Production and Quality Characteristics of Gluten-Free Noodles Made with Pregelatinized Rice Flour

The rice flour used in this study was the commercially available Nonghyup Gangwondo Rice Flour (Injekirin Nonghyup Co., Ltd., Gangwon-do, Korea). Noodles were produced by adding PR (Yoosung Food Co., Ltd., Gwangju-si, Gyeonggi-do, Korea) to ordinary rice flour at ratios of 0% (control), 10% (PR-10), 15% (PR-15), 20% (PR-20), and 25% (PR-25). Additionally, raw bean powder (Hamyang-Nonghyup Co., Ltd., Hamyang-gun, Korea), salt (Hanju Co., Ulsan, Korea), and HPMC (Cheminex Co., Ltd, CN10T, AnyAddy^R, Korea) were added to the noodles. The optimal conditions for flour dough formation were determined by setting the formulas through preliminary experiments.

To produce noodles with rice flour, blended rice flour was prepared through the addition of 10, 15, 20, or 25% PR, along with the raw bean powder, HPMC, and salt, which was dissolved in the mixing water prior to use (Table 1). The HPMC was added at a ratio of 1% based on the amount of rice flour to assist with dough conditioning. At room temperature, the blended flour and mixing water were added to a dough mixer (KMM020, Kenwood Ltd., Hertfordshire, UK) and mixed for 3 min. The prepared dough was rolled in a noodle maker (HSN-2, Hunwoo, Seoul, Korea) in a five-step rolling process using rolling steps of 7.5, 5.0, 4.0, 3.3, and 2.7 mm. The noodles were cut to produce a 3 mm width and 2.4 mm thickness. In the case of the non-PR rice flour (control), the noodle dough did not form. The characteristics of the noodles with the PR-blended rice flour and 100% wheat flour (wheat) were compared. Because of the high water absorption of rice flour, the dough was produced by decreasing the amount of water by 8.3% for every 5% decrease in the rice-flour content.

Table 1. Formulas for rice noodles with different pregelatinized rice flour contents

Download Excel Table

The color of the noodles before and after cooking was measured using a colorimeter (Model CM-5, Minolta Co., Tokyo, Japan) at the stage of the 2.4 mm noodle dough prior to cutting. The Hunter values of lightness (L), redness (a), and yellowness (b) were adjusted using the standard white plate for the color measurements. The color difference (∆E) was calculated using $\text{ΔE = }\sqrt{{\text{L}}^{\text{2}}{\text{ + a}}^{\text{2}}{\text{ + b}}^{\text{2}}}$, and images of the produced noodles were obtained by placing the noodles on a black stainless plate. The visible differences in appearance were observed by arranging the noodles in sequence.

To measure the texture of the cooked noodles, the Texture Analyzer^TM (TA-XT2, StableMicro System, Godalming, Surrey, England) was used. In the texture profile analysis (TPA) mode, five repeated measurements were taken for each sample using a cylinder probe (P/35, 35 mm dia, circle), and mean values were obtained. After cooking the noodles in 100 °C boiling water for 4 min, the noodles were cooled in running water for 1 min and left at room temperature for 3 min. The measurements were taken after removing the water. The cooking time was set as the time until the internal and external colors became equal. In the case of wheat, the noodles were cooked for 14 min until the internal and external colors became equal. Six strands of cooked noodles with a length of 5 cm were placed in parallel on a plate, and after two repeated compressions to induce a 70% change in the overall thickness from the surface, the hardness, springiness, cohesiveness, adhesiveness, gumminess, and chewiness were measured. Table 2 presents the details of the measurement conditions.

Table 2. Texture analyzer operating conditions for cooked noodles with different pregelatinized rice flour contents

Download Excel Table

The Texture Analyzer^TM (TA-XT2, StableMicro System, Godalming, Surrey, England) was also used to measure the tensile strength of the cooked noodles. For each sample, three repeated measurements were taken using a noodle tensile rig, and mean values were obtained. The noodles with PR-blended rice flour were cooked in 100 °C boiling water for 4 min, cooled in running water for 1 min, and left at room temperature for 3 min. The measurements were taken after removing the water. In the case of wheat, the noodles were cooked for 14 min, while all the other conditions remained identical. A strand of noodle was wrapped around a tensile rig from the top to the bottom in 20 mm intervals to measure the pulling force (N) required to break the strand, as well as the stretched distance (mm). Table 2 presents the details of the measurement conditions.

The methods of Kim et al. (1996), where the differences in the weights of noodles before and after cooking were compared, were used with modifications to measure the cooking characteristics of the noodles with rice flour. The weight of the cooked noodles was measured after cooking 25 g of noodles in 500 mL of boiling water for 4 min, cooling the noodles in running water for 1 min, and leaving the noodles at room temperature for 3 min. In the case of wheat, the cooking time was 14 min, while all the other conditions remained identical. After weighing, the cooked noodles were placed in a 250 mL graduated cylinder containing 150 mL of distilled water to measure the increase in volume. The water absorption of the cooked noodles was estimated using the following equation:

To measure the elution of cooked noodles, 25 g batches of noodles with rice flour and noodles with wheat flour were cooked in 500 mL of boiling water for 4 min and 14 min, respectively, after which 500 mL of water was added to measure the turbidity at 675 nm using a spectrophotometer (UV-1080, Shimadzu Co., Kyoto, Japan) and express the elution in terms of the absorbance.

All the experiments were independently repeated three times, with the textural properties of the noodles measured five times. The data were thus expressed as the mean±SD. To test the significance, an analysis of variance (ANOVA) was performed using SPSS 23.0 (SPSS Inc., Chicago, IL, USA), and Duncan’s multiple range test was performed. The level of significance was set at p<0.05.

