Retarding effects of organic acids, hydrocolloids and microwave treatment on the discoloration of green tea fresh noodles

LWT - Food Science and Technology 55 (2014) 176e182

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Retarding effects of organic acids, hydrocolloids and microwave treatment on the discoloration of green tea fresh noodles Ke-Xue Zhu*, Xin Dai, Xiaona Guo, Wei Peng, Hui-Ming Zhou State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, Jiangsu Province, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 February 2013 Received in revised form 27 May 2013 Accepted 20 August 2013

Superfine green tea powder (SGTP) was premixed with organic acids (ascorbic acid, citric acid) and hydrocolloids (sodium alginate, curdlan), and then mixed with microwave-treated wheat flour to produce green tea fresh noodles (GTFN). Darken-retardant effects of organic acids, hydrocolloids and microwave treatments on GTFN were evaluated, as well as pH, polyphenol oxidase activity, sensory and microstructure characteristics. The results revealed that organic acids exhibited a suppressive effect on discoloration, among which citric acid (CA) displayed more efficient influence with lower pH. After adding hydrocolloids and microwave treatments, retardant effects exhibited more significant (P < 0.05). Specifically, employing citric acid 0.6 g/100 g, sodium alginate 0.2 g/100 g, and 800 W microwave (MW) 50 s would contribute to lower darkening index DE* (24 h, 25
C) at 3.88  0.314, 4.94  0.297, 2.78  0.212, respectively. Furthermore, the combined effect of the above process restrained discoloring rate considerably (DE* ¼ 1.92  0.101), also provided pleasant sensory characteristics. The confocal scanning laser microscopy (CSLM) images demonstrated the microstructure of the noodle was strengthened compared with blank GTFN, and sodium alginate could serve as a binding agent to parcel SGTP and starch granules. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Fresh noodle Discoloration Organic acid Hydrocolloids Microwave

1. Introduction Noodle is a kind of popular staple food throughout history normally, and among which fresh noodle (FN) has gained preference wide spreadly for its delightful taste and mouthfeel, as well as convenience. Traditional FN is made from Triticum aestivum (common wheat) flour, water and salt, and reckoned to lack some essential nutrients, such as dietary fibers, minerals and vitamins (Choo & Aziz, 2010). With the fast pace of modern life, people are so overwhelmed with stress that junk foods which contain high levels of fat, salt and sugar are well received. This results in a rapid rise in the number and proportion of individuals who suffered from chronic disease such as diabetes, high blood pressure, high cholesterol, heart disease and stroke (Roberts & Barnard, 2005). According to World Health Organization (WHO, 2009), high blood pressure, abnormal blood glucose and overweight are three major risks for mortality in the world, respectively responsible for 13%, 6% and 5% of deaths globally. Therefore, with the intention of manufacturing healthy

* Corresponding author. Tel./fax: þ86 510 85329037. E-mail addresses: [email protected], [email protected] (K.-X. Zhu). 0023-6438/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2013.08.010

diet, additional ingredients which can serve as essential nutrients and fiber supplement can be introduced into prevalent stable foods, such as FN. Green tea is high in dietary fibers, minerals and vitamins, especially tea polyphenols, polysaccharides and proteins, all of which exhibit outstanding antioxidant and anti-carcinogenic activities (Lu, Lee, Maud, & Lin, 2010; Tsubaki, Iida, Sakamoto, & Azuma, 2008; Yang, Lambert, & Sang, 2009; Yu, Sheng, Xu, An, & Hu, 2007). As a superfine grinded food ingredient, superfine green tea powder (SGTP) with an average particle size of approximate 20 mm almost keeps the entire composition of organic green tea, such as tea polyphenols, polysaccharides and amino acids (Hu, Chen, & Ni, 2012). At the same time, SGTP has already been used as a new type of natural additive in wheat dough matrix. Besides, SGTP can also contribute to particular dough behaviors. Li et al. (2012) found that SGTP lead to slight but significant improvement of stability and viscoelasticity of wheat dough. However, FN is susceptible to time-dependent darkening, resulting in product’s undesirable appearance (Bhattacharya, Luo, & Corke, 1999; Hatcher, Symons, & Manivannan, 2004), which is a principal problem arising during the storage of FN, including common green tea fresh noodles (GTFN) products. In resent years, there is a universal consensus that discoloration occurred in FN

