EFFECTS OF MOISTURE, SUCROSE, NaCl, AND ARABINOXYLAN ON RELAXATION IN WHEAT DOUGH AS MEASURED BY DMTA

Abstract

The effects of sucrose, NaCl, and arabinoxylan on the α-relaxation of wheat doughs with different water contents were investigated using Dynamic Mechanical Thermal Analysis (DMTA). DMTA measurements were made at the heating rate of 2°C/min from at least 30°C below the observed onset of the α-relaxation (glass transition) to at least 30°C above the transition. The glass transition temperature, T_g , was taken from the onset temperature of the decrease in storage modulus (G′). The frequencies used were 0.1, 1, and 5 Hz and amplitude was 16 μm. The storage modulus, G′, showed α-relaxation in all doughs with added ingredients. Added ingredients decreased the glass transition temperature of dough. The T_g of doughs with different ingredients decreased with increasing water content of doughs over the whole a_w range used (0.113–0.753). Also, the T_g increased with increasing frequency.

Keywords:

• DMTA
• Glass transition
• Wheat dough

INTRODUCTION

The glass transition behavior of various cereal materials has often been studied using Differential Scanning Calorimetry (DSC). Zeleznak and Hoseney [1] studied the glass transition temperature behavior of wheat starch, Kalichevsky et al. [2] and Kalichevsky and Blanshard [3] of amylopectin, de Graaf et al. [4] of gliadin, Kalichevsky et al. [5], Noel et al. [6], and Cherian et al. [7] of wheat gluten. Baked products consist of polymer mixtures, mainly starch and gluten, showing a broad glass transition with a small change in heat capacity. Laaksonen and Roos 8-9 showed very clearly that DSC, as used in their studies, is not sensitive enough to observe state transitions, such as the glass transition, in frozen wheat doughs. Moreover, Wetton [10] and Rotter and Ishida [11] reported that DMA/DMTA is about 1000 times more sensitive in observing thermal transitions than DSC. The importance of the glass transition temperature (T_g ) in determining the stability and dynamic-mechanical properties of cereal materials has been recently investigated 8-9, 12-16.

Dynamic Mechanical Thermal Analysis (DMTA) is a technique for measuring mechanical properties of materials, including foods, as they are deformed under varying stress as a function of time, frequency, or temperature. The analysis measures three major properties of the material: 1) the storage modulus, G′, which measures the amount of energy stored in the material per cycle; 2) the loss modulus, G″, which is proportional to the amount of energy dissipated per cycle; and 3) the loss tanδ, which corresponds to the ratio of energy lost to the energy stored per cycle. The glass transition results in a structural mechanical event related to decreasing viscoelasticity. Therefore, the DMA is a very sensitive method for detecting second-order transitions resulting in large changes in viscoelastic properties occurring over the transition temperature region [17].

Water is an important plasticizer of amorphous polymers. It is well established that the plasticization by water affects the glass transition temperature (T_g ) of many biopolymers, including amorphous food materials, resulting in depression of T_g 18-22. The glass transition related relaxations of various cereal polymers have been widely studied as a function of water content using the DMA/DMTA[3], 13-15, 23-24.

Sugars added to cereal foods can also have a significant plasticization effect, because of their low glass transition temperature and low molecular weight [7], [25]. There are also opposite results concerning the effects of sucrose on the T_g [9], [26]. Salts increase the T_g of polymers, including food materials, as has been observed by a number of researchers [9], 27-29.

Water extractable arabinoxylans (WEA) are polysaccharides and form a part of pentosans fraction in cereals. The pentosan content of wheat flour amounts to 2–3% about which 30% is water extractable 30-32. The pentosans are particularly important in bread making because of their physical properties. It is well established that the pentosans are extremely hydrophilic. Even though the same farinograph method was used, different values for water-binding capacity of water extractable pentosans can be found in the literature, e.g. Kulp [30] found it was 11 times the mass of the water for extractable pentosans. Jelaca and Hlynca [33] showed that the water extractable pentosans adsorbed 9.2 times the mass of water. Kim and D'Appolonia [34] obtained a value of 4.4 for the water-binding capacity of water extractable pentosans. It is well established that addition of water extractable pentosans into baking systems result in a pronounced increase in bread volume 35-38. Pentosans have been shown to decrease the rate of recrystallization of starch by reducing the amount of starch components available for recrystallization [34], and therefore reduce staling rate of baked bread [34], [39]. However, no studies have been reported on the effects of pentosans on the glass transition on wheat doughs.

