WATER ABSORPTION AND TEMPERATURE EFFECTS ON VISCOELASTIC PROPERTIES OF BREAD WHEAT DOUGH Osorio Fernando, Gahona Eric, Alvarez Fresia Departamento de Ciencia y Tecnología de los Alimentos, Facultad Tecnológica, Universidad de Santiago de Chile, Avda. Ecuador 769, Santiago, Chile; Fax : 6 2 682 6 e-mail: fosorio@lauca.usach.cl ABSTRACT Three water absorption levels were applied, one to give a consistency of 00 Brabender Units (BU), other two levels were established at ± % of 00 BU. Temperature effects were analyzed in two ways: at constant temperature (20; 0; 0; and 8 C) and using a heating rate of 1 C/min applied from 10 to 90 C. Strain was found to be 0.2% for the region of linear viscoelasticity. Statistical analyses indicate that factors affecting G and G values were temperature, water absorption and frequency. Keywords: wheat flour; water absorption; viscoelastic behavior; dough rheology INTRODUCTION Wheat dough is prepared by mixing wheat flour, water, yeast, salt, other ingredients and energy (1,2). Wheat dough viscoelastic behavior is governed by the physics and chemistry of both gluten and starch (-8). Rheological characterization of wheat dough is usually performed with instruments such as farinograph, alveograph, extensigraph, mixograph (2) and lately micro-extensigraph (9); they give useful practical information but with limited empirical correlations (10), and due to doughs complex nature it is difficult to obtain reproducible results (11). One method of rheological characterization of wheat dough is performed using small amplitude oscillatory testing (12) with parallel plates geometry (1,1,6). Values where the linear viscoelastic region of wheat dough occurs are very variable and they are affected by factors such as type of material and composition (1). Previous researchers have determined the linear viscoelastic region of wheat dough in the order of 0.2% (1,1), 0.22% (1), 0.2% (16), 0.% (17), 0.8% (18). One of the basic flour quality parameters, besides moisture, gluten quantity and quality, damaged starch content and alpha amilase activity, is percentage of water absorption (also called farinograph dough consistency) (19). One problem of the Chilean industry is the control of percentage of water absorption; therefore, the objective of this study was to investigate the effects of degree of water absorption and temperature on the viscoelastic properties of wheat doughs samples prepared with respect to farinograph dough consistency. MATERIALS AND METHODS Flour samples The base flour was commercial bread flour, 0 kg bag, purchased from the Chilean local market and used for sample preparation The usual type of flour used in Chile for bread manufacture corresponds to wheat mixtures from different sources. Characteristics of wheat flour used in this study, sample preparation and analytical methods are published elsewhere (20). Rheological testing Small amplitude oscillatory measurements were carried out with a rheometer (Carri Med, CSL 2 100, TA Instruments, England) using a 0 mm diameter parallel plate fixture and mm gap. The lower plate is equipped with a Peltier temperature control system. Once wheat dough sample forming a sphere was placed on the lower plate, it was raised to the desired gap. Excess dough was retired to eliminate end effects. To stabilize internal tension produced during preparation and handling, dough samples were rested for min previous to each test (10). To ensure that the samples reach and maintain working temperature, a routine temperature stabilization provided by the instrument software was used. Silicon oil of 100000 cp was used to avoid dehydration by the plates ends, and at the end of each test it was verified that wheat dough was adhered to the plates as a way of controlling slippage absence (1). Preliminary tests were performed in strain sweep mode to determine the strain which assures the region of linear viscoelasticity. Temperature variation To study temperature effects on viscoelastic properties on wheat dough, temperature variations were applied in two ways: i) discrete temperature variations at 20; 0; 0; and 8 C ± 1 C with frequency sweep from 0.6 to 21. rad/s; and ii) temperature sweep from 10 to 90 C with a heating rate of
1 C/min (1,21) at 6.28 rad/s frequency. In both cases the strain applied was that corresponding to linear viscoelasticity. Statistical analysis A xx statistical design was applied and the results were analyzed by ANOVA and Pareto tests and response surface methodology with the aid of Statgraphics.0 plus for Windows software (Statistical Graphics Corp. 199-1997). The experimental factors were: temperature, water absorption and frequency; the response variables were storage (G ) and loss (G ) modulus. Results obtained are the average of three replicates which were analyzed with Microsoft Excel 2000 software (Microsoft Corporation, 198-1999). Treatment averages were considered significantly different at p<0.0. RESULTS & DISCUSSION Rheological results Due to the fact that wheat dough is very sensitive to strain, the first task of this study was to determine the region of linear viscoelasticity and strain was found to be 0.2%. This value is within those obtained by other researchers for wheat dough (1,1-16). The small region of non-destructive strain may be attributed to the high concentration of starch in dough (22). It was experimentally verified that this strain value did not significantly vary (p<0.0) over the temperature range applied in this study. Effects of water absorption and temperature Figure 1 shows the effect of frequency on storage (G ) and loss (G ) modules respectively, at different discrete temperatures for water absorption levels considered in this study. Storage modulus (G ) is higher than loss modulus (G ) for all temperatures under study; this type of behavior is characteristic for wheat doughs (). For all temperatures and water absorption values G and G increase with increasing frequencies. a) b) G'' (Pa) Figure 1. Variation of G and G modules with frequency at different water absorption levels (!=1.9%;!=6.9%;!=61.9%) for wheat dough at: a) 0 C, b) 8 C. For temperature range between 20 C to 8 C, in the analysis of each level of temperature, when water absorption increases G and G decrease, which is coincident with findings reported by other authors (10,2). When analyzing wheat dough rheological behavior at 8 C it was observed that the three curves corresponding to different levels of water absorption almost coincide in their G value, being exponentially higher than those values measured at C. This change is attributed to temperature effects on starch gelatinization which causes changes on the elastic properties of dough, this behavior is in agreement with the fact that dough is easily deformed when the water content increases.
