Properties of Bread Dough with Added Fiber Polysaccharides and Phenolic Antioxidants: A Review

1Bread is a staple processed food that dates back over 12000 y. Numerous studies have been devoted to various aspects of breadmaking—a process in which wheat flour, water, salt, sugar, and yeast are mixed in varying proportions into viscoelastic dough subjected to fermentation and baking. Bread is a leavened food produced via fermentation of wheat flour sugars derived from starch involving chemical interactions of various food components in the employed ingredients. These interactions can be adjusted to create desirable products, once the underlying chemical and physical processes are well understood.

2Consumer awareness of the importance of functional foods has greatly grown in the past years. The global market for functional food is expected to increase to €14.7 billions by 2013 (The Medical News, 2010, http://www.news-medical.net). Functional foods with elevated levels of antioxidants and dietary fibers (DFs) are of high demand because of their associated health benefits, including maintenance of health and protection from diseases, such as cancer, cardiovascular diseases, and degenerative diseases. As bread is a common component in western diet, it may be a convenient food to deliver fiber polysaccharides and phenolic antioxidants of high concentrations. In this article, we review the published literature on experimental and mathematical studies of breadmaking to identify the scopes of further investigation into functional bread, with particular reference to the incorporation of fiber and phenolic ingredients and associated technical challenges. We consider the chemical composition of the common ingredients used in breakmaking, with a specific focus on the impact of added fiber and/or phenolic constituents on dough functionality and bread quality, and with less emphasis on the optimization of bread processing and effects of processing on fortified ingredients. We also discuss the interactions among wheat proteins, fiber polysaccharides, and phenolic antioxidants during dough development and baking process, and the mechanisms associated with the changes in the structure and conformation of wheat proteins, fiber polysaccharides, and phenolic antioxidants.

3Wheat is normally used to make bread, pasta, and noodles, because among the cereal flours, only wheat flour has the ability to form cohesive doughs upon hydration. Wheat dough entraps gas, which is essential for the production of light and leavened products such as bread and pastry. These properties are mainly attributed to the gluten proteins that generate a continuous viscoelastic network during dough development. Protein constitutes only 7% to 15% of common wheat flour including albumins and globulins, and gluten accounts for 80% to 90% of flour proteins.

4Wheat proteins naturally occur as oligomers of different polypeptides containing more than 35% hydrophobic amino acid residues (isoleucine, leucine, tryptophan, tyrosine, valine, phenylalanine, and proline). There is 6% to 12% proline in wheat proteins. The structure of some amino acids in wheat proteins is shown in Figure 1. The molecular weight of protein generally ranges from thousands to millions, with those of wheat proteins being from 30000 to more than 10 million Daltons. Some protein chains are branched polymer. The presence of nonrotating N-C bond in the ring structure of proline facilitates a fixed Φ angle of 70° in the protein molecules, resulting in a random or aperiodic structure in the proteins containing higher levels of proline residues.

5When dough is washed in running water, the starch is removed and a viscoelastic rubbery mass is obtained that is called “gluten”. Gluten is a protein complex, with proline (10%), glycine (20%), and glutamine (approximately 35%) being the most abundant amino acids responsible for gluten development. Gluten proteins can be categorized based on their solubility into gliadins (alcohol–water soluble) and glutenins (insoluble). Gliadins and glutenins are well known for their influence on the properties of gluten. The gliadins create viscosity required for dough development, whereas the glutenins provide strength and elasticity of dough.

6Not only the structure of gluten protein but also the bonding within the protein plays important roles in the dough development and functionality. Proteins contain covalent and noncovalent bonds that contribute to dough formation and structure. Noncovalent bonds include hydrogen bonding, hydrophobic interactions, ionic bonds, and Van der waals interactions. Although hydrogen bonds are individually weak, they create stability to the dough when large numbers of bonds are established during dough development. Hydrophobic and ionic bonds, although present in very small amounts, play significant roles in the interactions among the biopolymers within bread dough that consequently promote dough stability. On the other hand, covalent bonds (namely peptide bonds and disulphide bonds) exist among the amino acids that normally remain unchanged during breadmaking. Although cystein accounts for only 2% of gluten protein, it can significantly influence the structure and functionality of gluten. The sulphydral group of a cystein can react with another cystein residue to form a disulphide bond (-S-S-). Two cystein residues, either derived from the same protein via intramolecular bonding or different protein chains via intermolecular bonding, can form a loop within the protein. During dough development, these disulphide bonds can be mobilized through disulphide interchange reactions. When gluten proteins are heated up to 75 °C, sulphydryl-disulphide interchanges are accelerated. Disulphide bonds among flour proteins form strong cross-links within and between polypeptide chains, which stabilizes other energetic bonding such as hydrogen and hydrophobic interactions. Another type of covalent bond form during breadmaking is tryosine–tryosine cross-link between gluten proteins and tryosine-dehydroferulic acid, or between gluten proteins and nonstarch carbohydrates—arabinoxylans from flour.

