Effects of Pleurotus eryngii (mushroom) powder and soluble polysaccharide addition on the rheological and microstructural properties of dough

Abstract Adding a certain proportion of Pleurotus eryngii can improve the nutritional value of wheat‐flour foods and enhance the utilization of this mushroom. In this research, partial wheat flour was substituted with P. eryngii powder (PEP) or soluble polysaccharide (SPPE) at different addition levels, and the effects of PEP and SPPE on the rheological and microstructural properties of dough were investigated. Farinographic assay results suggested that PEP significantly (p < 0.05) increased the water absorption of wheat flour but decreased the development time and stability of dough significantly (p < 0.05). Furthermore, it was capable of providing weaker extensographic characteristics and harder dough with the increasing of PEP addition levels. The dynamic oscillatory tests indicated that the PEP addition approximately increased the storage (G′) and loss (G″) moduli in the entire frequency range, while the tan δ roughly decreased with the increasing of PEP addition levels, which could be attributed to the low solubility and strong water‐trapping capacity of the dietary fiber in PEP. Due to the good water solubility and easy formation of hydrogen bonds, the addition of SPPE had inconsistent results with the PEP addition. The inner microstructure of dough showed that the continuity of gluten networks had been disrupted by PEP and SPPE addition and then resulted in a weaker extension and harder dough. This research could provide a foundation for the application of PEP in wheat‐flour foods, and PEP addition levels of 2.5%–5.0% are recommended.

anti-inflammatory, antihyperglycemia, hepatoprotective, and hypolipidemic effects (Chen et al., 2012;Jeong, Jeong, Gu, Islam, & Song, 2010;Ren, Wang, Guo, Yuan, & Yang, 2016;Sun et al., 2017;Yuan, Zhao, Rakariyatham, et al., 2017;Zhang et al., 2018). In recent years, many factories have been set up for the large-scale cultivation of P. eryngii. The annual output of P. eryngii in China has exceeded 1,000,000 tons, and it has become the second-largest mushroom variety under factory cultivation. However, there are few deep-processing technologies and products for P. eryngii at present in China, and the main consumption pattern is fresh eating.
As a result, the inconsistency between production and marketing is becoming increasingly acute with the fast growth of P. eryngii output. Therefore, developing deep-processing technologies and products is necessary for the healthy and sustainable development of the P. eryngii industry.
Gluten is the major protein source in wheat flour, which is extremely important to form the viscoelastic structure of dough (Mirsaeedghazi, Emam-Djomeh, & Mousavi, 2008). Rheological properties are closely associated with the viscoelasticity and mixing tolerance of dough, which also provide important reference for the processing properties of flour (Xu, Hu, Liu, Dai, & Zhang, 2017). The rheological properties of dough not only determine the processing characteristics during the manufacture of dough-based products but also affect the quality of final products (McCann, Le Gall, & Day, 2016). Exogenous additives, like proteins, polysaccharides, and fibers, can produce significant effects on the rheological behavior of doughs, which is likely due to the interactions between exogenous additives and wheat proteins, and further affect the integrity and stability of the gluten network (Mirsaeedghazi et al., 2008;Morris & Morris, 2012;Rubel, Pérez, Manrique, & Genovese, 2015).
Given its excellent nutritional value and bioactivity, adding P. eryngii into wheat-flour foods can improve the nutritional properties of products. In view of the huge consumption of wheat-flour foods, replacing part of the wheat flour with P. eryngii can also consume a large number of mushrooms and thereby resolve the inconsistency between the production and marketing of P. eryngii. P. eryngii contains polysaccharides, dietary fiber, protein, and other low-molecular-weight components, which would affect the rheological properties of dough and therefore influence the qualities of doughbased products. Studies have proved that mushroom powder can impact the rheological and structural properties of wheat dough and therefore the textural characteristics of products (Lu, Brennan, Serventi, Mason, & Brennan, 2016;Yuan, Zhao, Yang, McClements, & Hu, 2017). However, little information is available for the properties of wheat dough supplemented with single chemical component in mushrooms. Thus, the objective of this work was to investigate the effects of P. eryngii powder (PEP) and soluble polysaccharide on the rheological properties and microstructure of dough, so as to provide the basis for the processing of dough-based foods containing P. eryngii.

