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Thermal and dynamic rheological properties of wheat flour fractions
Food Research International 34 (2001) 329±335
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Thermal and dynamic rheological properties of wheat ¯our fractions K. Addo a,*, Y.L. Xiong b, S.P. Blanchard b a
Department of Nutrition and Food Science, University of Kentucky, Lexington, KY 40506, USA b Department of Animal Sciences, University of Kentucky, Lexington, KY 40506, USA Received 10 February 1999; received in revised form 8 May 2000; accepted 31 August 2000
Abstract A commercial high gluten ¯our (HGF) was fractionated into prime starch (PS), tailing starch (TS), and gluten (G). Fractions were examined alone or in various combinations. Dynamic rheological properties of samples were measured in an oscillatory rheometer (strain 0.02; frequency 0.1 Hz) during heating at 1 C/min. Thermal characteristics of samples were determined by dierential scanning calorimetry (DSC) at a heating rate of 10 C/min. The loss (G00 ) and storage (G0 ) moduli of PS and mixed G/PS, G/ TS, and G/PS/TS increased after 60 C, reaching peak values (e.g. 81, 301, 313, and 3000 Pa in G0 , respectively) around 75 C after which the moduli decreased. HGF showed a steady increase in G0 from 32 to 2490 Pa as temperature increased from 65 to 90 C, indicating continuous formation of elastic networks. Cooling increased G0 for G/PS/TS, decreased G0 for HGF, and produced no rheological transitions for all samples. TS and G alone did not exhibit appreciable viscoelastic responses to the heating and cooling temperatures. DSC measurements revealed a major endothermic transition in HGF. This transition, with a peak around 60 C, was due to starch gelatinization. The presence of G or TS resulted in reduced melting enthalpies of starch in the PS fraction. Gluten or TS fractions alone or in combination did not exhibit any endothermic transitions. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Rheology; Wheat ¯our
1. Introduction Rheological properties of wheat ¯our doughs are of primary importance because, the ¯ow and deformation behavior of doughs are recognized to be central to the successful manufacturing of bakery products (Menjivar, 1990). The rheological properties of the dough change greatly during mixing and during cooking where the starch granules and gluten proteins undergo structural alterations to form three-dimensional matrices via protein±protein and protein±carbohydrate interactions. Because of the direct relationship between the rheological behavior of a dough and the ®nal/physical (end-use) properties of the baked product, much of the research in bakery processing has been devoted to the testing of physical properties of fresh doughs (Dreese, Faubion & Hoseney 1988; Petrofsky & Hoseney, 1995). In comparison, relatively few studies have been conducted to monitor speci®c changes in viscoelastic properties of the dough during heating (baking) and subsequent cooling.
* Corresponding author. Tel.: +1-859-257-7784; fax: +1-859-2573707.
Most dough research has employed traditional dough testing instruments, such as the mixograph, extensigraph, and alveograph. These instruments provide useful and practical information about the relationships between rheological properties of ¯our doughs and baking quality, but are limited to empirical correlations (Berland & Launay, 1995). Analysis with these instruments involves large deformation of dough samples and produces practical information that may be empirically correlated to the true viscoelastic properties of the dough components. Dynamic rheological techniques, which are increasingly utilized in the last decade, on the other hand, can determine fundamental mechanical properties of dough materials. In particular, oscillatory dynamic testing, in which a sinusoidal small- amplitude strain is imposed onto a sample to produce a stress output, or vice versa, is a powerful means for examining the fundamental viscoelastic properties of dough (Petrofsky & Hoseney, 1995). Because the strain imposed is very small, samples are analyzed nondestructively, hence, the test can accurately probe mechanical properties of the dough during mixing and baking. Recent work by Petrofsky and Hoseney (1995), has shown the existence of starch±gluten±water interactions
0963-9969/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0963-9969(00)00171-X
330
K. Addo et al. / Food Research International 34 (2001) 329±335
The wheat ¯our used was a commercial high gluten ¯our (HGF) from Archer Daniels Midland Milling Company (Shawnee Mission, KS). It was unbleached, with 13.8% moisture, 13.8% protein, 0.53% ash, and a falling number of 257.
(Bohlin Instruments, Inc., Cranbury, NJ). Samples were loaded in the 1-mm gap between two parallel plates (upper plate diameter 3.0 cm). The sample perimeter was covered with a thin layer of silicon oil to prevent dehydration and the plates were insulated in a foamtype shell to reduce heat dissipation. After equilibration at the initial temperature (20 C) for 5 min, the sample was heated at a scan rate of 1 C/min by circulating water (from a programmable waterbath) around the lower sample plate. After reaching the ®nal temperature (80 C), the sample was cooled at 1 C/min to the initial temperature. During heating, the sample was sheared at a ®xed frequency of 0.1 Hz with a maximum strain of 0.02 and the shear force was registered via a torque element (91 g cm). The actual sample temperature was veri®ed using a thermocouple connected to the upper surface of the lower plate. Data was collected every 30 s during shearing measurement. The storage modulus (G00 , a measure of elastic response), and the loss modulus (G0 , a measure of viscous response) were continuously monitored during the dynamic rheological testing (Xiong, 1993).