K.-X. Zhu et al. / LWT - Food Science and Technology 55 (2014) 176e182

storage is primarily contributed by polyphenol oxidase (Baik, Czuchajowska, & Pomeranz, 1995; Fuerst, Anderson, & Morris, 2006; Mares & Campbell, 2001). Also, Asenstorfer, Appelbee, and Mares (2009) suggested that soluble protein fraction plays a role in the non-polyphenol oxidase (non-PPO) darkening course. In order to restrain PPO darkening during storage, extensive work has been done in the last few years. Previous studies have indicated that heat treatments on wheat flour and raw noodle could be used to fulfill this task because it would denature proteins and thereby stop enzyme activity (Asenstorfer et al., 2009; Neill, AlMuhtaseb, & Magee, 2012). Aside from heat treatments, a few organic acids such as ascorbic acid (AA) and citric acid (CA) could suppress the darkening of fresh-cut fruits and vegetables. These organic acids can reduce quinones to phenolic compounds, lower pH value of circumstance, and chelate cooper at the active PPO site (Iyengar & McEvil, 1992; Martinez & Whitaker, 1995). Pilizota and Sapers (2004) reported that a pH 2.9 dip consisted of AA and CA could retard the browning rate of fresh-cut apples. In addition, the improving effects of different hydrocolloids on noodles or wheat doughs have been reported. Cho, Shim, and Lee (2007) found that hydrocolloids together with appropriate amylase/amylopectin ratio would improve freeze-thaw stability of wheat dough, and Silva, Birkenhake, Scholten, Sagis, and van der Linden (2013) found that the hydrocolloids with high water binding capability could regulate dough rheology. Based on these benefits, hydrocolloids also contribute to noodle texture. Inglett, Peterson, Carriere, and Maneepun (2005) reported that by adding 10% Nutrim-5 (an oat cereal hydrocolloid), 50% rice flour could be incorporated in Asian noodles with satisfactory cooking loss and tensile strength. Similarly to research conducted by Oishi et al. (2009) who reported that hypoallergenic wheat flour noodles with sodium alginate and curdlan possessed better rupture strength and hardness. However, little attention was paid to the potential inhibiting effect of hydrocolloids on the discoloration of FN. The suppressive effects of particular organic acids, hydrocolloids and microwave treatments on the darkening of GTFN stored in 24 h under ambient conditions were investigated in order to develop a creative and feasible process for this FN product with healthy benefits. 2. Materials & methods 2.1. Materials High-protein T. aestivum (common wheat) flour was manufactured by China Oil & Foodstuffs Corporation (Qinhuangdao, China), and it’s moisture, protein and ash contents were 13.5  0.04, 13.1  0.14 (dry basis) and 0.57  0.04 g/100 g flour, respectively. Superfine green tea powder (SGTP) with an average particle diameter of 20  1.9 mm was supplied by Hangzhou Tea Research Institute of All China Federation of Supply and Marketing Cooperatives (Hangzhou, China). Citric acid (CA), ascorbic acid (AA), sodium alginate (SA) and curdlan (CL) of food-grade were supplied by Wuxi Shanzilingyun Trading Company. Sodium phosphate and pyrocatechol were provided by Sinopharm Chemical Reagent Co., Ltd (SCRC). Table salt was purchased from the local market. Fluorescein isothiocyanate (FITC) and Rhodamine B were produced by German Ruibio Chemicals Co., Ltd. 2.2. Methods 2.2.1. Premix treatment of superfine green tea powder SGTP (2 g/100 g flour), table salt (2 g/100 g flour), citric acid (0.2, 0.4, 0.6, 0.8, or 1.0 g/100 g flour) or ascorbic acid (0.2, 0.4, 0.6, 0.8, or