Moisture content influence on the mobility of dough components may be illustrated by comparison of derivative of log G with respect to log ω versus log ω (). According to Ferry (2), when the slope has a value near to 0 the material shows a rubbery-like behavior, whereas a liquid shows a slope with a value near to 2. For the frequency range used in this study the derivative of log G with respect to log ω was constant. When temperature increases from 20 C to 0 C the slope is constant at 0.2 and from 0 to 8 C the slope decreases to a value of 0.08 and wheat dough presents a more rubbery behavior than that at the temperature range from 20 C to 0 C. Effects of water absorption and sweep increase of temperature Figure 2 shows changes in G and G modules with a temperature sweep increase for different water absorption levels; from 10 to 60 C both G and G values show a small decrease, similar to what is expected for a solution, meaning a softening of the wheat dough, above 60 C there is a sharp increase for both G and G, with G values higher than G values, reaching a peak at 8 C, this behavior has been also reported by other authors (1,2). G'' (Pa) 1,E+02 0 20 0 60 80 100 T ( C) 1,E+02 0 20 0 60 80 100 T ( C) Figure 2. Effects of heating on G and G modulus for wheat dough at three of water absorption levels (!=1.9%;!=6.9%;!=61.9%) measurement at 1 Hz of frequency. Bars indicate standard deviation. When analyzing water absorption effects for temperature sweep, it can be observed that the wheat dough with lower water absorption (1.9%) presents higher G values at the lowest temperature (Figure 2), coincident with was obtained at discrete temperature (Figure 1a). The sharp increase of G and G modules in the temperature range from 60 to 8 C has been interpreted as the combine effect zone where starch gelatinizes and protein complex denaturizes, affecting wheat dough rheology. Some authors (2) indicate that gelatinization may give the possibility of increasing hydrogen bonds between gluten polypeptides and starch molecules. Statistical analyses and Response surface Applying both statistical analyses using ANOVA and Pareto charts (Figure ) over the experimentation area for G and G gave that factors: temperature, frequency and water absorption were significative (p<0.0); and combined factors were not important; Pareto charts showed no combined effects affecting G and G. Response surfaces (Figure ) for G and G at low (0.6 rad/s), medium (126 rad/s) and high (21. rad/s) frequencies were obtained and it was observed that the lowest G and G values are obtained when temperature is 20 C, water absorption level is the highest (61.9%) and at the lowest frequency.
A:Temperature C:Frequency B:Water absorption AC AB BC C:Frequency A:Temperature :Water absorption AC AB BC 0 2 6 8 10 Standardized effect a) 0 1 2 6 Standardized effect b) Figure. Pareto chart showing the importance of effects of : a) G modulus; b) G modulus on wheat dough, (p<0.0) For the lowest temperature the increase of G and G is more notorious when water absorption decreases. For lower water absorption, as temperature increases G increases exponentially, for G this increase is moderate; this effect is more notorious when rheological testing is performed at the highest frequency. This fact could indicate that at the lowest temperatures used in this study water has a plastizicing effect and is not forming part of the structure of the dough; at higher temperatures, including gelatinization, water is absorbed by the starch and therefore forming part of a more rigid structure. log... 61.9 20 8 1.9 Water absorption (% a) log G'' (Pa).. 2 20 8 1.9 61.9 Water absorption (%..... 61.9 2 20 8 1.9 20 Water absorption (% 8 1.9 61.9 Water absorption (% b) Figure. Surface response for estimated G and G response at different water absorption levels and temperature tests; a) low frequency(0.6 rad/s); b) medium frequency (126 rad/s). log log G'' (Pa)
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