The objective of the present study was to investigate the effects sucrose, NaCl, and water extractable arabinoxylan (WEA) on the glass transition of wheat dough at different water contents using Dynamic Mechanical Thermal Analysis (DMTA).

MATERIALS AND METHODS

Flour

Wheat (Torfrida, harvest 1997) was obtained from AVEVE (Landen, Belgium) and milled using a Buhler MLU-202 laboratory mill (Uzwill, Switzerland), according to AACC Method 26–31. Milling yield, protein content (% of dry matter), ash content (% of dry matter) and Farinograph water absorption (14% moisture basis) were 69%, 9.2%, 0.54%, and 62.0%, respectively. A micro-Kjeldahl procedure, according to AACC Method 46–13 (N × 5.7), was used for protein assessment. Ash content was measured according to AACC Method 08–01. Farinograph water absorption was determined according to AACC Method 54–21, and was based on dough consistency at the 500 BU (Brabender Unit) line.

Sample Preparation

Table 1 shows the composition of the various doughs. Sucrose and NaCl were dissolved in distilled water giving a proper concentration. The resulting solutions were added into a mixing bowl with the flour. The WEA was mixed with flour and water as a powder. Wheat flour doughs were mixed 3 min with the solutions to optimum consistency using the 10 g bowl Micro-Mixer (National Mfg. Co., Lincoln, Nebraska).

Table 1. Compositions of Various Wheat Doughs

	Dough (flour + water)	Dough + sucrose	Dough + NaCl	Dough + sucrose + NaCl	Dough + 1% WEA	Dough + 3% WEA
Flour	10 g	10 g	10 g	10 g	10 g	10 g
Water	5.4 mL	5.4 mL	5.4 mL	5.4 mL	5.4 mL	5.4 mL
Sucrose		0.6 g		0.6 g
NaCl			0.15 g	0.15 g
WEA					0.1 g	0.3 g

Dimensions of dough samples were 9 mm width, 0.9 mm thickness, and length varied from 15 to 20 mm. Samples were humidified at room temperature (24–25°C) in desiccators without vacuum over 11.3, 22.5, 32.8, 43.2, 57.6, 65.4, and 75.3% RH, as controlled by saturated solutions of LiCl, CH₃COOK, MgCl₂, K₂CO₃, NaBr, NaNO₂, and NaCl, respectively 40-41. Equilibrium occurred in about 3 weeks. The equilibrium water contents of the doughs were determined according to AACC Method 44–15A (at 130°C for 60 min).

Dynamic Mechanical Thermal Analysis (DMTA)

A Dynamic Mechanical Thermal Analyzer (Mark III DMTA, Polymer Laboratories, Loughborough, U.K.) was used to determine the α-relaxation of different doughs. At least triplicate samples were analyzed using DMTA at a heating rate of 2°C/min from at least 30°C below the observed onset temperature of the α-relaxation to at least 30°C above the transition temperature range. The glass transition temperature, T_g , was determined from the onset temperature of the α-relaxation, which resulted in a sharp decrease in storage modulus (G″). During dynamic heating, the samples were analyzed at frequencies of 0.1, 1, and 5 Hz and a strain setting was 0, which corresponded to an amplitude of 16 μm. The instrument measured each frequency for about 2 to 3 min and then changed to the next frequency during heating.

