Effects of Layering Milling Technology on Dough Properties of Highland Barley and Bread Qualities

,


Introduction
Highland barley (Hordeum vulgare), known as "Qingke" in China, is a cultivar of hulless barley which grows in Tibet [1]. Highland barley has the characteristics of drought resistance, barren resistance, short growth period, and special plateau adaptability. In local high-altitude areas above 4500 meters above sea level, highland barley is the only crop that can mature normally [2,3]. Te resources of barley varieties are abundant, with thousands of diferent colors and shapes. According to the number of edges, it can be divided into two-edge highland barley, four-edge highland barley, and six-edge highland barley [4,5]. According to diferent colors of highland barley, it can be divided into white highland barley, purple barley, blue barley, black barley, etc. [6,7]. Te structure of highland barley grain is the same as that of common barley. It is divided into husk, aleurone, endosperm, germ, and hypocotyl. Te husk is mainly composed of cellulose and hemicellulose, and the germ and hypocotyl are rich in vitamins and inorganic salts [8,9].
Highland barley, as a kind of coarse cereals, is rich in nutrients, such as, vitamin, soluble dietary fber, β-glucan, fat, and sugar [10,11]. Highland barley grain contains an average of 11.37% protein, which is lower than that of wheat and oats but is higher than that of other cereals [10,12]. It contains 18 amino acids, including 8 essential amino acids for the human body, and lysine can reach 0.36 g/100 g [12][13][14]. Te average content of dietary fber in highland barley is 20.32%, which is higher than that of corn and buckwheat. Dietary fber can reduce the absorption of glucose by the small intestine and reduce the starch hydrolysis rate [5,15,16]. Diferent varieties of highland barley have diferent β-glucan content due to their diferent growth environments, with an average content between 3.88% and 6.78%, which is higher than that in oats. Te consumption of foods with high β-glucan content (β-glucan >3.0 g) can reduce postprandial blood sugar and improve insulin sensitivity, and they are suitable for patients with high blood lipids and high cholesterol [17][18][19].
Bread is made of wheat four as the main raw material, added with yeast, sugar, shortening, eggs, etc., after a series of fermentation and molding processes, and then, it is baked. Bread is easy to carry and eat, easy to digest and absorb, and is highly recognized by the public [16,20]. Te rich nutrition and unique favor of highland barley make it favored by many researchers. Most researchers add highland barley wholegrain four to wheat four to study its quality changes, all of which can get the results of the deterioration of dough quality and the poor palatability of products. Te addition will increase the nutritional value of the product, but due to the high bran content, the product quality is poor, and it is not popular with consumers [3,17,[21][22][23][24]. Te nutrients in grains mainly exist in the cortex and aleurone layer. Te particle structure of highland barley is mainly composed of the cortex, aleurone, endosperm, and germ. Terefore, as the peeling rate gradually increases, its nutritional composition will also change [25][26][27][28][29]. In the previous study, we mainly analyzed the impact of diferent peeling rates on the physicochemical properties of highland barley [26]. In this study, it was proposed to remove the barley layers of different ratios with layering milling technology. Highland barley with diferent peeling rates was milled into four, and it was added to wheat four in the same proportion to make highland barley bread. Te impact of the proportion of barley grain four with varying degrees of hulling in the baking mixture on the fermentation properties and the quality of the dough and bread were assessed.

Preparation of Highland Barley Flour and Wheat-Highland Barley Blend Flours.
Te preparation of highland barley four and wheat-highland barley blend fours adopted the method described by Zong et al. [26]. Te peeling rates of highland barley were 4.59%, 7.20%, 9.22%, 11.15%, and 12.75% after each pass through the fexible peeling machine. Te estimation formula of the peeling rate was where m 1 represents the weight (g) before peeling and m 2 represents the weight (g) after peeling.

