Effect of potato flour on quality and staling properties of wheat–potato flour bread

Abstract To elucidate the impact of potato flour (PF) on quality changes and staling characteristics of the composite bread from wheat–potato flour (WPF), the physicochemical (specific volume, colority, sensory value, texture, and viscosity) properties, and staling (X‐ray diffraction and water migration) properties of bread were investigated. The quality of composite bread was comparable to wheat bread when addition level of PF at 20%, but decreased when the addition level increased to 30% or more, and became unacceptable at 50%. A chewy mouthfeel and an elastic and none‐crumbly texture were observed on composite bread, which had higher hardness than wheat bread, and could keep on both longer linear distance and higher linear force during compression test. It indicated that such new parameters other than hardness should be introduced to coordinate with the texture quality of composite bread. During storage, the higher addition level of PF significantly decreased crystallinity of composite bread and slowed water migration rate from the crumb to crust, suggesting that PF had antistaling effect on composite bread, which was further emphasized by the fact that the setback value of the WPF decreased with the increase of PF addition.

Bread staling results in loss of flavor and texture, and mainly leads to the increase of crumb firmness and loss of freshness, as well as severe waste (Fadda, Sanguinetti, Del Caro, Collar, & Piga, 2014). There are three main theories on the mechanism of bread aging: first, the transfer of the moisture in bread; second, the recrystallization of starch; and third, the interaction between starch and gluten in bread (Zhan, Ren, Min, & Liu, 2013). To restrict the bread staling caused by starch retrogradation, pregelatinized wheat, and maize starches could be used as antistaling additives (Hesso, Loisel, Chevallier, & Le-Bail, 2014). Moreover, bread with extruded potato starch was observed at a lower value of retrogradation rate (Gumul, Krystyjan, Buksa, Ziobro, & ZieBa, 2014).
The addition of potato flour to bakery products slowed down the staling rate as a fresh-keeping agent (Joshi et al., 2018). However, more information is needed to evidence the antistaling efficiency of potato flour and look inside the mechanism. Consequently, the objectives of the present study were to assess the impact of PF on the quality of the composite bread and to elucidate the staling mechanism explanation of composite bread.
Commercial potato flake (Shandong University of Technology) was milled using a universal high-speed smashing machine (FW-100D, Tianjin Xinbode Instrument Co., Ltd.) and passed a sieve of 200 mesh. Then, the potato flour (PF, 6.62% moisture, 59.26% starch, 9.16% protein, 0.29% ash, and 90.72% degree of gelatinization, which were determined under the method of Xiong (2000) for the degree of gelatinization, AACC Methods (2000) for moisture, protein, and ash content, AACC Method 76-11 (1999) for starch content) was packed in a resealable polyethylene bag for further study.
According to pre-elaborations, the amount of water for each formula was calculated by the sum of the 60% weight of WF and 120% of PF. A straight-dough method was performed with a five-speed dough mixer (SM-1688, Shepherd Wang Electrical Hardware Co., Ltd). Dry ingredients (WPF, instant dry yeast, and nonfat dry milk) were blended for 2 min at speed 1. Then, water and solution of sugar and salt were added to develop a dough by mixing for 1 min at speed 1, 1 min at speed 2, 1 min at speed 3 after the shortening was added, and 2 min at speed 5.
The resultant doughs were kneaded and rounded manually. After fermented for 90 min in a proofer Shanghai Jinghong Experimental Equipment Co.,Ltd.) at 30°C and RH of 85%, doughs were sheeted with the noodle pressing machine (300/100 type, Hubei WuRui Machinery Equipment Co., Ltd.) for three times in order to be bubble-expelled, and then rolled up manually and placed into baking tins (15 × 6 × 6.5 cm) for wakeup-proof for 20 min at 38℃ and RH of 85%. Bread billets were baked in the oven for 20 min at top temperature of 180℃ and bottom 190℃. After baking, the loaves were cooled for 1 hr at room temperature and stored in resealable polyethylene bags for further analysis. The bread preparation for each formula was performed in triplicate.

| Evaluation of the physical characteristics of bread
The specific volume After cooling for 2 hr, the weight and volume of bread were measured. Bread loaf volume was determined using the rapeseed displacement method of AACC International (2001). The specific volume was calculated by dividing the volume by the loaf weight (expressed as the loaf volume of 100 g of bread, i.e., cm 3 /g).

The color
The color of crust and crumb was measured using the colorimeter (X-rite color technology Co., Ltd., Ci7600, America) at an angle of 10° based on the CIE L*, a*, b* system. The C* value, which indicated the color saturation, and calculated by √ a* 2 + b* 2 was used to evaluated the colority as well. Crumb and crust color was determined at four different points on each piece of bread (Sun et al., 2019), and each measurement was performed in triplicate.

