The role of wheat, rye and oat dough aqueous phase constituents in bread making

Bread is an important staple food around the globe. In Europe and other parts of the world, it is most often made from wheat (Triticum aestivum L.) flour and mainly with a straight-dough process. This process starts by mixing flour, water, yeast, salt, and potentially a number of non-essential ingredients into viscoelastic dough. The dough is then fermented, which results in gas cell expansion and thus in an increased dough volume. Finally, the leavened dough is baked and the resultant bread cooled to room temperature. The loaf volume and crumb characteristics of bread are important quality characteristics which largely depend on the amount of gas cells incorporated during mixing and the degree to which they are stabilized throughout the bread making process. In wheat bread making, hydrated gluten proteins develop into a continuous, viscoelastic network which in the early stages of fermentation provides structural support to expanding gas cells and thereby stabilizes them. It has been suggested that this network fails to surround some areas of gas cells during the late stages of fermentation and early stages of baking as it ruptures as a result of dough expansion. From this moment onwards, proteins, surface-active lipids, and non-starch polysaccharides (NSPs) dissolved in a liquid film surrounding the gas cells supposedly take over their stabilization. These liquid films are believed to be part of the aqueous phase of dough. At least a fraction of this phase can be isolated from dough by ultracentrifugation. The supernatant obtained in this way is generally referred to as 'dough liquor' (DL). People today are aware of the potential health benefits of consuming mixed cereal breads. Indeed, partial replacement of wheat by for example rye (Secale cereale L.) or oat (Avena sativa L.) flour can increase bread dietary fiber and lysine (i.e. an essential amino acid) contents. However, mixed cereal breads are often of lower quality in terms of loaf volume and crumb structure than wheat breads because non-wheat cereals lack the typical wheat gluten proteins. Hence, it can be argued that the mechanism of gas cell stabilization by liquid films may be even more important in mixed cereal or non-wheat bread than in wheat bread making. To the best of our knowledge, no research has been conducted in this regard. Indeed, all studies available in literature today have focused on studying the chemical composition or functional properties of DL isolated from wheat dough. Thus, the potential of soluble constituents of non-wheat flour to stabilize gas cells in bread making has not yet been investigated, let alone that it would have been exploited. Against this background, the work in this dissertation was executed with the aim to explore the potential of soluble rye and oat flour constituents to stabilize gas cells in bread making. The work plan relied heavily on the use of DL as a model for the dough aqueous phase. In a first part, relations between (i) the chemical composition and (ii) the foaming and air-water (A-W) interfacial characteristics of wheat, rye, and oat DLs were established and hypotheses on the composition of DL stabilized A-W interfaces were brought forward. Wheat DL constituents produced a low amount of unstable foam. This was attributed to a low bulk phase viscosity and to them slowly developing a strongly viscoelastic mixed protein-lipid film at the A-W interface. In contrast, stirring rye DL solutions generated high volumes of foam ofpoor stability. The high initial foam volume was ascribed to a combined effect of a high bulk phase viscosity and a rapid formation of a strong predominantly viscous protein-dominated film at the A-W interface. The low initial foam volume produced from oat DL constituents was the result of lipids being the dominant constituents at oat DL stabilized A-W interfaces. This was deduced from a high total lipid content, very low surface tension, and absence of a viscoelastic film at the A-W interface of oat DL. As protein- or lipid-dominated A-W interfacial films are more resistant to deformations than mixed protein-lipid A-W interfacial films, rye and oat DL constituents seem to have more potential for stabilizing A-W interfaces than wheat DL constituents. In a second part, the hypotheses on the composition of the A-W interfaces stabilized by wheat, rye and oat DLs were tested and further refined by using DL modification strategies. First, the role of surface-active lipids in interfacial stabilization was studied by comparing the A-W interfacial properties of control and defatted wheat, rye, and oat DLs. Second, the role of NSPs was assessed by enzymatic depolymerization prior to studying DL bulk viscosity and A-W interfacial properties. Third, both treatments were combined to assess the extent to which the ability of DL NSPs to affect interfacial stability depends on the presence of lipids at the A-W interface. It was observed that NSPs contribute substantially to the bulk viscosity of wheat, rye, and oat DLs and thus likely also to the bulk viscosity of the aqueous phase in their respective doughs. In addition, it was established that by adsorbing at wheat and rye DL stabilized A-W interfaces lipids impair mutual interaction between adsorbed proteins. Surface tension measurements of control and defatted oat DL samples confirmed that lipids are the predominant DL constituent at oat DL stabilized A-W interfaces. Finally, irrespective of whether or not lipids were present at the A-W interface, wheat and rye DL arabinoxylan exerted a film weakening and strengthening effect respectively. This demonstrates that interaction between arabinoxylan and proteins at A-W interfaces in some but not all cases may improve their resistance to deformations. That proteins did not seem to be present at oat DL stabilized A-W interfaces supports the observation that oat DL β-D-glucan neither weakened nor strengthened the A-W interfacial film. Thus, wheat and rye DL stabilized A-W interfaces are composed of a mixed protein-lipid film with arabinoxylan acting as secondary layer, whilst