2.2. Fractionation of wheat ¯our
2.4. Dierential scanning calorimetry (DSC)
Flour was fractionated into gluten (G), tailing starch (TS) and prime starch (PS) according to a previously described procedure (Hoseney, Finney, Shogren & Pomeranz, 1969). Flour (100 g) was made into a dough (65% absorption), covered with water in a jar, relaxed for 30 min, and manually washed with ®ve 200-ml portions of water into G and a mixture of starch and water solubles. After screening (metal sieve with 0.425 mm openings) to recover small pieces of G, the mixture of water solubles and starch was centrifuged for 15 min at a force of 2000 g. Three fractions were obtained from top to bottom: water solubles, TS and PS. PS was dried under a fume hood at room temperature to a moisture content of about 10%. G and TS were freeze-dried to an average moisture content of approximately 5%. Dried PS and TS were ground with an Udy grinder (Udy Corporation, Fort Collins, CO) to pass a 0.50-mm sieve; G was ground to pass a 0.25-mm sieve. Unfractionated ¯our, individual ¯our components, and mixtures of ¯our components [G/TS (40:60), G/PS (13:87), TS/PS (17:83), G/PS/TS (10:74:16)] were used for rheological and thermal measurements. The ratios for the dierent ¯our components used in these mixtures closely represented those found in the HGF.
The thermal properties of the wheat ¯our and ¯our components were analyzed at least in triplicate with a Perkin-Elmer DSC-7 analyzer equipped with a thermal analysis data station (Perkin-Elmer Corp., Norwalk, CT). For DSC analysis of interactions among wheat ¯our components, mixtures of dried components were blended in a glass vial and vortexed for 2 min. Water (10 ml) was added to 5 mg of sample in an aluminum pan. The pan was hermetically sealed and the sample allowed to equilibrate for 1 h before analysis. Samples were heated at a rate of 10 C/min from 20 to 100 C. The onset temperature (TO), peak maximum temperature (Tp), and enthalpy of transition (H) were automatically computed.
in dough. The extent of these interactions was governed by the type of wheat (soft vs. hard), the source of starch (wheat versus non-wheat) and the gluten source (soft vs. hard wheat). However, measurements in the study were conducted at a constant temperature of 25 C, and the study only looked at the total starch fraction rather than subfractions. The goal of this study was to gain more understanding of thermally induced interactions among ¯our starches (prime and tailing) and gluten using both dynamic rheological and dierential scanning calorimetry methods. 2. Experimental 2.1. Wheat Flour
2.3. Dynamic rheological measurements Dynamic oscillatory measurements of 10% (w/v) suspensions of ¯our and ¯our mixtures in distilled water during heating and subsequent cooling were performed in duplicate samples using a Bohlin VOR rheometer
3. Results and discussion 3.1. Thermal properties Thermal transitions in HGF, ¯our components and their mixtures were detected by dierential scanning calorimetry (Fig. 1). All samples containing prime starch exhibited a single endothermic transition, with the corresponding temperatures and enthalpies of the transition shown in Table 1. No transition was discerned in samples containing G or TS alone or their combination. The lack of endothermal transition for TS could have been caused by mechanical damage to the starch granules or by interaction between starch and other TS components besides lipids (Erdogdu, Czuchajowska & Pomeranz, 1995). The lack of any endothermal transition for gluten
K. Addo et al. / Food Research International 34 (2001) 329±335
331
Table 1 Dierential scanning calorimetry characteristics of high gluten wheat ¯our and ¯our components at various combinations Samplea
Flour PSb TS G G/TS G/PS PS/TS G/PS/TS
Endotherm To ( C)
Tp ( C)
H (J/g)
60.0 56.7 ± ± ± 57.5 57.4 54.0
64.5 61.2 No peak No peak No peak 61.5 62.5 61.0
2.07 4.04 ± ± ± 1.18 2.02 1.56
a
PS, prime starch; TS, tailing starch; G, gluten. To, onset temperature; Tp, peak temperature; H, enthalpy. Values are means of triplicate samples. b
content, H is in¯uenced by the amount of available water (Eliasson, 1980). The lack of any endothermic ¯ow from the gluten fraction and the reduction in H caused by gluten, seem to indicate that the gluten proteins are not greatly aected during heating or baking. Interestingly, the gluten fraction also showed no appreciable viscous or elastic response during heating. Lower melting enthalpies than the HGF (2.07 J/g) were recorded when G was mixed with PS (1.18 J/g) and also with PS and TS. The addition of TS to PS also caused a reduction in enthalpy (Table 1). Fig. 1. Endothermic transitions in high gluten ¯our, ¯our components and mixtures of ¯our components as detected by dierential scanning calorimetry. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