7The polymers of glutenins are made up of high molecular weight (HMW) subunits (60 to 90 kDa) and low molecular weight (LMW) subunits (10 to 70 kDa). HMW subunits, accounting for 5% to 10% of total gluten protein, are formed among high molecular mass glutenin polymers (also called “glutenin macro polymers”). They range from 67.5 to 73.5 KDa (approximately 630 to 830 amino acids). These polymers commonly occur in allelic forms and influence gluten's viscoelasticity. Disulphide bonds significantly stabilize these polymers, functioning as interchain bonds between HMW subunits, and between HMW and LMW subunits. HMW subunits form an “elastic backbone” of head-tail polymer with interchain disulphide bonds, and the resultant backbone becomes the basis for LMW subunits to attach via disulphide bonds (Model HMW and LMW subunits as shown in Figure 2, ). Furthermore, gliadins can also interact with the glutenin polymers via noncovalent hydrophobic interactions and the glutamine residues via hydrogen bonds. Interchange of disulphide bond occurs during dough mixing, possibly via breaking, and reforming of disulphide bonds. Breaking and reforming disulphide bonds results in the formation of a network aligned along the direction of extension.

8“Loop and train model” was first proposed by to describe the behavior of gluten upon hydration (Figure 3). Hydrated gluten contains intermolecular β-sheet in addition to α-helix and β-turn structures, indicating the important role of HMW subunits in gluten elasticity. HMW glutenins comprise (1) extensive and repetitive sequences that can form loose β-reverse turns in solution and subsequently give a β-spiral structure, (2) short and nonrepetitive domain that are rich in α-helix.

9At low level of hydration (for example, <35%), most of the protein–protein interactions occur via interchain hydrogen bonding between glutamine residues in the β-spiral structures. When hydration level increases, plasticization of a system facilitates the formation of hydrogen-bonded structures between chains, allowing the orientation of the β-turns in adjacent to β-spiral to form “interchain”β-sheet. Further increase in hydration to high level leads to the formation of hydrogen bonds between water and glutamine, forming loops where they do not interact with each other. When hydration increases, the amount of train region in the β-sheet conformation is reduced, resulting in more loop regions. This loop and train model is consistent with the results from a later fourier transform infrared (FTIR) study on protein conformational changes during gluten extension and an investigation on the repetitive peptides of various numbers of HMW subunits at different levels of hydration.

10On the other hand, glutenin subunits form a disulphide-bonded network in dough. An extension on dough would result in a strain in the network, which can be described using a stretched loop and “unzipped” train regions. Water was proposed to have a major role in the loop and train model, and elevated water content leads to an easy-to-deform system. Increased train content in the dough system would exhibit an increased resistance to extension. The trains are associated with β-sheets conformation, and the resistance to extension depends on the proportion of HMW subunits in gluten proteins. Moreover, LMW subunits can make the dough stronger and less extensible. In summary, the length and nature of HMW subunits play a vital role in the viscoelastic properties of dough. The addition of glutenins with HMW and LMW subunits to dough would greatly influence rheological and baking properties. Table 1 lists some published review articles on the gluten structures.

11Wheat starch accounts for 63% to 72% of flour. The building block of starch and glucose can form both linear polysaccharides (called amylose) and branched polysaccharides (called amylopectin). The wheat starch commonly used contains approximately 25% amylose and 75% amylopectin. Wheat starch granules exist in 2 lens sizes: type B (spherical size, 1 to 3 μm diameter) and type A (larger granules, 20 to 45 μm diameter). Interactions between starch granules and gluten are possible, occurring via hydrogen bonding and preventing bread staling.

