Effects of Flour Strength,Baking Absorption,and Processing Conditions on the Structure and Mechanical Properties of Bread Crumb

The objective of this study was to determine the effects of flour type, baking absorption, variation in sheeting, and dough proofing time on the density, crumb grain (visual texture), and mechanical properties (physical texture) of bread crumb. All response variables were measured on the same bread crumb specimens. Bread loaves were prepared by a short‐time bread‐making process using four spring wheat flours of varying strength. After crumb density measurement, digital image analysis (DIA) was used to determine crumb grain properties including crumb brightness, cell size, cell wall thickness, and crumb uniformity. Tensile tests were performed on bone‐shaped specimens cut from the same bread slices used for DIA to obtain values for Young's modulus, fracture stress, fracture strain, and fracture energy. Proof time had the most profound influence on the bread with substantial effects on loaf volume, crumb density, crumb brightness, and grain, as well as crumb mechanical properties. Increasing proof time resulted in higher loaf volume, lower crumb density and brightness, coarser crumb with fewer and larger cells with thicker cell walls, and weaker crumb tensile properties. Varying flour type also led to significant differences in most of the measured crumb parameters that appeared to correspond to differences in gluten strength among the flour samples. With increasing flour strength, there was a clear trend to increasing loaf volume, finer and more uniform crumb grain, and stronger and more extensible bread crumb. Increasing baking absorption had virtually no effect on crumb structure but significantly weakened crumb strength and increased fracture strain. In contrast, varying the number of sheeting passes had a minor effect on crumb cellular structure but no effect on mechanical properties. The experimental data were consistent with a cause‐effect relationship between flour strength and the tensile strength of bread crumb arising as a result of stronger flours exhibiting greater resistance to gas cell coalescence, thereby having fewer crumb defects.     

Influence of Additives and Mixing Time on Crumb Grain Characteristics of Wheat Bread

The effect of additives and processing parameters on wheat bread were investigated objectively using image analysis (IA). Five different bread types were produced by varying the ingredients (standard, standard with fat, standard with emulsifiers) and changing the mixing times (90, 150, and 240 sec). A digital IA system for wheat bread was developed from generic commercial software. The system yielded reproducible results for a variety of bread crumb grain image features. Bread slices were scanned and evaluated using the IA system. Image characteristics were determined for each bread type. All data was statistically evaluated to detect significant differences between bread types. It was shown quantitatively that inclusion of fat or emulsifiers or extension of mixing time had a significant effect on crumb grain features such as mean cell area, total cell area, and number of cells/cm². The five bread types could be distinguished solely by crumb grain characteristics.     

French Bread Loaf Volume Variations and Digital Image Analysis of Crumb Grain Changes Induced by the Minor Components of Wheat Flour

A standard quality flour for French breadmaking was fractionated by extraction of water‐soluble components (6% db) and by defatting (<1%db) to study the impact of soluble components and lipids on bread quality in terms of loaf specific volume (v_s) and crumb structure. Addition of puroindolines (<0.2%) was also tested. Crumb cell structure was assessed by digital image analysis (DIA) according to erosion‐dilation and closing treatments. The fraction of cells area with size <1 mm (%d<1) was defined as an index of fineness of crumb structure. Both DIA procedures allowed differentiation of crumb structures obtained by various formulations and, in the range of composition modifications tested, variations by a factor of 2 of both criteria (v_s and %d1) were obtained. Soluble fraction increased v_s and decreased fineness. Defatting and adding puroindolines increased fineness with no effect on v_s. The possible role of molecular components of each flour fraction was discussed in terms of rheological and foaming properties. DIA methods and flour recipes tested in this work offer a valuable tool for further studies on the processing‐structure‐properties relationships of French bread dough and crumb.     

Bread Crumb Grain Development During Baking

Scanning electron microscopy was used to study gas cell size, shape, and distribution throughout the breadmaking process. Flours that produced bread with a relatively good grain and a relatively poor grain were used. Micrographs of the dough samples were taken at mixing; before and after each of two punches; before and after panning; after proofing; and after 12, 18, and 24 min (complete) of baking. No differences were found between the two flours at any dough stage. However, after 12 min of baking, the cell distributions were different between the doughs. These results suggest that the crumb grain differentiates during the early stages of baking. The changes documented during this time, i.e., cells becoming larger and the cell walls thicker, indicate that some gas cells coalesce during the early stages of baking and that this is reflected in the crumb grain of the bread.     

