Improved viscoelastic zein–starch doughs for leavened gluten-free breads: Their rheology and microstructure

Gluten-free bread was prepared from commercial zein (20 g), maize starch (80 g), water (75 g), saccharose, NaCl and dry yeast by mixing above zein's glass transition temperature (T_g) at 40°C. Addition of hydroxypropyl methylcellulose (HPMC, 2 g) significantly improved quality, and the resulting bread resembled wheat bread having a regular, fine crumb grain, a round top and good aeration (specific volume 3.2 ml/g). In model studies, HPMC stabilized gas bubbles well. Additionally, laser scanning confocal microscopy (LSCM) revealed finer zein strands in the dough when HPMC was present, while dynamic oscillatory tests showed that HPMC rendered gluten-like hydrated zein above its T_g softer (i.e. |G*| was significantly lower). LSCM revealed that cooling below T_g alone did not destroy the zein strands; however, upon mechanical impact below T_g, they shattered into small pieces. When such dough was heated above T_g and then remixed, zein strands did not reform, and this dough lacked resistance in uniaxial extension tests. When within the breadmaking process, dough was cooled below T_g and subsequently reheated, breads had large void spaces under the crust. Likely, expanding gas bubbles broke zein strands below T_g resulting in structural weakness.     

Improved viscoelastic zein–starch doughs for leavened gluten-free breads: Their rheology and microstructure

Gluten-free bread was prepared from commercial zein (20 g), maize starch (80 g), water (75 g), saccharose, NaCl and dry yeast by mixing above zein's glass transition temperature (T_g) at 40°C. Addition of hydroxypropyl methylcellulose (HPMC, 2 g) significantly improved quality, and the resulting bread resembled wheat bread having a regular, fine crumb grain, a round top and good aeration (specific volume 3.2 ml/g). In model studies, HPMC stabilized gas bubbles well. Additionally, laser scanning confocal microscopy (LSCM) revealed finer zein strands in the dough when HPMC was present, while dynamic oscillatory tests showed that HPMC rendered gluten-like hydrated zein above its T_g softer (i.e. |G*| was significantly lower). LSCM revealed that cooling below T_g alone did not destroy the zein strands; however, upon mechanical impact below T_g, they shattered into small pieces. When such dough was heated above T_g and then remixed, zein strands did not reform, and this dough lacked resistance in uniaxial extension tests. When within the breadmaking process, dough was cooled below T_g and subsequently reheated, breads had large void spaces under the crust. Likely, expanding gas bubbles broke zein strands below T_g resulting in structural weakness.     

Oxidation of commercial (α-type) zein with hydrogen peroxide improves its hydration and dramatically increases dough extensibility even below its glass transition temperature

To improve the rheological properties of zein doughs, α-type zein and zein-starch doughs were prepared with the oxidising agents, hydrogen peroxide and peroxidase, which strengthen gluten-based doughs by cross-linking. Hydrogen peroxide and peroxidase increased zein dough extensibility compared to preparation with water. Hydrogen peroxide prepared zein doughs were extensible and cohesive below zein’s glass transition temperature. The doughs did not exude water and maintained flexibility when stored. Confocal laser scanning microscopy revealed that in zein-starch doughs prepared with hydrogen peroxide a thin continuous zein matrix was formed around the starch granules, whereas doughs prepared with water exhibited clumps of granules. SDS-PAGE of zein doughs and films treated with the oxidising agents showed no evidence of zein polymerisation, nor did Fourier transform infrared spectrometry reveal any significant changes in secondary structure. Further, hydrogen peroxide treatment did not increase zein film glass transition temperature, but it did increase transition enthalpy, and film water uptake increased with hydrogen peroxide concentration. The greatly increased extensibility of hydrogen peroxide prepared zein doughs and their improved water-holding are not due to oxidative cross-linking. It is proposed that the effects are primarily due to hydroxylation of amino acid aliphatic side chains, improving hydration through hydrogen bonding.     

Effect of zein extrusion and starch type on the rheological behavior of gluten-free dough

Previous research has shown that zein, above its glass transition temperature, may adopt molecular structures that are able to form doughs with viscoelastic properties comparable to those of wheat gluten. It is hypothesized that extrusion can promote molecular changes in zein and favor interactions with starch that enhance dough viscoelasticity. Thus, the effects of extruding zein at 90–160 °C on the rheological properties of doughs prepared with potato, rice, and maize starches were determined.Formulations were optimized to provide similar mixing profiles to that of a standard wheat dough. For all zein samples, creep-recovery tests demonstrated that doughs prepared with maize and potato starches were less elastic when compared to doughs prepared with rice starch. Zein doughs produced using rice starch were comparable to wheat-dough. Extensional tests showed that zein extruded at 160 °C provided a larger increase in strain-hardening behavior, which is important for bread production. These samples also exhibited larger extensional stresses. Gel electrophoresis of zein extruded at 160 °C revealed an increase in protein aggregates and the presence of smaller peptides when compared to samples subjected at lower extrusion temperatures.Scanning electron micrographs of doughs containing zein showed starch granules embedded within an amorphous material and fibrous structures, which is attributed to elongated zein.     

