Effects of corn distillers dried grains on dough properties and quality of Chinese steamed bread

Abstract Chinese steamed bread (CSB) accounts for 30% of the wheat end‐use in China. CSB was studied as a platform for fiber and protein enrichment, employing corn distillers dried grains. Food grade distiller's grain (FDDG) processed from co‐products from the corn ethanol industry was used as the enrichment ingredient. Since CSB uses a lean formula with little or no added sugar or fat, it relies entirely on fermentation and steaming for flavor and texture development. FDDG was used to replace 0%–25% all‐purpose flour (APF) in CSB formulations. Effects of FDDG on dough properties and quality of CSB were evaluated by instrumental (Farinograph, Mixolab, and Texture Analyzer), nutritional, and sensory methods. Protein and dietary fiber contents showed significant increases to 18.8% and 15.3%, respectively, for 100 g of steamed bread (25% FDDG db). Fiber in 100 g of fresh FDDG CSB ranged from 2.8 to 7.7 g. FDDG fortified doughs demonstrated higher water absorption, while dough development time, dough stability, and dough extensibility decreased significantly with partial APF replacement. FDDG contributed to increased hardness and adhesiveness in the CSB. Crumb analysis revealed reduced number of gas cells at higher FDDG substitution. FDDG enrichment reduced brightness (L*) of flour blends and CSB. Rheological and sensory analysis showed an upper level of FDDG substitution of 15% was acceptable without detriment to dough functionality, texture, and taste.

loss in comparison to conventional oven baking. Conventional bread baked at 200°C leads to losses of lysine and vitamin B1. The protein efficiency ratio (PER) of steamed bread is higher than that of baked bread. Oven-baked bread is made using flour, salt, sugar, and fat. In contrast, the energy contained in CSB, with its leaner formula, is significantly lower than that of baked bread made with the same amount of flour. Steamed bread is thus more suited for diabetic diets. CSB, made with refined wheat flour, needs further improvement. Fortification with adjuncts such as distillers dried grain (DDG) shows potential for nutritional enhancement of conventional foods. Other workers have used okara, corn, rice bran, and wheat bran to replace part of the wheat flour in bread or steamed bread to improve the nutritional value of products (Liu, Singh, & Inglett, 2011;Lu, Cui, Liu, & Li, 2013;Rose, Inglett, & Liu, 2010).
Corn DDGs are co-products of the corn ethanol industry. The annual output of DDG is large (39 million MT in 2014). DDG serves predominantly as animal feed and is thus an underutilized, yet valuable resource (Stein & Shurson, 2009). Development of new applications for DDG has significant nutritional and economic benefits.
Dietary fiber has a beneficial effect in the prevention of coronary heart disease, colon cancer, obesity, and diabetes (Kaczmarczyk, Miller, & Freund, 2012). Adding DDG to wheat flour may not only add value to a corn co-product, but it can also improve the nutritional profile of wheat flour (Rosentrater & Krishnan, 2006).
Wheat flour, which is the principle ingredient of CSB, influences the quality of CSB (Wang, Hou, & Dubat, 2017). Optimal DDG substitution of wheat flour in bread formulations can change the quality of traditional baked bread owing to high levels of protein, dietary fiber, and pigments (Pourafshar, Rosentrater, & Krishnan, 2014a). In the present study, rheological behavior of the flour, image analysis, texture profile analysis, and quality analysis of steamed bread were determined to evaluate the effects of DDG on the quality of steamed bread. Two groups have reported on the rheological, texture profile analysis, and sensory properties of dough and steamed bread substituted with various dietary fibers. Fu, Chang, and Shiau (2015) reported that the addition of 3%-6% lemon fiber increased the hardness of steamed bread and decreased cohesiveness, specific volume, and elasticity. Wu and Shiau (2014) investigated the effects of different amounts of pineapple peel fiber on rheological and textural properties of dough and steamed bread.
They determined that steamed bread with 5%-10% pineapple peel fiber increased dietary fiber. Liu et al. (2011) reported that traditional corn bread with 30 g/100 g DDGS reduced textural quality. Tsen, Weber, and Eyestone (1983) and Pourafshar et al. (2014a), Pourafshar, Rosentrater, and Krishnan (2014b) used DDGS in conventional bread and tortilla production, respectively. There is a paucity of information on CSB made using corn DDG as a high fiber and high protein ingredient. Results from our study provide a scientific foundation for expanded use of DDG and enhancement of fiber and protein content of CSB.

| Materials
Distillers dried grain samples obtained from a commercial fuel ethanol plant were stored frozen at minus 80°C until further refinement.
All-purpose flour (APF) and active dry yeast, obtained from commercial sources locally, were stored frozen.

