Increasing the physicochemical stability of stored green tea noodles: Analysis of the quality and chemical components

Accepted Manuscript Increasing the physicochemical stability of stored green tea noodles: Analysis of the quality and chemical components Kun Yu, Hui-Ming Zhou, Ke-Xue Zhu, Xiao-Na Guo, Wei Peng PII: DOI: Reference:

S0308-8146(18)31944-7 https://doi.org/10.1016/j.foodchem.2018.11.012 FOCH 23820

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Food Chemistry

Received Date: Revised Date: Accepted Date:

14 May 2018 9 October 2018 1 November 2018

Please cite this article as: Yu, K., Zhou, H-M., Zhu, K-X., Guo, X-N., Peng, W., Increasing the physicochemical stability of stored green tea noodles: Analysis of the quality and chemical components, Food Chemistry (2018), doi: https://doi.org/10.1016/j.foodchem.2018.11.012

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Increasing the physicochemical stability of stored green tea noodles: Analysis of the quality and chemical components Kun Yu, Hui-Ming Zhou*, Ke-Xue Zhu, Xiao-Na Guo, and Wei Peng State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu 214122, People’s Republic of China *Corresponding author: Tel: +86 510 85329037; fax: +86 510 85329037 E-mail addresses: [email protected] Abstract Different methods used to process green tea powder noodles (GTPN) were compared by analyzing the quality and chemical components of the final products. Significant differences were observed in the discoloration rate, color loss, and lightness of all noodles. Polyphenol oxidase activity was effectively inhibited through the heat treatment, resulting in retardation of the GTPN discoloration rate. However, heat-exposed GTPN showed a higher surface lightness due to a greater loss of chlorophyll. The chlorophyll contents of GTPN prepared with a new method (a combination of water treatment and heat exposure) were prominently higher than those prepared by heat treatment alone, and exhibited less color loss. The heat-exposed noodles had relatively low free polyphenol contents, and their textural properties were not significantly decreased after 28 d. Therefore, combining water treatment and heat exposure in the preparation process has great potential for increasing the stability of GTPN. Keywords: processing method; green tea noodle; discoloration; polyphenol 1. Introduction Nearly 40% of the wheat consumed in Asian countries is processed into noodles. However,

the simple nutritional ingredient of wheat flour cannot meet the ever-growing nutritional demands of consumers. Therefore, several investigators have substituted part of the wheat flour with other natural components, such as green banana flour (Ramli, Alkarkhi, Yeoh, & Easa, 2009), bamboo leaf powder (Oh, 2004), milk protein (Li, Li, Gao, Hu, & Zan, 2017), oat bran (Mitra, James, Fenton, Cato, & Solah, 2016), and sweet potato (Menon, Padmaja, Jyothi, Asha, & Sajeev, 2016), in order to improve the nutritional and functional values of noodles. Tea, especially green tea, is claimed to contain an abundance of bioactive components with antioxidative (Rietveld, & Wiseman, 2003), anticarcinogenic (Khan, & Mukhtar, 2013), antimicrobial (Bansal, Choudhary, Sharma, Kumar, Lohan, Bhardwaj, Syan, & Jyoti, 2013), and anti-stress effects (Kimura, Ozeki, Juneja, & Ohira, 2007). Green tea has evolved to become one of the most popular drinks consumed worldwide. Because of this, the utilization of whole green tea (in the form of green tea powder (GTP)) has attained increasing attention with the development of tea research, whereby tea products containing GTP have gradually become a hot topic of research. The effects of GTP on the quality and functional characteristics of traditional foods such as noodles (Li, Zhang, Zhu, Peng, Zhang, Wang, Zhu, & Zhou, 2012), cookies (Ahmad, Baba, Wani, Gani, Gani, Shah, Wani, & Masoodi, 2015), and breads (Ning, Hou, Sun, Wan, & Dubat, 2017) have been reported, but numerous problems still exist in the quality of GTP-incorporated foods, such as the discoloration of green tea noodles. Part of the discoloration of green tea noodles may be caused by polyphenol autoxidation, enzymatic browning, non-enzymatic browning, and chlorophyll degradation. Polyphenol oxidase (PPO) is a crucial cause of the darkening of yellow alkaline noodles, albeit non-PPO darkening also occurs (Asenstorfer, Appelbee, & Mares, 2010). At present, although heat treatment is

