Effect of characteristics of different wheat flours on the quality of fermented hollow noodles

Abstract Hollow noodles, also known as Kongxin noodles in China, are traditionally hand‐made noodles produced by spontaneous fermentation. It is easy to cook, nutrient‐rich, and delicious. However, it is difficult to realize industrial production by spontaneous fermentation due to its complexity. More recently, new techniques have emerged for producing such noodles industrially using commercial yeasts. However, there are no reports on how to choose the raw materials for making fermented hollow noodles. Therefore, the suitability of eleven local varieties of wheat flour was determined by evaluating their physicochemical, rheological properties, and pasting properties. Flour and dough properties of wheat flour were also correlated with the quality characteristics of hollow noodles. The correlation coefficient data indicated that the color score was negatively correlated with ash content and positively correlated with starch content. Different from ordinary dried noodles, a negative correlation was observed between cooking time (CT) and protein content. Water absorption (NWA) of hollow noodles was negatively affected by extensograph properties. Water absorption of flour (FWA) and extensibility (E) were found to be highly correlated to hollow rate (Hol‐R), indicating that these two indexes could predict the fermentation status of hollow noodles. Results showed that wheat flours with higher swelling index of glutenin (SIG), FWA, E, and pasting temperature (PT) had better dough fermentation power and stability and thus were beneficial to the production of high‐quality hollow noodles. This study provides a simple method for the industrial production of hollow noodles and provides a basis for the selection of raw materials for their production.

Recently, studies were mainly focused on using non-conventional flours (e.g., legumes and pseudo-cereals) to improve the peculiar flavor, texture properties, and nutritional value of fermented noodles or pasta Lorusso et al., 2017;Marengo et al., 2015).
Additionally, sourdough fermentation is one of the oldest biotechnologies widely employed in bread and steamed bread production, as it decreases glycemic index, phytate content, and trypsin inhibitors (Coda et al., 2010(Coda et al., , 2011Moroni et al., 2012). To enhance nutritional and functional properties of pasta, researchers argued that sourdough-fermented ingredients are potentially applicable in pasta making (Montemurro et al., 2019). Specifically, pasta prepared by liquid sourdough decreased the slowly digestible starch content while increased the contents of inaccessible digestible starch and retrograded starch in noodles (Fois et al., 2017). In general, fermentation as an efficient and innovative tool can meet the requirements of modern consumers for nutritionally balanced and functional foods.
As a fermented noodle product, hollow noodles have been popular in China for hundreds of years.
Hollow noodles with smooth surfaces and small holes like silver wire inside are traditional and unique Chinese noodles. The hollow noodles are convenient, easy to cook, nutritious, and delicious, thus deeply loved by children and the elderly. At present, hollow noodles are mainly hand-made and spontaneously fermented based on previous experience by a complex microbiome dominated by yeast population. After repeated fermentation and stretching (necessary steps for making hollow noodles), a large number of microholes appear, which confers the hollow noodles with pleasant fermented flavor and good rehydration. Nevertheless, the manufacturing procedures of hand-made hollow noodles are various, complicated (up to 20 steps), and time-consuming (up to 70 hr). The fermentation process is also vulnerable to the weather. Therefore, the production of hand-made hollow noodles is limited and it is difficult to carry out large-scale industrialization.
More recently, new techniques have emerged for producing hollow noodles industrially using commercial yeasts. Yihai Kelly Group (China) has launched an industrial fermentation type hollow noodles which have been applied for a patent (Qiu et al., 2019). When the dough is mixing, yeast is added. After cutting, noodles are fermented in the pre-drying process. The key technology includes two fermentation stages (the first stage: humidity is 85%-95%, temperature is 20-35℃, and fermentation time is 0.5-1.5 hr; the second stage: humidity is 80%-85%, temperature is 33-40℃, and fermentation time is 1.0-2.0 hr). However, to date, the research on the hollow noodles has not been reported in detail. Therefore, raw materials, formulation, technology, and its quality formation mechanism need to be further investigated.
Wheat flour plays a dominant role in the quality of noodles.
Much research had been established to study the effects of flour and dough properties on the quality of noodles. For example, firmness of the noodles was positively correlated with wet gluten, dry gluten, and protein content (r = .676, .640, and .682, p < .05, respectively) . Presence of higher protein content might also result in lower swelling of starch granules that prevented the excessive leaching of amylose during cooking (Sissons & Batey, 2003). High peak viscosity and breakdown had positive effects on the appearance, texture (stickiness and elasticity), smoothness, and taste of dry white Chinese noodles (Liu et al., 2003). However, the production process of fermented noodles is unique, which is mainly reflected in the fact that yeast is required to produce gas and porous structure in noodles. In order to retain gas in the noodles, wheat flour with higher strength of gluten may be needed. On the other hand, different from the well-developed bread dough with high moisture content (40%-55%), an underdeveloped crumbly dough and dough sheet with low moisture content (28%-38%) were, respectively, formed in the mixing and sheeting processes of noodle making. And yeast growth, gas production, and gas-holding capacity in noodle dough may be different from that of bread dough. Therefore, the quality index of wheat flour for fermented noodles is expected to be different from that of other fermented products such as bread and steam bread. However, so far, there is a lack of research on the quality requirements of special raw materials for the new type of industrial fermented noodles-hollow noodles. It is necessary to find out which quality indexes of wheat flour can predict the quality of hollow noodles well.
In this work, the characteristics of 11 brands of wheat flour used for hollow noodles making were evaluated by physicochemical, rheological, and paste properties; then, a simple and industrializable production process is adopted for hollow noodles making and the correlation between characteristics of wheat flour and quality of hollow noodles were investigated. The objective was to provide a theoretical basis for the establishment of the raw material quality standard for making hollow noodles. All other reagents used in this experiment were of analytical grade.