12Other important components of flour are nonstarch fiber polysaccharides that are derived from wheat cell walls. These fiber polysaccharides account for 2% to 4% of flour. Both the flour from the whole grains and the flour that has been refined can be used for bread. The former is nutritionally preferable to the latter. Arabinoxylans and mixed 1,3-/1,4- β-D-glucans are 2 major wheat fiber polysaccharides or nonstarch polysaccharides. The structure of arabinoxylans varies significantly among types of cereal, containing xylan backbone branched with arabinosyl residues in various linkages. Arabinoxylans may also be linked to ferulic acids covalently through ester linkages to the arabinose as shown in Figure 4. The wheat fiber polysaccharides influence the functionality of gluten. Arabinoxylans significantly influence water balance and rheological properties of bread dough, as well as retrogradation of starch. Arabinoxylans are able to absorb high amounts of water, causing reduced water availability for gluten during dough development. The water-absorbing capacity of arabinoxylans also affects the distribution of moisture among dough constituents, resulting in altered dough rheological properties and prolonged mixing time. The redistribution of moisture among gluten and other macromolecules allows arabinoxylans to react directly with gluten molecules, generating a more complex network containing both gluten and arabinoxylans. Other molecules such as arabinogalatctan peptides also interact with gluten, resulting in a reduced water absorption and extensibility.

13Since the endogenous fiber polysaccharide content in wheat flour is only 2% to 4%, the incorporation of DF in bread would raise the health profile of the final bread products. Fibers in various forms have been previously used in breadmaking. The definition of DFs has been debated and has evolved in the past. A final agreement was only reached in November 2008 on a global DF definition for the Codex Alimentarius. Now the Codex defines DF as carbohydrate polymers with 10 or more monomeric units, which are not hydrolyzed by the endogenous enzymes in the small intestine of humans. DFs decrease intestinal transit time, increase stool bulk, reduce total and LDL cholesterol level in blood, decrease postprandial blood glucose and insulin level, buffer excessive acid in the stomach, and prevent constipation. Low DF intake has been associated with health problems such as diverticular disease, diabetes, obesity, coronary heart disease, and colorectal cancer. Various DF daily intake ranges are recommended in different countries, for example, 20 to 25 g in Japan, 20 to 35 g in America, 25 to 30 g in Australia and New Zealand (Natl. Health and Medical Research Council and Dept. of Health, http://www.thefreelibrary.com), and 30 to 40 g in France.

14In addition to associated health benefits, incorporation of DF to food products imparts a number of functional properties to the finished foods, including increased water holding, gel forming, stabilizing, texurizing, and thickening capacities. DF may stabilize or modify food physical structure and product density because of its fibrous nature. It was found that purified DFs from orange, pea, cocoa, coffee, wheat, and microcrystalline cellulose had pronounced effects on dough rheological behavior yielding higher water absorption and smaller extensibility than those obtained without fiber addition. Dry potato pulp Potex and 2 derived enzymatically treated fiber powders containing a high level of lignin and insoluble nonstarch polysaccharides had led to increased hardness, deformation modulus, and gumminess.

15Cereal grains contain intrinsic phenolic antioxidants. These compounds not only scavenge free radicals in the biological systems, but also prevent food spoilage. Free radicals derived from a wide range of biological reactions in the body can damage essential biomolecules. Excess of unscavenged free radicals cause unhealthy conditions as well as diseases, for example, reactive oxygen species (ROS) including superoxide (O2−), hydroxyl radical (OH), hydrogen peroxide (H2O2), and lipid peroxide radicals have been associated with chronic degenerative diseases such as cancer, inflammatory, aging, cardiovascular, and neurodegenerative disease. Natural antioxidants such as flavonoids, tocopherols, and phenolic acids (structure as shown in Figure 5) may inhibit lipid peroxidation in food and improve food quality. The modes of action include direct scavenging (for primary antioxidants, for example, α-tocopherol) and indirect scavenging (for secondary antioxidants, for example, dilauryl thiodipropionate and thiodipropionic acid).

16Cereal grains contain phenolic acids, saponins, phytoestrogens, and flavonoids. Ferulic, vanillic, and p-coumaric acids are the most abundant free phenolic acids in wheat exhibiting antioxidant activities. The antioxidant activity of wheat products depends on the nature of antioxidant species, wheat variety, extraction method, and type of antioxidant activity assay.