Impact of different isolation procedures on the functionality of zein and kafirin

Commercial corn prolamin (zein) aggregates in water at elevated temperatures into an extensible, viscoelastic gluten-like substance. This specific functionality of zein can be used in the production of gluten-free bread from true dough systems and not from batters. The present study examined laboratory-scale isolation of such functional zein from dry milled corn. RP-HPLC indicated that successful isolation procedures resulted in relatively pure α-zeins, with a maximum ratio of (β + γ)/α-zeins of about 10%. In the present study, such functional zeins were obtained by using 70% ethanol as the extractant, without added alkali or reducing agent in the main extraction step. In contrast, films could be cast from a wider range of zein isolates, also with higher ratios of (β + γ)/α-zeins. Isolation of the analogous prolamin (kafirin) from dry milled sorghum required a more hydrophobic extractant such as 83% isopropanol to achieve partial functionality. Such kafirin was able to aggregate in warm water, preferably when a reducing agent was added; however, it quickly became firm and lost its extensibility. The present study suggests that hydrophobic interactions rather than disulfide bonds are the key to gluten-like functionality of zein and kafirin.     

Elastic properties of gluten representing different wheat classes

Traditional instruments used to evaluate dough and/or gluten rheological properties do not provide unambiguous separation of elastic and viscous behaviors. Recovery after shear creep and cyclic large deformation cyclic tensile testing were used here to decouple elastic and viscous effects. A large variation in the recoverable shear strain (∼7.2% to ∼28%) was seen for glutens from 15 U.S. popular common wheat cultivars with varying HMW subunits. Sedimentation values ranged from 29 to 57 ml for 12 hard wheat cultivars and 15 to 22 ml for three soft wheat cultivars. The tensile force at 500% extension ranged from 0.12 to 0.67 N for hard wheat glutens and from 0.10 to 0.20 for soft wheat glutens. However, the recoverable work after large extension was less than 40% of the total work of extension. In addition, recoverable work in tensile testing was highly correlated with the total work of extension (r² = 0.97) and mixograph mix times (r² = 0.81). Good to excellent bread volume was obtained for several cultivars from this sample set. This suggests that optimizing water absorption for mixing doughs to achieve maximal bread volume compensates for the wide range of viscoelastic behaviors of gluten.     

Influence of gliadin removal on strain hardening of hydrated wheat gluten during equibiaxial extensional deformation

The aim of the present work has been to study the equibiaxial extensional deformation of doughs of gluten- and glutenin-rich fractions containing 40 wt% water subjected to lubricated squeezing flow with four different crosshead speeds at room temperature. The gluten dough shows strain softening and hardening in succession whilst the dough where the gliadins have been removed by alcohol extraction does not show strain hardening behavior but breaks immediately after strain softening. The equibiaxial extensional viscosity decreases with increasing strain rate at given strains, appearing as strain rate thinning behavior, which is stronger in the glutenin dough than in the gluten dough. The large extensibility with strain hardening in the gluten dough is due to the presence of gliadins acting as both plasticizers and promoters for the more extensible networks.     

Rheological properties of kafirin and zein prolamins

The possibility of forming dough from kafirin was investigated and laboratory prepared kafirin was formed into a viscoelastic dough system. Measurements with Contraction Flow showed that dough systems prepared from kafirin and from commercial zein had the required extensional rheological properties for baking of leavened bread. The extensional viscosity and strain hardening of the kafirin and zein dough systems were similar to those of gluten and wheat flour doughs. The kafirin dough system, however, unlike the zein dough system rapidly became very stiff. The stiffening behaviour of the kafirin dough system was presumed to be caused by cross-linking of kafirin monomers. SDS-PAGE showed that the kafirin essentially only contained α- and γ-kafirin, whereas the zein essentially only contained α-zein. Since γ-kafirin contains more cysteine residues than the α-prolamin it is more likely to form disulphide cross-links, which probably caused the differences in stiffening behaviour between kafirin and zein dough systems. Overall the kafirin dough system displayed rheological properties sufficient for baking of porous bread. Kafirin like zein appears to have promising properties for making non-gluten leavened doughs.     