| Physicochemical properties of wheat flour
The analyses were performed according to the method of AACC (2000). The flours were analyzed for moisture , protein (AACC 46-15), ash (AACC 08-01), wet gluten and its index (AACC 38-12), and Zeleny sedimentation value (AACC 56-61). The swelling index of glutenin in SDS solvent was carried out according to the previous study (Wang & Kovacs, 2002a). The color of different types of wheat flours, determined by L * , a * , and b * values, was performed in terms of the method of Kaur et al. (2015). Each measurement was repeated at least three times.

| Pasting properties of wheat flour
Pasting properties of different wheat flours were evaluated by a Rapid Visco-Analyzer (RVA-4, Newport Scientific Instruments).
Flour (3 g, 14% moisture basis) and 25 ml distilled water were added to an aluminum RVA sample canister and then run a heating and cooling cycle program: a heating step, from 50 to 95℃ at 12℃/min (after an equilibration time of 1 min at 50℃), a holding phase at 95℃ for 5 min, a cooling step from 95 to 50℃ at 12℃/min, and a holding phase at 50℃ for 2 min.

| Preparation of hollow noodles
The preparation process of hollow noodles in this study was not as complicated as traditional method and was more suitable for industrial production. The ingredients used in the preparation of the hollow noodles were 200 g wheat flour, 60 g distilled water, and 2 g yeasts. First, the yeasts and distilled water were mixed in a beaker and then preserved heat at 38℃ in a water bath for 10 min.
Dough was formed using a mixer (JHMZ 200,East Fude Technology Development Center,Beijing) which was operated at low speed for 3 min and medium speed for next 4 min. After mixing, the prepared dough was placed in a plastic bag and rest for 15 min in a thermostated container at 25℃. The noodle dough was then passed through a laboratory JMTD 168/140 noodle machine (East Fude Technology Development Center, Beijing, China) at the gap of 3 mm for 4 passes.
Each pass included folding the dough sheet to double the thickness.
After being doubled folded, the roll gap was adjusted to 2.0 mm for another pass. Following that, five more passes were made to reduce the gap progressively to 1 mm. Finally, the obtained sheet was cut into rectangles of dimensions of 25 cm ×0.2 cm ×0.1 cm. The resultant noodle strands were hung on rods and placed in an incubator at 38℃ and 85% RH for 2 hr (a fermentation step), then reduced relative humidity, and dried in incubator in two stages. In the first stage, the temperature was kept at 38℃ and relative humidity was at 65% for 10 hr. In the second stage, the temperature was kept at 25℃ for 4 hr and the relative humidity was at 65%. Finally, the moisture content of dried hollow noodles was at 12%-13%. The dried hollow noodles were cut into the length of 10 cm and stored in sealed plastic bags for the next studies.
Hollow noodles were prepared on two different days (2 independent trials), and two bags of hollow noodles were prepared for each flour type.