17Breadmaking is a complex process mainly consisting of mixing, fermentation, and baking, in which water evaporation, volume expansion, starch gelatinization, protein denaturation, and crust formation occur. Dough mixing transforms the mixture of flour and water into a homogenous viscoelastic dough for subsequent dough development and air occlusions. During dough mixing, the mechanical energy imparted induces conformational changes in wheat proteins, namely, breakage and formation of both covalent (-SS-) and noncovalent (hydrophobic and hydrogen) bonds. Water must be added to the optimal absorption level so that dough can reach a stage of “optimum development”. Baking is the last but also an important step involving heat and mass transfer, causing physical, chemical, and structural changes of dough components. Increased temperature would promote the formation of protein cross-links, causing setting of the loaf during baking. Leavening is accomplished through trapping the CO2 resulted from yeast fermentation in the dough system. The yeast is inactivated when temperature reaches approximately 54 °C. Starch gelatinization becomes noticeable at 65 °C and further enzymatic action stops at 76 °C. Wheat protein denaturation and starch gelatinization both influence water diffusion by releasing and absorbing water. Several changes occur simultaneously while baking. Some of the possible reactions and interactions are discussed below.

18During baking, the 3 hydroxyl groups of the glycosyl residue of wheat starch may form hydrogen bonding with water. The overall hydrogen bonding effect would become significant when starch is heated in water at 52 to 85 °C; crystallinity and birefringence of granules would be lost. This process is called gelatinization. When heating is prolonged, the starch granules break and are eventually disrupted. Amylose and amylopectin are dispersed. This is called pasting. On cooling and aging, the starch molecules can reassociate and form new crystals. This process is called retrogradation. Starch gelatinization, pasting, and retrogradation are common events in the baking process and are highly associated with rheological behavior. During breadmaking, water moves from hydrated gluten to starch granules causing gelatinization. Interactions between gelatinized starch granules and the gluten network occur in crumb, causing a loss of kinetic energy and subsequently an increase in firmness. Further to the study of on starch gelatinization using differential scanning colorimetry (DSC), used nuclear magnetic resonance (NMR) to characterize the dynamic starch-water interaction at heating temperatures ranging from 20 °C to 77 °C, and found that water molecules might have different relaxation rates, depending on their diffusive and chemical exchange with starch components.

19During baking, wheat proteins undergo structural changes because of their heat susceptibility. The decreased protein content is possibly due to gradual breakdown of protein during fermentation. A decrease in protein content of bread dough during fermentation has been detected by size exclusion-high-performance liquid chromatography (SE-HPLC). Polymeric proteins decrease and LMW proteins increase, and there is a reduction in protein solubility caused by aggregation or cross-linkage during baking. A cleavage of interchain disulphide bonds within glutenins would cause an increased amount of components with reduced molecule size. Therefore, intact disulphide bonds preserve large polymer structure of glutenin.

20During baking, temperature of the crumb starts from about 70 °C and can reach 100 °C within 30 min. Two thermally induced phenomena related to baking are starch gelatinization and protein denaturation. Gluten proteins are heat susceptible, and their performance or functionality during breadmaking would decrease when heat is applied and would be completely lost at 75 °C. During dough formation, proteins become hydrated. When temperature rises during baking, water migrates from gluten to the starch, promoting starch swelling. Increased cross-linking and polymerization of gluten polymers occur during baking, as a result of increased sulphydryl (SH) and disulphide (SS) interchange reactions. Elevated temperature during baking would cause protein cross-links promoting loaf setting. Therefore, increased cross-links enable protein molecules to aggregate closely. When the molecular size of the glutenin aggregated increases, the extractability of glutenin decreases. found that the functionality of gluten decreased during baking, and the solubility and extractability of gluten decreased rapidly between 70 and 90 °C. Surface hydrophobicity of gluten is initiated from 45 °C, indicating the exposure of hydrophobic groups of unfolded gluten polymers and consequently their decreased solubility. LMW and HMW glutenins and gliadins have different thermal stability. HMW glutenins polymerize below 100 °C, and gliadins can be polymerized directly into glutenins without any intermediate steps above 120 °C. FTIR, near infrared spectroscopy (NIRS), gel filtration, and SE-HPLC are commonly used methods to study protein loss during baking.