177

1.0 g/100 g flour), sodium alginate (0.05, 0.10, 0.15, 0.20, or 0.25 g/ 100 g flour) or curdlan (0.05, 0.1, 0.15, or 0.20 g/100 g flour) were mixed evenly, then dripped with 22 mL sterile water (deionized water was kept boiling in a pot for 15 min) meanwhile stirring to make a slurry. After that, the slurry was blended with a magnetic stirring apparatus (Model RHB1S25, IKA, Germany) for 30 min. Then, the slurry was stocked in a freezer for refrigerating overnight. 2.2.2. Microwave treatment of wheat flour Wheat flour (300 g) was evenly dispersed in a round plastic container, covered and then was treated in a conventional microwave oven (Model NJL07-3, Jiequan, China) at 800 W (2450 MHz) for 20, 40, 50, 60, 70, 80, or 90 s, or a ultraviolet-microwave oven (Model JHWB-MF4, Jiahua, China) at 2000 W (2450 MHz) for 20 or 40 s. Afterward, the container with flour was kept still until it felt cool. 2.2.3. Preparation of green tea fresh noodles Noodle dough was prepared in a dough mixer (Model 5K5SSWH, Kitchen Aid, USA) for 7 min to ensure uniform blending. The pre-formed green tea slurry was poured into the microwavetreated wheat flour slowly while starting the mixer, and then 6 mL of sterile water was added into it while stirring. After resting under ambient environment for 40 min, the dough was flattened gradually into a sheet with 0.9 mm in thickness by a noodle machine (Model JMTD-168/140, Dongfujiuheng, China), and cut into noodle strands of 22 cm in length. Noodles were stored in plastic bags at 25
C among different characteristics assessment periods. 2.2.4. pH measurement FN was mashed with a pestle in a mortar. Then 10 g noodle crumbs was homogenized with distilled water (90 mL) in an Erlenmeyer flask, using a magnetic stirring apparatus (Model RHB1S25, IKA, Germany) for 30 s. After a static period lasted 15 min, the pH value of the noodle was measured with a pH meter (Model FE20, Mettler Toledo, China). 2.2.5. Color measurement of noodle sheets The color of noodle sheet (CIE color parameters L*, a*, and b*) was measured using a Minolta Chroma Meter (Model CR-400, Minolta, Japan) equipped with D65 illuminant according to the description of Fuerst et al. (2006). All chroma assessments were conducted with the same white cardboard as background. L* denotes brightness (white-black) of noodle sheet, while a* and b* indicating red (þ)egreen () and yellow (þ)eblue () tendency, respectively. Samples were cut into pieces of 9  13 cm, and measured within 5 min straight after the noodle sheets were produced. Eight spots were chose randomly from both front and back sides of the sheet to be measured on. Color changes (DL*, Da*, Db*) were described by calculating the absolute difference between 24 h and zero-time readings during the storage period of FN sheets, and total color change DE* was calculated from the values of DL*, Da* and Db* as shown in Eq. (1).

Total color change index DE ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDL Þ2 þ ðDa Þ2 þ ðDb Þ2 (1)

2.2.6. Determination of polyphenol oxidase (PPO) activity The activity of polyphenol oxidase was measured with a spectrophotometric method employing by Yadav, Patki, Sharma, and Bawa (2008) with some modifications. FN (2 g) was ground with 30 mL sodium phosphate-citric acid buffer (pH 5.6) in a mortar. Then the mixture was shook for 12 h, using a constant temperature

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K.-X. Zhu et al. / LWT - Food Science and Technology 55 (2014) 176e182

bath reciprocating oscillator (Model SHA-B, Ronghua, China) set to 4
C and 100 rpm, followed by being centrifuged with a high-speed refrigerated centrifuge (Model CR21GⅢ, Hitachi Koki, Japan) at a speed of 12,000 rpm for 10 min at 4
C. Supernatant (crude PPO extract) of 1 mL and pyrocatechol (0.1 M, confected with pH 5.6 buffer solution) of 2 mL were incubated in a water bath for 5 min at 37
C, respectively. Afterwards, both of them were cooled in trash ice for 20 s, then mixed instantaneously with a vortex generator (Model lab dancer, IKA, Germany), and shifted into an optical glass cuvette with 10 mm path length rapidly. PPO activity was assayed at 420 nm with an ultra-violet and visible spectrophotometer (Model UV-2800, Unico, China) to monitor the absorbance of the solution for 5 min, using a blank solution with 2 mL buffer instead of crude PPO extract as correction. All PPO activities were analyzed in duplicate. One unit of PPO activity was defined as the increment of absorbance (at 420 nm) reached 0.001 in one min per gram sample. The PPO activity of noodle (U g1 min1) was calculated as shown in Eq. (2), where DA is the differences of absorbance within holding time t using buffer solution volume V2 for determination out of total amount V1 of buffer to prepare crude PPO extract, while m represents the quantity of noodle sample.

