Dough thermo-mechanical properties: influence of sodium chloride, mixing time and equipment

Journal of Cereal Science 41 (2005) 327–331 www.elsevier.com/locate/jnlabr/yjcrs

Dough thermo-mechanical properties: influence of sodium chloride, mixing time and equipment A. Angioloni*, M. Dalla Rosa Dipartimento di Scienze degli Alimenti, Universita` di Bologna, Corso di Laurea in Scienze e Tecnologie Alimentari, via Ravennate no. 1020, 47023 Cesena, (FC), Italy Received 8 June 2004; revised 13 October 2004; accepted 19 October 2004

Abstract Thermo-mechanical properties of doughs prepared from common wheat 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 effect 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 gelatinisation

1. Introduction 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 Go¨cmen, 2003). The production of wheat dough is a process in which raw 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 the 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 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 * Corresponding author. Tel.: C39 547 636120; fax: C39 547 382348. E-mail address: [email protected] (A. Angioloni). 0733-5210/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2004.10.004

type (Kilborn and Tipples, 1972; MacRitchie, 1986; Skeggs, 1985; Zheng et al., 2000). The time required for optimum dough development is positively 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 dough 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 breadmaking 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 10–15% of the energy generally imparted by conventional 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 up about 80% of its dry weight, influences dough rheological

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properties, especially upon heating in the presence of water when starch gelatinises (Li and Yeh, 2001). The gelatinisation process includes a number of changes: absorption of water and swelling of the granules, change in size and shape of the granules, 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 available (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 conditioning 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, are 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 Mulino Pivetti (Italy), sodium chloride, from Carlo Erba (Italy). AACC (2000) methods were used to determine moisture (44-19), ash (0801), 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 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 kneaded 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, 0–4.5%, dry basis (d.b.) was added, 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. Dough temperatures at the end of kneading were 26–28 8C for sample A and w35 8C for sample M; although the use of prototype mixer rapidly increased the dough temperature above this value. Thermomechanical tests were made using a controlled stress-strain rheometer (MCR 300, Physica/Anton Paar;

Table 1 Composition of experimental doughs Dough

Flour (g)

Salt (%)

Water (ml)a

A–M A–M A–M A–M

250 243.75 241.25 238.75

0 2.5 3.5 4.5

143.7 143.7 143.7 143.7

A, sample obtained using Alveograph mixer; M, sample obtained using prototype mixer. a Added water to obtaine dough with 50% moisture.

Messtechnik, Ostfildern, Germany), using parallel-plate geometry (25 mm plate diameter, 2 mm plate gap). The upper, serrated 25 mm plate was lowered until the thickness 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 during 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 obtained from a separately prepared batch of dough for each formulation and for the different mixers used. Significant differences in storage modulus (G 0 ) at 1 Hz were determined by Least Significant Difference analysis with P%0.05. All statistical analyses were performed with the Stat Soft Version 6.

3. Results 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. Table 2 Chemical composition and rheological properties of the flour Flour Proteina (%) Gluten wet (%) Asha (%) Moisture (%) Alveograph W (!10K4 J) P (height!1.1) (mm) Length (mm) P/L

11.83G0.15 31.10G0.87 0.40G0.01 10.84G0.06 104G3.3 29.63G0.96 118.00G3.61 0.25G0.01

a Corrected to 14% moisture content. Values represent mean of three replicatesGstandard deviation.

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Fig. 1. Oscillatory temperature ramp of dough A with different amounts of salt. The inset (a) shows the linear regression analysis of log G 0 versus temperature (see Table 3) between 55 and 70 8C.