21Maillard and caramelization reactions are important reactions observed in the processing of bakery products; for example, color of crust is associated with both Maillard and caramelization reaction products. The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, while caramelization is a complex group of reactions that take place when sugars are subjected to high temperatures in the absence of amino acids. Both Maillard and caramelization reactions can take place simultaneously and both reactions depend on temperature, water activity, and pH. Crust browning occurs when the baking temperature is greater than 110 °C. During baking, water is quickly removed from the dough surface, offering optimum conditions for a Maillard reaction. However, once the crust is formed, water vapor flow is restricted from pores to the dough surface. Free amino groups of lysine, peptides, or proteins could react with carbonyl groups of reducing sugars, initiating Maillard reactions under the baking conditions. At the early stage of the Maillard reaction, Amadori rearrangement products such as furosine are generated by acid hydrolysis of the Amadori rearrangement compounds (fructosyl-lysine and lactulosyl-lysine-lysine), and at later stages, florescence compounds and cross-linking products are produced. The end products of Maillard reactions are melanoidins that are responsible for browning. Temperature inside the dough is much lower than that of crust, but water activity is high, causing light coloration. Maillard reaction is also associated with the formation of toxic compounds such as acrylamide. It was found that the products of these browning reactions, especially caramelization intermediates, show antioxidant capacities.

22Attempts to add fiber into popular foods present challenges to develop products with a fiber level that meets the requirements of The Code of Federal Regulations (Title 21, Part 101.54), which allows “good source of fiber” and “excellent source of fiber” claims to be made for a product. Major technical challenges of incorporating fibers will be the maximum retention of functionality of added fibers in the final finished products. Therefore, the success of DF addition should be determined based on the biological and physical effects that DF may carry into the final products. Adding insoluble DF solely to baked products is limited because of its low functionality and fermentability, in comparison, soluble fibers can be fermented (by the large intestine microflora) leading to desired metabolic effects. Water holding capacity of soluble fibers (such as pectin and galactomannan) is greater than that of cellulose (insoluble fibers). Soluble fibers are normally added together with insoluble fibers to deliver the full spectrum of fiber functionality. Some characteristics of the added DFs, such as water solubility and coarseness, influenced the microbial ecology of the human colon and consequently digestion. Both undigested and fermentable fibers were important to digestive activities, with fibers of moderate fermentability are preferable.

23Added fibers have been found to influence the properties of other types of baked products, and these impacts may also occur to bread systems. Fibers derived from apple, lemon, and wheat have been added to cookies to replace the wheat flour on the levels of 15%, 20%, and 30% (w/w, based on the flour used), which showed that the in vitro protein digestibility decreased with an elevated fiber level. Decreased crude protein contents in muffins after the addition of apple skin powder on the levels of 4%, 8%, 16%, 24%, and 32% (w/w) to replace flour also suggested a dilution effect on wheat proteins. More interestingly, the addition of natural apple fiber (a mixture of soluble and insoluble fibers prepared using an aqueous method) into a snack bar recipe to replace 2.7% of the quick-cook rolled oats) appeared to facilitate higher contents of beneficial bioactive components including phenolics, pectic polysaccharides, and total DF. Also, the presence of these fibers (either inulin or apple DF) in the snack bar base had caused a reduced yellowness in color of the bar filling.

24The physical properties of fiber including water holding, oil holding, and swelling capacity, viscosity or gel formation significantly affect product processing and quality. The addition of fibers to dough would alter dough's water absorption, causing reduced water content in dough and poor viscoelastic property. The addition of fiber may or may not decrease dough stability. Decreased dough stability and prolonged dough development time (from 4.2 to 5.8 min) were possible after the use of mango peel powder (rich in pectins) to replace flour at a level of 10% (w/w). However, found that the added carob and pea (at the level of 3% w/w) did not alter dough development time or stability (except for the added inulin exhibiting an increasing effect). Different forms and levels of hazelnut (fine or coarse, dry or hydrated, 5% or 10% w/w) were added to bread dough, but only the added fine hazelnut powder at the level of 10% increased dough stability remarkably. Thus, the type of fiber determined its impact on dough stability, possibly due to the number of hydroxyl groups of fiber that interact with water through hydrogen bonding.

