Preparation of Matcha Fresh Noodles with Stable Color using Embedding Method and Microwave Treatment

Noodles are a staple of traditional grain-based cuisine in China, with approximately 40% of the wheat production in Asian countries being utilized for noodle processing¹. Nevertheless, the basic nutritional component of wheat flour is insufficient to satisfy the increasing nutritional needs of consumers. Therefore, several investigators have opted to replace a portion of the wheat flour in noodles with alternative natural ingredients, such as oat bran², milk protein³, sweet potato⁴, and citrus maxima⁵, in order to enhance the nutritional and functional qualities of the noodles. Matcha is an abundant bioactive compound possessing antioxidant and anti-inflammatory properties, which have the potential to mitigate the risk of cardiovascular disease and prevent chronic illnesses⁶. Consequently, there has been a burgeoning interest in investigating the integration of matcha into traditional culinary fare, including Chinese steamed bread, rice cake, and particularly fresh noodles.

However, fresh noodles are prone to time-dependent darkening, leading to unfavorable changes in the visual appearance of the product, which poses a significant challenge to the storage of fresh noodles⁷. It is widely agreed that the discoloration observed during the storage of fresh noodles is mainly caused by the presence of polyphenol oxidase (PPO)^7,8. Additionally, it was indicated that the soluble protein fraction is involved in the process of non-polyphenol oxidase (non-PPO) darkening⁹. Extensive efforts have been devoted in recent years to mitigate the darkening of PPO during storage. Previous studies have suggested that acid inhibitors and heat treatment applied to raw noodles could effectively achieve this objective by denaturing proteins and consequently inhibiting enzyme activity^10,11. Chlorophyll is susceptible to changes in pH, temperature, and heat, and the vibrant green hue of green tea noodles is primarily attributed to chlorophyll¹⁰. It is evident that there are limitations in effectively controlling the color of green tea noodles through the direct addition of acid inhibitors and heat treatment.

In addition to the thermal processing of wheat flour, the preservation of chlorophyll in matcha noodles is a critical factor to consider. Several methods have been proposed to prolong the storage time of chlorophyll and preserve its pigment, including the use of alkalinizing agents, copper complexations, and low-temperature storage¹². Unfortunately, the majority of processes necessitate a pH level close to nature in order to reduce the occurrence of unfavorable chemical reactions. The stability concern may be potentially mitigated by the copper complex of chlorophyll derivatives, which exhibit a green color reminiscent of natural chlorophyll. However, individuals exhibit a preference for natural chlorophyll over artificial colors. Microencapsulation techniques have emerged as a viable solution to the challenge of improving bioactive compounds' stability by providing barriers against environmental conditions such as oxygen, pH, ionic strength, and temperature^13,14,15. Until now, tea extract, catechins, and chlorophyll have been continuously studied for their stability and controlled release properties when embedded in different wall materials¹⁴. However, the incorporation of microcapsules into noodles has not yet been proposed¹⁵.

In this study, we described a method that embeds matcha with whey protein and carboxymethyl chitosan and microwave-treating wheat flour to obtain color-stable matcha fresh noodles. The addition of microencapsulated bioactive compounds to food facilitates the creation of novel functional food products while preserving the inherent qualitative attributes. We present the results obtained using this processing protocol to investigate alterations in color values of matcha noodles following storage. The specific objective of the study was to determine the optimal approach for preparing matcha noodles that demonstrate both exceptional color and flavor.

