Rehydration of Dried Wheat Noodles with Different Gluten Contents at Various Temperatures

Abstract

The rehydration process of dried wheat flour noodles (flat noodles) prepared by adding 0, 5, 10, and 15% wheat gluten by weight to wheat flour was measured at temperatures ranging from 40 to 80°C. Regardless of the gluten content, the rehydration process at any temperature was well described by an empirical hyperbolic equation regarding the rehydration time. The equilibrium moisture content and initial rehydration rate were smaller for the noodles with higher gluten content. The temperature dependence of equilibrium moisture content varied significantly between 50 and 60°C at any gluten content, reflecting the gelatinization temperature of the starch in the flour, and could be expressed by the van’t Hoff equation for the high-temperature region. The change in rehydration enthalpy at high temperatures was almost independent of the gluten content of noodles. The temperature dependence of the initial rehydration rate could also be expressed by the Arrhenius equation over all temperature ranges tested, and the activation energy for rehydration was almost independent of the gluten content.

1. Introduction

Wheat is one of the world’s three major cereals, along with rice and corn, and is the staple food for about 30% of the world’s population [1]. Wheat flour is not only consumed as a staple food such as bread, noodles, and naan but is also widely used as an ingredient in confectionery and cooking [2, 3]. The major components of wheat flour are starch and gluten, and usage of the flour varies depending on its gluten content. This study focuses on the use as noodles. The gluten content of pasta, Chinese noodles, and Japanese udon noodles, the major wheat flour noodles, is approximately 13%, 11%, and 9%, respectively, on a wet weight basis [3].

Noodles available in the market can be classified into three main types: fresh wet noodles, dried noodles, and frozen cooked noodles [4]. Dried noodles have excellent preservation and distribution efficiency and are used after being rehydrated before consumption [5]. The state of the starch grains [6], their swelling characteristics [7], and the moisture distribution within the noodle [8] play an important role in palatability [9].

Water movement in dried foods is often described by Fick’s second law [10], but the rehydration process of dried spaghetti, one of dried noodles, cannot be explained by a simple diffusion phenomenon based on a water concentration gradient [11], and gluten relaxation is the rate-limiting step [12]. In addition, drying conditions affect the rehydration process [13]. Furthermore, the gluten content of wheat flour noodles also affects the rehydration process. We reported that the rehydration process of fresh wheat flour noodles with different gluten contents in boiling water had two stages for the noodles with any gluten content: an early fast stage and a later slow stage, and that the latter stage was due to the relaxation of the gluten network [14].

The slow process observed in our previous report [14] was under the condition of overcooking in boiling water. To accumulate knowledge on the characteristics in the rehydration process of dried noodles, in this study, four kinds of dried wheat flour noodles with different gluten contents were prepared, and their rehydration processes during the early stage at various temperatures were analyzed kinetically.

2. Materials and Methods

2.1 Materials

Wheat flour for hand-pulled udon noodles (Marukou Shirotsubaki^®) was a product of Nisshin Seifun (Tokyo, Japan). The flour’s ash, crude protein, and water contents were 0.34%, 8.5%, and 14.0%, respectively, by wet weight [15]. Wheat gluten, dimethyl sulfoxide, and 4% paraformaldehyde-phosphate buffer were purchased from Nacalai Tesque (Kyoto, Japan). Fluorescein isothiocyanate (abbreviated as FITC) and rhodamine B (RhB) were purchased from FUJIFILM Wako Pure Chemical Corp. (Osaka, Japan).

2.2 Preparation of dried wheat noodles with different gluten contents

Wheat flour, gluten, and water were weighed into a bowl, as shown in Table 1. Water was added gradually over 1.5 min while mixing at speed 2 using a stirrer with a flat beater (KSM150PSGA type, KitchenAid, MI, USA). After that, the mixer’s beater was changed to a dough-hook, and the dough was kneaded at speed 2 for 8.5 min and further at speed 1 for 10 min. The kneaded dough was put into a Magica pasta machine (Bottene, Schio, Italy) and extruded through a Teflon No. 17 die (flat noodles: 1.8 mm W ×0.76 mm H; Bottene, Italy) to obtain fresh noodles. During extrusion, the pressure was reduced to 0.026-0.029 MPa (absolute pressure) using a DTC-60 diaphragm pump (ULVAC, Kanagawa, Japan). Fresh noodles were dried in an SH-641 small environment-controllable chamber (ESPEC, Osaka, Japan) until a constant weight was reached under adequately controlled humidity and temperature conditions.