typically used to inhibit enzymatic browning (Asenstorfer, Appelbee, & Mares, 2009; Zhu, Dai, Guo, Peng, & Zhou, 2014), the heat treatment of end products will result in the degradation of chlorophyll and polyphenol, thus lowering the sensorial and nutritional properties of green tea noodles. Previous studies eliminated the activity of PPO in wheat flour through microwave treatment (Zhu, Dai, Guo, Peng, & Zhou, 2014) or by adding PPO inhibitors (Asenstorfer, Appelbee, & Mares, 2009), which may be good ways to avoid the negative effects of heat treatment on noodles. However, the special equipment and reagents required would increase the production cost of noodles, where the added expense would most likely to be passed on to the consumer, resulting in decreased market consumption. Because the PPO molecules in GTP (Ozturk, Seyhan, Ozdemir, Karadeniz, Bahar, Ertas, & Ilgaz, 2016) and wheat flour are activated by the higher polyphenol content of green tea and the high moisture content from preparation of the noodles (Thanaraj, & Seshadri, 2010), the color of dried green tea noodles will darken and fade during its processing and storage. In this study, the objectives were to investigate the discoloration of dried green tea noodles and to put forward a feasible method to slow down such color change while retaining the functional components of green tea. 2. Materials and Methods 2.1 Materials Wheat flour (9.95% protein, 13.69% moisture, 8.65 mg/g free phenolic content) was obtained from a local supermarket (Wuxi, Jiangsu, China) and stored at 4 °C until further analysis. GTP (22.62% protein, 3.44% moisture, 1.56 g/g free phenolic content) was obtained from the Hangzhou Tea Research Institute of All-China Federation of Supply and Marketing Cooperatives.

All reagents used in the analytical procedures were from Sinopharm Chemical Reagent Co., Ltd., China National Pharmaceutical Group Corporation. 2.2 Pretreatments of the green tea powder The GTP was mixed with deionized water in a ratio of 1:10 (w/v), agitated on a vortex mixer at 2000 rpm for 10 min, and then centrifuged for 10 min at 1258 g. The supernatant was divided into green tea aqueous extract (TE) and green tea residue (TR) fractions, where the TR had absorbed approximately one quarter of the added water. The TE and TR samples were stored in a refrigerator at 4 °C prior to use. 2.3 Green tea noodle preparation GTPN were prepared with different processing methods according to the following specific steps: Step 1: Wheat flour (600 g) was first mixed with GTP (12 g) and then 192 mL of deionized water containing 6 g of salt was added to form a well-combined dough in a miniature mixer (Model HWJZ-5, Nanjing Yangzi Grain and Oil Food Processing Machinery Co., Ltd., Nanjing, China). Each sample was examined in duplicate (designated as “Dough 1” and “Dough 2”). Step 2: Instead of deionized water in Step 1, 168 mL of TE was added to wheat flour (600 g) to form TE dough. Each sample was examined in duplicate (designated as “Dough 3” and “Dough 4”). Step 3: All four doughs described above were respectively sealed in Ziploc bags and rested in a temperature and humidity chamber for 30 min at 25 °C. Step 4: The cured “Dough 1” was used directly in Step 6 and Step 7 to prepare GTPN (no treatment). The cured “Dough 2” and “Dough 3” were exposed to a thermal water bath (80 °C) for 15 min, followed by rapid cooling to 20(±5) °C. The heat-exposed “Dough 2” was used directly for the preparation of dried green tea noodles according to Step 6 and Step 7, and the product was designated as 80 °C GTPN (heat-exposed).

