Introduction
Noodle has been consumed as a traditional staple food, in a number of Asian countries since ancient times, and receives increasingly popularity globally for its convenience, nutritional quality, and palatability (Li et al., 2014). The noodle-related industry has become the second-largest worldwide, followed by the bread industry (Heo et al., 2013). Notably, noodle processing includes mixing, resting, sheeting, cutting and drying. These processing parameters are known to influence the development of dough by affecting the interaction with water, starch, and protein in the dough, thereby impacting the noodle-making and eating qualities (Fu, 2008). In China, the processing of traditional noodles is primarily predicated on experience. However, the development of the modern food industry increasingly diversifies the global food market, and consumers are paying close attention to the standard of food concerning the food they eat, which includes, but is not restricted to appearance and taste. Therefore, to improve the noodle quality, it is necessary for noodle processing to reduce its reliance on experience and attach more importance to science. Currently, research focusing on the influence of processing parameters on noodle processing and product quality is on the increase. In this context, mixing (Li et al., 2014; Liu et al., 2015) and drying (Wang et al., 2019; Yu, Wang, Zhang, Wadood, & Wei, 2018) processes have been widely investigated, but there is still limited information available regarding the effects of resting parameters on the processing quality of noodles.
After mixing process, dough resting is usually one of the key steps in noodle production and could have a significant impact on both noodle quality and the performance in processing. This step allows the crumbly mixture to rest for a period of time for further accelerating the hydration of flour particles and redistributing water within the dough system. In addition, during the sheeting process, dough resting can also improve processing properties and facilitate the formation of gluten by relaxing the gluten structure formed during the mixing process (Fu, 2008). So far, the majority of work has focused on the effect of resting time on the properties of dough rheology (Cuq, Yildiz, & Kokini, 2002; Kim et al., 2008; Sharma, Hanna, & Marx, 1993). In addition, some studies have been conducted on the effects of resting time on the fraction of gluten protein. As observed by Weegels, Hamer, and Schofield (1997), dough resting could end up increasing the proportion of high-molecular glutenin subunits in the glutenin macropolymer (GMP). Similarly, Aussenac, Carceller, and Kleiber (2001) demonstrated that GMP would be repolymerized after dough resting, as a result of which the GMP content in the dough increased. However, it is still unclear how the resting time could affect the structural change occurring to the gluten network in noodle dough during the sheeting process.
In terms of wheat dough, the protein network plays an important role in dough properties, such as consistency, cohesion, and rheology (Beck, Jekle, & Becker, 2012; He & Hoseney, 1991; Verheyen et al., 2015). Through the hydration of flour during mixing, the three-dimensional gluten network can be developed by the cross-linking of protein polymers, particularly through the formation of disulfide bonds, to obtain a continuous and viscoelastic phase (Bache & Donald, 1998; Singh & MacRitchie, 2001). A precise quantification of network attributes via image analysis is crucial to examining the changes occurring to the gluten network caused by ingredients, additives, or process variations. Currently, ImageJ and AgioTool software have been applied to process the image. The gluten protein area, average gluten width, average gluten length, branching rate, end-point rate and other parameters have been used to identify the differences of gluten network structure (Peighambardoust, Van der Goot, Van Vliet, Hamer, & Boom, 2006; Jekle & Becker, 2011; Li, Liu, Wu, & Zhang, 2017). During the noodle sheeting process, the gluten matrix in the machine-sheeted dough is aligned with the direction of sheeting, which makes the processing quality of dough sheet along the sheeting direction (longitudinal direction) evidently different from that perpendicular to the sheeting direction (transverse direction), as shown in Fig. 1. Comparatively, gluten network structure can develop in all directions in the hand-made noodle dough, with the superior texture of hand-made noodles largely dependent on the degree and mode of gluten formation (Fu, 2008). Therefore, it is essential to distinguish and quantitatively analyze the microstructure of the gluten network along the sheeting direction and perpendicular to the sheeting direction in the microstructure for improving the eating quality of machine-sheeted noodles. Up to now, however, there are still no reports on carrying out quantitative analysis of the structure of dough gluten network in longitudinal and transverse directions.
The objective of this paper was to improve our insights into the behaviors of protein polymerization and gluten formation in different sheeting directions within the noodle dough tending to be affected by resting during noodle processing. Low-field nuclear magnetic resonance (LF-NMR) and GMP content were applied to gain a better understanding of the enhanced gluten network after various resting times at the microscopic level, while the texture analysis of dough sheet was conducted to verify the development of gluten network in different sheeting directions at the macroscopic level.
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Materials
Wheat flour was purchased from Guangdong Jinhe Flour Co., Ltd., China, and stored at 25 °C. Moisture and protein content were determined by the AACC method (AACC, 2000) 44-15A and AACC 46-11A, and found to be 13.50% and 12.23%, respectively.
Preparation of dough sheet
Wheat flour (500 g total) was mixed with water by a vacuum mixer (SK-500, Ziranai Food Co., Ltd., Jiangsu, China) at −0.06 MPa for 10 min. While processing, 10 g of salt (containing 9 g NaCl and 1 g KCl) was dissolved in 165.38 mL of water, for the hydration
Water distribution in dough crumbs
Water is an essential ingredient for noodle processing. The status and distribution of water, as well as its interaction with other components are capable of influencing the physiochemical properties of noodle dough during processing (Li, Dickinson, & Chinachoti, 1996). The water redistributed in dough during resting process and LF-NMR was then used to determine the water distribution of dough crumbs after resting for 0, 30, 60, and 90 min, respectively, the results of which are presented in
Conclusions
Dough resting was observed to impart significant macroscopic and structural changes to the gluten protein in the noodle dough. It caused gluten protein to be combined with water tightly in dough crumbs, and the GMP content in the dough sheet increased significantly, thus leading the gluten network distributed more uniformly and densely in the dough sheet during the sheeting process. As revealed by the quantitative analysis conducted of the gluten network in the transverse and longitudinal
Conflicts of interest
The authors have no conflict of interest regarding the content of this paper.
CRediT authorship contribution statement
Shuyi Liu: Conceptualization, Methodology, Investigation, Writing - original draft. Quan Liu: Methodology, Investigation, Software. Xue Li: Methodology, Investigation. Mohammed Obadi: Writing - review & editing. Song Jiang: Methodology, Supervision, Writing - review & editing. Shiyan Li: Conceptualization, Methodology, Supervision, Writing - review & editing. Bin Xu: Conceptualization, Methodology, Supervision, Writing - review & editing, Funding acquisition.
Acknowledgements
The research was funded by the National Science Foundation of China (Grant No.
31771900
), and the National Key R&D Program of China (Grant No.
2018YFD0401004
).
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The noodle dough with a resting time more than 30 min had a higher resistance to extension compared to that with the resting time less than 30 min (Liu et al., 2021). This has been interpreted as being due to that a longer resting time contributes to the increase in glutenin macropolymers and the formation of gluten network with a more uniform structure (Liu et al., 2021). Due to the effects of longer resting time, the G* and δ of over-mixed doughs increased, and that of optimally-mixed doughs did not change significantly (Kim et al., 2008).
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This might be because that the protein was relaxed during the resting process, which led to slight gluten aggregation caused by weak molecular force (Lambrecht, Rombouts, Nivelle, & Delcour, 2017). However, Liu et al. (2021) reported that no significant differences were observed in the gluten network structure of dough sheeting between resting for 30, 60, and 90 min, this may be because the gluten network was further promoted by the stress during sheeting process. To further study the changes in gluten network structure upon different mixing and resting times of noodle dough, CLSM images were fitted by Angio Tool software.
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