1. Production of matcha-embedded suspension

1. Place 4 g of carboxymethyl chitosan (see Table of Materials) in 100 mL of distilled water to prepare a 0.04 g/mL of carboxymethyl chitosan stock solution. To dissolve carboxymethyl chitosan heat to 60 °C on constant temperature heating magnetic stirrer.
2. Dissolve 4 g of whey protein (see Table of Materials) in 50 mL of distilled water to prepare a 0.08 g/mL of whey protein stock solution.
3. After the solutions cool, store them in a freezer and refrigerate them overnight to fully saturate the polymer molecule.
4. Place 8 g of matcha and 4 g NaCl (see Table of Materials) in a beaker and add 50 mL of sterile boiled water to prepare matcha slurry. Assist the dissolution process using a glass stirring rod. Sterile water is deionized water that has been boiled in a pot for 15 min.
5. Mix matcha slurry with 50 mL of carboxymethyl chitosan solution and blend with a magnetic stirring apparatus at 300 rpm at room temperature.
6. Drip 25 mL of whey protein into the mixture and stir at 300 rpm for 30 min to create an embedded suspension. (see Figure 1). The suspension should be prepared concurrently with the noodle production and stored at a temperature of 4 °C on the same day.
   NOTE: The pH of the matcha-embedded suspension solution was left unadjusted, thus solely influenced by the matcha or wall material.

2. Microwave treatment of wheat flour

1. Distribute 300 g of wheat flour in a round plastic container. Cover the container with plastic wrap and place it in the microwave oven at 700 W for 60 s. Afterward, keep the container with flour outside until it feels cool.

3. Production of matcha noodles

1. Dough mixing: Pour 125 mL of pre-formed matcha-embedded suspension and 15 mL of water into 300 g of microwave-treated wheat flour slowly. Gently pull the flour into the center, bit by bit, until the embedded suspension is incorporated.
   1. For non-microwave-treated wheat flour, use 125 mL of water instead of matcha-embedded suspension to prepare noodles, which are labeled as blank noodles (control). Add 125 mL of water and 8 g of matcha to non-microwave-treated wheat flour to prepare noodles and designate these as matcha noodles (M-noodles).
2. Kneading the dough: Knead the noodle dough with a dough mixer for 7 min. The dough should be smooth and pliable, not wet or sticky when done.
3. Resting: Return the ball of dough to its mixing bowl and cover it with a damp tea towel. Let it rest for 30 min at room temperature.
4. Prepare noodles as described below.
   1. Divide the dough into 4 equal pieces, then rewrap 3 and set aside. Lightly flour the unwrapped piece, lifting it occasionally to ensure that it does not stick.
   2. Adjust the press roller spacing of the dough press to 3.5 mm. Place the dough on a dough press to sheet, and then compound. Press the dough sheet repeatedly until it reaches a smooth and pliable consistency.
   3. Thin the dough sheet to 1 mm. Starting from 3.5 mm, adjust the press roller spacing to 2 mm and then 1 mm. Press the sheet 3x in each roller spacing.
   4. Cover loosely with clingfilm, and then repeat steps 3.4.1-3.4.3 with the remaining dough.
      NOTE: In order to ensure that the pressing process of each batch of samples is consistent, the press roll spacing is adjusted once, and the next spacing is adjusted after each sample is passed.
   5. Slit the noodle strands into 22 cm long, 1 mm thick, and 3.0 mm wide samples (see Figure 2). Dust about 3.75 g semolina to make sure noodles do not stick together.

4. Sensory evaluation and physical property analysis

1. Sensory evaluation
   1. Place 30 g of noodles in 600 mL of boiling water and cook for 5 min. Immerse cooked noodles in cold water immediately, before testing.
   2. Perform sensory evaluation of noodle samples using quantitative descriptive analysis (QDA) as outlined in Li et al.¹². Ensure that the sensory characteristics of the samples are evaluated by 12 trained panelists using a nine-point hedonic scale with 9 denoting like extremely and 1 denoting dislike extremely.
2. Color measurement
   1. Cut cooked as well as fresh samples into pieces of about 10 cm and place them under the instrumental aperture of the colorimeter.
   2. Press the instrumental button to measure the color of the fresh noodles and repeat the test 6x for each sample.
3. Texture analyzer
   1. Determine the texture property of cooked as well as fresh noodles using a texture analyzer and calculate based on texture profile analysis (TPA).
   2. Place five strands of noodles on the test bench and repeat the test 6x for each sample.

