1、外文原文：http:/ 机械专业中英文文 献翻译 中文 3258字 Dough thermo-mechanical properties: influence of sodium chloride, mixing time and equipment A. Angioloni*, M. Dalla Rosa 出处： Journal of Cereal Science 41 (2005) 327331 Abstract Thermo-mechanical properties of doughs prepared from common whe
2、at flour were investigated under different kneading conditions and with different amounts of sodium chloride. Dynamic mechanical thermal analysis showed that high-speed mixing and the addition of salt to dough slowed heat-induced reactions such as starch gelatinisation and protein coagulation. The e
3、ffect of dough mixing technology was more significant than the amount of sodium chloride in modifying dough rheological characteristics. q 2004 Elsevier Ltd. All rights reserved. Keywords: Dough； Mixing； Sodium chloride； Thermo-mechanical properties； Starch gelatinization 1. Introduction &nbs
4、p; The first step in a baking process is mixing the dough； how the mixing is performed and the ingredients are incorporated and dispersed largely determine the final quality of the baked product (Aamodt et al., 2003； Basaran and Gocmen, 2003). The production of wheat dough is a process in which raw
5、materials (mainly flour, water, salt and yeast) are mixed and subjected to a large range of strain situations. Dough is a complex mixture of starch, protein, fat and salt. Mixing has three important functions: (i) it blends the ingredients into a macroscopically homogeneous mass, (ii) it develops th
6、e dough into a three-dimensional viscoelastic structure with gas-retaining properties and (iii) it incorporates air which will form nuclei for gas bubbles that grow during dough fermentation (Bloksma, 1990； Collado and Leyn, 2000； Dobraszczyk and Morgenstern, 2003； Hoseney and Rogers, 1990； Naeem et
7、 al., 2002). Both mixing intensity and mixing energy must be above a minimum critical level to develop the dough properly, the level varying with flour and mixer type (Kilborn and Tipples, 1972； MacRitchie, 1986； Skeggs, 1985； Zheng et al., 2000). The time required for optimum dough development is p
8、ositively correlated with the polymeric protein composition and the balance between protein polymers and monomers (Dobraszczyk and Morgenstern, 2003； MacRitchie, 1992； Millar, 2004). Rheological properties change during every stage of the dough making process； stress conditions are high when the dou
9、gh is mixed in high-speed mixers, to become an elastic and coherent mass. Mixing speed and energy (work input) must be higher than a certain value to develop the gluten network and to produce a suitable breadmaking dough. On the other hand, an optimal mixing time has been related to optimum breadmak
10、ing performance which varies depending on mixer type and ingredients (Dobraszczyk and Morgenstern, 2003； Mani et al., 1992). For example, kneading doughs to reach optimum development using elongational flow in sheeting, required only 1015% of the energy generally 机械专业中英文文 献翻译 imparted by conve
11、ntional high speed shear mixers, suggesting that much higher rates of work input can be achieved due to the improved strain hardening of dough under extension (Dobraszczyk and Morgenstern, 2003； Kilborn and Tipples, 1974； Millar, 2004) Starch, the major component of wheat flour, making upabout 80% o
12、f its dry weight, influences dough rheological properties, especially upon heating in the presence of water when starch gelatinises (Li and Yeh, 2001). The gelatinization process includes a number of changes: absorption of water and swelling of the granules, change in size and shape of the granules,
13、 loss of birefringence and X-ray diffraction pattern, leaching of amylose from the granules into the solvent and the formation of a paste (Atwell et al., 1988). At reduced water contents, such as in dough, the changes resulting from gelatinisation are strongly dependent on the amount of water availa
14、ble (Eliasson, 1983； Seetharaman et al., 2004). The increase in viscosity due to starch gelatinisation has been suggested to modify structural properties of dough. In addition, the presence of sodium chloride is known to affect dough properties； salt toughens the protein and helps in co
15、nditioning the dough by improving its tolerance to mixing； the addition of salt produced a more stable and stiff dough (Galal et al., 1978； Shiu and Yeh, 2001). Moreover it is known that when salt is added to the dough, heat-induced reactions such as starch gelatinisation and protein coagulation, ar
16、e slowed. The aim of the present work was to analyse the effects of increasing sodium chloride concentration and different kneading conditions on several dough thermo-mechanical properties, using a dynamical stress-strain controlled rheometer. 2. Materials and methods Commercial wheat flour was from
17、 Mulino Pivetti (Italy), sodium chloride, from Carlo Erba (Italy). AACC (2000) methods were used to determine moisture (44-19), ash (08-01), protein (46-10) and gluten (38-12) in the flour and its Alveograph characteristics. Dough samples with 50% moisture were prepared in accordance with Alveograph
18、 method AACC 54-30A (2000), using two different mixers and mixing times. In the first (sample A) the Alveograph mixer was used with standard conditions (250 g of flour was mixed with water for 7 min to form the dough). In the second (sample M) a prototype mixer was used where the ingredients were kn
19、eaded for only 15 s but at high-speed (1500 rpm). In this way high amounts of energy were transferred to the dough. The prototype mixer had a parallelpiped shape (12!8!12 cm) with two vertical arms operated by a 1.5 kW motor (Gamar s.r.l., VE-Italy). Sodium chloride, 04.5%, dry basis (d.b.) was adde
20、d, for each different kneading condition and mixer type (Table 1). Before rheological analysis all doughs were rested for 30 min at room temperature in a plastic container. Doughtemperatures at the end of kneading were 2628 8C for sample A and w35 8C for sample M； although the use of prototype mixer
21、 rapidly Thermomechanical tests were made using a controlled stress-strain rheometer (MCR 300, Physica/Anton Paar； Messtechnik, Ostfildern, Germany), using parallel-plate geometry (25 mm plate diameter, 2 mm plate gap). The upper, serrated 25 mm plate was lowered 机械专业中英文文 献翻译 until the thickne
22、ss of sample was 2 mm and excess was trimmed off. The exposed surface was covered with a thin layer of mineral oil to prevent moisture loss during testing. The sample was rested another 15 min in the rheometer, before each measurement, allowing relaxation of stresses induced durin
23、g sample loading to relax. All measurements were performed at a heating rate of 0.8 8C/min at fixed frequency of 1 Hz with the oscillation amplitude small enough to ensure linear viscoelasticity. The data are reported as means of measurements made on three samples, where each sample was obtain
24、ed from a separately prepared batch of dough for each formulation and for the different mixers used. Significant differences in storage modulus (G0) at 1Hz were determined by Least Significant Difference analysis with P%0.05. All statistical analyses were performed with the Stat Soft Version 6. 3. R
25、esults and discussion 3.1. Flour chemical and physical properties The chemical composition and rheological properties of the flour are shown in Table 2. Analysis of Alveograph data categorises the flour used as weak, and as seen by the P/L ratio, the gluten is richer in gliadins than in glutenins The resistance of a gluten dough to extension decreases and extensibility increases with an increasing gliadin to glutenin ratio (Grasberger et al., 2003； Kim et al., 1988； Uthayakumuran et al., 2000).