The mechanical test
The mechanical test was conducted by using a TA. XT Plus texture and linear force were defined as the abscissa and ordinate of the ending point respectively, whereas the chord modulus of elasticity was defined as the slope of the linear regression line (Figure 1).

| Sensory evaluation
The methodology of the test was reviewed and approved by Northwest A&F University and informed consent was obtained from each subject prior to their participation.
The 9-point hedonic scale (9 = like extremely, 5 = neither like, nor dislike, 1 = dislike extremely) was performed for the hedonic expression of data, which consist of panelists (three males and four females) who were experienced in descriptive sensory studies (Kim, Yun, & Jeong, 2015). The sensory attributes including color, cell uniformity, odor, taste, texture, and overall acceptability were defined by relevant literature (Jeddoua et al., 2017). For the sensory analysis, each bread sample was cut into slice (1 × 1 × 2 cm) and placed in a white plastic pan randomly encoded by three numbers.

| Observation of Water immigration during bread storage
Bread samples were packed into polyethylene resealable bags after cooling for 1 hr, and stored at 20 ± 2℃, RH of 35% for 1, 3, 5, and 7 days for further analysis. The bread being cooled for 1 hr after baking was used as the sample for 0 day.
The patterns of XRD were fitted using Origin 7.0. Crystalline peaks and amorphous areas were quantified with MDI Jade version 5.0 software. Each diffraction was performed in several times until the result was stable. The degree of crystallinity was described as total mass crystallinity (TC) which was calculated by formula 1 (Demirkesen, Campanella, Sumnu, Sahin, & Hamaker, 2014).
where Ic is the integrated intensity of the crystalline phase, and Ia is the integrated intensity of the amorphous phase.

| Statistical analysis
All the data were submitted to the one-way analysis of variance (ANOVA) and Turkey's test (p ≤ .05) which was used to describe means with 95% confidence intervals. Statistical analyses were performed using DPS software version 7.0.

| Effect of potato flour on the physical properties of breads
With the increasing of PF addition, the L* value of WPFB was increased, both a* and b* values were decreased (Table 1) process during PF production would affect the pigment content and darken the flour color (Martínez, Oliete, & Gómez, 2013).
With the increase of PF addition, the specific volume of bread decreased from 2.76 to 1.41 (Table 1). During leavening, the carbon dioxide could not be retained in the dough effectively due to PF addition, which diluted the gluten content and reduced the dough gas-holding capacity. Therefore, an inadequate expanded gluten network in dough resulted in a decreasing of bread specific volume (Pongjaruvat, Methacanon, Seetapan, Fuongfuchat, & Gamonpilas, 2014).

| Sensory analysis of composite breads
The spider plot (Figure 2) demonstrated the organoleptic quality analysis of bread samples. For overall acceptable, the scores of WPFB20 were comparable with WFB, achieving the highest score of 7.43, and all WPFB samples were acceptable since the scores were higher than 5, except WPFB50 with a score of 3.57.
WPFB20 obtained the highest score for all sensory characteristics except the color and cell uniformity. Similar dependence was studied by Kim et al. (2015), which illustrated that rice bread was fortified with 20% potato starch showed the highest scores for overall acceptability. But when the substitutional level increased to 30% or more, the quality of the WPFB decreased in color, texture, and taste, and became unacceptable at the addition level of 50%.   all WPFBs had a higher water content than WFB during 7 days storage. The starch in PF was in the gelatinized state, which had a high water-binding ability, so the PF could be used as a humectant to prevent moisture loss during bread aging (Joshi et al., 2018). On the other hand, high water content of bread could decrease hardening rate (He & Hoseney, 1990).

| Effect of potato flour on texture properties during bread storage
The hardness of the WPFB crumb ( Figure 4) increased significantly with PF addition (p < .05). It may due to the low specific volume of WPFB, which gave bread a firmer crumb, leading to the high bread hardness. Moisture change would accelerate the starch-gluten and starch-starch interaction thus hardening the bread crumb (Ozkoc, Sumnu, Sahin, & Turabi, 2009). Because of the stronger water absorption of PF, the formula of WPFB contained higher water content, which led to the full swelling and dissolving of starch, promoting the contact of the surface between starch and gluten, and thus increased the hardness of bread crumb (Gumul et al., 2014).
As mentioned in 3.6, during bread staling, the crystallinity of stale bread declined with PF addition, whereas the hardness increased. Moreover, during the experiment we have found that the texture of the fresh and stale bread crumb of WPFB were chewy and elastic, rather than crumbly compared to WFB.
Based on the interesting finding, we further analyzed the compression curve of the bread samples ( Figure 5). The linear distance, linear force, and chord modulus of elasticity, i.e. the slope of linear range curve were shown in Table 2. For both fresh and staled bread samples, which contained more PF had both longer linear distance and higher linear force at the same time. It was inferred that the addition of potato flour could prevent bread crumb from slagging, and give bread a chewy mouthfeel. Crystallinity grade analysis was used to characterize the aging degree of stored bread (Song & Tong, 2017). As shown in Figure 7, with storage time went on, the total mass crystallinity (TC) of all bread samples was significantly increased (p ≤ .05). The crystallization grade increased rapidly in the former 24 hr and then slowed down (Figure 7). It was mainly caused by the retrogradation of amylose occurred in a few minutes to the first few days of storaging at a faster rate, and the amylopectin was gradually regenerated at a slow rate in the late storage (Sullivan, Hughes, Cockman, & Small, 2017). The sample with the highest degree of crystallinity was found to be the WFB after stored for 7 days. In the same storage time, with the increase of the PF addition, the crystallinity decreased significantly, which strongly indicated that the addition of PF could inhibit the staling of bread. This was probably because the PF broke the gluten network in the system, hindered the process of water diffusion and transfer, as well as the interaction of starch and protein (Joshi et al., 2018). This point was strengthened by the decreasing of setback values mentioned in 3.6 section (Table 3).