Activity equation of polyphenol oxidase ðPPOÞ activity ¼

D A  V1 0:001  t  m  V2

(2)

2.2.7. Confocal scanning laser microscopy Fresh noodle samples of three ingredients (control FN, blank GTFN and treated GTFN) were cut with a dissecting blade with the dimensions of 3  3  1 mm (approximately), before they were placed on glass slides and then post-stained with a solution of 3.5  104 g/mL Fluorescein isothiocyanate (FITC) and 1.3  105 g/ mL Rhodamin B. Images were acquired in 1024  1024 pixel resolution within an hour after staining period with a laser scanning confocal microscope (Model LSM 710, Zeiss, Germany) equipped with a set of four visible light lasers and an invert microscope Observer Z1. FITC and Rhodamin B were used to non-covalent label starch (green) and protein (red), respectively, permitting discrepant observation of starch and protein by CSLM (Peighambardoust, van der Goot, van Vliet, Hamer, & Boom, 2006; Peressini & Sensidoni, 2009; Silva, Scholten, van der Linden, & Sagis, 2012). The images were analyzed using ZEN2011 software with a 10 (Plan e Apochromat 10  10.45 ph1 M27) magnification to figure out the microstructure of noodle samples. The excitation wavelengths for FITC and Rhodamin B were 488 and 561 nm, respectively.

2.2.8. Statistical analyses Data were presented as means and compared by one-way analysis of variance (ANOVA) using SPSS 17.0. Means were compared to test for significant differences (P < 0.05) using the least significant difference statistic (LSD). Three replications were made for physical measurement and chemical analysis. 3. Results and discussion 3.1. Effect of organic acids on the discoloration and pH of green tea fresh noodles Chromatic aberration and pH values of GTFNs treated with different organic acids (AA or CA) were shown in Table 1. As expected, the pH values of GTFN were largely decreased with the increase of organic acids. However, the pH values went through a rapid decline at first, and then showed lower rates of descent. By using LSD’s test, no significant difference (P < 0.05) was observed among noodle samples. Table 1 presented pH values of GTFNs added AA were higher than those of GTFNs added CA at the same level, which is consistent with acidity differences reported by Blanco, Ronda, Perez, and Pando (2011). No significant difference of DL* values was observed among GTFNs with different kinds or levels of organic acids. For total color variance DE*, it seemed that GTFNs added CA 0.6 g/100 g, 0.8 g/100 g, or 1.0 g/100 g got the smallest variation (with no significant difference), followed by one with CA 0.2 g/ 100 g. However, adding CA up to 1.0 g/100 g led pH value to drop below 4.0, which would bring obvious sour to the GTFN. Therefore, the addition of CA 0.6 g/100 g was optimal for GTFN since it could restrain discoloration without tasting too sour. According to the report of Baik et al. (1995), AA or CA treatment could delay darkening of FN effectively, since the pH value of noodle was deviated optimal value (near 6.0) of wheat PPO. Furthermore, as antioxidant agents, organic acids, such as ascorbic acid, can reduce benzoquinones back to dihydroxyphenols before the former compounds turn to melanins nonenzymatically (Iyengar & McEvil, 1992). However, ascorbic acid can be spontaneously oxidized to dehydroascorbic acid (Baik et al., 1995; Lombardi & Zaritzky, 1996) during the storage of noodle, leading to short retarding period. 3.2. Effect of hydrocolloids on the discoloration of green tea fresh noodles Table 2 indicated that discoloring rate of GTFN decreased obviously at SA or CL 0.05 g/100 g, while the performance of the former

Table 1 Chromatic aberration and pH values of GTFNs with different organic acids. Composition (g/100 g)a Cont. AA 0.2 AA 0.4 AA 0.6 AA 0.8 AA 1.0 CA 0.2 CA 0.4 CA 0.6 CA 0.8 CA 1.0 a b