| Materials
Fresh P. eryngii was obtained from Xinxiang Kanghong Edible Mushrooms Co., Ltd. Commercial wheat flour with 12.0% moisture, 1.5% fat, and 9.0% protein was obtained from COFCO Co., Ltd. Salt was purchased from the local market. High temperature-resistant alpha-amylase solution (30 U/mg) and glycosylase solution (100 U/ mg) were purchased from Shanghai Ruji Bio-Technology Co., Ltd.

| Preparation of Pleurotus eryngii powder
After removal of the root, the sliced fresh P. eryngii were sun-cured outside and dehydrated to about 70%. Semidried P. eryngii were then dried in a 75°C draft drying cabinet for 5-6 hr. The dried mushrooms were milled using a laboratory-scale pulverizer (Beijing Zhongxingweiye Instrument Co., Ltd.) and then screened (>80 mesh).

| Preparation of soluble polysaccharide of Pleurotus eryngii
PEP was extracted with petroleum ether at 80°C for 2 hr to remove lipids and pigments. The residue was then extracted with hot distilled water (ratio of water to material of 20 ml/g) at 90°C for 1 hr and then centrifuged at 3,000 g for 15 min to remove the precipitate. This process was repeated twice, and the two extraction solutions were merged. Crude polysaccharide extraction solution was precipitated with 3-fold volume anhydrous ethanol (ethanol final concentration, 75%) and then kept at 4°C for 24 hr. After centrifugation at 3,000 g for 15 min, the precipitate was washed with anhydrous ethanol and acetone, respectively, and then lyophilized as crude soluble polysaccharide of P. eryngii (SPPE).

| Farinographic assays
The farinographic assay was carried out according to the AACCI approved method 54-21 (AACC, 2000) using a farinograph equipment (Brabender, Duisburg, Germany) with a mixing bowl for 300 g flour.
The farinographic parameters measured were water absorption, development time, and stability time.

| Dynamic rheological assays
Dough for dynamic rheological assays was kneaded for 5 min using a 50 g farinograph (Brabender), and water absorption was optimized to make the consistency at the end of mixing centered in the range of 480-520 B.U. The dough was then covered with a plastic film immediately to avoid water losses, and cylindrical pieces of 3 cm diameter and 2 mm height were obtained from dough. Dynamic oscillatory tests were carried out in a Haake MARS III controlled stress oscillatory rheometer (Haake) at 25 ± 0.1°C using a 1 mm gap plate-plate sensor system. Test parameters referenced the method of Correa, Añón, Pérez, and Ferrero (2010). Storage modulus (G′), loss modulus (G″), and tan δ (G″/G′) were obtained as a function of frequency.

| Scanning electron microscope observation of dough
Microstructures of doughs prepared with different levels of PEP and SPPE were performed using a Quanta 200 environmental scanning electron microscope (SEM; FEI, Hitachi). Dough samples were cut into 5 mm height and 3 cm diameter cylindrical pieces, which were dried by vacuum freeze drying. The dried dough slices with natural fracture surfaces were fixed by conducting resin and coated with gold and then observed at a voltage of 20 kV and high vacuum condition.

| Statistical analysis
All experiments were performed at least in duplicate. Statistical analysis was performed by SPSS version 17.0 software for Windows

| The chemical composition of PEP
The moisture content of fresh P. eryngii was 90.26%. After drying, milling, and sieving, the mushroom powder (>80 mesh) was obtained.
The chemical composition and mineral element contents of PEP are shown in Table 1. The results showed that the primary components of PEP were polysaccharides, dietary fiber, and protein. The potassium content of PEP was high, and the ratio of potassium to sodium was 143. A diet high in potassium and low in sodium is conducive to preventing hypertension (Oliver, Cohen, & Neel, 1975) and cardiovascular disease (Chang et al., 2006). The zinc content of PEP was high, which is helpful for human growth (Ploysangam, Falciglia, & Brehm, 1997).
The composition of amino acids in PEP is shown in Table 2. PEP contained 16.59 g amino acids per 100 g proteins, and the E/T value (ratio of essential amino acids to total amino acids) was 39%.
Polysaccharides are the major bioactive component in P. eryngii, and many studies have demonstrated that they have many biological activities, such as antioxidant (Sun et al., 2017), anti-inflammatory (Yuan, Zhao, Rakariyatham, et al., 2017), immunomodulatory (Jeong et al., 2010), and antitumor (Ren et al., 2016) activities. The analytical results suggested that adding a moderate amount of PEP could improve the deficiencies of wheat-flour foods in nutrients such as dietary fiber, lysine, and potassium.