Farinograph Analysis.
According to the Al Ansi method [30] with some modifcations, the efect of highland barley four blends with diferent peeling rates on the farinograph properties of dough was studied using Brabender farinograph (Duisburg, Germany). 300 g highland barley four blends were added to the kneading bowl, and immediately (within 25 seconds), water was added. Te water absorption, dough development time, dough stability time, and softening degree of the dough were measured.

Extensograph Analysis.
Te efect of highland barley four blends with diferent peeling rates on dough extensograph properties after 45 min, 90 min, and 135 min resting time was determined according to the AACC Method 54-10 [28]. Firstly, diferent dough types were prepared using Farinograph AT (Brabender, Duisburg, Germany) to achieve a consistency of 500BU. Ten, tensile properties were analyzed using Extensograph-E (Brand, Duisburg, Germany). Diferent dough made from wheat-highland barley blend fours were rolled into cylinders, and we placed them in a test chamber at 30°C for 45 min. Subsequently, their resistance and extensibility were determined.

Rheo-Fermentation Analysis of Dough.
Te rheofermentation analysis of dough from wheat-highland barley blend fours were determined using a rheofermentometer F3 (Chopin, Villeneuve-La-Garenne Cedex, France) with the method of Omedi et al. [31] with some modifcations. Te dough was prepared as follows: 250 g four blends, 5 g NaCl, 3 g active dry yeast, 80% of the water absorption, and then 250 g of dough wheat-highland barley blend fours were placed in the fermentation chamber, and a 2000 g weight was put on the top. Te dough was evaluated at 30°C for 3 h. Te characteristics of gas generation and dough development were recorded.

Confocal Laser Scanning Microscopy (CLSM).
Dough samples from highland barley four blends with diferent peeling rates were treated using the bulk water technique described by Zhang et al. [32] with some modifcations. Te dyeing agent was prepared by dissolving rhodamine B and fuorescein isothiocyanate (FITC) with dimethylformamide, respectively, preparing 0.1% rhodamine B solution and 0.1% FITC solution, and mixing the two solutions at a ratio of 1 : 1. Te dough was cut into 10 mm × 10 mm × 0.7 mm and transferred to a glass slide, and 2 drops of mixed dye were added dropwise to the surface of the dough. After soaking for 10 min, the foating color on the surface was washed away with distilled water and the water was dried in the air for observation under a microscope by using a Fl UOVIEW FV3000 LSCM (Olympus Corporation, Japan).

Bread-Making
Procedure. Te preparation of highland barley bread adopted a one-time fermentation method described by Cakir et al. [20] with some modifcations. Based on 200 g wheat-highland barley blend fours, the bread formula was 6.0% sugar, 4.0% milk powder, 3.0% shortening, 1.8% active dry yeast, and 1.5% salt, and water was added according to the results of 500 FU powder quality determination. Te above mixed substances in the bowl of a needle-type dough mixer (JHMZ 200, Beijing, China) at a low speed for 20 s. Shortening and water was added with a continuous blending at a high speed for about 6 min. After standing for 40 min at 25°C, the dough was fermented for 90 min at 35°C in a proofer with a relative humidity of 85% and then baked in an oven (SEC-2Y, Guangzhou, China) at 190°C for 20 min. Te wheathighland barley bread was cooled for 60 min and stored for further analysis.
2.6. Bread Analysis 2.6.1. Specifc Volume of Bread. Specifc volume of highland barley bread was measured 24 h after baking using the seed displacement method [33]. Te specifc volume of highland barley bread (mL/g) was calculated as Specific loaf volume � Volume of loaf (mL) Weight of loaf (mg) .
(2) 2.6.2. Te Color of Bread. Te color of highland barley bread was measured with a colorimeter (Spectrophotometer CR-400, Konica Minolta, UK) and expressed as color space values using L * (lightness), a * (redness) and b * (yellowness).