DL*b

pHb 6.14 5.73 5.33 4.90 4.76 4.61 5.07 4.60 4.36 4.06 3.96

          

a

0.02 0.01a 0.01a 0.02a 0.01a 0.01a 0.01a 0.01a 0.02a 0.01a 0.01a

6.62 7.04 6.45 6.62 5.67 5.62 5.12 5.33 3.58 3.47 3.28

Da*b           

a

0.212 0.178a 0.414a 0.332a 0.104a 0.280a 0.295a 0.414a 0.332a 0.104a 0.280a

1.90 2.12 1.87 1.73 1.37 1.56 1.54 1.68 1.30 1.47 1.34

Db*b           

h

0.184 0.151i 0.066h 0.049g 0.040cd 0.077ef 0.074ef 0.066fg 0.049c 0.040de 0.077ef

Cont., control; AA, ascorbic acid; CA, citric acid. Least square means in the same column with same letters (a, c, d, e, f, g, h, i) are not different (P < 0.05).

1.11 0.50 0.13 0.25 0.05 0.04 0.38 1.22 0.75 0.70 0.71

DE*b           

f

0.284 0.231e 0.111cd 0.270d 0.080c 0.158c 0.234ef 0.111f 0.270f 0.133f 0.158f

6.98 7.37 6.71 6.84 5.83 5.83 5.36 5.72 3.88 3.83 3.61

          

0.208f 0.193g 0.408f 0.314f 0.108e 0.280e 0.297d 0.408e 0.314c 0.117c 0.280c

K.-X. Zhu et al. / LWT - Food Science and Technology 55 (2014) 176e182

6.62 5.29 5.31 5.02 4.71 6.61 6.07 4.90 4.53 5.19

         

0.212e 0.391d 0.442d 0.924cd 0.302cd 0.259e 0.462e 0.275cd 0.501c 0.925cd

1.90 1.33 1.20 1.14 1.09 1.86 1.36 1.00 1.01 1.25

Db*b          

0.184f 0.284de 0.219cde 0.172cde 0.051cd 0.142f 0.184de 0.052c 0.134c 0.208de

1.11 1.70 1.30 1.55 0.94 1.86 2.20 1.48 0.74 1.52

DE*b          

0.284cde 0.541def 0.743cde 0.750cdef 0.380cd 0.625ef 0.975f 0.585cdef 0.452c 0.868cdef

6.98 5.74 5.63 5.41 4.94 7.14 6.64 5.23 4.72 5.60

         

0.208f 0.452e 0.505de 0.968cde 0.297cd 0.354f 0.710f 0.359cde 0.497c 1.007de

a

Cont., control; SA, sodium alginate; CL, curdlan. Least square means in the same column with same letters (c, d, e, f) are not different (P < 0.05). b

was better. With the increase of the additive amount, the chromatic aberration of GTFN in 24 h was decreased, and when adding SA 0.20 g/100 g or CL 0.15 g/100 g, the minimal color change was found. However, further addition resulted in an excursion of color change which was larger than that of the control sample. Similar phenomenon was found in the samples added CL. Both SA and CL are the major food ingredients, while the former is more commonly used because of low price. Thus, adding SA into GTFN at 0.20 g/ 100 g was more desirable. Nowadays, various ingredients have been introduced to wheat products to develop foods with particular healthy benefits, such as vegetable (Lee et al., 2002), starch (Huang & Lai, 2010), nuts (Gomez, Oliete, Caballero, Ronda, & Blanco, 2008), surimi (Kim, Huang, & Carpenter, 1990). According to the report of Zhou et al. (2009), green tea may contain abundant hydroxyl groups components, contrary to normal ingredients, which can lead to strong water absorption ability (Li et al., 2012). In addition, SA and CL have also been utilized to increase rupture strength and elasticity in Asian noodle industry (Oishi et al., 2009). Thus, it may be concluded that the premixing SGTP with hydrocolloids (SA and CL) enable tea powder to possess high water binding capacity and limit the swelling of SGTP by competing to assimilate water (Silva et al., 2013). On the other hand, hydrocolloids can wrap around SGTP by non-covalent bonds, protect susceptible compounds from oxygen. Moreover, SA and CL can be used as wall material for encapsulating of sensitive pigments, even in the environment of pH lower than 3.0 (Ferreira, Faria, Grosso, & Mercadante, 2009). Based on these, discoloration can be retarded by SA or CL. 3.3. Effect of microwave-treatment on the discoloration of green tea fresh noodles Microwave is thought to have an inhibitory effect on enzyme activity for the heat effect and non-heat effect (Edwards, 1964; Matsui, Gut, Oliveira, & Tadini, 2008; Yadav et al., 2008). Two kinds of microwave treatment (conventional microwave and ultraviolet-microwave) were adopted in this paper. Compared with the control, the darkening rate of GTFN made from microwave-treated flour was highly inhibited except one treated by microwave for 20 s. Presumably, because the acting time was so short that it raised the temperature of flour and promoted the activity of enzyme PPO. As treatment time went on, total chromatic aberration of GTFN was restrained at a lower level, and minimum discoloration occurred when microwave-treated flour for 90 s, even which the noodle sheet turned brown during storage, partly because of Maillard reaction (Vadlamani & Seib, 1996). Although the