| DDG preparation for food applications
Food grade DDG specific for this study was prepared using solvent extraction, multiple washing, drying, and fine-grinding protocols developed in our university. A Retsch Mill (GmbH and Co.KG, 5657 HAAN1) operated at 20,000 rpm and with a 0.5-mm sieve, ensured fine milling. DDG was also steam-sterilized and controlled for particle size. Two-kilogram batches of DDG/APF blends were prepared for each of the four treatments (10%, 15%, 20%, and 25% DDG fortification levels). A control flour sample made up of 100% all-purpose wheat flour served as 0% DDG level. Samples were stored at 4°C.

| Flour blends preparation
A twin-shelled blender facilitated preparation of DDG and APF blends (Peterson Kelly Co. Inc). This instrument, operated for 1 hr, accomplished homogenous distribution of DDG within the wheat flour. Thus, four types of blends (10%, 15%, 20%, and 25% food grade distiller's grain [FDDG] in APF) were prepared along with the 100% APF.

| Preparation of steamed bread
There were five treatments reflecting five levels of fiber substitutions in wheat flour (0% DDG served as the control). Five flour blends were produced (0%, 10%, 15%, 20%, and 25% DDG/APF). Each treatment received three separate bread-making tests. This means that each blend yielded three 200 g dough pieces. Each 200 g dough piece, which was further subdivided into yield two portions of 100 g bread dough, went through the final steaming process. Therefore, each treatment was represented by six loaves of steamed bread. In total, 30 loaves of steamed bread were prepared to study the effects of five levels of DDG substitution in all-purpose flour. This level of replication is adequate for determination of effects of fiber and protein fortification on nutritional composition and sensory quality of steamed bread.
Steamed bread with and without DDG was prepared as prescribed by Chinese Standard GB/T17320-2013. Rheological analysis using the Mixolab provided information on the amount of water needed for production of optimum dough from each flour. Dry yeast (2 g, 1%), dissolved in water (30°C), was stored in a proofing cabinet.

| Mixolab analysis
Rheological behavior of dough was determined with the Mixolab ® (Chopin Technologies). The "Chopin+" protocol included an initial equilibration at 30°C for 8 min, heating to 90°C at a rate of 4°C/ min and holding at 90°C for 7 min, cooling to 50°C at a rate of 4°C, and then holding at 50°C for 5 min. Mixing speed was 80 rpm.

| Dough extensibility
Dough extensibility and resistance to extension were determined using the Texture Analyzer (TA-XT2i, Stable Micro Systems) equipped with a Kieffer dough and gluten extensibility rig (A/KIE) and a 5-kg load cell operated in tension mode. Ten-gram dough balls placed onto the oiled grooved mold were pressed into dough strips.

| Texture profile analysis
Texture profile analysis (TPA) of steamed bread was performed using a Texture Analyzer (TX.XT-Plus, Stable Micro Systems) equipped with a 5-kg load cell and a 25-mm-diameter cylindrical probe (P/25). The TPA of steamed bread was accomplished 24 hr after steaming. Steamed breads sliced at their largest circumference yielded uniform slices of 25 mm thickness. The texture analyzer employed a pretest speed of 1 mm/s, test speed of 1 mm/s, post-test speed of 2 mm/s, a compression strain of 40%, and an auto trigger force of 5 g (Lu et al., 2013).

| Quality analysis of steamed bread
A sensory panel scored the steamed bread after reheating in the steamer (6-8 min) and cooling for 3-5 min. Thirty loaves of steamed bread were prepared representing five treatment flour blends. Six steamed bread loaves represented each of the five treatments. The quality scoring system was set up according to China Standard GB/T 17320-2013 (Table 1). Height of streamed bread was determined using a Vernier caliper. The volume of steamed bread was recorded using rapeseed displacement (AACCI Method 10-05.01). The steamed bread samples placed in sampling cups were coded using random numbers and presented to a trained panel of judges (six males and six females). This panel evaluated samples of steamed bread containing 10% DDG, 15% DDG, 20% DDG, and 25% DDG, including a control (0% DDG). Investigators replicated sensory analysis to insure that there was consistency in their responses. Panelists evaluated the steamed breads for acceptability of the specific volume, height, surface color, surface structure, exterior appearance, interior structure, elasticity, chewiness, stickiness, and flavor ( Figure 1). An overall total score based on 100 points was used to rate the steam bread as excellent, good, average or acceptable, and poor.

| Scanning electron microscopy (SEM)
A scanning electron microscope (SEM; S-3400N, Hitachi Co.) enabled the study the crumb structure of steamed bread. Freeze-dried steamed bread (0.5 cm 3 ) was mounted on circular aluminum stubs with double-sided tape and coated with gold prior to electron microscopy (Kim, Morita, Lee, & Moon, 2003).