5. Data analysis

1. Analyze the data by variance analysis (ANOVA) and consider the difference significant when p < 0.05.

This protocol allowed for the sensory and physical property analysis of processed matcha-incorporated foods and noodles, beginning with matcha treatment and continuing through the intermediate stages of processing to the final product. This protocol was coupled with embedding and microwave to produce matcha noodles (Figure 3). The fresh noodles with unencapsulated matcha, with embedded matcha and microwave treatment, and without any matcha were marked as M-Noodles, ME-Noodles, and control, respectively. In terms of sensory evaluation of three noodle samples, it was observed that microwave and addition of wall materials did not have any discernible negative impact on the sensory attributes of matcha noodles, including their aroma and flavor (Figure 4). The ME-Noodles sample showed a moderate level of fresh color, while its cooked color and smoothness were superior to those of the control, and its flavor and firmness received the highest praise.

The control group, fresh M-noodles group, and fresh ME-noodles group were characterized by a colorimeter, determining the absolute variance (Δ) between the 24 h and initial readings throughout the duration of storage. The L (black-white), a (redness-greenness), and b (yellowness-blueness) values for evaluating the surface color of each sample were recorded using a chromameter. ΔE* was calculated as follows:

(ΔE*= )

The values of cooked groups were described by calculating the absolute difference between cooked noodles and fresh noodles (Figure 5). The total color variance (ΔE*) of fresh M-Noodles in 24 h decreased, while a minimal color shift was found in fresh ME-Noodles. The results indicated that the fresh ME-Noodles retained color better (ΔE*= 1.98 ± 0.12) than the M-Noodles (ΔE*= 6.98 ± 0.21). A similar phenomenon was found in the cooked samples. As shown in Table 1, the hardness, springiness, and chewiness of the ME-Noodles group were significantly higher than those of the M-Noodles group and control, while there was no significant difference in the adhesiveness among all 3 samples. The above results indicate that the texture quality of fresh noodles was improved by adding embedded matcha and microwave treatment.

Figure 1: Schematic diagram of production of matcha-embedded suspension. The figure shows (1) carboxymethyl chitosan solution, (2) whey protein solution, (3) matcha suspension, and (4) pre-formed matcha-embedded suspension. Please click here to view a larger version of this figure.

Figure 2: Schematic diagram of the production of matcha noodles. (A) Microwave treatment of wheat flour; (B) production of matcha noodles. Please click here to view a larger version of this figure.

Figure 3: Photograph of noodle samples. (A) Control, fresh noodles without matcha; (B) fresh noodles with unencapsulated matcha; (C) fresh noodles with embedded matcha and microwave treatment; (D) cooked noodles without matcha; (E) cooked noodles with unencapsulated matcha; (F) cooked noodles with embedded matcha and microwave treatment. Please click here to view a larger version of this figure.

Figure 4: Sensory quality characteristics of noodle samples. The control group had noodles without matcha, the M-Noodles group had noodles with unencapsulated matcha, and the ME-Noodles group had noodles with embedded matcha and microwave treatment. Please click here to view a larger version of this figure.

Figure 5: Chromatic aberration of noodles samples. (A) The value of ΔL* of noodles samples with different treatments. (B) The value of Δa* of noodles samples with different treatments. (C) The value of Δb* of noodles samples with different treatments. (D) The value of ΔE* of noodles samples with different treatments. Error bars represent mean ± SD (n= 6). Alphabet indicates a significant difference at p <0.05 using ANOVA. Please click here to view a larger version of this figure.

Table 1: Physical quality characteristics of cooked noodle samples. Values represent mean ± SD (n= 6). Alphabet indicates a significant difference at p <0.05 using ANOVA.

Compared to instant noodles, fine-dried noodles, and other similar products, fresh noodles have a greater capacity to preserve their original taste and flavor, making them highly promising in the market. A previous study has shown that green tea could enhance the overall quality of fresh noodles to a certain degree¹⁶. Therefore, incorporating tea into the flour product system of fresh noodles aims to prioritize both high quality and health benefits, in line with the contemporary trend of promoting natural and pursuing a green and healthy diet. However, compounds such as polyphenols and pigments from tea become more susceptible to oxidation or degradation by microorganisms in the ambient storage environment of tea-infused fresh noodles¹⁰. This not only leads to a reduction in the nutritional and health-promoting properties of tea-infused fresh noodles but also significantly impacts their visual appeal and other quality attributes, which may lower consumer acceptance. To avoid the above-mentioned problems caused by adding tea, we present a method for color and quality stabilization of matcha noodles in this study.