3.4. PPO activity assessment of green tea fresh noodles Fig. 1 showed the changes of PPO activity within a relatively short storage period at ambient temperature. A progressive increase of PPO activity was observed in all the samples. It could be concluded that the activity of PPO in GTFNs decreased slightly during the first 12 h storage, and then increased. However, as for GTFN with CA 0.6 g/100 g, enzyme PPO was highly motivated and its activity increased obviously, since the optimal pH for PPO of wheat was found to be near acerbic pH values, according to Altunkaya and Gokmen (2012). Similar conclusion could be drawn from two GTFN samples under MW treatment for 50 s. The GTFN sample treated by microwave for 50 s without acid had the lowest PPO activity among the four samples, since PPO was largely depressed. This could partly explain the lower discoloration degree of GTFN made from microwave-treated flour during the same storage period. The effect of microwave treatment on PPO activity and temperature of wheat flour was also investigated, and the results were shown in Fig. 2. The PPO activity of wheat flour was decreased by almost 20% when flour temperature reached at approximate 80
C under the treatment of MW for 50 s, which was analogous to the result of Moreno, Chiralt, Escriche, and Serra (2000). Now, it has been well accepted that PPO plays a vital part in the discoloration of Asian noodles. As PPO was inactivated to some degree, time-

350

300 -1

Cont. SA 0.05 SA 0.10 SA 0.15 SA 0.20 SA 0.25 CL 0.05 CL 0.10 CL 0.15 CL 0.20

Da*b

-1

Composition DL*b (g/100g)a

ultraviolet-microwave (UVM) treatment got a much larger power than conventional microwave, color variation of GTFN made from flour treated with the former was more evident than that of the latter process under the same treatment time. This maybe attributed to the difference of work capability between the two microwave ovens with the same workload. It seemed that MW 50 s is the optimal microwave treatment. As treatment time went beyond 50 s, the moisture of flour became lower (date were not shown), and the flour was probably to be overheated. The protein and other compositions were likely to be denatured, which would bring undesirable consequences to the quality of noodle. As a kind of protein, PPO enzyme can be easily denatured by heat treatment. According to Yadav et al. (2008), microwave (900 W, 2450 MHz) heating of wheat grains for 80 s would decrease PPO activity to 7% and thus reduce color change of whole wheat dough stored under refrigerated.

PPO activity (U·g ·min )

Table 2 Chromatic aberration of GTFNs with different hydrocolloids.

179

250

200

150

100 0

12

24

36

48

Time (h) Fig. 1. The changes of PPO activity in 4 types of GTFNs during the storage for 48 h. Blank GTFN, CC 0.6 g/100 g GTFN, MW 50 s GTFN, MW 50 s þ CC 0.6 g/100 g GTFN.