The heat-exposed “Dough 3” was used for the next step of processing (Step 5). Step 5: The heat-exposed “Dough 3” and cured “Dough 4” were respectively mixed with 36 g of TR using a miniature mixer. These two prepared doughs were then processed into green tea noodles according to Step 6 and Step 7 and the products were designated as 80 °C (TE&TR)N (a combination of water treatment and heat exposure) and (TE&TR)N (water treatment only), respectively. Step 6: All four doughs were cooled to room temperature and then passed through the roller unit of an experimental noodle machine (Model JMTD-168/140, Dongfang Fude Technology Development Center, Beijing, China) with a roller gap of 2–0.8 mm. Step 7: All the green tea noodles were finally dried with intelligent experimental equipment (Model SYT-030, China Packaging and Food Machinery Co., Ltd., Beijing, China), where the drying process was carried out at 36(±2) °C, 40(±2) °C, 45(±2) °C, 40(±2) °C, and 30(±2) °C, with a relative humidity of 80%, 70%, 60%, 60%, and 60% for 30, 40, 180, 30, and 30 min, respectively. The samples were dried to a final moisture content of 11–12%. 2.4 Packaging and storage The untreated GTPN, 80 °C GTPN, (TE&TR)N, and 80 °C (TE&TR)N were cut into a length of 20 cm and packed into aluminum foil bags, which were then sealed with a vacuum packing machine (Model DZ-300/5SA, Shineye, Henan, China). The weight of each package was about 50 g. The packaged samples were stored in a thermostatic incubator at 37 °C. For each group of samples, three packages were taken out for analysis of the physical and chemical properties after 0, 1, 3, 5, 7, 14, 21, and 28 d of storage. According to the test requirements, all GTPN were smashed and ground into 80-mesh powders. To monitor the color variation of the noodles during storage, the four GTPN were fixed on a whiteboard that was then sealed in an aluminum foil bag.

2.5 Analytical determinations 2.5.1 Physicochemical parameters The water contents of the GTPN were measured according to national standards for food safety (GB 5009.3-2016) by direct drying and then maintained at nearly the same value (about 11%). The water activity (aw) was determined using a Novasina Thermoconstanter Model Labswift-a w meter (Novasina, Lachen, Switzerland). 2.5.2 Discoloration rate and color loss The L (lightness: 0 = black & 100 = white), a (redness-greenness: + = green & – = red), b (yellowness-blueness: + = yellow & – = blue), and △E (△E=

) values

for evaluating the surface color of each sample were recorded using a Minolta chromameter (Model CR-400, Minolta Camera Co., Osaka, Japan). The ratio of the –a/b parameters is usually applied in research focused on the color changes of processed green vegetables. Three replicates were conducted for each sample. The discoloration rate of each sample was calculated by plotting the regression lines of the logarithms of the ratios of (–a/b)t and (–a/b)0 as a function of storage time in days (d). The formula for calculating the kinetic constant (k) of the GTPN discoloration rate was as follows:

where

is the color parameter at any time t,

is the initial color value, and

is the kinetic rate constant (d–1). The color loss of the GTPN was calculated using the value of (a/b) before and after drying, as follows:

where the the

value of 80 °C GTPN was obtained at the time before heat treatment, and value of the other samples was obtained after sheeting. The

values

were all obtained after the drying process. 2.5.3 Image scanning and sensory evaluation Images of the green tea noodles stored for 0, 21, and 28 d were recorded with a scanner (Model LiDE220, Canon Inc., Vietnam). During the sensory evaluation, ranking and marking by dual comparison were used to assess the noodle color of preference. A consumer panel (untrained) consisting of 19 judges was selected for this evaluation. All the samples were coded by unordered numbering (e.g., “823”, “936”, “365”, “583”) to eliminate distraction by a simple figure, and the panelists were asked to give a score in the order from the worst to the best color. In this test, “1” represented the most unacceptable color, “5” a poor color, “7.5” an acceptable color, and “9” the favorite color. A score of less than “7.5” represented non-acceptance. All tests were carried out at the same place and under the same illumination, and the results were analyzed with SPSS software. 2.5.4 Textural and thermal properties GTPN of 20 cm in length were cooked to the optimum cooking time, which aimed to gelatinize all the starch (i.e., the time end point when the raw starch in the center of the cooked noodles has just disappeared). The surface moisture of the cooked GTPN was removed by blotting with filter paper. The textural properties of the cooked noodles were measured using a TA.XTplus Texture Analyser (TPA; Stable Micro Systems, Godalming, UK) with the following settings: Pre-test speed: 0.8 mm/s; Test speed: 0.8 mm/s; Post-test speed: 0.8 mm/s; Strain: 75%; Trigger force: 10 g; Time: 1 s.