The colors of the gluten-free noodles with PR-blended rice flour are presented in Table 3. The lightness (L) of the uncooked control noodles (with ordinary rice flour) was the highest at 86.3, followed by the wheat noodles at 84.0. However, the lightness did not vary significantly after cooking for the noodles with PR-blended rice flour (p<0.05). Jung (2020) reported that the L value of rice flour before gelatinization was high (96.47-96.64), with a white color. In agreement with this, the control in this study, which had the highest content of ordinary rice flour, displayed the highest lightness value. The L values of the noodles with the PRblended rice flour were low (78.3-78.6), which coincided with the results reported by Choi & Jung (2021) that the L value of PR varied according to the rice cultivar, with an influence from the yellow tint of rice. An increasing trend was found for the yellowness (b) as the PR content increased, which was also presumed to be due to the differences in the color of the rice cultivar and PR. The color of the cooked noodles could not be measured for the control because the noodle dough could not be formed. This coincided with the results reported by Park & Eun (2021) that noodle dough could not be formed when the added amount of rice flour increased to a certain level. The brightness of the sample decreased to 69.4 in PR-10 and 55.2 in PR-25. The yellowness increased to 4.71 in PR-10 and then decreased to 1.96 in PR-15. Wadchararat et al. (2006) reported that the viscosity and color varied according to the method of physical treatment used to process α-rice flour, such as the water content and heat-treatment temperature. The PR in this study was yellowish in color, but the yellowness seemed to decrease during the heat treatment. Fig. 1 presents images of the noodles with PR-blended rice flour. Noodle dough could be formed using PR-10, and no significant variation in color or appearance was observed across the groups, except for the wheat group. Thus, the addition of PR was shown to improve the inadequate dough conditioning ability of rice flour and allow noodle dough formation.

Table 3. Hunter’s color values of cooked noodles with different of pregelatinized rice flour contents

Download Excel Table

Fig. 1. Appearances of noodles with different pregelatinized rice flour contents.

Download Original Figure

The texture profiles of the noodles with the PR-blended rice flour are shown in Table 4. As the PR content increased, the hardness increased compared to Wheat (84.6 N), reaching PR- 10 (97.0 N), PR-15 (108.0 N), PR-20 (121.0 N), and PR-25 (141.0 N). This agreed with Lee et al. (1995), where an increase in the content of α-rice flour led to an increase in hardness. The result may also be attributed to the strong viscosity of PR. However, the texture approached the level of rice cake as a higher amount of PR was added, which lent support to the trend of a significant increase in adhesiveness, or the force causing a substance to adhere to surfaces. Foods such as rice cake are known to have unique textural properties, including their adhesiveness and tensile strength (Kohyama et al., 2007). Chewiness increased to 0.030 N in PR-10 and 0.059 N in PR-25. Gumminess increased to 1.85 N, 2.28 N, 2.65 N, and 2.82 N in line with an increase in the added amount of PR. This agreed with the results reported by Han et al. (1988) that the hardness, chewiness, and gumminess increased with the added amount of α-rice flour. The springiness and cohesiveness did not vary significantly (p<0.05). The PR-10 and PR-15 resembled the wheat noodles to the highest degree with respect to texture. Thus, PR was shown to play a key role in the formation of noodle dough as an alternative to gluten.

Table 4. Textural profiles of noodles with different pregelatinized rice flour contents

Download Excel Table

The tensile profiles of the noodles with the PR-blended rice flour are presented in Table 5. The tensile strengths of the noodles with the PR-blended rice flour were as follows: wheat (0.312N ), PR-10 (0.061 N), PR-15 (0.138 N), PR-20 (0.092N) , and PR-25 (0.078 N). Thus, the group with the closest tensile strength to that of wheat was PR-15, while noodle dough could not be formed using the control. In producing noodles with a gluten-free material, the appearance, taste, aroma, and texture may show quality degradation (Jo et al., 2020). In this study, the addition of PR and HPMC to overcome the inadequate processability was shown to improve the textural properties. Jung et al. (2015) reported that noodle molding was difficult using gluten-free dough because noodles with reduced tensile strength tended to break. Thus, the addition of PR in this study was shown to improve the noodle molding and noodlebreaking phenomenon.

Table 5. Tension profiles of cooked noodles with different of pregelatinized rice flour contents

Download Excel Table

The cooking characteristics of the gluten-free noodles are shown in Table 6. The results demonstrated that the water absorbance values of the noodles with gluten-free rice flour were as follows: wheat (142.2%), PR-10 (72.8%), PR-15 (50.4%), PR-20 (43.8%), and PR-25 (36.9%). Thus, the absorbance decreased as the added amount of PR increased. The volume of the cooked wheat noodles was 211 mL, and the following decreasing trend with an increase in PR was observed: PR-10 (193 mL), PR-15 (188 mL), PR-20 (186 mL), and PR-25 (185 mL). The weight of the cooked noodles also tended to decrease as the added amount of PR increased. This was presumably because the texture of the rice flour and HPMC became more elaborate as the added amount of PR increased, which reduced the overall water absorbance, volume, and weight of the cooked noodles. The turbidity increased as follows: wheat (0.16), PR-10 (0.20), PR-15 (0.26), PR-20 (1.16), and PR-25 (1.49). As an indicator of cooking loss through the elution of noodles during cooking (Park & Eun, 2021), the turbidity was reported to increase as the solid content during cooking decreased (Jeong et al., 2019). Thus, a suitable level of PR addition was necessary as an increase in the added amount of PR led to an increase in cooking loss despite an increase in the dough conditioning of the rice flour.

Table 6. Cooking characteristics of noodles with different pregelatinized rice flour contents

Download Excel Table