| Pasting characteristics of flours
The indexes of viscosity evolving of WF, PF, and WPF during the programmed pasting process were shown in Table 3  in the mixture flours could prevent the formation of network within starch structure. Meanwhile, the nonstarch polysaccharides also compete to combine water, which obstructed the swelling of the starch, and thus reduced the gelatinized viscosity value (Guo, Li, & Zhang, 2015). The cooking process of PF could cause the starch to lose ordered structure and decrease paste viscosity (Martínez et al., 2013). The setback value of the WPF decreased with the increase of PF addition, indicating better stability of cold paste, slower retrogradation rate, and antistaling potential of PF (Yan et al., 2016).

| Correlation analysis
The hardness showed a significant linear correlation with the crumb it is reasonable to suppose that the gelatinized starch changed the structure of the gluten network. This was probably because the PF did not have gluten network and thus hindered the process of water diffusion and transfer, as well as the interaction of starch and gluten (Joshi et al., 2018), moreover, the water redistribution could affect F I G U R E 6 XRD patterns of bread crumb after stored for different times (0, 1, 3, 5, and 7 days). WFB: wheat flour bread; WPFB20, WPFB30, WPFB40, and WPFB50: wheat-potato flour bread with 20%, 30%, 40%, and 50% potato flour addition, respectively F I G U R E 7 Total mass crystallinity grade (TC) of different storage time bread. WFB: wheat flour bread; WPFB20, WPFB30, WPFB40, and WPFB50: wheat-potato flour bread with 20%, 30%, 40%, and 50% potato flour addition, respectively the localized amylopectin recrystallization kinetics (Besbes, Jury, Monteau, & Bail, 2014). Mehran, Behzad, Mostafa, and Saman (2018) found that there was a significant positive correlation between the moisture and homogeneity of the bread crumb. Therefore, it was preliminarily speculated that hardness is not necessarily an accurate judgment indicator in the process of bread staling. The relevant mechanism needs further experimental proof.

| CON CLUS IONS
This study evaluated the quality and staling properties of composite bread, which formulated with different levels (0%, 20%, 30%, 40%, and 50%) of potato flour. With the addition of PF, the specific volume of composite bread decreased and the crust color became lighter.
According to sensory evaluation, the composite bread was comparable to wheat bread when addition level of PF at 20%, but decreased when the addition level increased to 30% or more. From the comprehensive analysis of the sensory evaluation and texture tests, a chewy mouthfeel, an elastic and none-crumbly texture of composite bread were observed, which had higher hardness value than wheat bread, and could keep on both longer linear distance and higher linear force, indicating that such new parameters other than hardness should be introduced to coordinate with the texture quality of composite bread.
The results of setback value from pasting behavior of the flours, as well as water migration and XRD during bread storage, evidenced that the addition of potato flour has an efficiency of antistaling of the composite bread. These indicators of bread staling characteristics showed high correlation coefficients between each other. The crumb moisture was negatively correlated with crystallinity and hardness, while crystallinity was positively correlated with hardness.

ACK N OWLED G M ENTS
We would like to show our deepest gratitude to Dr. Hongjun Li of Shandong University of Technology, who has provided potato flakes used for experiments.

E TH I C A L R E V I E W
This study was approved by the Institutional Review Board of Northwest Agriculture and Forest University.

I N FO R M E D CO N S E NT
Written informed consent was obtained from all study participants. Note: WF: wheat flour; PF, potato flour; WPF20, WPF30, WPF40, and WPF50: premixed flours of WF and PF with potato flour substitutive level at 20%, 30%, 40%, and 50%, respectively. Different superscript letters at the same column indicate significantly different(p < .05).