might be explained by endothermic and exothermic events canceling each other, or by the absence of signi®cant cooperativity in the protein structure (Eliasson, 1990). The result also indicates that the gluten preparation contained little, if any, residual prime starch (Eliasson & Hegg 1980). The onset (To) and peak (Tp) temperatures of gelatinization were highest for the HGF than for the individual components or mixtures of the components. PS, which consists of large, relatively pure and undamaged granules (Pomeranz, 1988) showed a greater H (4.04) than the high gluten ¯our (2.07) or mixtures of the ¯our components. Several factors to explain the higher H value for PS include: size of the starch granule, total starch content, amylose-amylopectin ratio, and interactions between starch and other components (Erdogdu et al., 1995). Addition of gluten to the PS fraction (G/PS), caused a slight increase in both To and Tp of gelatinization, but a reduction in H. The reduction in H could be attributed to a competition for water between gluten and the starch molecules. Whilst both To and Tp seem to be independent of the water
3.2. Dynamic rheological properties None of the ¯our fractions showed a signi®cant viscous response (G00 ) during the dynamic shearing test at temperatures below 50 C (Fig. 2). However, as the heating temperature was raised above 55 C, the loss modulus of G/PS/TS increased rapidly, reaching a maximum (750 Pa) at about 65 C (Table 2). The G00 value decreased steadily during further heating at higher temperatures. Similar viscous characteristics were observed in PS, or samples containing mixed G/PS or G/TS, which all showed a peak G00 value within 64±67 C. HGF, which had a gluten to starch ratio of approximately 1:8, exhibited a continuous increase in G00 in the temperature range of 58±78 C. There was also an increase in G00 for tailing starch heated to above 69 C. However, G alone did not show any viscous change during the entire course of heating. It was conspicuous that the thermally induced increases in loss modulus resulted from starch swelling. The onset melting point of the prime starch was 56.7 C (Table 1). Amylose and amylopectin that leached out from the starch granules would increase the viscosity and therefore, the G00 value, to the extent that all starch granules had substantially lost their structure. The G00 reduction beyond 65 C for mixed gluten and starch samples and not for the ¯our seems to indicate
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K. Addo et al. / Food Research International 34 (2001) 329±335
Fig. 2. Loss modulus (G00 ) values generated during heating of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten. Table 2 Mean shear moduli of wheat ¯our and ¯our components at various combinations during heating and coolinga Sampleb
TM
G. . . ( C)
Loss modulus (G00 ), Pa
Storage modulus (G0 ), Pa
Heating
Flour PS TS G G/TS G/PS GPTS a b
± 65.5 66.4 67.3 55.4 67.3 65.5
Cooling
Heating
Cooling
Initial
Peak
Final
Initial
Final
Initial
Peak
Final
Initial
Final
0 1 0 5 2 2 9
± 69 18 ± 50 135 1161
299 5 202 4 47 17 233
280 2 271 4 33 15 204
0 1 82 5 42 22 448
3 1 2 5 3 0 97
± 98 21 27 25 382 3000
0 8 265 39 137 63 904
2540 9 279 41 135 69 881
1710 2 116 70 245 134 142
Values are means of two duplicate samples. PS, prime starch; TS, tailing starch; G, gluten.
that some other ¯our components, e.g. lipids and watersoluble proteins, may be involved in the rheological changes. G, which is described as consisting of globular aggregates with hydrophobic cores and hydrophilic surface zones forming a continuum (Hermasson & Larsson, 1986), was not aected by heating. Inter-peptide disul®de bonds, which presumably were formed during heating, apparently did not have a major eect on the protein viscosity. Changes in storage modulus (G0 ) of the dierent ¯our fractions generally followed similar patterns with similar respective magnitudes to those in G00 (Fig. 3; Table 2). The increase in G0 from 60 to 65 C for most samples
suggested formation of elastic matrices which apparently involved gluten. It is of interest to note that neither gluten nor starch alone produced any appreciable elastic response during the shear stress testing, indicating that structure formation of baked products is due to a concerted eort by both the protein and the starch fractions. The results also suggest that some minor ¯our components (lipids and solubles, etc.) were probably involved in the formation of elastic protein-carbohydrate composite networks in high gluten ¯our. In the absence of these minor constituents, the gluten-starch networks appeared to be loosely bound, and at high temperatures (>65 C), they tended to be disrupted
K. Addo et al. / Food Research International 34 (2001) 329±335
333
Fig. 3. Storage modulus (G0 ) values generated during heating of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
Fig. 4. Loss modulus (G0 ) values generated during cooling of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
probably due to unfavorable protein-carbohydrate interactions or competition for water between gluten and starch. Another hypothesis is that the starch granules were initially entrapped simply as ®llers in the gluten
network. As these granules began to swell upon heating, they reinforced the protein ``gel'' network thereby increasing the G0 value of the gluten±starch mixture up to 65 C. The temperature range during which major
