Pasting, thermo, and Mixolab thermomechanical properties of potato starch–wheat gluten composite systems

Abstract This research investigated the viscosity, thermal, thermomechanical, and microstructural properties of potato starch–wheat gluten composite systems with different starch/gluten ratios under mechanical shear and heating conditions. Results showed that the peak, trough, and final viscosities increased with the increase in potato starch fraction. The breakdown and setback values of samples decreased with increasing gluten content, and the endothermic enthalpy showed a similar variation trend. The gelatinization temperature of the samples increased significantly as the gluten proportion increased. Morphological observation showed that the gluten protein was wrapped around potato starch granules and the starch granules have a diluting effect on gluten network. Moreover, gluten formed a water diffusion barrier out of the starch granules, this barrier effect and the competitive hydration between starch and gluten could primarily explain the delayed gelatinization temperatures.

. For instance, adding 20% potato flour to wheat flour will maintain the rheological properties and improve the nutritional value of the noodle and steamed bread (Liu, Mu, Sun, Zhang, & Chen, 2016;Xu, Hu, Dai, Hu, Dai, & liu, Q., Huang, Y., & Zhang, H., 2017;. Potato starch at a level of 5%-15% is often used to improve the textural properties of wheat-based alkaline instant noodles (Noda et al., 2006;Zaidul, Yamauchi, Matsuura-Endo, Takigawa, & Noda, 2008). Potato starch has high water-binding capacity and swelling power, which differ from those of other commercial starches such as corn, wheat, and rice starches (Zhang, Lim, & Chung, 2019). Compared with starches from other botanical sources, potato starch contains larger quantities of phosphate groups which can hydrate quickly under hydrothermal treatment, thus has a higher viscosity and forms a more clearly gel (Noda et al., 2006;Yusuph, Tester, Ansell, & Snape, 2003).
Although the effect of potato flour and potato starch on dough formation has been widely studied, most of the tested dough systems contain a variety of ingredients (i.e., wheat starch, potato starch, potato protein, and wheat gluten), making it difficult to know the effects of starch on gluten network formation. A common simplifying approach in investigations of the role of gluten and starch in dough systems is to use gluten-starch model dough samples (recombine the extracted starch and gluten). This allows precise control of the gluten content and also excludes complex effect of other ingredients in the dough systems (Jekle, Mühlberger, & Becker, 2016;Zhang, Mu, & Sun, 2018). Particularly, the interaction between wheat starch and wheat gluten was investigated by using starch-gluten model dough system. Research showed that gluten can form 3D matrices through protein-protein and protein-carbohydrate interactions when gluten is subjected to certain external conditions of mixing and heating; such interactions also largely affect the functional qualities of flour products (Bock & Damodaran, 2013;Singh & Singh, 2013). Starch granules in dough systems act as filler particles within gluten matrix. Previous works have also explored the interaction between starch and protein from other different perspectives (Espinosa-Dzib, Ramírez-Gilly, & Tecante, 2012;Homer, Kelly, & Day, 2014;Ravindra, Genovese, Foegeding, & Rao, 2004). Dutta reported that silk sericin and rice starch can be thermo-mechanically conjugated and casted into continuous 2D films through evident molecular interaction between starch and sericin (Dutta, Dutta, & Devi, 2018). Both starch, protein, and their interaction are responsible for the macroscopic properties of food matrix (Jekle et al., 2016).
However, as the two main ingredients, the function of potato starch and wheat gluten in potato-based staple foods still need to be studied. Understanding the physicochemical properties of potato starch-wheat gluten composite systems and their interactions under thermal and mechanical treatments would help researchers to achieve better designs of the formulation and processing conditions of food systems, in which these two biopolymers are the major ingredients.