RESULTS AND DISCUSSION

The water content of the different dough samples as a function of water activity are given in Table 2. As seen, they varied from 1.73 to 10.40 g H₂O/100 g dm over a_w range of 0.113–0.753 for the plain dough (flour + water). Figure 1 shows water sorption isotherms for the different doughs. The water sorption isotherm is an important tool in the characterization of relationships between water content and water activity of food materials. Water contents in the doughs with added sucrose were higher over the whole a_w range used when compared to the water contents of the plain dough (Table 2 and Figure 1). The higher water-binding capacity of dough with added sucrose was probably due to more OH-groups available for same total weight of dry matter. Water contents of doughs with added NaCl were slightly higher than water contents of doughs with added sucrose (Table 2). This was probably due to capability of NaCl to partially open the gluten network and therefore allowing more water to be adsorbed. Figure 1 and Table 2 also show that doughs with added sucrose + NaCl had the highest water contents among the doughs studied. These water contents were from 2.98 to 12.64 g H₂O/1002g dm over the whole a_w range used (0.113–0.753). Although it is well known that pentosans have a high water-binding capacity, Table 2 shows that addition of 1% WEA (% of flour weight) did not increase the water content of the doughs over the a_w range used. This shows that either the amount of WEA was too minor to increase the water-binding capacity of dough or that the WBC Constant Dough Farinograph method for water binding capacity does not truly measured adsorbed water at lower water activities. If it were so, each one gram of pentosan would acount for an additional 9 grams of water. As seen, the addition of 3% of WEA increased the water contents at the same level as doughs with added sucrose and NaCl (Table 2).

Table 2. Water Contents (g H₂O/100 g Dry Matter) for Wheat Doughs with Different Ingredients

Aw	Dough (Flour + water) g H2O/100 g dm	Dough + sucrose g H2O/100 g dm	Dough + NaCl g H2O/100 g dm	Dough + sucrose + NaCl g H2O/100 g dm	Dough + 1% WEA g H2O/100 g dm	Dough + 3% WEA g H2O/100 g dm
0.113	1.73 ± 0.10	2.48 ± 0.19	2.51 ± 0.03	2.98 ± 0.12	1.72 ± 0.12	2.50 ± 0.15
0.225	3.36 ± 0.15	3.56 ± 0.08	3.69 ± 0.13	4.88 ± 0.08	3.29 ± 0.07	4.12 ± 0.18
0.328	4.12 ± 0.20	5.22 ± 0.10	5.29 ± 0.11	6.04 ± 0.11	4.44 ± 0.17	4.89 ± 0.08
0.432	5.86 ± 0.03	6.69 ± 0.13	6.89 ± 0.20	7.11 ± 0.09	5.59 ± 0.20	6.37 ± 0.03
0.576	7.60 ± 0.18	8.18 ± 0.17	8.98 ± 0.19	9.23 ± 0.08	7.98 ± 0.19	9.11 ± 0.15
0.654	9.04 ± 0.09	9.14 ± 0.09	9.70 ± 0.10	10.16 ± 0.17	9.88 ± 0.09	10.01 ± 0.20
0.753	10.40 ± 0.11	10.93 ± 0.02	11.94 ± 0.14	12.64 ± 0.09	10.41 ± 0.14	11.42 ± 0.16

Figure 1. Comparison of water isotherms at 24–25°C for dough (flour + water) (•), dough + sucrose (+), dough + NaCl (▵), dough + sucrose + salt (○), dough + 1% WEA (♦), and dough + 3 WEA (□).

Figure 2 shows typical changes in the dynamic-mechanical properties of the wheat dough (flour + water) stored at a_w of 0.225, as a function of temperature measured by DMTA at 0.1, 1, and 5 Hz. A sharp drop in the G′ can be observed depending on frequency. This change in G′ corresponded to the T_g of the material. Moreover, at the glass transition, a material becomes softened and its ability to store energy is partially lost resulting in a decrease in G′. The T_g was also frequency dependent as seen in Figure 2. Dependence on frequency of the T_g is well known [8], 42-44, but its significance to dough rheology is unclear.

Figure 2. The storage modulus (G′) and loss tan delta of the wheat dough (flour + water) stored at a_w of 0.225, as a function of temperature, as measured by DMTA at 0.1, 1, and 5 Hz.

The plastization effect of water observed on the T_g is very clear as seem from the decrease in T_g (Table 3, Figures 3 and 4). The T_g dropped 5–10°C per 1% water (dm), which is typical for various food materials [5], [15], [45]. The T_g values decreased with increasing water contents of the samples at all frequencies, from 57 to −10°C, from 58 to −8°C, and from 60 to −5°C at 0.1, 1, and 5 Hz, respectively. Both water contents and T_g values of the samples agreed very well with the results of Nikolaidis and Labuza [15] for cracker dough measured by DMTA with a heating rate of 3°C/min at 1 Hz. Hoseney et al. [46] and Noel et al. [6] reported higher T_g values for gluten and gluten proteins, respectively, than observed in the present study. Also, native and gelatinized starch had higher T_g values [1]. It should be noted that all these prior studies were done by DSC with a heating rate of 10°C and T_g was taken from the midpoint of the glass transition. It is well known that the faster heating rate and midpoint of the glass transition gives higher T_g values. Anyhow, it seems that the T_g of gluten dominated the glass transition of dough given the closest T_g values.