| Statistical analysis
All data were determined in triplicate and expressed as mean ± standard deviation. A one-way analysis of variance (ANOVA) and Duncan's multiple range tests enabled determination of significant difference of means at p < .05 employing SPSS software. Table 2   Exterior appearance 10 Symmetrical, spherical = 7-10; Flat, unsymmetrical = 4-7

| Physicochemical properties
Interior structure 15 Tiny pores, uniform = 12-15; Close pores, uniform or separated between the edges and the epidermis = 8-12; Large pores, rough structure = 5-11 Elasticity 10 Rebound quickly, can be compressed more than 1/2 = 7-10; Rebound slowly or does not rebound = 3-7; Difficult to compress, very hard = 2-6 Chewiness 10 Chewy = 7-10; Tender, crumble, low elasticity = 4-7 Stickiness 15 Does not stick to teeth = 8-10; Slightly sticky or sticky = 3-7  Protein content also showed a linear increase from 12.3% in the control to 18.8% in the 25% DDG steamed bread. Fat in steamed bread remained below 0.7%. CSB is essentially a low-fat food product based on the formulation. As 15% and 20% DDG steamed breads did not show significant differences in protein or fiber content, the upper fortification limit could be placed at 15% DDG. This is a modest fortification level while retaining sensory and food functional traits similar to the control. Table 4

| Dough extensibility analysis
The addition of DDG into wheat flour had a significant impact on dough strength and dough extensibility (Table 6). Increasing the level of DDG substitution up to 25% in APF doubled the resistance to extension and reduced the extensibility by one-half. The addition of DDG reduced the relative content of gluten in wheat flour, which affected the formation and stability of the dough, and decreased the extensibility of the dough. Fu et al. (2014) also pointed out that DDG is rich in rigid dietary fiber that hindered the formation of the gluten network, resulting in the decreased extensibility and the increased stiffness.  and Fu et al. (2015) also observed the detrimental effect on extensibility of dough. to 938.7 g. Lu et al. (2013) also demonstrated that adhesiveness increased with the addition of fiber-enriched okara in steamed bread while springiness, cohesiveness, and resilience showed reduction. Note: Means ± standard deviation values in the same column that followed by different letters are significantly different (p < .05).

| C-Cell image analysis
Abbreviation: NDF, neutral detergent fiber. † TDF: total dietary fiber as consumed in a 100 g portion of steamed bread.

| Sensory analysis of steamed bread
Figure 3 provides images of CSB prepared with varying levels of DDG. Table 9 provides the sensory scores of quality analysis of CSB with DDG. With increasing levels of DDG, the specific volumes of steamed bread decreased significantly from 14.75 (0% DDG) to 3.08 (25% DDG). This phenomenon was due to the DDG contribution to gluten dilution and resultant deterioration of gas retaining ability (Abbott, O'Palka, & McGuire, 1991). Specific volume reflects the magnitude of volume expansion of the dough and is, therefore, a desirable trait in steamed breads. Specific volume influenced the elasticity and chewiness of the bread. From Table 9, it is notable that the elasticity and chewiness decreased with high levels of DDG, as did CSB height and stickiness. Amir et al. (2013)  or less, the quality score of steamed bread is more than 70, which indicated that the quality of steamed bread was acceptable. Almeida, Chang, andSteel (2013) andO'Shea et al. (2015) reported that the addition of dietary fiber reduced the specific volume and crumb luminosity of bread significantly. Sabanis, Lebesi, and Tzia (2009) observed similar effects of maize fiber on the specific volume. Liu et al. (2011) also demonstrated that the increased content of DDGS darkened bread's appearance. Tsen et al. (1983) found that the replacement at 20% DDG reduced the grain score. Fu et al. (2014) found that the bread with the highest dietary fiber decreased its overall acceptability.

| Scanning electron microscopy (SEM)
The structure of the gluten network is important for the rheological properties of dough. The microstructure of the gluten matrix can be assessed by using SEM. Figure 4 provides SEM of control

| CON CLUS IONS
Distillers dried grain is a low-cost functional food ingredient owing to its large output as a co-product of ethanol production (39 million tons/year) as well as its richness in dietary fiber and protein content. The higher substitution (20% and 25%) of DDG in APF caused a significant reduction in CSB scores according to a panel of trained judges.
The 10% and 15% DDG substitutions yielded steam breads that were acceptable by a trained sensory panel. CSB formulations are particularly lean, containing only flour, yeast, and water as principle ingredients. Such lean formulations permit modest enrichment levels (up to 15%) without detriment to rheology and food quality. Results from this study demonstrate food applications of DDG and the development of fiber-rich and protein-rich steamed bread. Improvement of texture, mouth-feel, and flavor of steamed bread may benefit from use of sugar and dough improvers in CSB formulation.

ACK N OWLED G M ENTS
The authors gratefully acknowledge financial support from the

CO N FLI C T O F I NTE R E S T
The authors declare that they have no conflict of interest to report in this publication.