Preparation and quality evaluation of potato steamed bread with wheat gluten

Abstract The present study aimed to study the preparation and quality evaluation of potato steamed bread by using potato flour, wheat flour, and gluten at the presence of yeast and inorganic additives. As the rheological properties of the potato–wheat formulated flour negatively related to the potato flour, the potato–wheat formulated flour with 35% potato flour was set as the basic flour (100%). The effects of wheat gluten on the rheological properties of the dough were also evaluated, and gluten addition amount was set at 6.5%. The effects of yeast, sodium bicarbonate, citric acid, and monocalcium phosphate addition on steamed bread properties have been studied and optimized by orthogonal test. The obtained potato steamed bread formula was 100% basic flour (potato/wheat mass ratio of 35:65), 6.5% wheat gluten, 1.1% yeast, 1.4% NaHCO3, 0.75% citric acid, and 0.50% Ca(H2PO4)2. The prepared potato steamed bread has good sensory and texture properties, with natural potato flavor.

reported (Liu, Mu, et al., 2019). However, there was no study about the potato steamed bread with wheat gluten.
In this study, the preparation and quality evaluation of potato steamed bread with wheat gluten have been studied. The influence of potato flour ratio on the rheological properties of the dough has been studied. With a fixed potato/wheat mass ratio, the effect of wheat gluten on rheological properties of the dough was evaluated. The influences of yeast, sodium bicarbonate, citric acid, and monocalcium phosphate addition on steamed bread properties have been studied and optimized by orthogonal test. At last, a new potato steamed bread formula by using potato flour, wheat flour, and gluten at the presence of yeast, NaHCO 3 , citric acid, and Ca(H 2 PO 4 ) 2 has been developed.
The formulated flour of S35 with a potato mass ratio of 35% was selected to be the basic flour. Gluten was added to the basic flour at levels of 6.0%, 6.5%, and 7.0%, referred as S35+W6.0, S35+W6.5, and S35+W7.0, respectively. The basic flour and gluten were, respectively, weighted according to the proportion and then thoroughly incorporated in the mixer and sealed preserved at 4°C.

| Preparation method
The production of steamed bread followed a previous method with some modification (Zhu, 2014). The formulated flour of S35 with potato mass ratio of 35% was selected to be the basic flour, and the gluten addition amount was set at 6.5%. First, 100 g of the based flour S35 (65 g of the wheat flour and 35 g of the potato flour), 6.5 g of gluten, 1.1 g of NaHCO 3 , 0.6 g of citric acid, and 0.3 g of Ca(H 2 PO 4 ) 2 were mixed homogeneously. Second, 1.0 g of dried yeast was dispersed in 50 g distilled water (40°C) for 10 min before mixing with the flour.
Then, the dough was prepared by mixing the ingredients in a Swanson mixer (National Co., Ltd) at a speed of 105 rpm for 5 min. The dough was sheeted for ten times on a noodle machine, hand-kneaded for 1 min, and shaped. The proofed dough was fermented at 37°C and 55% relative humidity for 40 min in a controlled fermentation cabinet (MXF-A, Mingsheng), then steamed in a steaming chamber for 20 min, and cooled at room temperature for 0.5 hr before evaluation.

| Single factor experiment
With fixed basic flour amount of 100% and gluten addition amount of 6.5%, the effects of inorganic additives on steamed bread properties were studied. As Table 1 shown, four factors were selected: yeast, NaHCO 3 , citric acid, and Ca(H 2 PO 4 ) 2 addition.

| Orthogonal test
As Table 2 shown, the effects of inorganic additives on steamed bread properties were then optimized by four-factor and three-level orthogonal test. Specific volume determination and sensory evaluation were conducted.

| Specific volume determination
The cooled steamed bread was weighed, and its volume was measured by the millet replacement method. The specific volume of the steamed bread was the result of volume divided by weight.

| Sensory evaluation
Sensory evaluation of the steamed bread was carried out by ten trained panelists from Henan University of Technology according to the Chinese standard method SB/T 10139-1993 with some modification (Liu, Solah, et al., 2019). Sensory items are shown in Table 3.