Iron-Enriched Bread with Karkade (Hibiscus sabdariffa) and Wheat Flour

Karkade (Hibiscus sabdariffa) was blended with wheat flour to make bread. When 0.5% Karkade was blended with wheat flour, maximum bread height and specific volume (cm³/g) were obtained (pH 4.8–5.0); these properties gradually deteriorated with increased Karkade. The pH of the bread crumb decreased with increased Karkade, reaching pH 3.35 when blended with 10% Karkade. The pH of Karkade alone was 2.5, which was adjusted to ≈5.0 by the addition of alkali just before blending with wheat flour and making bread. Control of the Karkade pH resulted in bread height and specific volume recovering to the original optimal levels. In addition, the deep reddish purple color (513 nm) of the bread crumb changed to a brownish color crumb. The Fe content was 0.14 mg of Fe/gram of dry crumb, or 6.22 mg of Fe/60 g of fresh bread when 5% Karkade was blended with wheat flour.     

Understanding and Modeling the Processing‐Mechanical Property Relationship of Bread Crumb Assessed by Indentation

Measuring fundamental mechanical parameters such as Young's modulus and critical stress is a straightforward and valid approach to evaluating the physical texture of breadcrumb. The objectives of this study were to evaluate whether such fundamental mechanical properties could be measured by indentation techniques such as the AACC crumb firmness method, and then to alter breadmaking conditions so as to model the relationship between these indentation mechanical properties as a function of crumb moisture content and crumb density. Bread was baked according to a short dough process using Canadian western red spring (CWRS) wheat flour. Factors considered in the design of experiments were proofing time, water absorption, crosshead speed, and indenter diameter. Young's modulus and critical stress, measured with 12‐ and 20‐mm cylindrical indenters, were well covalidated with those obtained from a standard compression test. With increases in proofing time and water absorption, a more porous and compliant bread texture led to decreasing Young's modulus and critical stress. Our results revealed a good mapping of mechanical properties to crumb moisture content and density that were correlated to breadmaking conditions, thus permitting more precise prediction of the mechanical properties that determine bread texture.     

Effects of Varying Weight Ratios of Large and Small Wheat Starch Granules on Experimental Straight‐Dough Bread

One commercial bread wheat flour with medium strength (11.3% protein content, 14% mb) was fractionated into starch, gluten, and water solubles by hand‐washing. The starch fraction was separated further into large and small granules by repeated sedimentation. Large (10–40 μm diameter) and small (1–15 μm diameter) starch fractions were examined. Flour fractions were reconstituted to original levels in the flour using composites of varying weight percentages of starch granules: 0% small granules (100% large granules), 30, 60, and 100% (0% large granules). A modified straight‐dough method was used in an experimental baking test. Crumb grain and texture were significantly affected. The bread made from the reconstituted flour with 30% small granules and 70% large granules starch had the highest crumb grain score (4.0, subjective method), the highest peak fineness value (1,029), and the second‐highest elongation ratio (1.55). Inferior crumb grain scores and low fineness and elongation ratios were observed in breads made from flours with starch fractions with 100% small granules or 100% large granules. As the proportion of small granules increased in the reconstituted flour, it yielded bread with softer texture that was better maintained than the bread made from the reconstituted reference flour during storage.     

Quantitative Assessment of Gas Cell Development During the Proofing of Dough by Magnetic Resonance Imaging and Image Analysis

The structure of bread crumb is an important factor in consumer acceptance of bakery products. The noninvasive monitoring of the gas cell formation during the proofing of dough can aid in understanding the mechanisms governing the crumb appearance in the baked product. The development of gas cells during the proofing of dough was monitored in a noninvasive manner using magnetic resonance imaging (MRI) at 4.7‐T. The acquired MRI time series were analyzed quantitatively using image analysis (IA) techniques. The effects of both kneading temperature and mechanical damage by molding were studied. When additional rheological stress was introduced during molding, a more heterogeneous (coarse) gas cell size distribution was observed, and the dough had a smaller specific volume (as measured by MRI). These characteristics were preserved in the bread crumb structure after baking. The fast‐deformation during molding also resulted in an isotropic growth of the dough during proofing, whereas slow‐deformation during molding resulted in anisotropic growth. This can be related to a better conservation of stress in the dough under a moderate molding operation. A higher temperature during kneading also resulted in a coarser distribution of the gas cells and a smaller MRI specific dough volume. No effect of kneading temperature on the growth anisotropy could be detected, however. This indicates that temperature has a smaller effect on the conservation of stress in the dough than molding. The current work illustrates the capability of MRI/IA for understanding and predicting the influence of food processing parameters on consumer‐relevant features in a food product (bread).     