The quality of fresh noodles is influenced by the raw materials, processing technology, and storage conditions. The complex composition of fresh noodles can lead to various physicochemical and biochemical reactions during storage, resulting in browning. Enzymatic^1,17 and non-enzymatic^9,18 browning may cause darkening, or black spots in the color of wet noodles, and physical factors can also affect their appearance. Currently, the techniques for inhibiting enzymatic browning in noodles encompass the addition of inhibitors, microwave treatment, superheated steam treatment, ultrasonic treatment, and other methods. As the optimal pH for PPO activity in wheat was determined to be 6.5, the enzymatic activity of PPO can be effectively inhibited by adjusting the pH conditions through the addition of acidity regulators such as citric acid and malic acid¹⁵. Moreover, heat treatment is a widely employed technique for the inactivation of enzymes, typically achieved by raising the temperatures to 50-90 °C¹⁹. In contrast to wheat noodles, matcha noodles contain a high concentration of chlorophyll. The processing of matcha noodles leads to unavoidable interaction between PPO in wheat flour and chlorophyll, as well as exposure to water and oxygen. This disrupts the stability of the two independent systems and facilitates enzymatic browning during processing, ultimately leading to an unstable coloration of tea noodles.

This protocol outlines a highly efficient approach that utilizes low-intensity microwave-assisted heat treatment to effectively alleviate the color deterioration of matcha noodles while simultaneously improving pigment retention and passivating enzymes. When subjected to microwave radiation, wheat flour demonstrates not only non-thermal effects, i.e., in the absence of significant temperature elevation, the electromagnetic field effect of microwaves can elicit a robust biological response, thereby eradicating bacteria, preventing mold growth and deactivating enzymes - but also absorbs electromagnetic energy, resulting in a certain degree of thermal effect that deactivates PPO²⁰. Furthermore, the degradation of polyphenols is exacerbated by the prolonged cooking of noodles. Microwave heat treatment of dough and gelatinization at the core of the noodles can effectively shorten the required cooking time^21,22. This approach not only shortens the cooking process for matcha noodles but also enhances their microstructure, thereby promoting the retention of polyphenols.

The vibrant green hue of matcha noodles is primarily attributed to chlorophyll. However, chlorophyll is susceptible to changes in pH, temperature, and chemical reactions¹⁰. It is evident that the direct addition of acid inhibitors and heat treatment for color control in green tea noodles presents certain limitations. Currently, the color protection technology for tea beverages encompasses antioxidants, enzyme preparations, and metal ions. In the technology of ionic color protection, zinc ions are typically paired with chlorophyll to augment the thermal stability of chlorophyll²³. The key feature of this protocol is that embedding techniques have been employed to enhance the stability of matcha noodles' color. Whey proteins and carboxymethyl chitosan, as well as other carbohydrates, have great compounding and binding properties, and they can enhance the properties of complexes through synergistic action²⁴. Moreover, this protocol also demonstrates that microwave and embedding treatment does not have any detrimental impact on the physical property and sensory traits of matcha noodles.

This protocol could be easily adapted to simulate the production of other processed matcha food products. For example, matcha-infused suspensions are utilized in the manufacturing of various traditional foods, as well as dumpling wrappers or steamed bread^15,25. An important limitation of this protocol is that the preparation of matcha-infused suspension requires water as the medium. If the production process of the product does not necessitate the use of water, this method may not be appropriate.

In conclusion, this protocol presents a methodology for the production of fresh matcha noodles with enhanced flavor and consistent color through the utilization of embedding and microwave treatment. Furthermore, a reduction of over 70% in discoloration appears to be a strong threshold that ensures the stability of matcha noodles. However, scaling up to industrial conditions needs to be tested to ensure that the product has the same characteristics as the noodles obtained in this protocol.