Table 3. Glass Transition Temperatures (T_g ) of Wheat Dough with Different Ingredients Measured by DMTA as Storage Modulus (G′) at 0.1, 1, and 5 Hz

Aw	Frequency (Hz)	Dough (Flour + water) Tg (°C)	Dough + sucrose Tg (°C)	Dough + NaCl Tg (°C)	Dough + sucrose + NaCl Tg (°C)	Dough + 1% WEA Tg (°C)	Dough + 3% WEA Tg (°C)
0.113	0.1	57 ± 1	52 ± 1	54 ± 2	52 ± 1	53 ± 0	50 ± 2
0.113	1	58 ± 2	53 ± 1	56 ± 3	53 ± 1	55 ± 1	51 ± 2
0.113	5	60 ± 1	55 ± 1	59 ± 2	57 ± 1	58 ± 1	54 ± 2
0.225	0.1	52 ± 1	46 ± 1	47 ± 1	47 ± 1	46 ± 1	37 ± 2
0.225	1	53 ± 1	47 ± 0	48 ± 1	48 ± 0	48 ± 1	41 ± 3
0.225	5	55 ± 1	50 ± 0	50 ± 2	51 ± 1	50 ± 1	44 ± 1
0.328	0.1	40 ± 3	37 ± 1	39 ± 3	42 ± 0	46 ± 0	35 ± 1
0.328	1	43 ± 1	38 ± 1	40 ± 2	43 ± 0	47 ± 0	40 ± 1
0.328	5	46 ± 1	40 ± 1	42 ± 3	45 ± 1	49 ± 1	43 ± 0
0.432	0.1	39 ± 3	30 ± 2	35 ± 1	36 ± 2	36 ± 3	31 ± 2
0.432	1	41 ± 2	32 ± 1	37 ± 1	38 ± 1	38 ± 2	35 ± 1
0.432	5	44 ± 3	35 ± 1	40 ± 1	40 ± 1	40 ± 3	39 ± 2
0.576	0.1	26 ± 1	16 ± 1	22 ± 0	20 ± 3	12 ± 1	11 ± 1
0.576	1	28 ± 0	18 ± 0	24 ± 1	22 ± 2	15 ± 2	13 ± 0
0.576	5	32 ± 1	20 ± 0	26 ± 0	24 ± 2	17 ± 2	15 ± 1
0.654	0.1	1 ± 1	− 2 ± 2	0 ± 2	− 1 ± 0	− 1 ± 1	− 5 ± 1
0.654	1	3 ± 1	0 ± 1	2 ± 1	2 ± 1	3 ± 2	− 2 ± 2
0.654	5	9 ± 1	3 ± 1	7 ± 1	5 ± 1	6 ± 2	0 ± 1
0.753	0.1	− 10 ± 2	− 15 ± 3	− 13 ± 1	− 15 ± 1	− 19 ± 1	− 24 ± 0
0.753	1	− 8 ± 2	− 13 ± 2	− 10 ± 2	− 13 ± 1	− 17 ± 1	− 22 ± 1
0.753	5	− 5 ± 0	− 10 ± 2	− 7 ± 1	− 10 ± 0	− 15 ± 1	− 20 ± 1

Figure 3. Comparison of the glass transition temperatures (T_g ) as a function of water activity (aw) at 24–25°C for dough (flour + water) (•), dough + sucrose (+), dough + NaCl (▵), dough + sucrose + salt (○), dough + 1% WEA (♦), and dough + 3% WEA (□), as measured by DMTA at 1 Hz.

Figure 4. Comparison of the glass transition temperatures (T_g ) as a function of water content (g H₂O/100 g dry matter) for dough (flour + water) (•), dough + sucrose (+), dough + NaCl (▵), dough + sucrose + salt (○), dough + 1% WEA (♦), and dough + 3% WEA (□), as measured by DMTA at 1 Hz.