| Farinographic behavior of composite flours
We observed that the water absorption of composite flour supplemented with 2.5% PEP increased significantly (p < 0.05) compared to wheat flour, and the increase tended to be subtle until the 10% addition of PEP, finally reaching significance (p < 0.05) at 12.5% and 15.0% addition levels ( Figure 1a)

| Extensographic behavior of composite dough
The extensographic properties of dough samples containing PEP are shown in Figure 2. After 45 min of fermentation, the area (energy) of the dough decreased significantly (p < 0.05) at a 2.5% PEP level, increased more subtly at a 5.0% level, and increased nonsignificantly thereafter. The Ex of the dough with 2.5% and 5.0% PEP supplementation were both higher than that of the control group, and these properties decreased when the level of PEP was increased to 15.0%, which significantly (ρ = −0.978, p < 0.01) negatively correlated with the PEP addition levels ( This suggests that the addition of PEP weakened the extensographic properties of dough, overall. A possible reason is that partial wheat flour was substituted by PEP, which relatively decreased the wheat gluten content (dilution effect). It is well known that proteins in PEP cannot participate in the dough formation, which disrupts the welldefined protein-starch complex in wheat-flour dough, resulting in a weakening of dough (Sun, Zhang, Hu, Xing, & Zhuo, 2015).

| Viscoelastic properties of composite dough
The viscoelastic properties of dough with PEP addition are presented in Figure 3. The results showed that G′ and G″ of all doughs increased with the increase of frequency (Figure 3a,b). All dough samples displayed higher G′ than G″ in the range of frequencies used in this study, indicating that all doughs exhibited more elastic behavior compared to viscous behavior with or without PEP addition (Shi, Wang, Li, & Adhikari, 2013). Meanwhile, G′ and G″ of dough first decreased at the 2.5% PEP level and then increased with the increase of the PEP addition level (Figure 3a,b), which both significantly (ρ = 0.849, p < 0.05; ρ = 0.866, p < 0.05) positively correlated with the PEP addition levels ( Table 3). The tan δ values of dough at 2.5%, 5.0%, and 7.5% PEP levels were higher than those in the con-  (Table 3).
Polysaccharides are one of the primary components and the most important bioactive substances of P. eryngii. Therefore, watersoluble polysaccharides of P. eryngii were prepared to investigate its effect on the viscoelastic properties of dough. SPPE addition levels were designed according to PEP content. The viscoelasticity of dough with SPPE addition is presented in Figure 4. The results showed that G′ and G″ of doughs first decreased at the 1.0% SPPE level and then increased with increasing SPPE level (Figure 4a,b), while tan δ of doughs first increased at the 1.0% and 2.0% SPPE levels and then increased as the SPPE level increased (Figure 4c).

| Microstructure of composite dough
The microstructures of the doughs with different addition levels of

| CON CLUS ION
This study demonstrated that different addition levels of PEP and SPPE affected the rheological properties and microstructures of dough systems. Farinographic assay results suggested that PEP could increase the water absorption and decrease the development time and stability of dough, while extensographic assays indicated that it was capable of providing weak extensographic characteristics and harder dough. The dynamic oscillatory tests revealed that the PEP addition roughly increased G′ and G″ but decreased tan δ in the entire frequency range, while the addition of SPPE had results inconsistent with those of PEP addition. The microstructure of dough samples was observed by SEM, which showed that the PEP and SPPE addition both decreased the integrity and continuity of gluten network in the dough. Overall, adding PEP produced a significant effect on the quality of dough, and relatively low addition levels of 2.5%-5.0% are recommended. This research could provide a foundation for the application of PEP in wheat-flour foods. Further research will be conducted to investigate the influence of PEP addition on dough-based foods like steamed bread, noodles, and biscuits, and products with high nutritional and edible qualities will be developed.

This work was supported by Innovation Scientists and Technicians
Troop Construction Projects of Henan Province (2017JR0006).

CO N FLI C T O F I NTE R E S T
The authors declare that they do not have any conflict of interest.

E TH I C A L S TATEM ENT
This study does not involve any human or animal testing.