Effects of (1→3)(1→4)-β-d-Glucans of Wheat Flour on Breadmaking

Water-soluble nonstarch polysaccharides were extracted from commercial hard red winter wheat flour and separated into three fractions by graded ethanol precipitation. The three fractions, F15, F40, and F60, varied in polysaccharide composition. Fraction F15 was rich in watersoluble (1→3)(1→4)-β-d -glucans, and fractions F40 and F60 were rich in arabinoxylans. Addition of individual fractions to a bread formula did not affect bread loaf volume. Addition of fraction F15 to the formula improved bread crumb grain. Treatment of (1→3)(1→4)-β-D -glucan-rich fraction F15 with lichenase before its addition to the bread formula resulted in bread with poor crumb grain. Treatment of the F15 fraction with β-xylanase before its addition to the bread formula resulted in bread with slightly improved crumb grain. Presumably, the (1→3)(1→4)-β-D -glucans in fraction F15 improved crumb grain by stabilizing air cells in the bread dough and preventing coalescence of the cells. Addition of pentosan-rich fractions F40 and F60 to the bread formula did not improve crumb grain and interfered with the improving effect of (1→3)(1→4)-β-D -glucan-rich fraction F15. Hydrolysis of the arabinoxylans in flour by adding β-xylanase to the bread formula resulted in improved crumb grain.     

Size Distribution and Properties of Wheat Starch Granules in Relation to Crumb Grain Score of Pup‐Loaf Bread

Twelve hard winter wheat flours with protein contents of 11.8–13.6% (14% mb) were selected to investigate starch properties associated with the crumb grain score of experimentally baked pup‐loaf bread. The 12 flours were classified in four groups depending on the crumb grain scores, which ranged from 1 (questionable‐unsatisfactory) to 4 (satisfactory). Flours in groups 1, 2, 3, and 4 produced breads with pup‐loaf volumes of 910–1,035, 1,000–1,005, 950–1,025, and 955–1,010 cm³, respectively. Starches were isolated by a dough handwashing method and purified by washing to give 75–79% combined yield (dry flour basis) of prime (62–71%) and tailing (7–16%) starches. The prime starch was fractionated further into large A‐granules and small B‐granules by repeated sedimentation in aqueous slurry. All starches were assayed for weight percentage of B‐granules, swelling power (92.5°C), amylose content, and granular size distribution by quantitative digital image analysis. A positive linear correlation was found between the crumb grain scores and the A‐granule sizes (r = 0.65, P < 0.05), and a polynomial relationship (R² = 0.45, P < 0.05) occurred between the score and the weight percentage of B‐granule starch. The best crumb grain score was obtained when a flour had a weight percentage of B‐granules of 19.8–22.5%, shown by varietal effects.     

Effects of Oat Grain Hydrothermal Treatments on Wheat-Oat Flour Dough Properties and Breadbaking Quality

Hydrothermal treatments, which are routine in oat processing, have profound effects on oat flour dough rheological properties. The influence of roasting and steam treatments of oat grain on dough mixing and breadbaking properties was investigated when hydrothermally treated oat flour was blended with wheat flour. Roasting of oat grain (105°C, 2 hr) resulted in oat flours that were highly detrimental to wheat flour dough mixing properties and breadbaking quality. Steaming (105°C, 20 min) or a combination of roasting and steaming of oat grain significantly improved the breadbaking potential of the oat flours. The addition of oat flours increased water absorption and mixing requirements of the wheat flour dough and also decreased bread loaf volume. However, at the 10% substitution level, steamed oat flours exhibited only a gluten dilution effect on bread loaf volume when wheat starch was used as a reference. Oat flour in the breadbaking system decreased the retrogradation rate of bread crumb starch. The results indicate that adequate hydrothermal treatments of oat grain are necessary for oat flour breadbaking applications. Steamed oat flours used at a 10% level retarded bread staling without adversely affecting the loaf volume.     

