 2014 American Chemical Society

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MATERIALS AND METHODS

Materials. Commercial European bread wheat flour (Crousti) [moisture content, 14.2%; protein content (N × 5.7), 11.1%, as is basis] was from Dossche Mills (Deinze, Belgium). Methods. Dough Making, Fermentation, and Proofing. Bread was prepared according to a straight-dough breadmaking procedure.10 Wheat flour (100.0 g, 14.0% moisture basis) was mixed for 4 min in a Received: Revised: Accepted: Published: 6445

April 17, 2014 June 19, 2014 June 20, 2014 June 20, 2014 dx.doi.org/10.1021/jf501856s | J. Agric. Food Chem. 2014, 62, 6445−6453

Journal of Agricultural and Food Chemistry

Article

100 g pin mixer (National Manufacturing, Lincoln, NE, USA) with 57.0 mL of deionized water (mixograph-derived water absorption), 6.0 g of sucrose, 5.3 g of compressed yeast (Bruggeman, Ghent, Belgium), and 1.5 g of salt. The mixed dough was transferred to a lightly greased bowl and put in a fermentation cabinet (National Manufacturing) at 90% relative humidity and 30 °C. Fermentation took 90 min, with intermediate punching after 52 and 77 min using a dough sheeter (National Manufacturing). After 90 min, dough was punched a last time and molded. It was then either placed in a lightly greased metal baking pan for conventional baking or put in a plexiglass ERO baking chamber. In both cases, dough was then transferred to the fermentation cabinet for a final proof stage of 36 min. Conventional Baking. Proofed dough was baked at 215 °C for 24 min in a National Manufacturing rotary oven. After baking, depanning, and 120 min of cooling to room temperature, breads were weighed, packed in a hermetically sealed plastic bag to prevent moisture loss, and stored at room temperature until further analysis. Baking in an Electrical Resistance Oven. Heating of dough by an ERO is based on the principle of Joule’s first and Ohm’s laws. Joule’s first law expresses the relationship between the heat (Q) generated by the current (I) flowing through a resistance (R) for a time (t): Q = I2 × R × t. Joule heating, also known as ohmic or resistive heating, is the process by which the passage of an electric current through a resistance releases heat. According to Ohm’s law, the applied voltage (V) equals the current (I) × the resistance (R). In line with the above, at a given dough resistance, the intensity of the current rises and, as a result, the temperature in dough increases at a higher rate when the applied voltage is increased. Evidently, the heating rate also depends on dough/bread resistance. The ERO was based on a model described by Hoseney1 with slight modifications (baking chamber made of plexiglass plates of 10 mm thickness with internal dimensions of 12.5 × 10.0 × 22.0 cm). Figure 1 shows the system, with indications of the positions of the thermocouples (see below). Dough was placed between two stainless steel plates (distance between the plates = 6.5 cm, plate thickness = 1.5 mm), which were coupled to a variable transformer (Hossoni, Hongbao, China; 0−250 V). A perforated (126 holes, each with a diameter of 1.5 mm) plexiglass plate (12.4 × 6.4 cm, plate thickness 10 mm), which could easily slide upward when the dough/bread expanded, was placed on top of the dough during proofing and baking to keep the surface of the bread flat. This ensured good contact of the baking dough/bread with the electrode plates and reproducible baking. ERO baking was programmed to simulate the temperature profile of the center of dough/bread during conventional baking. The heating rate, and thus also dough temperature, was controlled by adjusting the applied voltage by a variable transformer. Temperature was monitored by thermocouples (see below) inserted into dough through a hole in the baking chamber 4.0 cm above the bottom of the oven or through holes in the perforated plate. The voltage−time profile applied during baking was chosen to simulate the temperature profile in the center of conventionally baked dough/bread and was as follows: 50 V (0−1 min), 100 V (1−8 min), 120 V (8−14 min). After baking, bread was removed from the oven and allowed to cool for 120 min. Afterward, it was weighed and packed individually in a hermetically sealed plastic bag to prevent moisture loss. It was then stored at room temperature for 7 days. Analyses at day 0 were those on breads cooled to room temperature and stored for 2 h. Temperature Measurements during Baking. Temperature measurements were performed using a Datapaq (Cambridge, UK) temperature data logger (MultiPaq 21) with type T thermocouples. In conventional baking, a stainless steel thermal barrier (Datapaq) protected the MultiPaq temperature data logger. Temperature measurements were performed in triplicate. Determination of Moisture Contents. Moisture contents were measured for samples withdrawn at eight different places (indicated in Figure 5) in fresh and aged bread according to AACC International method 44-15A.11 Differential Scanning Calorimetry (DSC). DSC was carried out with a Q1000 differential scanning calorimeter (TA Instruments, New Castle, DE, USA). Bread crumb samples were freeze-dried, ground,

