Thin film drainage dynamics of wheat and rye dough liquors and oat batter liquor

Abstract

To obtain bread with a high loaf volume and homogeneous crumb, gas cells require sufficient stability until the discontinuous gas structure of dough is transformed into the continuous structure of bread. At the end of fermentation, dough gas cells are supposedly regionally separated by two air-water (A-W) interfaces and a liquid film. ‘Dough liquor’ (DL) or ‘batter liquor’ (BL), i.e. the supernatant after ultracentrifugation of dough and batter, respectively, is a model system for the fluid in these films. We report here on drainage dynamics of free-standing thin films (TFs) prepared from wheat and rye DLs and oat BL after applying a driving pressure of either 50 or 200 Pa at bulk concentrations lower than and equal to (‘native’) those in the respective supernatants obtained by ultracentrifugation. At lowered bulk concentrations, wheat and rye DL TFs were surrounded by planar and dimpled A-W interfaces, respectively, both of which were composed of weakly viscoelastic mixed protein-lipid films. In contrast, lipids stabilized dimpled oat BL TFs by migrating along their A-W interfaces and thus by generating Marangoni stresses. Liquid drainage and subsequent film rupture occurred rapidly in these TFs. At native bulk concentrations, TFs generally drained much slower and they were even found to be stable at small thicknesses (<100 nm). Here, lipids stabilized dimpled wheat DL and oat BL TF A-W interfaces by Marangoni effects or by forming a densely-packed layer, respectively. In addition, wheat DL and oat BL TFs at their native concentrations exhibited stratification due to layered structuring of micelles, which contributed to TF stability by exerting oscillatory structural forces. Thus, it can here be concluded that wheat DL and oat BL constituents at their native concentrations may contribute substantially to stabilizing gas cells in dough.