Table 1 Composition of wheat noodles with different gluten contents.

Wheat flour [g]	500	475	450	425
Gluten [g]	0	25	50	75
Water [g]	160	160	160	160
wG [g-gluten/g-d.m.]	0.099	0.151	0.202	0.252

w_G: Weight fraction of gluten in the dried wheat noodle

The weight fraction w_G of gluten in the dried noodles was calculated assuming that all the protein in the flour was gluten and that the moisture content of the added gluten was negligibly small. Each dried noodle with different gluten contents was prepared three times.

2.3 Image analysis of a cross-section of dried noodles

Dried noodles were cut into 10-mm lengths, immersed in 4% paraformaldehyde-phosphate buffer, and fixed at 30°C for 90 min with shaking at 60 strokes/min. The fixed noodles were washed twice with distilled water and divided into two or three pieces with a scalpel. A mixed aqueous solution was prepared with a final concentration of both FITC and RhB of 0.001 wt%. FITC was dissolved in dimethyl sulfoxide at 0.0035 wt% in advance of the preparation of the aqueous solution. Noodle pieces were dipped into the aqueous solution of FITC and RhB, stained for 10 min with gentle shaking, and then washed with distilled water.

Starch and gluten in the noodles were fluorescently labeled with FITC and RhB, respectively, and imaged with a BZ-X810 fluorescence microscope (Keyence, Osaka, Japan) equipped with a 20X objective lens (Plan Fluorite 20X LD PH, NA ＝ 0.45, WD ＝ ～8.8). The optical sectioning function provided with the microscope was used to obtain fluorescence images of the focal plane. FITC and RhB were excited at 450-490 nm and 532.5-557.5 nm, respectively. The images were saved in 16-bit tiff format.

To determine the amount of gluten inside the noodle, the image obtained by exciting at 532.5-557.5 nm was binarized by the Otsu method [16] using ImageJ to represent protein (gluten) as white image, and the percentage of white areas was calculated.

2.4 Rehydration process of dried wheat noodles

A four-hole heat block (inner diameter 52 mm, height 66 mm) was heated using a THB-1 dry block bath (AS ONE, Osaka, Japan). A hundred milliliters of distilled water (milli-Q water) was poured into each heat block. A wooden plate about 2 cm thick covered with food-grade wrapping film was placed on top of the block to suppress heat loss from the top. The water temperature in the heat blocks was measured with a K-type thermocouple and an AD-5605H digital thermometer (A & D, Tokyo, Japan) and was regulated at a constant value from 40 to 80°C.

After the water temperature reached the predetermined value, dried noodles cut into approximately 4 cm lengths were placed in the block to start the rehydration. The surface of dried noodles, such as spaghetti, has minute cracks generated during the drying process, and water seeps into these cracks immediately after the start of rehydration, resulting in a rapid increase in moisture content [17]. Therefore, the moisture content was measured at 1-min intervals until the first 5 min of rehydration, and after that measured at 10, 15, 20, 30, 45, 60, 75, and 90 min. Since dried noodles of each gluten content were prepared three times, the rehydration processes were measured for each dried noodle. Two samples were taken out at each time, the surface water on the samples was lightly wiped off with a Kimtowel (Nippon Paper Crecia, Tokyo) and KimWipe (Nippon Paper Crecia, Tokyo). They were placed on a heat-resistant sheet (AS ONE, Osaka) with an identification number, the weight of which had been weighed (tare), then put on an electronic balance (TX423N, Shimadzu Corp., Kyoto, Japan). The weight of the tare was subtracted to estimate the wet weight of the samples w_w. The noodles were dried in a DO-300FA dryer (AS ONE, Osaka) at 130°C for about 2 h, and then weighed again. The weight of the tare was subtracted to obtain the dry weight w_d. From these weights, the moisture content X_T based on dry weight at the rehydration time t was calculated using Eq. (1).