The thermal properties of the GTPN were analyzed by differential scanning calorimetry (DSC; Model 8500, PerkinElmer, Waltham, MA, USA). The green tea noodle powder (80 mesh) or freeze-dried dough powder (80 mesh) (i.e., the dough obtained before making noodles) was mixed with deionized water at the ratio of 1:3 to form a suspension. Approximately 6–10 mg of the suspension was weighed into an aluminum pan, which was then sealed and kept for 24 h at 4 °C before DSC analysis. An empty pan was used as a reference. The sample was heated from 20 to 100 °C at a rate of 10 °C /min. 2.5.5 Confocal laser scanning microscopy The effects of processing on the GTPN microstructure were studied by confocal laser scanning microscopy (CLSM; Model LSM710, Carl Zeiss AG, Oberkochen, Germany). The GTPN were first enclosed in Leica glue and stored –20 °C before being sliced. The enclosed samples were cut into 10-μm-thick slices with a freezing microtome (Model CM1950, Leica, Wetzlar, Germany) and then placed on a slide and stained with a solution of 0.25% fluorescein 5-isothiocyanate (FITC, tagging wheat starch green), 0.025% rhodamine B (tagging protein red), and calcofluor-white (undiluted liquid, tagging tea fiber blue) at the volume ratio of 1:1:1 (Huang, Zhao, Zhu, Guo, Peng, & Zhou, 2017). The stained samples were decolorized with deionized water and observed by CLSM. The excitation wavelengths for FITC, rhodamine B, and calcofluor-white were 488, 561, and 405 nm, respectively. The obtained images were analyzed using ZEN 2012 software to reveal the microstructures of the GTPN. 2.5.6 Determination of total chlorophyll, chlorophyll a, and chlorophyll b contents The chlorophyll of the 80-mesh green tea noodle powders (2.5 g) was extracted with 10 mL of 80% acetone in a mortar for 10 min, and continuously extracted with 10 mL of 80% acetone

until the greenness of the samples had disappeared. The extract was filtered through a filter paper into a colored volumetric flask, and the final volume was made up to 25 mL with 80% acetone. The absorbance of the chlorophyll extract was measured using a UV/VIS spectrophotometer (Model TU-1810, PERSEE, Beijing, China) at 663 and 645 nm, respectively. The values of the total chlorophyll (TC), chlorophyll a (Ca), and chlorophyll b (Cb) contents were calculated using the equations below: Ca (mg/L) = 12.7D663 – 2.69D645 Cb (mg/L) = 22.9D645 – 4.68D663 TC (mg/L) = 20.21D645 + 8.02D663 The absorption coefficients of Ca and Cb at 663 and 645 nm were 12.7 and 2.69, and 22.9 and 4.68, respectively. 2.5.7 Free phenolic content Determination of the free phenolic content (FPC) was carried out according to the methods of Levent (2017), with slight modifications. The 80-mesh green tea noodle powders (0.2 g) were extracted in 4 mL of 70% methanol at 70 °C for 30 min, and the mixtures were then shaken vigorously for 10 min and centrifuged at 1258 g for 10 min. The extraction procedure was repeated twice. All supernatants were collected and diluted to 10 mL with 70% methanol. The polyphenol extracts were stored at 4 °C before analysis. Free polyphenols were determined using the Folin-Ciocalteu reagent. In brief, 1 mL of polyphenol extract was mixed with 4.5 mL of Folin-Ciocalteu reagent. After 3 min of reaction, 5 mL of 7.5% Na2CO3 solution was added and the reaction mixture was kept for 1 h in the dark. The absorbance was measured at 765 nm using a UV/VIS spectrophotometer. The polyphenol content was quantified with gallic acid as a