Effects of Sucrose

Added sucrose decreased the T_g of the dough 5 to 10°C, over the a_w range of 0.113–0.753, compared to the plain dough (flour + water) as seen in Table 3. Similar results have obtained by Cherian et al. [7] for wheat gluten films with added sucrose, and Georget and Smith [47] for wheat flakes with added fructose and sucrose. Moreover, according to Levine and Slade [48] and Slade and Levine [49] small molecule weight carbohydrates can plasticize food materials, resulting in a drop in T_g. The T_g increased with increasing frequency, as observed in the plain dough. The T_g values decreased with increasing water contents of the samples over a_w range of 0.113–0.753 (2.48–10.93 g H₂O/100 g dm, Table 2) at all frequencies, from 52 to −15°C, from 53 to −13°C, and from 55 to −10°C at 0.1, 1, and 5 Hz, respectively.

Effects of NaCl

The effect of frequency on the T_g was clearly observed in doughs with added NaCl, with an increase in the T_g with increasing frequency. The T_g values were slightly higher than those in the doughs with added sucrose, however it was lower than those in the plain doughs over the whole a_w range used. Thus, added NaCl had a plasticization effect on the wheat dough. This was mostly due to the higher water contents of the dough samples with added NaCl as compared to the plain dough. There are several studies, which reported that added salts increase the T_g of polymers. James et al. [27] have seen increased in the T_g of poly(propylene oxide) with the addition of cobalt chloride. Cowie and Martin [28] have observed that with the addition of lithium perchlorate and LiT to poly(vinyl methyl ether). Both groups have reported that the elevation in T_g levels off beyond a certain salt concentration. Moreover, Olsen and Koksbang [29] showed that the T_g increased dramatically with increasing LiAsF₆ concentration for propylene carbonate-and hybrid polymer electrolyte.

Effects of Sucrose-NaCl-Mixture

The T_g values of the dough with added sucrose and NaCl were intermediate, compared to doughs with added sucrose or NaCl separately. At some water activities and frequencies, the T_g was the same as in doughs with added sucrose. While the addition of sucrose had the plasticization effect on doughs decreasing the T_g , NaCl conversely increased the T_g (Table 2). The T_g values of the doughs with added sucrose + NaCl decreased with increasing water contents of the samples over a_w range of 0.113–0.753 at all frequencies, from 52 to −15°C, from 53 to −13°C, and from 55 to −10°C at 0.1, 1, and 5 Hz, respectively.

Effects of WEA

Addition of 1% of WEA decreased the T_g at all frequencies and over the whole a_w range used. The decrease in the T_g was approximately at the same level or lower than in doughs with other added ingredients. Addition of 3% of WEA decreased the T_g giving the lowest T_g values of all doughs with or without added ingredients. Doughs with WEA had a decrease in the T_g 10–15°C compared to the plain dough. Addition of 3% of WEA caused lower T_g values for dough than addition of 1% WEA, over whole a_w range used, probably because of higher water contents in doughs with 3% WEA (Tables 1 and 2).

CONCLUSION

The present study of effects of different ingredients on the T_g of wheat doughs confirmed that doughs are plasticized by water. Dynamic Mechanical Thermal Analysis (DMTA) was appropriate and sensitive enough to observe the α-relaxation (glass transition) of all doughs studied. When the water sorption isotherm and state diagram of various doughs are known, processing and environmental conditions during storage can be controlled. The lowering effect of added sucrose and NaCl on the T_g of dough was similar as reported for other polymers in a literature. Even though, addition of WEA decreased the T_g more than expected. The change in water binding capacity was small.

ACKNOWLEDGMENTS

This study has been carried out with financial support from the Finnish Food Research Foundation, the Foundation of Tiura, the Commission of the European Communities, Agriculture and Fisheries (FAIR) specific RTD program, CT97–3609, Use of Wheat Water Extractable Arabinoxylans (WEA) to Improve Stability of Frozen Doughs and Quality of Bread, the University of Minnesota, Agriculture Experiment Station, Project 18–72, and the Institute of Food Technologist Marcel Loncin Research Prize. The study does not necessarily reflect the Commission's views and in no way anticipates the EC Commission's future policy in this area. The flour and flour analysis were kindly provided by Professor Jan Delcour, Katholieke Universiteit, Leuven, Belgium.