25The apparent negative effects of incorporated fibers on the final bread quality include reduced loaf volume, increased crumb firmness, darkened crumb appearance, and possibly tastes. The incorporation of apple fiber into bread might increase product density as a result of the water-binding capacity of fiber. Bread volume was reduced after the addition of hazelnut fiber, sugar beet fiber, and apple fiber. This phenomenon was possibly a result of the fiber weakening or crippling dough structure and reducing CO2 gas retention. Moreover, appreciable amounts of water could have strongly bound to the added fibers during breadmaking, so less water was available for the development of the starch-gluten network, causing an underdeveloped gluten network and reduced loaf volume. Therefore, the 2 mechanisms causing reduced loaf volume are the dilution of gluten, and the interactions among fiber components, water and gluten. The dilution of gluten is evident by electron microscopy (Figure 6), cell walls of control bread showing a fine structure composed of numerous thin filaments connecting to adjacent cell (6A), and a reduction in fine structure in fibrous added breads, the bread crumb filaments and sheets are coarse and massive. Bread hardening effects observed after the addition of fibers results from the dilution of gluten content. The increased dough development time and decreased dough stability caused by added apple fiber were possibly associated with slowed water hydration rate and gluten development due to increased fiber content. Increased mixing tolerance and extension value may be possible, due to interactions between fibrous materials and gluten. The disruption of breadcrumb structure is due to the impairment in gas retention. Fiber addition caused shortened and low resistance to dough extension, and increased concentration of insoluble and soluble cell wall materials have been shown to partially disrupt the gluten network.

26Phenolic antioxidants are one of the major antioxidants in wheat and can form complexes with proteins and/or polysaccharides. Such a complexation can occur reversibly via hydrogen bonding between hydroxyl groups of phenols and the carbonyl group of peptide residue of proteins. The resultant complexes can be further stabilized via other types of bonds such as the covalent bonds and ionic bonds between phenolate and anion or cationic site of protein molecules. Hydrophobic interactions can be another means of complexation, by which polyphenol molecule attach on to the protein surface, or cross-link with different protein molecules. Figure 7 shows one type of the hydrophobic interactions between the aromatic ring of tannins and the hydrophobic region of proteins. The formation of complexes between phenolic antioxidants and polysaccharides is similar to that between phenolic antioxidants and proteins that is mediated by H-bonding and hydrophobic interactions. The hydrophobic interactions are facilitated by hydrophobic cavities. The resultant affinity is influenced by the molecular size, conformational flexibility of phenolic antioxidants, and water solubility of phenols.

27Processing including heating may alter the phenolic antioxidants in foods to different extents and in different ways. Previous study on ultra high temperature (UHT) milk drink containing high level of added phenolic antioxidants has shown that the stability of phenolic antioxidants could be influenced by production process and storage. In the case of baking, there was an increase in the phenolic content of biscuits when mango peel powder was incorporated, whereas no change in total phenolic content was detected when apple or lemon fiber was added to cookies. Moreover, the addition of different levels of apple skin powder had led to a range of incorporated phenolics in muffin such as flavonols, dihydrochalcones, phenolic acids, cyanidin-3-O-galactoside, and flavan-3-ols, and the baking process affected the level and composition of phenolics in baked muffin (for example, cyanidin-3-O-galactoside was the most affected phenolic compound with a 15.7% recovery). After a green tea extract (rich in catechins) was added to the bread dough on the levels of 50, 100, and 150 mg per 100 g flour, varied stability was detected among the individual phenolics and 83% to 91% of the total phenolics were retained. The chemical structure of some phenolics is shown in Figure 5.

28Several mechanisms may be involved underlying the varied behaviors of phenolic compounds upon food processing. While the loss of catechins could be a result of the combined effect of oxidation, isomerization/epimerization, and degradation during breadmaking, the reduction of phenolic content could possibly be associated with the interactions between phenolic antioxidants and wheat proteins via hydrogen bonding during dough preparation. The detected antioxidant composition or capacity of a final baked product would derive from the intrinsic phenolic compounds of flour, added phenolic ingredients, other ingredients naturally containing phenolics, intermediate phenolic products newly generated during baking (for example, via Maillard reactions), thermal-induced degradative products, and/or polyphenol-polysaccharides complexes.

29It is worth noting that the use of high level of added phenolic antioxidants in food formulations may lead to negative effects on the sensory attributes of finished foods such as increased bitterness and astringency. However, if the level of added phenolic antioxidants is well monitored, especially when the phenolic antioxidants co-exist naturally with other active compounds such as pectic polysaccharides, the consumer acceptability of the finished food might increase due to the beneficial effects on sensory attributes derived from the interactions among phenolic antioxidants and other compounds.

30Rheological properties of dough are very important indices for product development in terms of product quality and process efficiency. Rheology concerns the flow and deformation of a material. The various approaches and understandings of the molecular basis of dough rheology have been well discussed and reviewed. The rheological properties of gluten particularly elasticity are associated with texture, shape, and expansion of a finished product. There is a strong relationship between rheological properties such as viscoelasticity and the amount of HMW gluten polymers. Gluten polymer network has flexible or semiflexible chains between junction gaps of 20 nm.