Figure 1. Schematic representation of baking chamber of the electrical resistance oven (ERO). The baking chamber is made from plexiglass. The voltage is applied by a variable transformer over the two electrode plates. A perforated plexiglass plate is used to keep the surface of the bread flat during final proof and oven rise. The produced gases can escape from the system through the pores. The numbers indicate the places where the thermocouples are placed. and accurately (2.5−4.0 mg) weighed in three separate aluminum pans (PerkinElmer, Waltham, MA, USA). An excess of deionized water was added [1:3 w/w sample dry matter (dm)/water]. The pans were hermetically sealed. After equilibration at room temperature for at least 20 min, the pans were heated from 0 to 120 °C at 4 °C/min. An empty pan was used as reference. Calibration was with indium. TA Universal Analysis software was used to determine the onset (To), peak (Tp), and conclusion (Tc) temperatures and melting enthalpy (ΔH) of retrograded amylopectin. Polarized Light Microscopy. The gelatinization process during baking using both conventional and ERO technologies was visualized using polarized light microscopy. For these analyses, samples obtained from dough before baking and from fully baked bread were prepared by putting a small piece of dough/crumb collected from the center of the dough/bread on a carrier glass. After the addition of a drop of deionized water, a cover glass was placed on it, and the samples were examined with an Olympus BH2 microscope (Olympus, Tokyo, Japan) in the polarization mode. Proton Nuclear Magnetic Resonance Measurements. Measurements of proton distribution in fresh and stored bread crumbs were performed with a low-resolution proton nuclear magnetic resonance (1H NMR) spectrometer (Minispec mq 20, Bruker BioSpin, Ettlingen, Germany) at an operating resonance frequency of 20 MHz for 1H (magnetic field strength of 0.47 T) according to the method of Bosmans et al.12 This technique detects differences in mobility of protons that are in different environments.13−16 During measurements, the probe head was kept at 25 °C using an external water bath. Samples (approximately 0.3 g) were accurately weighed in a Bruker NMR tube (10 mm external diameter) and sealed to prevent moisture 6446

dx.doi.org/10.1021/jf501856s | J. Agric. Food Chem. 2014, 62, 6445−6453

Journal of Agricultural and Food Chemistry

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Figure 2. Temperature evolution of dough/bread during baking in a conventional rotary oven, measured at five different places.

Figure 3. Temperature evolution in dough/bread during baking in an ERO, measured at nine different places in dough/bread, differing in length (A), height (B), and width (C). loss. Three NMR tubes were filled to about 8 mm height with tightly compressed bread crumb (extracted from the loaf center) to avoid large air holes. Triplicate measurements were performed on each NMR tube. Transverse relaxation times (T2) were measured. The transverse relaxation curves were developed using a single 90° pulse [free induction decay (FID)] and the Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence. The pulse lengths were 2.86 and 5.42 μs, respectively. An acquisition period of 0.5 ms was used for FID

measurements, and 500 data points were collected. The inhomogeneity of the static magnetic field affected the output for the most mobile FID population (around 0.5 ms). Therefore, this fraction was excluded from the analyses. The CPMG sequence had an interpulse spacing of 0.1 ms, and 2500 data points were acquired during the measurements. To increase the signal-to-noise ratio, 32 scans were performed with a recycle delay of 3.0 s for both measurements. The transverse relaxation curves were transformed with an inverse Laplace 6447

dx.doi.org/10.1021/jf501856s | J. Agric. Food Chem. 2014, 62, 6445−6453

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transformation to continuous distributions of T2’s using the Contin algorithm of Provencher (Bruker software).17 Fast-relaxing protons, for example, protons of solids or protons strongly associated with solids, have a short transverse relaxation time. These can be observed in FID measurements. CPMG measurements are used to study protons with higher transverse relaxation times, such as water protons and solid protons in strong interaction with water.14,16,18 The areas under the curves of the different proton populations are proportional to the quantity of protons in these populations for both FID and CPMG measurements. The areas were calculated by taking into account the exact weight of the bread crumb samples. Measurements were performed in triplicate to confirm the reproducibility.

lost their Maltese crosses (Figure 4C,D) and thus were gelatinized.