Textural Characteristics of Bread.
Te texture profle analysis (TPA) of highland barley bread was carried out using a TA-XT analyzer from Stable Micro Systems (Guildford, UK) with its Pasta Firmness/Stickiness Rig probe (P/25). Te highland barley bread was cut into pieces of 10 mm thickness. Te test parameters of the texture analyzer were set as follows: the pretest speed was 1.0 mm/s; the test speed was 1.0 mm/s; the posttest speed was 1.0 mm/s; the trigger force was 5 g; the compression degree was 50%; and the two compression pause time was 5 s.

Statistical Analysis.
All experiments were performed in triplicate except for the fermentation characteristics of dough. Te results were expressed as the mean ± standard deviation (SD). Te fnal results were evaluated using SPSS 19.0 statistical analysis software. Te signifcance level was determined to be p < 0.05. Te statistical analysis was performed using software Origin 9.0 (Origin Lab Co., USA).  Figure 1. Te results showed that the area under the extension curve (A) and extensibility (E) of the highland barley four blends dough were both smaller than those of the control group in Figures 1(a) and 1(b). Te results showed that maximum resistance to extension (Rm) and the ratio of maximum resistance and extensibility (Rm/E) of the highland barley four blends dough were greater than those of the control group as shown in Figures 1(c) and 1(d). Since highland barley four itself was difcult to form gluten, adding to the bread four afected the formation and stability of the gluten network structure of the dough, the ductility and viscoelasticity of the dough was reduced, and the extensograph properties of the dough were generally weakened. With the increase of the peeling rate, the area under the extension curve, extensibility, and the ratio of maximum resistance and extensibility frst increased and then decreased, and the extensograph gradually decreased. Te results showed that the reduction of the bran in the highland barley had a favorable efect on the stability of the dough, and its plasticity has also been improved, which had a certain improvement in the production of four products. Table 2, the gas holdup rate decreased, indicating that the Journal of Chemistry  gluten quality of the four blends deteriorated and the air cells were formed by fermentation. Te gas could no longer be retained, and the gas retention of the dough becomes poor. Some studies have shown that the cross-linking of dietary fber and gluten protein was the main reason for the weakening of its gluten [21,34]. Compared with the peeled highland barley four blends' dough, the fermentation characteristics of QK0-35% were the weakest in all indicators. Te results showed that the removal of the cortex of highland barley four was benefcial to its fermentation characteristics, the comprehensive capacity of gas production and gas holding has been improved, and the maximum fermentation height and retention coefcient were both at QK2-35%, while the gas production at QK4-35% was higher than other samples.

Confocal Laser Scanning Microscopy (CLSM) Analysis of Dough.
In order to reveal the microstructure of dough, CLSM is crucial for understanding the interactions and structure-function relationships between dough components [32,38]. Te efect of adding highland barley four with diferent peeling rates on the dough network structure has been observed by using the laser confocal microscope in Figure 2.
FITC and rhodamine B stained the starch and protein in green and red, respectively. Te yellow or orange-red in Figure 2 was the part where protein was combined with starch granules, and the black part was water, pores, or other substances. In Figure 2, the images A1 and A2 of the control group showed that the dough had a continuous and uniform gluten network structure, and the starch granules were well embedded in it. After adding highland barley four, larger holes appeared in B1 and B2, which proved from the microscopic aspect that the internal structure of the dough was seriously damaged, and the gluten structure was rough and not continuous and complete. Te results were the same as the results of farinograph and extensograph properties. Te production of the dough would have adverse efects during the fermentation process. With the increase of the peeling rate, it could be seen from the images of C2, D2, E2, F2, and G2 that the pores gradually became smaller, and the structure of the dough was relatively tight, which was consistent with the cortex. Te reduction of substances such as medium fbers was inseparable. Figure 3 is a slice diagram of highland barley bread with diferent peeling rates. Te results showed that compared with the control group, the perimeter of highland barley bread slices decreased and the color became darker. Meanwhile, uneven size holes appear on the slice surface, but with the increase of the peeling rate of highland barley four, the color of the bread gradually became lighter. It could be seen from Figure 4 that the specifc volume of highland barley bread was signifcantly lower than that of the control group. Te addition of highland barley four with diferent peeling rates hindered the formation of gluten protein; meanwhile, dietary fber and other substances could not be decomposed by yeast, which weakened the fermentation process [39]. With the increase of the peeling rate, the specifc volume of bread did not change signifcantly. Although the cortex of highland barley four was partially removed, the addition of highland barley four had a more signifcant efect on the specifc volume of bread.