  

$X_{\text{T}} = (w_{\text{w}}-w_{\text{d}})/w_{\text{d}}$(1)

The moisture content was measured as a function of rehydration time t in triplicate, and the mean and standard deviation were calculated.

3. Results and Discussion

3.1 Cross-sectional images of dried noodles

Figure 1 (a) shows fluorescence micrographs of cross-sections of dried wheat noodles with different gluten contents. The blue and green images show starch and gluten, respectively. It can be seen that the gluten formed a network structure surrounding the starch granules. The larger the amount of gluten added, the larger the percentage of the white area indicating the presence of protein through binarization of the image (Fig. 1(b)), and the more gluten network formed. The planar image obtained here corresponded to the 2D image across the 3D network image of gluten in spaghetti [18]. The white area does not strictly reflect the area of the gluten itself since the area of the white area depends on the exposure time and sectioning parameters used in the image measurement. However, the percentage of the white area correlated well with the gluten content (Fig. 1(b)), indicating that the gluten was not localized but homogeneously distributed within the noodles.

Fig. 1

(a) Cross-sectional images of dried wheat noodles with the gluten weight fractions of (a-1) 0.099, (a-2) 0.151, (a-3) 0.202, and (a-4) 0.252. Blue and green images indicate starch and gluten, respectively. (b) Relationship between the fraction of white area in the binarized images and the weight fraction of gluten in the dried noodle, w_G. The circle and bar represent the mean ± standard deviation, respectively.

3.2 Change in the moisture content over rehydration time

Figures 2(a)-(d) shows the changes in the moisture content X_T over time based on dry weight during the rehydration processes of wheat flour noodles with different gluten contents. Although many equations have been proposed to describe the rehydration process of dried materials [11], the hyperbolic equation (Eq. (2)), which well described the rehydration process of dried spaghetti with the highest gluten content among wheat flour noodles [19], was used to correlate X_T and rehydration time t for each wheat noodle.

  

$X_{\text{T}} = at/(b + t) + X_{\text{T}0}$(2)

where a and b are constants. Equation (2) has the same mathematical meaning as Peleg’s equation [20], an empirical equation widely used to describe dried foods’ moisture sorption and rehydration processes. As mentioned above, water seeps into minute cracks immediately after rehydrating dried noodles and the moisture content increases rapidly. Therefore, the initial moisture content X_T0 immediately after the rehydration was estimated by approximating the moisture content from 1 to 5 min (measured at 1-min intervals) with a linear function of rehydration time t and extrapolating to t = 0. Parameters a and b were determined to minimize the sum of the residual squares of the measured X_T and the value calculated by Eq. (2) using the Solver function of Microsoft Excel^®. The solid curves in Fig. 2 were calculated using the estimated a, b, and X_T0 values. The equilibrium moisture content X_Te after a sufficiently long period of rehydration is expressed by Eq. (3).

  

$X_{\text{Te}} = a + X_{\text{T}0}$(3)

The initial rehydration rate immediately after the rehydration dX_T/dt|_{t = 0} is given by Eq. (4).

  

$dX_{\text{T}}/dt|_{t = 0} = a/b$(4)

b represents the rehydration time required for the moisture content X_T to reach (X_Te+X_T0)/2.

Fig. 2

Changes in the moisture content, X_T, over rehydration time, t, of dried wheat noodles with the gluten weight fractions of (a) 0.099, (b) 0.151, (c) 0.202, and (d) 0.252 at (□) 40°C, (▪) 50°C, (△) 55°C, (▴) 60°C, (○) 65°C, (●) 70°C, and (◇) 80°C. Bars for representing standard deviation are behind the symbols.

The moisture content increased faster at higher temperatures for all noodles with different gluten contents. Equation (2) expressed the trend of the measured values well at every temperature and for the wheat noodle with any gluten content. The rehydration curve, which represents the relationship between moisture content and rehydration time, showed a large shift significantly around 55°C for all the noodles. This should reflect the gelatinization of starch, as described below.