a lipid film is present at oat DL stabilized A-W interfaces. In a third part, the composition of wheat, rye, and oat DLs and the A-W interfacial properties of their constituents were related to the loaf volume and crumb structure of breads prepared from their respective flours. In terms of loaf volume, wheat bread had a high specific volume despite the poor foaming and A-W interfacial properties of wheat DL constituents. This was of course mostly due to the viscoelastic gluten network which by displaying strain hardening acted as the primary gas cell stabilizing entity. In contrast, even though rye and oat DL constituents seemed to have more potential for stabilizing A-W interfaces than wheat DL constituents, the volumes of rye and oat bread loaves were much lower than that of wheat bread. Thus, assuming that rye and oat dough aqueous phase constituents contribute to gas cell stability in rye and oat bread making, they cannot match the efficiency of the combined contributions of the gluten network and dough aqueous phase constituents in terms of stabilizing gas cells in wheat bread making. However, in terms of crumb structure more gas cells per surface unit were observed in rye than in wheat and oat bread crumbs. Bread making experiments in which a xylanase preferentially hydrolyzing the water-extractable arabinoxylan population of rye flour was used revealed that arabinoxylan contributes substantially to the fine grained crumb of rye bread. Indeed, arabinoxylan enzymatic hydrolysis resulted in rye bread crumbs with considerably larger mean gas cell areas and lower numbers of cells per surface unit than was the case for control rye bread. This implies that rye flour arabinoxylan delays gas cell coalescence during rye bread making presumably because of its contribution to the bulk viscosity of the dough aqueous phase. To further assess the contribution of DL constituents to bread loaf volume, breads were prepared from doughs containing blends of commercial wheat gluten and commercial wheat starch, with and without addition of wheat, rye, or oat DL constituents. Overall it was observed that wheat, rye, and oat DL constituents result in a pronounced increase in the volume of such model breads. This implies that not only wheat gluten proteins but also DL constituents contribute to gas cell incorporation and/or stabilization in bread making. However, it should be mentioned that the addition of DL constituents likely changed the bulk rheology of the model doughs which in turn may have contributed to the above mentioned bread volume increase. Notable was that the addition of wheat DL constituents resulted in the most pronounced bread volume increase. This did not match our expectations based on the foaming and A-W interfacial characteristics of wheat, rye, and oat DLs. Thus, the mechanism by which DL constituents contribute to gas cell stabilization in bread making remained unclear at this point. In this context, it is important that stability of gas cells is not only determined by the characteristics of the interfaces surrounding them, but also by those of the liquid films between them. Moreover, A-W interfacial properties can often only be studied at concentrations lower than that found in the supernatant after ultracentrifugation (i.e. the 'native concentration'). Therefore, to better understand the role of dough aqueous phase constituents in bread making, in a fourth part the drainage dynamics of free-standing DL thin films (both at lower and at native concentrations) were assessed. Comparison of the drainage times and interferometry images of DL thin films at lower and native bulk concentrations demonstrated that the DL bulk concentration has a drastic impact on the structure and stability of the obtained thin films. Whereas protein aggregates dispersed in mixed protein-lipid A-W interfacial films were characteristic of wheat DL thin films at low bulk concentrations, lipids were the dominant constituent at A-W interfaces of wheat DL thin films at their native concentration. Moreover, they stabilized it by diffusing along the A-W interfaces and thus by exerting Marangoni-type effects. Lipids also stabilized oat DL thin film A-W interfaces both at low and at their native bulk concentrations by exerting Marangoni effects and presumably by forming an immobile monolayer, respectively. In addition, wheat and oat DL thin films at their native concentrations exhibited stratification. This essentially means that the thin films were made up of stacked layers of supramolecular structures, in this case likely lipid micelles. If at least two of such layers are present, the layered structuring provides thin films with an additional degree of stability as it increases their disjoining pressure. Furthermore, protein aggregates in rye DL thin films at low bulk concentrations were surrounded by a relatively thick film. In addition, adsorbed proteins contributed to thin film stability by exerting steric and/or electrostatic repulsive protein-protein interactions. Finally, A-W interfaces of DL thin films at low bulk concentrations merged rapidly after drainage was forcibly induced, whilst DL thin films at their native concentrations were stable for up to at least three minutes of monitoring. This most important observation implies that DL constituents may contribute to the stability of gas cells in both wheat and non-wheat bread making. In conclusion, in this doctoral dissertation it was demonstrated that soluble wheat, rye, and oat flour constituents seem to have great potential for stabilizing gas cells in bread making. That wheat and oat DL thin films at their native concentrations had excellent stabilities combined with the observation that wheat, rye, and oat DLs increased the volume of model breads implies that gas cell stabilization by dough aqueous phase constituents is of importance both in wheat and non-wheat doughs. However, the volumes of rye and oat bread loaves were still much lower than that of wheat bread. This illustrates that the loaf volume of bread depends on the combined contributions of gluten proteins and of dough aqueous phase constituents.status: publishe