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K.-X. Zhu et al. / LWT - Food Science and Technology 55 (2014) 176e182 1000

120

900 100

-1

PPO activity (U·g ·min )

800 700

500

60

400

Temp(°C)

-1

80

600

40

300 200

20

100 0

0 Control MW 20s

MW 40s

MW 50s

MW 60s

MW 70s

MW 80s

MW 90s

UVM UVM 20s 40s

Fig. 2. The PPO activity and center temperature in wheat flours treated with two kinds of microwave. PPO activity (m/(g min)), Temp (
C).

dependant darkening of FN was effectively depressed, which was accordance with the indication as shown in Fig. 1. 3.5. Effect of combination treatment on the discoloration of green tea fresh noodles Table 3 showed the synergic effect of organic acid, hydrocolloids and microwave on the discoloration of GTFN. As for control GTFN, L*(0 h) ¼ 77.13  0.09, a*(0 h) ¼ 8.19  0.03, b*(0 h) ¼ 24.39  0.18; while as for CeSeM GTFN, L*(0 h) ¼ 74.58  0.20, a*(0 h) ¼ 5.53  0.05, b*(0 h) ¼ 21.47  0.29. Table 3 indicated that CeSeM GTFN sample had the minimal color change (slightly more evident than the samples treated with MW 90 s but without significant difference) compared with the control sample, and much lower than the samples single treated with microwave, organic acids or hydrocolloids. Thus, it could be concluded that retarding effect on the GTFN discoloration would be remarkably amplified by adopting organic acids, hydrocolloids and microwave treatments, especially when it came to CA and SA. 3.6. Sensory evaluation With respect to sensory assessment of five GTFN samples, Fig. 3 showed that microwave treating for 50 s and adding CA 0.6 g/100 g had no apparent detrimental effects on the sensory qualities of GTFN, either on its smell or flavor. Similar results were obtained on Table 3 Chromatic aberration of GTFN with different microwave treatment.

DL*b

Da*b

Cont. 6.62  0.212g MW 20 S 10.16  0.646h MW 40 S 3.86  0.572f MW 50 S 2.70  0.224e MW 60 S 2.93  0.320e MW 70 S 2.15  0.127d MW 80 S 2.82  0.197e MW 90 S 1.36  0.795c UVM 20 S 10.55  0.652h UVM 40 S 4.15  0.242f CeS-M 1.82  0.076cd

1.90 2.33 0.86 0.68 0.69 0.50 0.48 0.34 2.24 0.91 0.33

Samplea

Db*b           

0.184g 0.171h 0.060f 0.026e 0.062e 0.103d 0.037d 0.090c 0.194h 0.105f 0.062c

1.11 1.68 0.42 0.15 0.15 0.18 0.74 0.03 1.12 0.60 0.45

enhanced-quality green tea (Gulati, Rawat, Singh, & Ravindranath, 2003), wheat chapattis (Yadav et al., 2008) and gluten-free bread (Blanco et al., 2011). As for fresh or cooked color of GTFN, CeSeM GTFN sample maintained an intermediate level, while its viscoelasticity and chewiness scored higher than that of the control, and its flavor got the highest credit. Although fresh color of blank GTFN scored highest, cooked color of control FN was more popular, partly because of traditional dietary habits. And CeSeM sample scored higher than CC 0.6 g/100 g or MW 50 s GTFN, owning to the complementary effect. In view of viscoelasticity and chewiness, CeSeM GTFN sample gained the majority of attention, followed by blank GTFN, while GTFN of MW 50 s scored lowest. This could attribute to the reinforced effect of sodium alginate on wheat dough matrix as the report of Rosell, Rojas, and Benedito de Barber (2001). On the contrary, microwave could partly denature gluten protein of wheat (Edwards, 1964), leading to the weakness of dough network. To sum up the above indexes, overall acceptability indicated that CeSeM sample and CC 0.6 g/100 g sample scored as high as blank GTFN, that is, higher than that of the control. Accordingly, adding CC 0.6 g/100 g would cause no deterioration effect on the sensory quality of GTFN, meanwhile adding SA 0.2 g/100 g could compensate for the destructive effect imposed by MW 50 s. In other words, CeSeM combination would be a potential process to produce GTFN with high quality. 3.7. Observation of noodle microstructure with CLSM Microstructure of stained samples was viewed with CLSM technology for detecting effect of CeSeM combination treatment on the behavior of dough, as well as distribution of starch granules and protein network in fresh noodles. CeSeM combination was chosen because it was the most cost-efficient productive procedure that could depress time-dependent darkness of GTFN as revealed in Section 3.5. Images of noodle sheets taken from CSLM technology at 10 magnification were shown in Fig. 4aec. Starch granules were clearly observed as green round regions, while red zones represented gluten network existing in the noodle system as described by Peressini and Sensidoni (2009) and Peighambardoust et al. (2006), also some bright yellow areas could be found in the pictures indicating some compounds of wheat flour and SGTP that could be labeled by both FITC and Rhodamin B. In the Fig. 4a, it could be found that the starch granules were dispersed evenly in the gluten network. Fig. 4b described the blank GTFN sheet, indicating gluten was diluted to some degree, and starch granules had