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RESULTS AND DISCUSSION Breadmaking: From Dough to Fresh Bread. Temperature Changes during Baking. Monitoring the temperature at different places in dough/bread during conventional baking revealed that there were large gradients from the outside to the center of the dough (Figure 2). The closer to the dough center, the longer is the lag phase and the longer the temperature rise is postponed. In crumb, a maximum temperature of about 100 °C is reached. In contrast, the temperature increases to above 100 °C at the surface, when water vaporization has finished. For ERO baking, He and Hoseney8 observed that the resistance of dough changes during the baking process. At about 65 °C, the resistance of yeasted dough increases gradually. This is caused by starch gelatinization, which makes less free water available to the ionic solution to conduct the electric current.7,8 In the present case, the total baking time (14 min) was shorter than in conventional baking (24 min) by omitting the isothermal phase around 30 °C (0−5 min) in the center of the bread crumb, which exists during conventional baking, and by shortening the final isothermal phase. The latter was done to avoid shrinking of the breads in the ERO and thereby losing contact with the electrode plates. Nevertheless, the baking process was considered to be complete because the bread crumb structure was set and no crumb collapse (“keyholing”) was observed. The temperature gradients during conventional baking are much larger than those during ERO baking. They were measured in the length, width, and height, as shown in Figure 3. Nearly no temperature gradients at points differing in height (measuring points 3, 6, and 7; Figure 3B) were noted. The differences in temperature between these measuring points were limited to maximally 2.3 °C. However, temperature gradients ran up to 13.1 °C near the surface (measuring point 9, Figure 3B). The differences were much lower (3.0 ± 0.7 °C) when the temperature was measured approximately 12 mm under the surface (measuring point 8, Figure 3B). In the length axis of the bread (measuring points 1−4 and 7, Figure 3A) and across the width (measuring points 3−5, Figure 3C), temperature gradients were limited to maximum differences of 2.8 and 2.5 °C, respectively. These small differences showed that ERO baking is almost gradient-free. At the end of baking (e.g., probes 1, 2, and 9, Figure 3), the temperature decreased when shrinking occurred and caused the bread to lose contact with the electrode plates at some places. Starch Gelatinization during Baking As Visualized by Polarized Light Microscopy. Starch granules in dough are present as native semicrystalline granules with a typical Maltese cross when viewed under polarized light.19 During the initial stages of both conventional and ERO baking, starch granules are still present as native granules (Figure 4A,B). Irrespective of the heating mode, at some point during baking, starch granules

Figure 4. Polarized light microscope images of conventional (A, C) and ERO dough/bread (B, D) before baking (A, B) and after finishing the baking process (C, D).

Bread Moisture Content after Baking. During conventional baking, water evaporation occurs at the surface. Water vapor is lost through the crust into the surrounding hot air while another part of the water moves toward the cooler regions inside the crumb and condenses there.20,21 In general, conventional bread had a lower weight than ERO bread, that is, 135.7 (±0.6) and 149.8 (±0.9) g, respectively. In conventional bread, the moisture content in the different parts of the crumb was relatively constant (approximately 43%). Possibly, moisture had redistributed during cooling. The moisture content in the crust of conventional bread was much lower, that is, approximately 12.9% (Figure 5). During ERO baking, water vapor condenses on the relatively cold surface of the bread and the plexiglass plates in contact with the ERO bread, whereas the air in a conventional oven is warmer and water vapor stays in the gas phase during the entire baking process. As a result, a small, relatively wet, and leathery layer was formed on top of ERO bread rather than a dry and crisp crust, as observed in conventional baking. Furthermore, the baking time of ERO bread is slightly shorter than that in conventional baking, which shortened the time span over which water could evaporate. Although the total amount of water was higher in ERO breads, the moisture contents in the different parts of the bread crumb were generally lower for ERO bread than for conventional bread. In addition the water was unequally distributed (Figure 5). The moisture content ranged from 33.0% (long side, bottom) to 41.0% (center, top). Overall, the center of ERO bread had a slightly higher moisture content than its outer parts. However, moisture content was higher in the “crust” of ERO breads, as described earlier by Martin et al.,9 who reported that the exterior 5−10 mm of ERO bread contains approximately 47% moisture.9 These observations can be explained by differences in water transport during ERO and conventional baking due to the differences in temperature gradient. In ERO bread, temperature gradients are small and the outer parts of the bread are in contact with relatively cooler surfaces (plexiglass and metal electrode plates). Water evaporation and subsequent condensation occur on the colder 6448

dx.doi.org/10.1021/jf501856s | J. Agric. Food Chem. 2014, 62, 6445−6453

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Figure 5. Moisture content during storage (days 0, 3, and 5) of conventional (A, B) and ERO (C, D) baked breads measured at four different places in the upper (A, C) and lower parts (B, D) of the breads.

Amylopectin Retrogradation during Storage of Bread. Retrogradation of amylopectin was monitored by DSC analysis of crumb samples. Recrystallized amylopectin melted between 40 and 65 °C (peak temperatures around 50 °C). Figure 6 shows the increase in enthalpy of recrystallized amylopectin melting in crumb during storage of conventional and ERO baked breads. For both samples, very low enthalpy values were

plexiglass plates of the baking chamber or water evaporates and escapes to the surrounding air. In this way, a water distribution opposite that in a conventional bread (i.e., high moisture content on top of the ERO bread) was reached, similar to what has been observed earlier.8,9 As a result, the moisture content of the upper part of ERO bread was higher than that of the lower parts. Changes during Storage of Breads. Macroscopic Changes in Moisture Gradient during Storage of Breads. During storage of conventional bread, water migrates from the crumb to the crust. Figure 5 shows the evolution of the moisture content in different parts of the bread during storage. Moisture content decreased in every part of the crumb, whereas that of the crust increased. The largest shifts occurred during the first days of storage. After 5 days of storage, the moisture content of the crust seemed to have stabilized. In contrast to what one might expect on the basis of these data, only a slight weight loss occurred, that is, approximately 1.7 g on a total of 136 g. In contrast to conventional bread, freshly baked ERO bread does not have a dry and crisp crust. Previous studies showed the exterior layer (5−10 mm) of ERO bread to have a moisture content of approximately 47%,9 whereas the moisture content of the crust in a conventional bread is