31Molecular weight and structure of gluten polymers are closely linked to their rheological behaviors and, ultimately, their performance in the end product. Polymer characterization techniques that require solubility in water or solvent such as SE-HPLC are unable to examine gluten polymers with little solubility in water or solvent. The problems recognized in rheological studies include the measurements are not carried out in a region of extension that is appropriate to the actual mixing processes. Another way is to extract the material from dough during mixing and relate the properties to the mechanical changes occurring during mixing.

32There are several ways to evaluate the rheological behaviors of a baked product, with G′ (elastic modulus), G″ (viscous modulus), η′ (dynamic viscosity), η″ (complex viscosity), and loss tangent tan δ (G″/G′=η′/η″) being the commonly used terms. G′ and G″ of glutens show significant positive correlations with loaf volume, and the delta value is helpful to understand behavior of a material. If a material is an ideal elastic material, the stress and strain are in phase and δ= 0. Therefore, G″ and η′ are also equal to 0 because there is no viscous dissipation energy. The term strength is used to describe the type of flour, and the terms “strong flour” and “weak flour” indicate flour from hard and soft wheats, respectively. Strong flours are preferred for breadmaking and weak flours for cakes and biscuits. Strong flour has a higher proportion of protein with its gluten having a good elasticity. Dough made from good quality (strong) flour has lower tan δ values than that from poor quality (weak) flour. Soluble fractions in dough play important roles in breadmaking, and less tan δ and greater G′ may occur in the absence of the water-soluble fractions. Addition of pentosan could increase tan δ and reduce G′. Hydrocolloids have been incorporated in dough imparting different textural impacts on bread, for example, alginate enhances dough strength and κ-carrageenan reduces breadcrumb firmness. The tan δ values of glutens are ranked in the decreasing order as weak gluten > strong > extra strong glutens, while G′ and G″ values show the reverse trend. The high tan δ value of doughs that were made from poor quality flour could be a result of fewer entanglements or entanglements that were easily dissociated (due to hydrophilic interaction between the gluten proteins). reported that G′ value for gluten varies from 2000 to 8000 Pa, depending on the temperature used. G′ increased at higher temperatures (for example, >60 °C), possibly due to gelatinization of residual starch, and/or formation of new cross-links via disulphide bond formation. Therefore, gluten rich in HMW subunits show low G′ and G″. For glutens rich in HMW subunits, G′ and G″ show a slight frequency dependency and heat treatment does not significantly influence the rheological behavior. Gluten rich in LMW subunits is viscous before baking, while heat treatment might completely change it into an elastic material. Gliadins can act as plasticizers, and an elevated gliadin content in gluten would result in a decrease in elasticity.

33This particular review brings to light the recent interests in nutrition and disease prevention that may drive a consumer demand for functional bread with enhanced fiber and phenolic antioxidant contents. A review of the published literature revealed that (1) breadmaking may alter the protein structure; (2) the addition of different types of fibers may affect nutritional values, antioxidant status, rheological properties, and sensory attributes of baked products; (3) baking may influence added phenolic antioxidants in free forms or as components of added ingredients; (4) research approaches of previous studies are mainly include the measurement of color, volume expansion, moisture content, fiber content, and phenolic content of the finished baked products; (5) different fibers have been incorporated into baked products, and the fibers were in pure form such as pectin or a complex form with other macro-/micro-food components such as bound phenolic antioxidants; (6) the phenolic antioxidants included in the incorporated ingredients were at relatively low level.

34As pointed out throughout the text of this article, there are challenges of investigating deeper into the process of breadmaking. For example, when new bread formulations like those containing high levels of fiber polysaccharides and phenolic antioxidants are demanded, the correlation between individual phenolic compounds as a function of structure needs elucidation. The “order of addition” may be another important concern for future work when both polyphenol concentrate and fiber ingredients are required to be added into dough. The change in the “order of addition” may result in different interactive mechanisms and reaction kinetics among the dough components including wheat proteins, fiber polysaccharides, and phenolic antioxidants, which consequently alter the dough properties and bread attributes. Therefore, in order to answer the fundamental questions on the bread-improving mechanisms, future research should focus on modes of action of different ingredients and/or additives including those newly required for addition. Examination of the molecular interactions among the key dough components, and the resultant changes in polymeric network and bulk characteristics of dough and bread is required.