| Raw materials
Shepody potatoes were provided by the Dingbian Science and Technology Bureau of Shaanxi Province, China. Potato starch was isolated from Shepody potatoes containing 4.24% moisture, 95.76% starch, and 1.95% ash. Wheat gluten was purchased from Tian Long Wheat Flour Co., Ltd. (Henan, China). The wheat gluten contained 7.22% moisture, 89.34% protein, 9.82% starch, and 0.82% ash. The production way of the wheat gluten was as follows: Wheat flour was mixed with water and then separated by pumping the flour paste into a series of hydrocyclone. The obtained flow was then washed and sieved to get the wet gluten. After that, a drying process was applied before packing.
Moisture, protein, and ash contents were determined using AOAC official methods (Association of Analytical Chemists, 2000), and starch content was determined using a total starch assay kit (Megazyme, K-TSTA 04/2009). All these measurements were performed in triplicate.

| Potato starch isolation
Potato starch was obtained using the method previously reported by Gani (Gani et al., 2014). Potatoes were washed, peeled, and then ground into paste by using a blender (KM005, Kenwood, England).
The resultant slurry was sieved through a muslin cloth to remove potato residues. The resultant starch suspension was left to stand overnight, and the solid residue was washed ten times with distilled water. The starch dispersion was sieved through a 150 μm mesh sieve, and the supernatant liquid was discarded. Finally, purified starch was freeze-dried for 36 hr in a pilot-scale vacuum freeze dryer (Genesis™ SQ, Virtis, America).
The thermal, microstructural, and thermomechanical properties of control samples and the potato starch-wheat gluten composite systems were studied.

| Paste viscosity
The viscosity properties of native potato starch, wheat gluten, and starch-gluten composite systems were analyzed using Rapid Viscosity Analyser (RVA, super 4, Newport Scientific, Australia).
The tests were conducted under fixed shear conditions with a completely controlled temperature cycle of heating, holding, and cooling.
This procedure ensures the generation of highly reproducible gelatinization and pasting profiles. Samples (2 g each) were suspended in 25 ml of water to prepare suspensions to be measured by the RVA.
The constant rotating speed (160 rpm) and fixed heating up and cooling down process were adopted for the measurement. The heating temperature was maintained at 50°C for 1 min, followed by a ramped-up temperature to 95°C at a rate of 12°C/min; the temperature was held at 95°C for another 2.5 min and then cooled down to 50°C at the same rate (12°C/min). Finally, the temperature was held again at 50°C for 2 min. The peak viscosity, trough viscosity, final viscosity, peak time, break down, setback, and pasting temperature were recorded during the heating and subsequent cooling process.

| Differential scanning calorimetry (DSC)
The thermal properties of potato starch, wheat gluten, and starchgluten composite systems were investigated using a differential scanning calorimeter (TA Q200, TA Instruments, New Castle, USA).
Samples (3 mg) were weighed into stainless steel pans, and 10 μl of distilled water was added before the pans were hermetically sealed.
The pans were equilibrated at 4°C for 12 hr before heating from 25 to 100°C at a rate of 5°C/min by DSC. The onset temperature (T 0 ), peak temperature (Tp), peak width at half height (ΔT), and endothermic enthalpy (ΔH) were calculated automatically. A sealed empty pan was used as reference.

| Thermomechanical measurements
Mixing and pasting properties of potato starch, wheat gluten, and their mixtures were studied using a Mixolab analyzer (Chopin Technologies, Villeneuve-la-Garenne, France). Samples were placed in a bowl with two kneading arms, and the amount of water added was determined by the water absorption capacity of the samples. The torque (Nm) was obtained as a function of time, thus allowing evaluation of the thermochemical properties of dough samples. The Chopin + standard protocol was used to determine the water absorption of flour by the dosage of water until the dough was able to reach the maximum torque of 1.1 ± 0.05 Nm (equivalent to 500 Farinograph units). The quality of protein network and the starch behavior of dough were analyzed using the standard procedure during heating and cooling. First, the mixture was held at 30°C for 8 min. Second, the temperature was increased F I G U R E 1 The RVA curves for potato starch (PS, a), wheat gluten (WG), and PS-WG composites (b) with different starch fractions to 90°C at a rate of 4°C/min and then maintained for another 8 min.
Afterward, the samples were dried and sprayed with gold. Potato starch granules and wheat gluten were used for observation without pretreatment. ESEM was used to observe and photograph the samples at 500× magnification.