| Texture analysis of steamed bread
The texture analysis was conducted on the steamed bread after steaming for 20 min and stored at room temperature for 1 hr.
The texture properties of the steamed bread were analyzed as described by using a TA-XT2i Texture Analyzer equipped with a probe of P/36R (Stable Microsystems). Texture profile analysis (TPA) was set as: pre-and post-test speed of 2.0 mm/s, test speed of 1.0 mm/s, trigger force of 5 g, compression distance of 25 mm, and pause between two compression of 1 s. Hardness, stickiness, springness, cohesiveness, chewiness, and resilience were measured.

| Statistical analysis
All experiments were repeated in triplication, and results were expressed as mean ± SD. The ANOVA procedure and Duncan's multiple range method were used to evaluate the significant differences between treatments (p < .05).

| Mixing behavior of the formulated flour
Chemical composition of the formulated flour was characterized in our study (Table S1). As Figure 1 shown, the wet gluten content in the formulated flour declined as the mass ratio of potato flour increased, due to no gluten existed in potato flour (Xu, Hu, Liu, Dai, & Zhang, 2017).
But the gluten index gradually increased at the ratio increased, indicating improvement of the gluten flexibility. Table 4 shows the farinograph parameters of the formulated flour. The water absorption rate of the formulated flour increased with the potato ratio, because of the stronger water absorption ability of potato flour. It has been reported that the water absorption rate of potato flour was 148%, much higher than that of the wheat flour (62.8%) (Zeng et al., 2019). The wheat protein was high in glutenin and gliadin, which were responsible for the dough strength, while the potato protein was mainly composed of globulin and alkali-soluble protein (Muneer, Johansson, Hedenqvist, Plivelic, & Kuktaite, 2019). The gluten content of the formulated flour decreased with the increase of the potato mass ratio, resulting in decline of the dough strength. As Table 4 shown, the dough stability times of the formulated flour with different contents of potato flour were significantly lower than that of the control (6.63 min). However, as the ratio increased, the stability time first decreased and then increased. Pu reported it has been related to the change of water absorption and the uniformity of the formulated flour . Meanwhile, as the ratio increased, the weakness exhibited an increasing tendency, and the evaluation score declined significantly (Table 4).
The farinograph properties of the formulated flour are shown in Table 5. As the mass ratio of potato flour increased, the extensible area reduced from 78.0 to 9.0 cm 2 , and the extensibility of the flour decreased from 164.0 to 113.1 mm. Also, the extensible resistance and the max-extensible resistance reduced significantly. But the extensible rate and the max-extensible rate first decreased and then increased as the ratio increased. The results indicated decline of the dough strength and degradation of the processing properties of the dough.

| Effect of gluten on formulated flour
Addition of gluten in low protein flour has been proved to be able Baik, 2017). In this study, the formulated flour with 35% potato flour and 65% wheat flour was selected to be the basic flour. Effects of gluten (5.0%-7.0%) on the farinograph and extensograph properties of the dough have been studied.
As Table 4 shown, as the gluten addition amount increased, farinograph properties of the dough showed an increasing tendency. According to the requirements of SB/T10137-93 for making steamed bread, wet gluten content of the flour should be

| Effect of yeast on steamed bread
As shown in Table 6, with the increase of the yeast amount, the specific volume of the steamed bread gradually increased, while the scores of appearance, interior texture, and aroma all first increased and then decreased. The maximum sensory score (88.5) was obtained at 1.0% addition. During the dough fermentation, the CO 2 produced by yeast fermentation made the dough to be looser, resulting in increase of the specific volume. However, excessive CO 2 would lead to formation of so many irregular holes, broken of the wheat gluten network, even collapse or deformation of the steamed bread (Gao, Tay, Koh, & Zhou, 2017). Therefore, the yeast addition of 1.0%-1.1% was feasible.