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). 3.2. Thermo-mechanical properties of doughs The same amounts of salt (0, 2.5, 3.5, 4.5% d.b.) were added to both samples (A; M) to check the effect of salt and different kneading conditions on sample behaviour during dynamic mechanical thermal analysis. This measurement simulates the physicochemical changes that take place during thermal treatment of dough. Figs. 1 and 2 show the effect of salt addition on the storage modulus (G 0 ) during an

oscillatory temperature ramp. Below 55 8C, G 0 , for both samples, gradually decreased as temperature increased, indicating softening of the dough. Thereafter, the storage modulus increased from 55–60 to 80 8C and then slowly decreased. The abrupt increase can be attributed to the gelatinization of starch; the swelling and distortion of starch granules during gelatinization were responsible for the rapid increase of G 0 not only by their action as a filler in the gluten network, but also by promoting effective cross-linking in the system (Dreese et al., 1988). The glutenin fraction of gluten has been found to be more sensitive to heat than the gliadin fraction; on heating up to 75 8C glutenin proteins unfolds and disulphide/sulphydryl interchange reactions are promoted, thus increasing the molecular size of

Fig. 2. Oscillatory temperature ramp of dough M with different amounts of salt. The inset (a) shows the linear regression analysis of log G 0 versus temperature (see Table 3) between 55 and 70 8C.

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Table 3 Linear regression analysis of log G 0 versus temperature Samples

Slope

ra

A A 2.5% salt A 3.5% salt A 4.5% salt M M 2.5% salt M 3.5% salt M 4.5% salt

0.060 a 0.055 b 0.056 b 0.052 c 0.052 a 0.047 b 0.041 c 0.042 c

0.984 0.995 0.995 0.991 0.994 0.990 0.990 0.982

Within column, values with the same following letter do not differ significantly from each other (P%0.05). a Correlation coefficient.

the aggregates (Dreese et al., 1988; Peressini et al., 1999). The increase of storage modulus during heating has been reported (He and Hoseney, 1991) to be proportional to the starch content of the dough; indicating the physicochemical changes in heated dough are essentially due to changes in the starch fraction. For both samples the transition temperature range of salted dough appeared to be shifted to higher values than doughs made without salt (Figs. 1 and 2) as reported previously by Dreese et al. (1988) and Peressini et al. (1999). Moreover, a comparison of the slopes obtained from the linear regressions over the temperature range (55–70 8C) where the G 0 increased, showed that, in all cases, the slopes for salted dough were significantly lower than for unsalted doughs (Figs. 1(a) and 2(a) and Table 3). The effect of sodium chloride in delaying the starch gelatinization has been reported (Chiotelli et al., 2002; Galal et al., 1978; Peressini et al., 1999; Preston, 1989) and different explanations for this phenomenon proposed. When salt is added to dough, it lowers water activity and increases the energy necessary for chemical and physical reactions involving water (Kim and Cornillon, 2001; Seetharaman et al., 2004). Table 4 compares the slopes obtained from linear regressions at the different kneading conditions (in the temperature range from 55 to 70 8C) with respect to starch Table 4 Linear regression analysis of log G 0 versus temperature Samples

Slope

ra

A M A 2.5% salt M 2.5% salt A 3.5% salt M 3.5% salt A 4.5% salt M 4.5% salt

0.060 a 0.052 b 0.055 a 0.047 b 0.056 a 0.041 b 0.052 a 0.042 b

0.984 0.994 0.995 0.990 0.995 0.990 0.991 0.982

Within column, values with the same following letter do not differ significantly from each other (P%0.05). a Correlation coefficient.

gelatinization. For each salt concentration it can be seen that the slopes sample M are lower that those for sample A, consequently the type of mixing seems to be relevant to the delay phenomenon. The doughs prepared using short time and high-speed mixing conditions, sample M, where high energies were transferred to the dough, were probably less hydrated and developed than sample A, therefore for starch gelatinization, for which water is indispensable, requires higher energy. The dough structure created in these kneading conditions could decrease the capability of water being effectively involved in starch granule swelling and therefore the gelatinization process is delayed.

4. Conclusions The experiments showed that a rapid increase in G 0 between 55 and 70 8C due to gelatinization of starch. They also showed that the onset of starch gelatinization and protein coagulation for doughs was delayed with increasing salt concentrations and even more for doughs kneaded at high speeds in a prototype mixer. Thus not only salt addition but also mixing technology used can change the thermomechanical properties of doughs. These results are sufficiently promising, to suggest lines of research directed to finding ways to make a more stable dough structure at pasteurisation temperatures as used in preparation of long shelf life, ready-to-cook bakery products in which partial starch gelatinization results in a plastic-like product with poor organoleptic properties.

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