Surface rheological properties of bread dough components in relation to gas bubble stability

The surface rheological properties of dough (components) were determined in order to estimate the effect of these properties on disproportionation and coalescence of gas bubbles in bread dough. Three different systems were studied as a model for the gas-dough interface: a diluted aqueous dough dispersion, gluten and wheat lipids spread on water. The surface dilational modulus, E, and tanϑ of these systems were determined as a function of frequency using a modified Langmuir trough. Values of E and tanϑ found were: 35–100 mN/m and 0·7–0–2, resp., for dough dispersions, 20–45 mN/m and 0·4–0·15, resp., for gluten, and 20–90 mN/m and 1·3–0·1, resp., for lipids in the frequency range tested at room temperature. On the assumption that the gas-dough interface is comparable either to the surface of the dough dispersions tested or to a water surface with spread gluten, it was shown that disproportionation of gas bubbles in dough can be retarded but not prevented. Wheat lipids present in the right concentration in the surface can prevent this foam stabilising mechanism to a larger extent. The surface dilational modulus as well as the surface tension during continuous expansion of dough dispersions were also determined at 45°C. The surface dilational modulus of a dough dispersion at 45°C was 7–25 mN/m, which was approximately 5 times smaller than at room temperature. Results of surface tension measurements during continuous expansion in a Langmuir trough showed that values for surface tension were only slightly higher than at equilibrium (ca. 2 mN/m) at 45°C and at deformation rates of the surface comparable to those at oven rise. These results suggest that thin dough films at higher temperatures will be less stable than at room temperature. Implications in relation with coalescence in dough are discussed. No significant differences in surface rheological properties of dough dispersions of wheats with different bread-making qualities were found in the sinusoidal oscillation tests nor in the continuous expansion tests. Surface rheological properties, therefore, appear not to be the main factor responsible for differences in baking quality amongst different wheats.     

pH-, Temperature- and Time-dependent Activities of Endogenous Endo-β-D-Xylanase, β-D-Xylosidase and α-L-Arabinofuranosidase in Extracts from Ungerminated Rye (Secale cereale L.) Grain

Activities of endogenous β-D-xylosidase (EC.3.2.1.37), α-L-arabinofuranosidase (EC. 3.2.1.55) and endo-β-D-xylanase (EC.3.2.1.8) were quantified in extracts from ungerminated rye (Secale cereale Lvar. Amando) grain. pH- and temperature optimum and stability of the unpurified enzymes were examined. The activity of β-xylosidase and α-arabinofuranosidase against p-nitrophenyl-glycosides was 180 and 346 pkatal/g grain, respectively (pH 4·5; 30 °C) and that of the endo-xylanase against RBB-xylan was 11 pkatal/g grain (pH 4·5; 40 °C). The pH optimum of each of the enzymes was 4·5, which is similar to the pH of a rye dough. The temperature optima for the enzymes were 40 °C (endo-xylanase), 70 °C (β-xylosidase) and 60 °C (α-arabinofuranosidase), respectively. Sodium chloride (0·5–3·0%, w/v) had no effect on enzyme activities. The enzymes were relatively stable at pH 4·5 and room temperature for at least 24 h. The enzymes showed no significant decrease in activity when extracts were incubated at pH 4·5 and 30–40 °C for 80–120 min. Incubation at temperatures higher than 40 °C resulted in a significant decrease in activity. The rate of hydrolysis of the respective p -nitrophenyl glycosides and RBB-xylan was high under conditions typical for the rye dough production stages but decreased at temperatures typically encountered in the baking process.     

Improvement of zein dough characteristics using dilute organic acids

The replacement of gluten in dough products poses a major challenge. Preparing zein doughs in dilute acetic acid and lactic acid, such as produced during sourdough fermentation, was investigated. Increasing acid concentrations (0.7, 1.3 and 5.4% [v/v]) increased zein extensibility and reduced the stress and related parameters. Preparation of zein-maize starch/-rice doughs in dilute organic acids improved dough properties to the extent that the doughs could hold air and be inflated into a bubble by Alveography. Further, they exhibited similar Stability (P), Distensibility and deformation energy (W) to wheat flour dough. Confocal laser scanning microscopy revealed an ordered linear fibril network in zein and zein-rice flour doughs prepared in the dilute acids, which became uniform with increasing acid concentration. SDS-PAGE showed that the acids did not hydrolyse or polymerise the zein. FTIR indicated that the acidic conditions slightly increased the proportion of α-helical conformation in the zein doughs, possibly as a result of deamination. This conformational change may be responsible for the considerably improved zein dough properties. Zein doughs prepared in dilute organic acids show potential as a gluten replacement in gluten-free formulations.     