| Hollow rate of hollow noodles
The fermentation state of hollow noodles was determined by hollow rate, and not all noodles were fermented into hollow ones.
Satisfactory fermentation states were generally along with high hollow rates. Fifty noodles were randomly selected from each type of wheat flour and the number of hollow noodles (L) was counted. The hollow rate of hollow noodles was expressed as: % hollow rate of hollow noodles =L/50 × 100.

| Sensory evaluation of hollow noodles
Ten trained food engineering students participated in the sensory evaluation of the hollow noodles, including color, firmness, viscoelasticity, smoothness, and flavor. A 9-point scale (1 = dislike extremely and 9 = like extremely) (ISO 11136:2014) was used for this evaluation. During sensory evaluation, each sample was placed in the same plate coded with a digit number, and drinking water was provided to clean mouths during tasting.

| Statistical analysis
The data reported in all tables are averages of at least three replications. Pearson correlation (r) between different properties and analysis of variance (ANOVA) between different wheat flours by Duncan's test was conducted using software 19.0 (SPSS Inc.). Significance was defined at p < .05.

| Physicochemical properties
Wheat flours showed a variation in the quality characteristics (Table 1). The moisture content was observed to be 12.36%-14.79%, and the ash content was 0.35%-0.60%. Ash content is related to the degree of flour refinement, but must be cautious to use it as a flour refinement indicator, because the ash content is different among wheat varieties (Symons & Dexter, 1991). Wheat starch, occupied about 70% of wheat flour, is contributed to the quality of noodle products (Guo et al., 2003). Much research had also shown that the components of flour (protein, starch, fat, ash, etc.) are inextricably linked with the quality of noodles, especially the protein and starch have the greatest impact on the quality of noodles (Lindsay, 2012;Liu et al., 2003). Starch content ranged from 76.90% to 83.03%, and there was no significant difference among the samples (p > .05).
The protein content, wet gluten content (WG), and gluten index (GI) were in the range from 8.35% to 12.48%, 21.89% to 33.13%, and 52.38% to 95.26%, respectively. GI indicates the information about quantity and quality of wet gluten, which is strongly influenced by environmental factors (ŠIMIĆ et al., 2006). The Sany flour (12.48%) displayed the highest protein content while Fengzheng HG (8.35%) showed the lowest, but Fengzheng HG had the highest GI (95.26).
This result indicated that GI did not depend on protein content.

Zeleny sedimentation value (ZeSV) varied from 51.75 ml in Xuejian
WC to 38.25 ml in Fengzheng HG. The values of SIG ranged from 367.04 to 452.57. SIG can be used to predict insoluble glutenin content and wheat gluten strength (Wang & Kovacs, 2002). L * , a * , and  Abbreviations: GI, gluten index; SIG, swelling index of glutenin; WG, wet gluten; ZeSV, zeleny sedimentation value.

| Rheological properties
Rheological index of wheat dough is an important basis for predicting the quality of final wheat-based products. Farinographic property reflects the dynamic changes of the consistency of dough during stirring (Mis et al., 2012). The FWA ranged between 59.8% and 65.1% (Table 2). The variation in FWA might be attributed not only to the type of protein but also to the amount of starch (Sarker et al., 2008). Extensographic properties reflect the resistance to stretching of dough. As shown in

| Pasting properties
Pasting properties, analyzed by a rapid visco analyzer (RVA), indicate the swelling and crystal collapse of starch granules during heating and cooling process in a system (Hormdok & Noomhorm, 2007). As shown in Table 3