calibration standard. 2.5.8 Polyphenol oxidase activity The PPO activity was determined according to the method described by Yadav, Patki, Sharma, and Bawa (2008), with minor modifications. In brief, the 80-mesh green tea noodle powders (4 g) were mixed with 10 mL of phosphate-buffered saline (0.1 M, pH 6.0), and the mixture was shaken at 4 °C for 48 h and then centrifuged at 10,615 g for 20 min at 4 °C. The supernatant was used as the crude PPO extract. For PPO activity detection, 250 μL of the crude enzyme extract was added into the wells of a 96-well plate and 50 μL of 0.1 M catechol solution was then added to each well. Detection was carried out in a preheated Epoch 2 microplate spectrophotometer (Model EPOCH2T, BioTek Instruments Inc., Winooski, Vermont, USA). The dynamic program for detection of PPO activity was set to a time interval of 1 min, wavelength of 420 nm, and time length of 30 min. PPO activity (U/g

) was defined as the increase in absorbance by 0.001 per

gram of sample in 1 min, as calculated by the following equation: PPO activity = where

A represents the changes in the absorbance of the reaction solution;

the buffer solution in the crude enzyme extract solution (mL); enzyme solution in the reaction solution (mL);

is the volume of

is the volume of the crude

is the mass of the sample (g); and

is the

reaction time (min). 2.6 Statistical analysis The results from three replicates were subjected to one-way analysis of variance, and their mean values and standard deviations were obtained. Differences between means were analyzed by Tukey’s test, where a level of α ＜ 0.05 was considered significant. All figures were plotted using

OriginPro 2016 (OriginLab Corporation, Northampton, MA, USA). 3. Results and Discussion 3.1 Discoloration rate and color loss Heat treatment of the dough and water treatment of the GTP in the preparation of GTPNs decreased the discoloration rate of the noodles along the storage period (Fig. 1b, c, & d). The color stability depended on various factors: namely, the PPO activity, chlorophyll content, water activity, and polyphenol content (Ortiz, Ferruzzi, Taylor, & Mauer, 2008). The discoloration rate of untreated GTPN increased to 0.03097 until 7 d of storage, representing an approximately 20.12% decrease from the initial color (Fig. 1a). However, the discoloration rate of 80 °C GTPN was only about 62% that of untreated GTPN, which was likely due to the 80 °C heat treatment. Because of the water treatment, the discoloration rate of (TE&TR)N was markedly lower than that of untreated GTPN. The discoloration rate of 80 °C (TE&TR)N was significantly retarded during the initial 7 d and remained similar to that of 80 °C GTPN thereafter. The similar discoloration rates between (TE&TR)N and 80 °C (TE&TR)N could be attributed to residual enzyme in the tea residue and to the bad combination of the tea residue and heated dough in 80 °C (TE&TR)N. The difference in the vertical coordinates in Fig. 1a, b, c, & d is related to the amount of color variation during storage, where it can be seen that the Y-axis value of untreated GTPN is significantly higher than that of the other samples. The distinction in the amount of color variation for 80 °C GTPN, (TE&TR)N, and 80 °C (TE&TR)N became smaller until 7 d later (Fig. 1b, c, & d). A previous study had reported that non-enzymatic browning, as well as chlorophyll degradation, polyphenol autoxidation, and the Maillard reaction, had contributed to the discoloration of peach puree during the heating process and long-term storage (Gerza, Ibarz, Pagan, & Giner, 1999).