Dough/crumb transition during French bread baking

The modifications occurring during dough to crumb (D/C) transition of French bread (350 g) were studied in an instrumented pilot-scale oven for doughs with different contents of minor components, soluble, lipids and puroindolines. Internal temperature measurements showed that, for most compositions, complete D/C transition occurred between 55 and 70 °C, after 5 min of baking, and coincided with maximum loaf expansion. Differential scanning calorimetry (DSC) in excess of water performed on samples taken during baking (3 and 5 min) showed that starch gelatinization and melting developed continuously during D/C transition for various contents of the soluble fraction in dough. Dynamic thermomechanical analysis (DMA) on dough showed that dough stiffened between 60 and 70 °C, as seen by the increase of elastic modulus E′ by more than one decade, for all dough compositions. Relating these changes to the results of baking experiments, D/C transition was assigned first to gluten reticulation and, to a lesser extent, to continuous starch granule swelling.     

Improvement to baking quality of frozen bread dough by novel zein-based ice nucleation films

Freezing deteriorates the baking quality of frozen bread dough. This study revealed the protective effects of zein-based ice nucleation films (INFs) on the baking quality of frozen dough. INFs were prepared by immobilizing biogenic ice nucleators on the surface of zein films, which consequently revealed ice nucleation activity and increased the ice nucleation temperature of water from −15 °C to −6.7 °C. By using these films to wrap frozen dough during five freeze/thaw cycles, the specific volume of bread was increased by up to 25% compared to the bread from control frozen dough. The reason was attributed to 40% more viable yeast cells preserved by INFs. In addition, zein-based INFs also reduced the water loss by frozen dough resulting in higher water content in bread crumb. Combining the protective effects on both specific volume and water content from zein-based INFs, the obtained bread showed 68% lower firmness and fracturability and 2.4 times higher resilience compared to the control. The INFs were also superior in that for zein-based INFs, biogenic ice nucleators showed desirable affinity with the surface to sustain at least fifteen repetitive uses on freezing water.     

脱脂和重组对面团流变学特性的影响

选用3个筋力不同的小麦品种(系)为材料,采用脱脂面粉、重组面粉和原面粉相比较的方法,研究了小麦脂类对面粉主要基础品质指标、面团拉伸和粘度特性的影响。结果表明:(1)脱脂后面粉沉淀值、降落值下降,面粉白度提高;(2)脱脂后鲁麦14和01-35的面团最大拉伸阻力、拉伸面积和最大拉伸比变大,延伸度变小,但PH3259表现为相反的趋势;(3)与原面粉相比,脱脂后面团粘度变小,附着功减少,粘聚性下降;(4)重组面粉与原面粉面团流变学特性无明显差异。     

Understanding the link between GMP and dough: from glutenin particles in flour towards developed dough

Clear correlations exist for glutenin macropolymer (GMP) quantity and rheological properties vs. wheat quality and dough rheological properties, but real insight in understanding these links is still missing. The observation that GMP consists of glutenin particles opens up new possibilities to reveal the underlying mechanism linking glutenin network properties with dough preparation. GMP was isolated from flour of three wheat varieties: Estica, Soissons and Baldus, strongly varying in their mixing requirements (expressed as time-to-peak, TTP). Decrease of GMP quantity and G′ vs. mixing energy was confirmed. More detail was obtained by studying the changes in GMP particles when mixing flour into dough. Mixing leads to a decrease in the average size of the particles. Interestingly, the TTP coincided with the work-input at which all particles just became soluble in SDS. At TTP, the average size of the GMP particles was the same for each variety. During mixing particles lost their globule shapes and appeared ruptured. Particle size analysis confirmed that particles were still present near TTP. Analysis of the change in particle size vs. energy input using physical principles revealed the following: (1) mixing energy is the predominant actuator in decreasing GMP particle size; (2) the initial GMP particle size in flour strongly determines the practical mixing requirements; and (3) the derived mixing energy vs. GMP particle size relationship was shown to be applicable for both Mixograph and Farinograph mixing. Our results demonstrate that, for the flour samples used, glutenin particle size determines TTP and GMP rheology, showing that glutenin particle properties could be a new key to understand the link between GMP and dough properties.     