| Quality of hollow noodles made by different wheat flour
The cooking properties include the optimum CT, NWA, and cooking loss (CL). Overcooked noodles are undesirable for too soft and sticky taste. In contrast, a raw flour taste and hard texture are found for undercooked noodles during biting and, hence, cooking time is a key indicator for texture quality of noodles (Park & Baik, 2004). As shown in   Abbreviations: AD, adhesiveness; CL, cooking loss; CO, cohesiveness; CT, cooking time; HD, hardness; Hol-R, hollow rate; NWA, noodle water absorption; SP, springiness; Ten-D, tensile distance; Ten-F, tensile force.
The instrumental texture test of cooked noodles, as a valuable research tool, is suitable for the quality monitoring of noodles (Ross, 2006). As shown in table 4, textural properties were influenced significantly by flour brands (p < .05). The hardness (HD), adhesiveness (AD), springiness (SP), and cohesiveness (CO) varied from 4,231.15 to 5,204.14 g, 91.67 to 191.64 g, 0.84 to 0.93, and 0.68 to 0.73, respectively. Hollow noodles made from Xiangxue HG flour exhibited excellent textural properties as evident from the highest HD, AD, and SP values and higher CO value, while Sany flour had lower AD, the lowest SP, and lower CO values. A high degree of gluten network formation would completely encircled starch granules and formed a compact internal structure of noodle strands, which largely contribute to these results (Park & Baik, 2004).
Moreover, tensile test assesses the ability of noodles to resist breaking, including tensile force (Ten-F) and tensile distance (Ten-D). The results of Ten-F and Ten-D ranged from 10.66 to 14.77 g and 56.30 to 82.78 mm, respectively (Table 4). Hollow noodles from Sany flour required the maximum force to break and that from Wudeli flour showed the far most tensile distance, while Jinyuan flour exhibited the minimum Ten-F and Ten-D.
The Hol-R of hollow noodles, determined by the proportion of noodles with hollows in total noodles, represents the degree of fer- was no significant differences among these samples (p > .05), the scores of firmness, viscoelasticity, and smoothness varied from 6.45 to 7.50, 7.00 to 8.00, and 5.10 to 6.00, respectively. The pleasant degree and uniqueness of the fermented flavor of hollow noodles were determined by the flavor scores. According to table 4, the hollow noodles from Xuejian MP and Xiangxue HG flours had higher flavor scores, which might be attributed to higher Hol-R of these noodles.

| Relationship between wheat flour characteristics and hollow noodle quality
The correlation coefficients between physicochemical properties of different wheat flour and the quality of hollow noodles are presented in Table 5. Among the physicochemical characteristics, the moisture content of flour was negatively correlated to the Ten-F of cooked hollow noodles (r = −.632, p < .05). The ash content of flour showed a negative correlation to the color score of cooked hollow noodles (r = −.617, p < .05), indicating that the flour with higher ash content could darken the brightness and color of noodles. A similar relationship between ash content and noodle color had been reported (He et al., 2004). Although p < .05, starch content was positively correlated to color score (r = .632). Brighter or whiter appearance might be attributed to more light reflected from more starch granules. A slight negative correlation was observed between protein content and CT (r = −.556, p > .05), which were not in agreement with the results reported by Kaur et al. (2015Kaur et al. ( , 2016. It has been reported that more proteins in flour might inhibit swelling of starch granules, thus prolonging the cooking time and reducing the cooking loss of noodles (Grzybowski & Donnelly, 1979).

| CON CLUS IONS
In this study, based on a set of commercial samples, the quality of hollow noodles was associated with physicochemical characteristics, rheological properties including farinograph and extensograph and pasting properties of wheat flours. The CT of hollow noodles was negatively related to protein content while positively related to pasting properties of flour. Extensographic properties had a considerable effect on NWA. The FWA and E were found to be highly correlated to the Hol-R. A significant positive correlation was observed between SIG and viscoelasticity, PT, and SP. Furthermore, farinographic properties substantially affected the firmness of sensory evaluation. Thus, flour with higher SIG, FWA, E, and PT resulted in hollow noodles with better fermented flavor and better textural quality. So far, there are few studies on the hollow noodles, including formula, quality changes during storage, quality formation mechanism, and further investigations need to be conducted.

ACK N OWLED G M ENTS
This work was supported by the National Natural Science

E TH I C A L A PPROVA L
This study does not involve any human or animal testing. The authors have declared no conflicts of interest for this article.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available.