Under the different processing methods, the lightness and color loss were considerably different among the various GTPN (Fig. 1e). The lightness of 80 °C GTPN was drastically increased after drying, which was caused by the great loss of chlorophyll. By contrast, the color loss of 80 °C (TE&TR)N was about 34% less than that of 80 °C GTPN, and its lightness was also lower than that of 80 °C GTPN. However, the lightness of untreated GTPN was the lowest among the samples, which might be due to enzymatic browning of the polyphenols (Mcevily, Iyengar, & Otwell, 1992). 3.2 Sensory evaluation and scanned images Visual images of the untreated GTPN, 80 °C GTPN, (TE&TR)N, and 80 °C (TE&TR)N at the storage times of 0, 21, and 28 d are presented in Fig. 2a. A significant difference in the surface color was observed from all the green tea noodles at 0 d, while the browning of untreated GTPN increased gradually with storage time. Among the four green tea noodles, untreated GTPN had the lowest acceptable level in the sensory evaluation (Fig. 2b), which resulted mostly from its darkened color during storage. The sensory score for 80 °C GTPN followed that of GTPN owing to the great color loss, which almost crossed the acceptable line. 3.3 Analysis of thermal and textural properties The effects of the different processing methods on the thermal behavior of the doughs and GTPN were measured by DSC (Table 1). Both 80 °C GTPN and 80 °C (TE&TR)N had a wider gelatinization temperature range (the difference value between TC and T0) than that of untreated GTPN and (TE&TR)N. Moreover, the enthalpy values (△H) for 80 °C GTPN and 80 °C (TE&TR)N were lower than those of untreated GTPN and (TE&TR)N. These results could be attributed to the gelatinization of starch due to the heat treatment, resulting in aging of the starch.

Previous research studies have reported that retrograded starch has a lower enthalpy value and a wider gelatinization temperature range (Ronda, Quilez, Pando, & Roos, 2014). The hardness and chewiness of the cooked GTPN were examined by texture profile analysis (Table 1). The degree of hardness in noodles is related to the gelatinization of starch, which contributes to the good texture of the noodle. When heat treatment was involved in noodle processing, the hardness and chewiness of 80 °C GTPN and 80 °C (TE&TR)N were significantly increased compared with those of untreated GTPN and (TE&TR)N, due to starch aging. These results underpin the fact that tea polyphenols, a component of GTP, contribute to the cross-linking between proteins and starch and enhance the gel properties of tea-based foods, as reported in previous studies (Budryn, Żyżelewicz, Nebesny, Oracz, & Krysiak, 2013). After storage at 37 °C for 28 d, the hardness and chewiness of the cooked GTPN had increased significantly, except for those of untreated GTPN that had reduced slightly. This might be due to enzymes in the untreated GTPN, which can soften the texture of noodles during the storage period (Zhu, Cai, & Corke, 2010). 3.4 Confocal laser scanning microscopy image analysis The microstructure of all green tea noodles was observed by CLSM (Fig. 3), where the natural shape and structure of the starch in 80 °C GTPN could be seen to have nearly disappeared as a result of the heat treatment. However, complete starch morphology was evident in the magnified image of 80 °C (TE&TR)N. Combined with the enthalpy (△H) values of the doughs used for 80 °C GTPN and 80 °C (TE&TR)N, the results indicate that these phenomena might be attributed to the tea polyphenols inhibiting the starch gelatinization process. Further observation revealed that the GTP molecules appeared in different positions between the proteins and starch,

where they were mostly wrapped within the gel structure of the gelatinized starch in the images of 80 °C (TE&TR)N. The different positions of the GTP molecules would be related to the stability in discoloration of the green tea noodles, in accordance with the results of the discoloration rates presented in section 3.1 above. 3.5 Changes and differences in the chlorophyll contents of GTPN during storage The total chlorophyll, chlorophyll a, and chlorophyll b contents of the four GTPN were determined at 37 °C during 28 d of storage (Fig. 4a, b, & c). Untreated GTPN and 80 °C GTPN (storage at 0 d) showed the greatest loss in chlorophyll. It could be seen that chlorophyll a was more sensitive than chlorophyll b to heating; therefore, after drying, the loss of chlorophyll a was greater than that of chlorophyll b. The total chlorophyll contents of untreated GTPN and (TE&TR)N decreased gradually with prolongation of the storage time; conversely, those of 80 °C GTPN and 80 °C (TE&TR)N were relatively stable (Fig. 4c). These results showed that heat treatment delayed the loss of chlorophyll during the storage period. Nevertheless, the chlorophyll content of 80 °C GTPN was significantly lower than that of 80 °C (TE&TR)N owing to the direct heat exposure in the former, confirming that the processing method used for preparing 80 °C (TE&TR)N had decreased the amount of chlorophyll lost. 3.6 Changes and differences in the FPC, PPO activity, and water activity As observed in Table 2, the FPCs of the heat-exposed green tea noodles (80 °C GTPN and 80 °C (TE&TR)N) were sharply lower than those of untreated GTPN and (TE&TR)N at 0 d of storage, and the FPCs of GTPN and (TE&TR)N showed a decreasing trend thereafter. These results were in accordance with those of the study of Mcevily, Iyengar, and Otwell (1992), which reported that the PPO activity and autoxidation of free polyphenols contributed to enzymatic