Effect of Gaseous Acetic Acid on Dough Rheological and Breadmaking Properties

Breads baked from wheat flours (protein contents 14.1–16.5% at 14.0% mb) that were pretreated with 2–3 mL of gaseous acetic acid per kg of wheat flour, showed maximum bread height and specific volume (cm³/g). Flour-water suspension and the crumb pH values were gradually decreased with increased amounts of acetic acid. Gas generation and dough expansion tests with bread dough showed that the addition of the same amount of acetic acid, which achieved maximum specific volume, also showed the highest rate of gas generation and dough expansion. However, increasing acetic acid decreased these values. Scanning electron microscope (Cryo-SEM) observation showed that the bread dough made from the same acetic acid-treated flour indicated continuum and no cracks in the dough matrix. Evaluation of mixograms showed the decrease of mixing stability with increased acetic acid levels. Viscosity and water binding capacity of flour-water suspensions were sharply increased by the addition of acetic acid at pH 5.0–3.5.     

Effect of Zinc and Aluminum Ions in Breadmaking

Zinc and aluminum ions as chloride or sulfate salts at 50–500 ppm metal ion (flour basis) had no detrimental effect on fermentation of yeastleavened dough. Increased mixing times (≈10–50%) due to addition of aqueous solutions of zinc (250–500 ppm) or aluminum (150–250 ppm) ions to a bread formula was overcome by withholding salt until the final mixing stage. Breads made from commercial flours (12.5% protein) containing zinc (250–500 ppm) or aluminum (150–250 ppm) ions and no oxidant had improved loaf volume and crumb grain when compared with control bread, and no off-taste. Additionally, breads with added zinc or aluminum had better crumb grains and slower firming rates when compared with breads containing optimum l -ascorbic acid (50 ppm) or potassium bromate (20 ppm). Breads made from commercial flours (11.1% protein) and three laboratory flours (11.4–13.6% protein) containing zinc (250 ppm) or aluminum (150 ppm) ions also had improved loaf volumes and crumb grains. Zinc or aluminum ions in combination with l -ascorbic acid, but not potassium bromate, had a detrimental effect on bread quality. Scanning electron microscopy of freeze-dried bread doughs revealed that zinc and aluminum ions enhanced the film-coating property of gluten. One serving (one slice, 28 g) of bread made with 250 ppm zinc ion would provide 25% of the adult recommended dietary allowance of zinc.     

Effects of Glycerol and Moisture Redistribution on Mechanical Properties of White Bread

Effects of glycerol and moisture redistribution on mechanical properties of bread were investigated. Firmness increased in all bread crumb over storage time but firming rate was dependent on the initial moisture content, storage method (stored with and without crust), and the presence of glycerol. Faster firming was observed when bread crumb had low initial moisture content and high glycerol level, and was stored with crust. The effect of glycerol was more pronounced when stored with crust, suggesting a critical role of water loss. Firmness showed a good correlation (r² = 0.95) with the scale factor (C₁) from a mathematical model. Recoverable work rapidly decreased in first three days of storage and then remained relatively unchanged thereafter. Hardening of aged bread (but not fresh bread) by glycerol may be explained by local dehydration of bread polymer due to osmotic dehydration or competition for water, which in turn promote more rapid amorphous network formation but less amylopectin recrystallization.     

Factors Controlling Gas Cell Failure in Bread Dough

Stress relaxation in the wall of a gas bubble, as measured by the alveograph, was used to study surface tension at the gas-dough interface of doughs from flours producing differing bread crumb grains. The surface tensions in the various wheat flour doughs were not different. Dough rheological properties, as measured by both dynamic oscillatory rheometry and lubricated uniaxial compression, were not different for doughs made from wheat flours that gave breads with different crumb grains. However, when the effect of starch granule size on gas cell wall stability was tested, the presence of a greater proportion of large starch granules in wheat flour dough was sufficient to result in gas cell coalescence and open crumb grain in the final baked product. This suggests that starch granule size is at least one of the factors that affects the crumb grain of bread.     