334
K. Addo et al. / Food Research International 34 (2001) 329±335
Fig. 5. Storage modulus (G0 ) values generated during cooling of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
rheological changes occurred was consistent with the gelatinization (melting) temperature for the prime starch (Table 1), thus, supporting the above hypothesis. The ``®lling'' eect by swollen starch granules is somewhat similar to that by small inert or active ``beads'' which are added to polymer matrices to produce rubber materials with varying elasticities and strengths. Studies conducted by Brownsey, Ellis, Ridout and Ring (1987), have produced evidence that deformable swollen starch granules interpenetrate the amylose gel matrix thereby reinforcing the gel. However, as the starch molecules started to leach out and the starch granules largely lost their integrity, which presumably occurred at about 65 C based on the starch melting temperature (Fig. 1; Table 1), the gluten network was markedly weakened due to the removal of the matrix-reenforcing substances. The eect of the protein to starch ratio on viscoelasticity of doughs has been controversial. Some studies (Hibberd, 1970) have shown that an increase in the protein to starch ratio decreases the storage modulus, while the opposite was observed in other studies (Smith, Smith & Tschoegl, 1970). At an equal total solid content, the mixture of gluten and prime starch (13:87) and that of gluten and tailing starch (40:60) produced comparable viscoelasticity values (G00 and G0 ; Figs. 2 and 3). On the other hand, with a comparable gluten to starch ratio at a 10% total solid basis, the rheological characteristics of the G/PS/TS (10:74:16) mixture markedly diered from
those of the HGF (gluten:starch=10:80). Thus, it seems unreliable to predict the rheological behavior of dough based solely on the gluten to starch ratio. Ostensibly, some minor ¯our components were critically involved in forming the viscoelastic protein-polysaccharide network in wheat ¯our dough. The viscoelastic attributes (G00 and G0 ) of heated high gluten ¯our dough and mixtures of the various ¯our components exhibited only small changes during cooling. Mixed gluten and starches (G/PS/TS) experienced a continuous increase in G00 and G0 during cooling (Figs. 4 and 5), suggesting that ordered starch and gluten composite gel networks, presumably stabilized by the H-bonds, were formed. References Berland, S., & Launay, B. (1995). Rheological properties of wheat ¯our doughs in steady and dynamic shear: eect of water content and some additives. Cereal Chem., 72, 48±52. Brownsey, G. J., Ellis, H. S., Ridout, M. J., & Ring, S. G. (1987). Elasticity and failure in composite gels. J. Rheology, 31, 635±640. Dreese, P. C., Faubion, J. M., & Hoseney, R. C. (1988). Dynamic rheological properties of ¯our, gluten, and gluten-starch doughs. I. Temperature-dependent changes during heating. Cereal Chem., 65, 348±353. Erdogdu, N., Czuchajowska, Z., & Pomeranz, Y. (1995). Wheat ¯our and defatted milk fractions characterized by dierential scanning calorimetry. I. DSC of ¯our and milk fractions. Cereal Chem., 72, 70±75.
K. Addo et al. / Food Research International 34 (2001) 329±335 Eliasson, A.-C. (1990). Rheological properties of cereal proteins. In H. Faridi, & J. M. Faubion, Dough Rheology and Baked Product Texture (pp. 67±110). New York: Van Nostrand Rheinhold. Eliasson, A.-C. (1980). Eect of water content on the gelatinization of wheat starch. Starke, 32, 270±272. Eliasson, A.-C., & Hegg, P. O. (1980). Thermal stability of wheat gluten. Cereal Chem., 57, 436±437. Hermansson, A.M. and Larsson, K. (1986). The structure of gluten gels. Food Microstruct. 5, 233±239. Hibberd, G. E. (1970). Dynamic viscoelastic behavior of wheat ¯our doughs. III. The in¯uence of starch granules. Rheol. Acta, 9, 501± 504. Hoseney, R. C., Finney, K. F., Shogren, M. D., & Pomeranz, Y. (1969). Functional (breadmaking) and biochemical properties of wheat ¯our components, III. Characterization of gluten protein fractions obtained by ultracentrifugation. Cereal Chem., 46, 126±129.
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Menjivar, J. A. (1990). Fundamental aspects of dough rheology. In H. Faridi, & J. M. Faubion, Dough rheology and baked product texture (pp. 1±28). New York: Van Nostrand Rheinhold. Petrofsky, K. E., & Hoseney, R. C. (1995). Rheological properties of doughs made with starch and gluten from several cereal sources. Cereal Chem., 72, 53±58. Pomeranz, Y. (1988). Composition and functionality of wheat ¯our components. In P. Pomeranz, Wheat chemistry and technology (pp. 231±241). St. Paul, MN: American Association of Cereal Chemists. Smith, J. R., Smith, T. L., & Tschoegl, N. W. (1970). Rheological properties of wheat doughs. III. Dynamic sheer modulus and its dependence on amplitude, frequency and dough composition. Rheol. Acta, 9, 239±242. Xiong, Y. L. (1993). A comparison of the rheological characteristics of dierent fractions of chicken myo®brillar proteins. J. Food Biochem., 16, 217±220.