browning, resulting in the content changes of polyphenols. From the perspective of PPO activity, that of the green tea noodles was well suppressed through heat treatment and did not change obviously with storage time. Comprehensive analyses of the PPO activity and FPC revealed that the FPC of 80 °C (TE&TR)N remained at a relatively low level, and its PPO activity was significantly less reactive. As for the water activity (aw) (Table 2), only slight fluctuations were evident for all four green tea noodles during the storage period. 4. Conclusions Heat treatment of the dough used for preparing green tea noodles (80 °C GTPN) notably retarded the discoloration of the product during storage; however, the chlorophyll contents of these noodles were greatly diminished, resulting in poor sensory scores. The discoloration rate of green tea noodles prepared with water-treated GTP was retarded during the storage period. The color of untreated GTPN was significantly browned after drying, and darkened with prolonged storage time. Enlarged CLSM images revealed that the gelatinization of starch in 80 °C (TE&TR)N had tightly wrapped the GTP molecules, and the complete particle morphology of starch was observed. Thus, the processing method of 80 °C (TE&TR)N, which effectively combines water treatment of the GTP and heat exposure of the dough, resulted in the lowest PPO activity and loss of chlorophyll, realizing protection of the tea compounds. However, further study of the key role of GTP is necessary to improve the color stability of green tea noodles. Acknowledgments This work was supported by the Postgraduate Research & Practice Innovation Program of Jiangsu Provence (Grant number KYCX17_1405). The authors express cordial gratitude to our laboratory colleagues and to Dr. Mohammed Obadi for polishing the English in the article.

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Fig. 1 Discoloration rate of green tea noodles (GTPN (a), 80 °C GTPN (b), (TE&TR)N (c), 80 °C (TE&TR)N (d)) during storage, and the color loss and lightness of green tea noodles (e) after drying.

0d

21 d

28 d

(a)

(b) Fig. 2 Scanning pictures on the facades of GTPN, 80 °C GTPN, (TE&TR)N, 80 °C (TE&TR)N at the storage time of 0 d, 21 d and 28 d (a), and the sensory scores of green tea noodles at 28 d (b).

Fig. 3 Confocal laser scanning microscopy (CLSM) images of GTPN, 80 °C GTPN, (TE&TR)N, 80 °C (TE&TR)N from top to bottom

Fig. 4 Chlorophyll a (a), Chlorophyll b (b) and Total chlorophyll (c) content of GTPN, 80 °C GTPN, (TE&TR)N, 80 °C (TE&TR)N during storage.

Table 1 Thermal and textural properties of green tea noodles during storage. DSC

GTPN dough

80 °C GTPN noodle

abc

60.591±0.14

dough ab

60.54±0.17

(TE&TR)N noodle

ab

60.43±0.07

dough

ab

60.77±0.41

noodle

a

60.18±0.04bcd

T0 (°C)

60.33±0.44

Tp (°C)

64.30±0.36a

64.38±0.01a

64.52±0.30a

64.99±0.06a

64.66±0.28a

64.30±0.02a

TC (°C)

67.96±0.07b

67.99±0.12b

67.57±1.02b

71.13±1.62a

68.03±0.01b

68.43±0.09b

△H (J/g)

4.804±0.24ab

4.850±0.46ab

5.06±0.35a

4.251±0.177bc

4.565±0.37abc

4.326±0.06bc

TC - T0 (△T)

7.63±0.37b

7.395±0.04b

7.03±0.85b

10.7±1.69a

8.00±0.94b

8.05±0.15b

(°C) TPA Storage time Hardness (g) Chewiness (g)

GTPN 0d

28 d b

4930.15±133.25 2823.40±117.10

80 °C GTPN

a

0d b

4813.79±162.44

b

2642.72±194.62

(TE&TR)N 28 d

5364.23±207.12

a

2928.62±194.94

a

0d

5599.57±174.44 3114.88±184.80

a

a

5584.09±110.

a

3289.90±153.