Efect of the Peeling Rate on the Color of Highland Barley
Bread. Te color of bread is an evaluation index for consumers to evaluate the quality of bread, which is related to the quality of raw materials and the baking process [26,30,40,41]. Table 3 shows the measurement results of the core color of the highland barley bread with diferent peeling rates. Te color of the bread in the control group was the largest L * , the smallest a * , and the largest b * . Te results showed that from QK0-35% to QK5-35%, the L * signifcance of the highland barley bread increased from 56.31 to 70.88. All indicators of QK5-35% were close to the control group, indicating that after a certain cortex was removed, the color of highland barley bread has been optimized. Te main reactions that afect the color value are Maillard and caramelization. Tese reactions are usually infuenced by the four composition or other substances used in bread making, metabolites accumulated during dough fermentation, dough pH value, baking temperature, and time [42].

Efects of the Peeling Rate on Texture Properties of Highland Barley Bread.
Texture properties refect the quality of bread's tissue structure, softness, and chewing taste through the size of various indicators such as hardness, elasticity, and cohesion [43]. Table 4 shows the efect of the peeling rate on the texture of highland barley bread. Te hardness, adhesion, and chewiness of highland barley bread were much higher than those of the control group, and the elasticity, cohesion, and resilience were lower than those of the control group. With the increase of the peeling rate, the hardness, adhesion, and chewiness of highland barley bread decreased signifcantly. Tere was no signifcant change in the hardness of the bread, and the adhesion decreased by 25.98%. Tere was no signifcant change in the adhesion from QK3-35% to QK5-35%, and the chewiness decreased by 35.82%. With the gradual increase of the peeling rate, the elasticity, cohesion, and resilience of the highland barley bread from QK3-35% to QK5-35% were not signifcantly changed, and the cohesion was not signifcantly changed from QK1-35% to QK5-35%. It could be seen from the abovementioned analysis that the peeling rate of highland barley was negatively correlated with hardness, adhesion, and chewiness, but it was positively correlated with elasticity, cohesion, and resilience. Te deterioration of the quality of highland barley bread was due to the rough taste caused by the fbers and other substances in the highland barley four, and the highland barley four itself was not easy to form gluten, which made the bread's elasticity to deteriorate, there were no uniform and fne pores, and the interior was too tight. After the highland barley four was used, the hardness, elasticity, chewiness, and resilience of the bread were improved [44,45].

Conclusion
Te particle structure of highland barley is mainly composed of aleurone, endosperm, cortex, and embryo, so as the peeling rate gradually increases, its nutritional composition will also change. Our study reports, for the frst time, that the efect of the highland barley peeling rate on dough properties of wheat-highland barley blend fours and bread qualities was studied. In the highland barley four blends, QK4-35% was ideal for the farinograph properties. Te results showed that choosing QK4-35% as the best peeling rate of highland barley four blends could not only retain the nutritional value of highland barley bread but also optimize the quality of bread to a certain extent, which can attract consumers and has a good development prospect. To retain more nutrients, only a small amount of cortex was removed from highland barley. In subsequent studies, more cortex could be removed and barley-refned four could be used as a comparison to study the changes in nutrients and physicochemical properties. In this article, the additional amount of highland barley four was set at 35%. If the proportion of highland barley four is less, better quality barley bread could be prepared.

Data Availability
Te data used to support the fndings of this study are available from the corresponding authors upon request.  Means with diferent letters within the same row are signifcantly diferent (p < 0.05). Journal of Chemistry 7