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Thermal and dynamic rheological properties of wheat ¯our fractions K. Addo a,*, Y.L. Xiong b, S.P. Blanchard b a
Department of Nutrition and Food Science, University of Kentucky, Lexington, KY 40506, USA b Department of Animal Sciences, University of Kentucky, Lexington, KY 40506, USA Received 10 February 1999; received in revised form 8 May 2000; accepted 31 August 2000
Abstract A commercial high gluten ¯our (HGF) was fractionated into prime starch (PS), tailing starch (TS), and gluten (G). Fractions were examined alone or in various combinations. Dynamic rheological properties of samples were measured in an oscillatory rheometer (strain 0.02; frequency 0.1 Hz) during heating at 1 C/min. Thermal characteristics of samples were determined by dierential scanning calorimetry (DSC) at a heating rate of 10 C/min. The loss (G00 ) and storage (G0 ) moduli of PS and mixed G/PS, G/ TS, and G/PS/TS increased after 60 C, reaching peak values (e.g. 81, 301, 313, and 3000 Pa in G0 , respectively) around 75 C after which the moduli decreased. HGF showed a steady increase in G0 from 32 to 2490 Pa as temperature increased from 65 to 90 C, indicating continuous formation of elastic networks. Cooling increased G0 for G/PS/TS, decreased G0 for HGF, and produced no rheological transitions for all samples. TS and G alone did not exhibit appreciable viscoelastic responses to the heating and cooling temperatures. DSC measurements revealed a major endothermic transition in HGF. This transition, with a peak around 60 C, was due to starch gelatinization. The presence of G or TS resulted in reduced melting enthalpies of starch in the PS fraction. Gluten or TS fractions alone or in combination did not exhibit any endothermic transitions. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Rheology; Wheat ¯our
1. Introduction Rheological properties of wheat ¯our doughs are of primary importance because, the ¯ow and deformation behavior of doughs are recognized to be central to the successful manufacturing of bakery products (Menjivar, 1990). The rheological properties of the dough change greatly during mixing and during cooking where the starch granules and gluten proteins undergo structural alterations to form three-dimensional matrices via protein±protein and protein±carbohydrate interactions. Because of the direct relationship between the rheological behavior of a dough and the ®nal/physical (end-use) properties of the baked product, much of the research in bakery processing has been devoted to the testing of physical properties of fresh doughs (Dreese, Faubion & Hoseney 1988; Petrofsky & Hoseney, 1995). In comparison, relatively few studies have been conducted to monitor speci®c changes in viscoelastic properties of the dough during heating (baking) and subsequent cooling.
* Corresponding author. Tel.: +1-859-257-7784; fax: +1-859-2573707.
Most dough research has employed traditional dough testing instruments, such as the mixograph, extensigraph, and alveograph. These instruments provide useful and practical information about the relationships between rheological properties of ¯our doughs and baking quality, but are limited to empirical correlations (Berland & Launay, 1995). Analysis with these instruments involves large deformation of dough samples and produces practical information that may be empirically correlated to the true viscoelastic properties of the dough components. Dynamic rheological techniques, which are increasingly utilized in the last decade, on the other hand, can determine fundamental mechanical properties of dough materials. In particular, oscillatory dynamic testing, in which a sinusoidal small- amplitude strain is imposed onto a sample to produce a stress output, or vice versa, is a powerful means for examining the fundamental viscoelastic properties of dough (Petrofsky & Hoseney, 1995). Because the strain imposed is very small, samples are analyzed nondestructively, hence, the test can accurately probe mechanical properties of the dough during mixing and baking. Recent work by Petrofsky and Hoseney (1995), has shown the existence of starch±gluten±water interactions
0963-9969/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0963-9969(00)00171-X
330
K. Addo et al. / Food Research International 34 (2001) 329±335
The wheat ¯our used was a commercial high gluten ¯our (HGF) from Archer Daniels Midland Milling Company (Shawnee Mission, KS). It was unbleached, with 13.8% moisture, 13.8% protein, 0.53% ash, and a falling number of 257.
(Bohlin Instruments, Inc., Cranbury, NJ). Samples were loaded in the 1-mm gap between two parallel plates (upper plate diameter 3.0 cm). The sample perimeter was covered with a thin layer of silicon oil to prevent dehydration and the plates were insulated in a foamtype shell to reduce heat dissipation. After equilibration at the initial temperature (20 C) for 5 min, the sample was heated at a scan rate of 1 C/min by circulating water (from a programmable waterbath) around the lower sample plate. After reaching the ®nal temperature (80 C), the sample was cooled at 1 C/min to the initial temperature. During heating, the sample was sheared at a ®xed frequency of 0.1 Hz with a maximum strain of 0.02 and the shear force was registered via a torque element (91 g cm). The actual sample temperature was veri®ed using a thermocouple connected to the upper surface of the lower plate. Data was collected every 30 s during shearing measurement. The storage modulus (G00 , a measure of elastic response), and the loss modulus (G0 , a measure of viscous response) were continuously monitored during the dynamic rheological testing (Xiong, 1993).