5010.90±182.50 2829.06±171.19

Different lowercase letters show significant differences (α＜0.05) among different samples and different storage time.

28 d b

Table 2 Free phenolic content (FPC), polyphenol oxidase (PPO) activity and water activity (aw) of green tea noodles. Sa

Storage time (days)

mples 0

1

3

5

34.65±1.1

33.96±0.0

35.44±0.0

7

14

21

28

33.19±0.0

34.55±1.4

34.36±0.7

33.11±0.2

FPC (mg/g) GT PN

36.72±1.6 1

80 °C

a

ab

30.98±1.4 4

c

b

ab

8

3

0

32.56±0.3

37.24±0.1

37.57±0.2

c

ab

ab

3

1

5

9b

38.19±0.0

37.04±0.6

36.39±1.5

35.24±0.4

2

4

6b

39.01±0.4

39.08±0.2

38.09±0.1

36.85±1.0

36.82±0.4

3b

3b

6bc

8c

6c

26.49±1.0

29.93±1.5

32.88±0.1

28.59±1.2

27.84±1.3

6

3

39.50±0.7

39.08±0.4

40.58±0.1

2ab

9b

0a

25.67±0.2

31.05±0.9

25.80±0.5

a

ab

ab

0

3

a

b

ab

GTPN (T E&TR) N 80 °C

6

e

ab

2

e

9

2

de

bc

1

a

cd

3

1

1cde

(TE&T R)N PPO activity (U/g GT

83.33±12. a

PN 80 °C

in) 71.43±13. a

72.20±17. a

73.33±18.

34

55

22

26

34.00±16.

36.25±8.5

44.00±13.

85b

7b

56ab

95.00±15.

95.71±7.2

82.50±12.

a

68.89±16. a

76.33±11.

78.00±11.

9a

42.73±10.

38.33±15.

31.67±10.

52ab

72b

67b

90.00±8.1

88.00±14.

94.28±23.

10

61.00±13.

33.64±14.

75a

94b

87.50±9.6

88.33±17.

a

72.5±12.9

66

18

a

GTPN (T E&TR)

a

a

a

00

8

24.00±16.

23.75±13.

a

a

99

8

18.75±11.

37.50±16.

a

a

72

6

20.00±13.

22.50±14.

70

21a

27.50±8.2

25.00±13.

N 80

a

°C

25

a

a

a

17

66

39

0.466±0.0

0.467±0.0

0.444±0.0

a

a

23

79

0.413±0.0

0.436±0.0

a

84a

9

(TE&T R)N Water activity GT

0.454±0.0 d

PN 80

c

e

03

00

04

02

0.468±0.0

0.510±0.0

0.447±0.0

0.472±0.0

b

°C

c

a

c

07

02

11

0.434±0.0

0.445±0.0

0.446±0.0

00

b

g

f

01

02

0.452±0.0

0.446±0.0

c

c

02

00

0.430±0.0

0.446±0.0

0.484±0.0 a

0.476±0.0

00

02b

0.508±0.0

0.432±0.0

a

00

02d

0.480±0.0

0.435±0.0

GTPN (T E&TR)

c

b

b

0.430±0.0 c

01

02

00

00

0.474±0.0

0.454±0.0

0.450±0.0

0.452±0.0

c

b

01

00

0.464±0.0

0.440±0.0

a

00

01c

0.487±0.0

0.434±0.0

N 80 °C (TE&T R)N

02

a

03

d

02

d

02

d

00

c

00

e

00

b

00f

Different lowercase letters show significant differences (α＜0.05) during storage within the same sample.

Highlights 

The discoloration of green tea noodles was investigated and improved.



One processing method inhibited the polyphenol oxidase activity.



Chlorophyll loss and degradation were decreased by this processing method.



CLSM revealed the microstructures of the green tea noodles.