Similarities and differences in secondary structure of viscoelastic polymers of maize α-zein and wheat gluten proteins

The secondary structure of a dough-like zein polymer was compared to the structure present in a wheat viscoelastic system using FT-IR spectroscopy. When zein was mixed at 35 °C, which is above its glass transition temperature (T_g), changes in its secondary structure suggested that the protein loses its native structure, mainly composed of α-helices (68%), and a viscoelastic system is formed by a structural rearrangement that favors β-sheet structures. This rearrangement is very similar to the structural changes observed in gluten viscoelastic polymers. Upon removal of shear stress, the zein polymer showed a rapid decrease in the proportion of β-sheet structures (from 48% to 28% after the first 3 min) in favor of unordered structures. At the same time, the viscoelasticity of the polymer decreased rapidly. In contrast, gluten, in a similar viscoelastic system and held at the same temperature, showed a fairly constant high content of β-sheet structures (49%) coinciding with the slow relaxation time typical of gluten networks after the removal of shear. We speculate that the addition of a protein capable of causing extensive and stable β-sheet formation in the zein–starch viscoelastic polymer could increase the stability and relaxation time of the zein system and, thereby, create the possibility of a zein dough with similar functionality to a wheat viscoelastic system.     

Soaking of Maize Determines the Quality of Aflata for Kenkey Production

Aflata is a gelatinised maize paste, serving as intermediate in the manufacture of kenkey, a traditional cooked fermented maize product of Ghana. The effect of water uptake during soaking of whole or dry-milled maize, the extent of starch damage, dough pH, fermentation time, and of endogenous and added enzymes on pasting and set-back viscosities of aflata dough were studied. Water uptake by coarsely dry-milled maize (grits) reached 0·63 mL/g dry matter in just 1 h, compared with 0·50 mL/gin 3 days for whole grain. High endogenous activity of proteases and carbohydrases were recorded in both grits and whole maize when soaked at 4 °C or 25 °C. These were significantly reduced after soaking at 60 °C. Soaking of grits at 60 °C with a heat-stable protease, or wet fine-milling of fermented grits resulted in significant (P<0·05) increases in pasting viscosities. Peak viscosities increased with fermentation time up to 24 h. Pasting viscosities decreased with repeated wet milling of fermented dough.     

The composition of floury and vitreous endosperm affects starch digestibility kinetics of the whole maize kernel

Maize grain starch is the major energy source in animal nutrition, and its high digestion and utilization largely depend on endosperm traits and the structure of the starch-lipoprotein matrix. The aim of this work was to determine floury and vitreous endosperm traits and its relation to starch digestibility rate. In total, kernels of 30 hybrids were manually dissected, and amylose, total zein and starch and non-starch lipids were determined in both vitreous and floury endosperm. Starch digestibility of the whole kernel was determined based on glucose released during a two-step in vitro pig model of enzymatic digestion, and starch digestibility rate was calculated according to the first-order kinetics. The vitreous endosperm of tested hybrids had higher contents of amylose (204.6 vs 190.4 g/kg), zein (63.2 vs 40.4 k/kg) and starch lipids (5.6 vs 4.9 g/kg), and lower content of non-starch lipids (7.3 vs 9.6 g/kg) than floury endosperm. Digestibility coefficients varied among hybrids, and starch digestibility rate varied from 0.73 to 1.63 1/h. Lipids in both vitreous and floury endosperm negatively correlated with the most of digestion coefficients, whereas zein correlated in vitreous and amylose in the floury endosperm (P < 0.05). Starch digestibility rate negatively correlated with all traits, except amylose content in vitreous endosperm. As a result, a linear regression model with four variables including contents of zein and starch lipids in vitreous and zein and amylose in floury endosperm can predict more than 65% variability of starch digestibility rate of tested hybrids.     

Gliadin and zein show similar and improved rheological behavior when mixed with high molecular weight glutenin

Small amplitude and lubricated squeezing flow tests were performed to investigate the effect of high molecular weight glutenin (HMWG) on the rheological properties of gliadin and zein dough composites. It was hypothesized that addition of small amounts of HMWG to zein cause changes in its viscoelastic properties in the same way as its addition to gliadin. Starch (87%, w/w) and protein/protein composites (13%, w/w), and water were mixed into dough. The water content of prepared dough was in the 41.7–45.3% range. Composites were gliadin-HMWG and zein-HMWG. Phase angle and complex modulus (G*) were obtained from frequency sweep tests at 0.5% strain amplitude over a 0.01–100 rad/s frequencies. Lubricated squeezing flow test were carried out at a range of strain rates to determine the sample extensional viscosities.