color(fresh)

overall acceptability

color(cooked)

flavor

viscoelasticity

DE*b           

0.284f 6.98  0.208g 0.676g 10.56  0.703h 0.138cde 3.98  0.600f 0.064cd 2.78  0.212e 0.012cd 3.02  0.324e 0.058cd 2.21  0.117d 0.173ef 2.96  0.238e 0.009c 1.40  0.484c 0.593f 10.95  0.636h 0.199def 4.29  0.256f 0.012cde 1.92  0.101cd

a Cont., control; MW, microwave; UVM, Ultraviolet microwave; CeSeM, citric acid 0.6 g/100 gesodium alginate 0.2 g/100 gemicrowave 50 s. b Least square means in the same column with same letters (c, d, e, f, g, h, i) are not different (P < 0.05).

chewiness Fig. 3. Sensory analyses of five different types of FN. GTFN, CC 0.6 g/100 g GTFN, MW 50 s GTFN,

Control FN, Blank CeSeM GTFN.

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Fig. 4. LSCM images of noodle sheet from (a) control FN, (b) blank GTFN, and (c) CeSeM GTFN.

more possibility to aggregate some particles with smaller size. These two pictures suggested the dilution effect of SGTP on the matrix of GTFN dough, similarly with the report of Wang, Zhou, Yu, and Chow (2006) for green tea extracts played as a tea catechins fortified agent in bread but with a reducing effect in bread volume. Fig. 4c demonstrated the CeSeM treatment on the microstructure of noodle sheet. In the Fig. 4c, it could be found that starch granules were not separated so clearly from gluten network just as shown in Fig. 4a. Instead, some starch granules seemed to be wrapped up by some kind of binding agent. And gluten network spread more homogeneously compared with blank GTFN displayed in Fig. 4b. Since using this stain package could not distinguish hydrocolloids which could be labeled by both dyes (Silva et al., 2013), and size ranges of starch granules (2e35 mm) and SGTP (average particle diameter 20  1.9 mm) overlap to a certain extent. Moreover, SGTP consisted with protein, dietary fiber, polysaccharides, polyphenols and so on (Lu et al., 2010). Thus, it could be inferred that SGTP could not be discerned in the images. On the other side, some starch and SGTP granules could be concluded to be entangled by sodium alginate, which corresponded to assumption in Section 3.2. This led to the formation of enhancement in the structure of GTFN. 4. Conclusions By premixing SGTP with two kinds of organic acids, hydrocolloids, or microwave treatments, the rate of discoloration of GTFN was significantly inhibited (P < 0.05) compared with the control. Furthermore, sensory characteristics of the creative processed FN

product which is comprised CA 0.6 g/100 g, SA 0.2 g/100 g and MW 800 W 50 s treatments (CeSeM), didn’t exhibit unfavorable items. On the contrary, this preferable procedure could result in slight improvement of the overall acceptability, while darkening rate of GTFN was considerably retarded. This could be proved by the inhibition of PPO activity of the final product in contrast to the control. In addition, images obtaining by CSLM revealed that sodium alginate could entangle SGTP and starch granules, and CeSeM combination treatment could not only protect SGTP from oxidation-related darkening, but also reinforce dough matrix. Therefore, the innovative combined process could be regarded as a promising path to retard darkening of GTFN. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No.31371849), and the National Key Technology R&D Programs (Grant No. 2012BAD36B06 and No. 2012BAD37B04). The authors are grateful to Hangzhou Tea Research Institute of All China Federation of Supply and Marketing Cooperatives for the contribution of superfine green tea powder. Also the authors wish to thank Chen Chen for assistance in confocal scanning laser microscopy analysis of noodle samples. References Altunkaya, A., & Gokmen, V. (2012). Partial purification and characterization of polyphenoloxidase from durum wheat (Triticum durum L.). Journal of Cereal Science, 55(3), 300e304.

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