Studies on proofing of yeasted bread dough using near- and mid-infrared spectroscopy

Dough proofing is the resting period after mixing during which fermentation commences. Optimum dough proofing is important for production of high quality bread. Near- and mid-infrared spectroscopies have been used with some success to investigate macromolecular changes during dough mixing. In this work, both techniques were applied to a preliminary study of flour doughs during proofing. Spectra were collected contemporaneously by NIR (750-1100 nm) and MIR (4000-600 cm(-1)) instruments using a fiberoptic surface interactance probe and horizontal ATR cell, respectively. Studies were performed on flours of differing baking quality; these included strong baker's flour, retail flour, and gluten-free flour. Following principal component analysis, changes in the recorded spectral signals could be followed over time. It is apparent from the results that both vibrational spectroscopic techniques can identify changes in flour doughs during proofing and that it is possible to suggest which macromolecular species are involved.     

Application of Response Surface Methodology in the Development of Gluten‐Free Bread

The formulation of gluten‐free (GF) bread of high quality presents a formidable challenge as it is the gluten fraction of flour that is responsible for an extensible dough with good gas‐holding properties and baked bread with good crumb structure. As the use of wheat starch in GF formulations remains a controversial issue, naturally GF ingredients were utilized in this study. Response surface methodology was used to optimize a GF bread formulation primarily based on rice flour, potato starch, and skim milk powder. Hydroxypropylmethylcellulose (HPMC) and water were the predictor variables. Analyses of the treatments from the design were made 24 hr after baking. Specific volume and loaf height increased as water addition increased (P < 0.01). Crumb firmness decreased as water levels increased (P < 0.01). Significant interactions (P < 0.01) between HPMC and water were found for the number of cells/cm². The number of large cells (>4 mm²) decreased with increasing levels of HPMC and water. Optimal ingredient levels were determined from the data obtained. The optimized formulation contained 2.2% HPMC and 79% water flour/starch base (fsb) and measured responses compared favorably to predicted values. Shelf‐life analysis of the optimized formulation over seven days revealed that, as crumb firmness increased, crust firmness and crumb moisture decreased.     

Pyranose Oxidase from Trametes multicolor Impacts Dough and Bread Microstructure

Pyranose oxidase (P₂O) improves wheat flour dough stability and bread quality. We related its effect on dough spread behavior to that on dough and bread crumb structure. Increasing P₂O addition levels gradually reduced dough flow. High P₂O addition levels further increased dough strength, significantly increased dough cell wall thickness, and decreased bread loaf volume. Taken together, affecting dough spread behavior impacts dough and bread (crumb) structure, and dough structure largely determines bread crumb structure.     

Effects of Rice Flour Blends on Bread Texture and Staling

The effects of blending rice flour with wheat flour on bread texture and staling were investigated with three rice varieties with different amylose contents. For the texture analysis of bread crumb, the compression test, puncture test, and tensile test were performed. A flour blend containing rice flour suppressed the recovery of the crumb after compression. For the puncture test, blends with rice flour increased the distance to penetrate the crumb. The rupture strain measured with the puncture test decreased with staling of the bread crumb for all samples tested. The fresh bread crumb sample containing waxy rice flour had much greater extensibility in the tensile test than the other samples tested, but it was dramatically decreased after one day of storage. Endothermic enthalpies corresponding to retrograded amylopectin, which is part of the staling process, were also measured. The enthalpy of bread crumb from the blended flour was lower than that of wheat flour bread up to three days but was higher on day 4. A blend of rice flour thus reduced amylopectin retrogradation during early storage, but it was accelerated later. Bread blended with waxy rice flour showed the lowest enthalpy during storage.     

Influence of Bran Particle Size on Bread‐Baking Quality of Whole Grain Wheat Flour and Starch Retrogradation

The influence of bran particle size on bread‐baking quality of whole grain wheat flour (WWF) and starch retrogradation was studied. Higher water absorption of dough prepared from WWF with added gluten to attain 18% protein was observed for WWFs of fine bran than those of coarse bran, whereas no significant difference in dough mixing time was detected for WWFs of varying bran particle size. The effects of bran particle size on loaf volume of WWF bread and crumb firmness during storage were more evident in hard white wheat than in hard red wheat. A greater degree of starch retrogradation in bread crumb stored for seven days at 4°C was observed in WWFs of fine bran than those of coarse bran. The gels prepared from starch–fine bran blends were harder than those prepared from starch–unground bran blends when stored for one and seven days at 4°C. Furthermore, a greater degree of starch retrogradation was observed in gelatinized starch containing fine bran than that containing unground bran after storage for seven days at 4°C. It is probable that finely ground bran takes away more water from gelatinized starch than coarsely ground bran, increasing the extent of starch retrogradation in bread and gels during storage.