| Effect of NaHCO 3 on steamed bread
The effect of NaHCO 3 on sensory properties of steamed bread was studied. As Table 6 shown, the specific volume, scores of appearance, and texture of the steamed bread increased with the NaHCO 3 addition amount. However, when NaHCO 3 addition amount was higher than 1.3%, the color, texture in mouth, and aroma qualities of the steamed bread became worse. For instance, with NaHCO 3 addition of 1.5% (Sample SB-SB-4), the color of the steamed bread became yellower, unpleasant alkali aroma could be detected, and the aroma of the potato flour and the steamed bread fermentation could not be detected. NaHCO 3 can react with organic acid produced by lactic acid bacteria during proofing and steaming, at the same time producing more gas (Guo, Yang, & Zhu, 2019). It has been reported that NaHCO 3 addition on steamed bread could promote protein cross-linking, therefore improving the strength of the gluten network and enhancing gas retention ability of the dough (Li, Guo, Zhu, & Zhou, 2018). Therefore, the NaHCO 3 addition of 1.3% was feasible for potato spontaneous steamed bread formula.

| Effect of citric acid on steamed bread
In this study, the effect of citric acid addition (0.5%-0.9%) on sensory properties of steamed bread was tested. The results showed that there was no significant difference in specific volume and appearance score between steamed breads with different addition amounts of citric acid. The aroma score firstly increased and then decreased as the citric acid amount increased. With addition of 0.8% or 0.9%, the steamed bread had an unpleasant sour flavor. It has been reported that citric acid in steamed bread could not only enhance the taste property of the steamed bread, but also prolong the retention period of the steamed bread with good antioxidant ability (Blanco, Ronda, Perez, & Pando, 2011;Su et al., 2019). As the highest score was obtained at 0.7% addition (90.3), that amount was thought to be suitable.

| Optimization of the steamed bread preparation
Optimization of the steamed bread preparation was obtained by the L9(34) orthogonal design method, as Table 2 shown. Two evaluation indexes were selected, specific volume and sensory score of steamed bread. If specific volume was taken as the evaluation index, the effects sequence was: NaHCO 3 (B) > yeast (A) > citric acid (C) > Ca(H 2 PO 4 ) 2 (D), and the optimal combination was A 2 B 3 C 3 D 3 .
If sensory score was taken as the evaluation index, the effects sequence was: NaHCO 3 (B) > Ca(H 2 PO 4 ) 2 (D) > citric acid (C) > yeast (A), and optimal combination was A 3 B 2 C 3 D 3 .

| Model validation
Afterward, the model validation was evaluated. As Table 7 shown, specific volume of the steamed bread at experiment A 2 B 3 C 3 D 3 was 2.18 ml/g, and sensory score was 90.6. Specific volume of the steamed bread at experiment A 3 B 2 C 3 D 3 was 2.21 ml/g, and sensory score was 92.7. Therefore, the optimal steamed bread preparation additions was 1.1% yeast, 1.4% NaHCO 3 , 0.75% citric acid, and 0.50% Ca(H 2 PO 4 ) 2 . The prepared steamed bread had good sensory and texture properties. As Figure 2 shown, the prepared steamed bread possessed homogeneous interior texture, but looked a little yellow. The steamed bread also tasted a natural potato flavor. Note: Results are presented as mean value ± SD. Different letters within a column mean significant difference (p < .05).

Combination
F I G U R E 2 Photograph of the potato steamed bread

| CON CLUS IONS
As the rheological properties of the dough negatively related to the potato flour, the potato-wheat formulated flour with potato mass ratio of 35% was set as the basic flour. The addition of 6.5% wheat gluten could effectively improve rheological properties of the formulated flour. The L9(34) orthogonal design results showed that the optimal steamed bread preparation addition was 1.1% yeast, 1.4% NaHCO 3 , 0.75% citric acid, and 0.50% Ca(H 2 PO 4 ) 2 . Therefore, the new potato steamed bread formula was 100% basic flour (potato/wheat mass ratio of 35:65), 6.5% wheat gluten, 1.1% yeast, 1.4% NaHCO 3 , 0.75% citric acid, and 0.50% Ca(H 2 PO 4 ) 2 . The prepared steamed bread had good sensory and texture properties.

ACK N OWLED G M ENTS
This work was supported by Scientific Research Project of Universities in Henan Province (19B550001).

CO N FLI C T O F I NTE R E S T
All authors declare no conflicts of interest related to this article.

E TH I C A L A PPROVA L
There was no ethical issue related to his manuscript.

I N FO R M E D CO N S E NT
Written informed consent was obtained from all study participants.