| Statistical analysis
The data reported were averages of triplicate observations and expressed as mean ± standard deviation. ANOVA and Tukey's test at .05 significance level were conducted to evaluate the significant differences among sample means. ANOVA was performed using SPSS 18.0 (SPSS Inc., Chicago, USA).

| Viscosity properties
The pasting profiles for potato starch, wheat gluten, and their composite systems at different starch/gluten ratios were examined during heating and cooling periods by RVA. As shown in Figure 1, the pasting curves with higher content of potato starch were obviously higher than that of the samples with lower content of potato starch. The peak viscosity of potato starch-wheat gluten composite pastes decreased as the protein fraction increased.

| Thermal properties
The DSC thermograms of potato starch, wheat gluten, and their composites with different starch fractions were shown in Figure 2.
The phase transition peaks of all the samples ranged between 50 and 70°C, and such peaks were believed to be associated with starch gelatinization. No peak was observed of the wheat gluten sample within the tested temperature range. A thermodynamic transition occurred in the starch-gluten composite systems during the heating process. When water was added, the potato starch began to swell. Under heat treatment, the starch molecules began to acutely vibrate, and the intermolecular hydrogen bond was broken followed by the disappearance of the starch crystalline region.
In this process, the state of the starch molecules changed with the alteration of energy. With the increase of wheat gluten fraction in potato starch-wheat gluten composites, obvious decrease in the areas under the thermal transition peak above the extrapolation can be observed and the DSC curves tend to be flat with the addition of wheat gluten.
The gelatinization parameters of samples are listed in Table 2.
The gelatinization onset (T 0 ) and peak temperatures (

| Thermomechanical properties
The thermomechanical properties of samples during mixing, temperature rising, pasting, and subsequent cooling were shown in  curves. Such measurement process is equivalent to the determination of dough characteristics in the whole process of converting powder into products (Huang et al., 2010). Samples with different potato starch/wheat gluten ratios showed significant differences in their thermomechanical curves. Parameters obtained from the curves were shown in Table 3. Results showed that the water absorption capacity of the potato starch-wheat gluten composite dough samples increased obviously with increasing gluten content. This result confirmed that the gluten protein demonstrates a stronger water absorption capacity than starch. The samples containing larger amounts of gluten displayed higher gelatinization temperatures. This changing trend was also consistent with the previous result obtained by RVA. Gluten's water absorption capacity was stronger than that of starch; hence, the competitive hydration between polymers hindered the swelling and gelatinization processes of potato starch. However, the gelatinization temperatures of the samples obtained by Mixolab were not exactly the same with the temperature obtained by RVA. This discrepancy might be ascribed to the different magnitudes of the imposed deformation between the two methods (Kim et al., 2014).

| Morphological analysis
The pristine surfaces of the native potato starch granules and wheat gluten were observed through ESEM (Figure 4a and b). The images clearly show that the wheat gluten condensed into blocks, and almost no starch granule can be observed. The potato starch granules were characterized by an oval or elliptical shape with a smooth surface. Similar morphological features also observed in previous studies (Lovedeep, Narpinder, & Navdeep, 2002  In addition, the gluten was wrapped around the starch granules, thus forming a barrier for the hydration of the starch granules. The diffusion of water into the starch granules was hindered by the barrier effect; this may explain the delayed gelatinization temperature of starch-gluten composites as previously measured by RVA and DSC. The competitive hydration between wheat gluten and potato starch also plays an important role in the delayed gelatinization temperature because gluten demonstrates a stronger water absorption capacity than starch as previously measured by Mixolab. When additional potato starch granules were exposed, the barrier effect of gluten was decreased and gelatinization temperature was lower.

| CON CLUS IONS
The ratio of potato starch to wheat gluten has significant effect on

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 A PPROVA L
This study does not involve any human or animal testing.