2.2. Fractionation of wheat ¯our
2.4. Dierential scanning calorimetry (DSC)
Flour was fractionated into gluten (G), tailing starch (TS) and prime starch (PS) according to a previously described procedure (Hoseney, Finney, Shogren & Pomeranz, 1969). Flour (100 g) was made into a dough (65% absorption), covered with water in a jar, relaxed for 30 min, and manually washed with ®ve 200-ml portions of water into G and a mixture of starch and water solubles. After screening (metal sieve with 0.425 mm openings) to recover small pieces of G, the mixture of water solubles and starch was centrifuged for 15 min at a force of 2000 g. Three fractions were obtained from top to bottom: water solubles, TS and PS. PS was dried under a fume hood at room temperature to a moisture content of about 10%. G and TS were freeze-dried to an average moisture content of approximately 5%. Dried PS and TS were ground with an Udy grinder (Udy Corporation, Fort Collins, CO) to pass a 0.50-mm sieve; G was ground to pass a 0.25-mm sieve. Unfractionated ¯our, individual ¯our components, and mixtures of ¯our components [G/TS (40:60), G/PS (13:87), TS/PS (17:83), G/PS/TS (10:74:16)] were used for rheological and thermal measurements. The ratios for the dierent ¯our components used in these mixtures closely represented those found in the HGF.
The thermal properties of the wheat ¯our and ¯our components were analyzed at least in triplicate with a Perkin-Elmer DSC-7 analyzer equipped with a thermal analysis data station (Perkin-Elmer Corp., Norwalk, CT). For DSC analysis of interactions among wheat ¯our components, mixtures of dried components were blended in a glass vial and vortexed for 2 min. Water (10 ml) was added to 5 mg of sample in an aluminum pan. The pan was hermetically sealed and the sample allowed to equilibrate for 1 h before analysis. Samples were heated at a rate of 10 C/min from 20 to 100 C. The onset temperature (TO), peak maximum temperature (Tp), and enthalpy of transition (H) were automatically computed.
in dough. The extent of these interactions was governed by the type of wheat (soft vs. hard), the source of starch (wheat versus non-wheat) and the gluten source (soft vs. hard wheat). However, measurements in the study were conducted at a constant temperature of 25 C, and the study only looked at the total starch fraction rather than subfractions. The goal of this study was to gain more understanding of thermally induced interactions among ¯our starches (prime and tailing) and gluten using both dynamic rheological and dierential scanning calorimetry methods. 2. Experimental 2.1. Wheat Flour
2.3. Dynamic rheological measurements Dynamic oscillatory measurements of 10% (w/v) suspensions of ¯our and ¯our mixtures in distilled water during heating and subsequent cooling were performed in duplicate samples using a Bohlin VOR rheometer
3. Results and discussion 3.1. Thermal properties Thermal transitions in HGF, ¯our components and their mixtures were detected by dierential scanning calorimetry (Fig. 1). All samples containing prime starch exhibited a single endothermic transition, with the corresponding temperatures and enthalpies of the transition shown in Table 1. No transition was discerned in samples containing G or TS alone or their combination. The lack of endothermal transition for TS could have been caused by mechanical damage to the starch granules or by interaction between starch and other TS components besides lipids (Erdogdu, Czuchajowska & Pomeranz, 1995). The lack of any endothermal transition for gluten
K. Addo et al. / Food Research International 34 (2001) 329±335
331
Table 1 Dierential scanning calorimetry characteristics of high gluten wheat ¯our and ¯our components at various combinations Samplea
Flour PSb TS G G/TS G/PS PS/TS G/PS/TS
Endotherm To ( C)
Tp ( C)
H (J/g)
60.0 56.7 ± ± ± 57.5 57.4 54.0
64.5 61.2 No peak No peak No peak 61.5 62.5 61.0
2.07 4.04 ± ± ± 1.18 2.02 1.56
a
PS, prime starch; TS, tailing starch; G, gluten. To, onset temperature; Tp, peak temperature; H, enthalpy. Values are means of triplicate samples. b
content, H is in¯uenced by the amount of available water (Eliasson, 1980). The lack of any endothermic ¯ow from the gluten fraction and the reduction in H caused by gluten, seem to indicate that the gluten proteins are not greatly aected during heating or baking. Interestingly, the gluten fraction also showed no appreciable viscous or elastic response during heating. Lower melting enthalpies than the HGF (2.07 J/g) were recorded when G was mixed with PS (1.18 J/g) and also with PS and TS. The addition of TS to PS also caused a reduction in enthalpy (Table 1). Fig. 1. Endothermic transitions in high gluten ¯our, ¯our components and mixtures of ¯our components as detected by dierential scanning calorimetry. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