3.3 Temperature dependence of kinetic parameters

Figure 3(a) shows the temperature dependence of equilibrium moisture content X_Te. Regardless of the gluten content of the wheat flour noodles, the plots were represented by two straight lines at high- and low-temperature regions with a boundary around 55°C, which corresponds to the gelatinization temperature of wheat starch. The straight line indicates that the temperature dependence of X_Te can be expressed by van’t Hoff equation (Eq. (5)).

  

$d\text{log}X_{\text{Te}}/d(1/T) = - \Delta H_{\text{T}}/\left( 2.30R \right)$(5)

where T is the absolute temperature, ∆H_T is the enthalpy change during the rehydration process of dried noodles, and R is the gas constant. The X_Te values below the gelatinization temperature (low-temperature region) were only at 40°C and 50°C, and the standard deviation of the plots was large, so only the results for the high-temperature region are discussed here. X_Te became smaller as the gluten content of the wheat flour noodles increased. This would be ascribed to the smaller equilibrium moisture content of gluten compared to starch [12]. The higher the gluten content w_G, the smaller the value of ∆H_T tended to be, but the values were very small, ranging from 0 to 1 kJ/mol. This suggests that the endothermic heat by rehydration is small above the starch gelatinization temperature.

Fig. 3

Temperature dependencies of (a) equilibrium moisture contents of the wheat noodles, X_Te, and the wheat starch, X_Se, and of (b) the initial rehydration rate of wheat noodles, a/b. The weight fractions of gluten in dried wheat noodles are (□) 0.099, (△) 0.151, (○) 0.202, and (◇) 0.252. The symbol ● represents the X_Se. The bars represent the standard deviations for triplicated measurements.

As mentioned above, a/b represents the initial rehydration rate. The temperature dependence of the initial rehydration rate of spaghetti could be expressed by the Arrhenius equation (Eq. (6)) [19].

  

$d\text{log}(a/b) = - E_{\text{T}}/(2.30R)$(6)

As shown in Fig. 3(b), the higher the gluten content of the flour noodles, the lower the initial rehydration rate. However, Eq. (6) could express its temperature dependence over the entire temperature range tested, regardless of the gluten content. The values of activation energy E_T tended to be larger for the noodles with higher gluten content but did not differ significantly, ranging from 3.6 to 6.0 kJ/mol.

3.4 Estimation of equilibrium moisture content of wheat starch

Figure 4 shows the relationship between X_Te and w_G at various temperatures. Although there was some variation, the relationship between X_Te and w_G at each temperature could be approximated by a straight line. The equilibrium moisture content of wheat starch, X_Se, was estimated as the value of the intersection of the vertical axis of the line extrapolated to w_G→0. The temperature dependence of X_Se is shown in Fig. 3(a). As in the case of wheat flour noodles, the plot changed stepwise between 50°C and 60°C, indicating that the gelatinization temperature of starch of the wheat flour used was in this temperature range, which is almost the same as the value obtained for wheat flour noodles containing gluten. This suggests that the thermal contribution of gluten to the rehydration of wheat flour noodles was insignificant. The extrapolated value of X_Te at w_G ＝ 1 in Fig. 4, in principle, gave the equilibrium moisture content of gluten, X_Ge, and these values ranged from -4 to 1 g-H₂O/g-d.m. X_Ge must be positive. This unreasonable value seems to be due to extrapolating from a plot with variation in the range below w_G < 0.252 to estimate a value at w_G ＝ 1, which is a large distance away. However, the tendency for X_Ge to be considerably smaller than X_Se was consistent with our previous result [12].

Fig. 4

Relationship of the equilibrium moisture contents, X_Te, and the weight fractions of gluten w_G at (□) 40°C, (▪) 50°C, (△) 55°C, (▴) 60°C, (○) 65°C, (●) 70°C, and (◇) 80°C.

Acknowledgments

We thank Mr. M. Kohsaka for his technical assistance.

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