might be explained by endothermic and exothermic events canceling each other, or by the absence of signi®cant cooperativity in the protein structure (Eliasson, 1990). The result also indicates that the gluten preparation contained little, if any, residual prime starch (Eliasson & Hegg 1980). The onset (To) and peak (Tp) temperatures of gelatinization were highest for the HGF than for the individual components or mixtures of the components. PS, which consists of large, relatively pure and undamaged granules (Pomeranz, 1988) showed a greater H (4.04) than the high gluten ¯our (2.07) or mixtures of the ¯our components. Several factors to explain the higher H value for PS include: size of the starch granule, total starch content, amylose-amylopectin ratio, and interactions between starch and other components (Erdogdu et al., 1995). Addition of gluten to the PS fraction (G/PS), caused a slight increase in both To and Tp of gelatinization, but a reduction in H. The reduction in H could be attributed to a competition for water between gluten and the starch molecules. Whilst both To and Tp seem to be independent of the water
3.2. Dynamic rheological properties None of the ¯our fractions showed a signi®cant viscous response (G00 ) during the dynamic shearing test at temperatures below 50 C (Fig. 2). However, as the heating temperature was raised above 55 C, the loss modulus of G/PS/TS increased rapidly, reaching a maximum (750 Pa) at about 65 C (Table 2). The G00 value decreased steadily during further heating at higher temperatures. Similar viscous characteristics were observed in PS, or samples containing mixed G/PS or G/TS, which all showed a peak G00 value within 64±67 C. HGF, which had a gluten to starch ratio of approximately 1:8, exhibited a continuous increase in G00 in the temperature range of 58±78 C. There was also an increase in G00 for tailing starch heated to above 69 C. However, G alone did not show any viscous change during the entire course of heating. It was conspicuous that the thermally induced increases in loss modulus resulted from starch swelling. The onset melting point of the prime starch was 56.7 C (Table 1). Amylose and amylopectin that leached out from the starch granules would increase the viscosity and therefore, the G00 value, to the extent that all starch granules had substantially lost their structure. The G00 reduction beyond 65 C for mixed gluten and starch samples and not for the ¯our seems to indicate
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Fig. 2. Loss modulus (G00 ) values generated during heating of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten. Table 2 Mean shear moduli of wheat ¯our and ¯our components at various combinations during heating and coolinga Sampleb
TM
G. . . ( C)
Loss modulus (G00 ), Pa
Storage modulus (G0 ), Pa
Heating
Flour PS TS G G/TS G/PS GPTS a b
± 65.5 66.4 67.3 55.4 67.3 65.5
Cooling
Heating
Cooling
Initial
Peak
Final
Initial
Final
Initial
Peak
Final
Initial
Final
0 1 0 5 2 2 9
± 69 18 ± 50 135 1161
299 5 202 4 47 17 233
280 2 271 4 33 15 204
0 1 82 5 42 22 448
3 1 2 5 3 0 97
± 98 21 27 25 382 3000
0 8 265 39 137 63 904
2540 9 279 41 135 69 881
1710 2 116 70 245 134 142
Values are means of two duplicate samples. PS, prime starch; TS, tailing starch; G, gluten.
that some other ¯our components, e.g. lipids and watersoluble proteins, may be involved in the rheological changes. G, which is described as consisting of globular aggregates with hydrophobic cores and hydrophilic surface zones forming a continuum (Hermasson & Larsson, 1986), was not aected by heating. Inter-peptide disul®de bonds, which presumably were formed during heating, apparently did not have a major eect on the protein viscosity. Changes in storage modulus (G0 ) of the dierent ¯our fractions generally followed similar patterns with similar respective magnitudes to those in G00 (Fig. 3; Table 2). The increase in G0 from 60 to 65 C for most samples
suggested formation of elastic matrices which apparently involved gluten. It is of interest to note that neither gluten nor starch alone produced any appreciable elastic response during the shear stress testing, indicating that structure formation of baked products is due to a concerted eort by both the protein and the starch fractions. The results also suggest that some minor ¯our components (lipids and solubles, etc.) were probably involved in the formation of elastic protein-carbohydrate composite networks in high gluten ¯our. In the absence of these minor constituents, the gluten-starch networks appeared to be loosely bound, and at high temperatures (>65 C), they tended to be disrupted
K. Addo et al. / Food Research International 34 (2001) 329±335
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Fig. 3. Storage modulus (G0 ) values generated during heating of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
Fig. 4. Loss modulus (G0 ) values generated during cooling of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
probably due to unfavorable protein-carbohydrate interactions or competition for water between gluten and starch. Another hypothesis is that the starch granules were initially entrapped simply as ®llers in the gluten
network. As these granules began to swell upon heating, they reinforced the protein ``gel'' network thereby increasing the G0 value of the gluten±starch mixture up to 65 C. The temperature range during which major
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K. Addo et al. / Food Research International 34 (2001) 329±335
Fig. 5. Storage modulus (G0 ) values generated during cooling of high gluten ¯our, ¯our components and mixtures of ¯our components. HGF = high gluten ¯our; PS = prime starch; TS = tailing starch; G= gluten.
rheological changes occurred was consistent with the gelatinization (melting) temperature for the prime starch (Table 1), thus, supporting the above hypothesis. The ``®lling'' eect by swollen starch granules is somewhat similar to that by small inert or active ``beads'' which are added to polymer matrices to produce rubber materials with varying elasticities and strengths. Studies conducted by Brownsey, Ellis, Ridout and Ring (1987), have produced evidence that deformable swollen starch granules interpenetrate the amylose gel matrix thereby reinforcing the gel. However, as the starch molecules started to leach out and the starch granules largely lost their integrity, which presumably occurred at about 65 C based on the starch melting temperature (Fig. 1; Table 1), the gluten network was markedly weakened due to the removal of the matrix-reenforcing substances. The eect of the protein to starch ratio on viscoelasticity of doughs has been controversial. Some studies (Hibberd, 1970) have shown that an increase in the protein to starch ratio decreases the storage modulus, while the opposite was observed in other studies (Smith, Smith & Tschoegl, 1970). At an equal total solid content, the mixture of gluten and prime starch (13:87) and that of gluten and tailing starch (40:60) produced comparable viscoelasticity values (G00 and G0 ; Figs. 2 and 3). On the other hand, with a comparable gluten to starch ratio at a 10% total solid basis, the rheological characteristics of the G/PS/TS (10:74:16) mixture markedly diered from
those of the HGF (gluten:starch=10:80). Thus, it seems unreliable to predict the rheological behavior of dough based solely on the gluten to starch ratio. Ostensibly, some minor ¯our components were critically involved in forming the viscoelastic protein-polysaccharide network in wheat ¯our dough. The viscoelastic attributes (G00 and G0 ) of heated high gluten ¯our dough and mixtures of the various ¯our components exhibited only small changes during cooling. Mixed gluten and starches (G/PS/TS) experienced a continuous increase in G00 and G0 during cooling (Figs. 4 and 5), suggesting that ordered starch and gluten composite gel networks, presumably stabilized by the H-bonds, were formed. References Berland, S., & Launay, B. (1995). Rheological properties of wheat ¯our doughs in steady and dynamic shear: eect of water content and some additives. Cereal Chem., 72, 48±52. Brownsey, G. J., Ellis, H. S., Ridout, M. J., & Ring, S. G. (1987). Elasticity and failure in composite gels. J. Rheology, 31, 635±640. Dreese, P. C., Faubion, J. M., & Hoseney, R. C. (1988). Dynamic rheological properties of ¯our, gluten, and gluten-starch doughs. I. Temperature-dependent changes during heating. Cereal Chem., 65, 348±353. Erdogdu, N., Czuchajowska, Z., & Pomeranz, Y. (1995). Wheat ¯our and defatted milk fractions characterized by dierential scanning calorimetry. I. DSC of ¯our and milk fractions. Cereal Chem., 72, 70±75.
K. Addo et al. / Food Research International 34 (2001) 329±335 Eliasson, A.-C. (1990). Rheological properties of cereal proteins. In H. Faridi, & J. M. Faubion, Dough Rheology and Baked Product Texture (pp. 67±110). New York: Van Nostrand Rheinhold. Eliasson, A.-C. (1980). Eect of water content on the gelatinization of wheat starch. Starke, 32, 270±272. Eliasson, A.-C., & Hegg, P. O. (1980). Thermal stability of wheat gluten. Cereal Chem., 57, 436±437. Hermansson, A.M. and Larsson, K. (1986). The structure of gluten gels. Food Microstruct. 5, 233±239. Hibberd, G. E. (1970). Dynamic viscoelastic behavior of wheat ¯our doughs. III. The in¯uence of starch granules. Rheol. Acta, 9, 501± 504. Hoseney, R. C., Finney, K. F., Shogren, M. D., & Pomeranz, Y. (1969). Functional (breadmaking) and biochemical properties of wheat ¯our components, III. Characterization of gluten protein fractions obtained by ultracentrifugation. Cereal Chem., 46, 126±129.
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Menjivar, J. A. (1990). Fundamental aspects of dough rheology. In H. Faridi, & J. M. Faubion, Dough rheology and baked product texture (pp. 1±28). New York: Van Nostrand Rheinhold. Petrofsky, K. E., & Hoseney, R. C. (1995). Rheological properties of doughs made with starch and gluten from several cereal sources. Cereal Chem., 72, 53±58. Pomeranz, Y. (1988). Composition and functionality of wheat ¯our components. In P. Pomeranz, Wheat chemistry and technology (pp. 231±241). St. Paul, MN: American Association of Cereal Chemists. Smith, J. R., Smith, T. L., & Tschoegl, N. W. (1970). Rheological properties of wheat doughs. III. Dynamic sheer modulus and its dependence on amplitude, frequency and dough composition. Rheol. Acta, 9, 239±242. Xiong, Y. L. (1993). A comparison of the rheological characteristics of dierent fractions of chicken myo®brillar proteins. J. Food Biochem., 16, 217±220.