Thermodynamic aspects of dough formation and functionality

Abstract

Thermodynamic and microrheological approaches providing information on mechanisms involved in dough formation and functionality are proposed. The phase behaviour of biopolymer mixtures and excluded volume effects of macromolecules are critical factors influencing the functionality of dough. Milk proteins and flour proteins have similar structures and functionalities, therefore skimmed milk protein—polysaccharide systems can be used to model wheat dough. During mixing of flour with water, albumins, globulins, water-soluble starch (from damaged starch granules) and pentosans form a liquid aqueous phase. This is immiscible with glutelins and gliadins which form a separated gluten phase. Aggregation of the gel particles of the gluten phase minimizes contact with the non-wetted liquid phase and results in the formation of dough structure. At first, bread dough contains two continuous protein phases: a gluten thixotropic gel phase and a liquid phase. Phase equilibrium of the co-existing phases is controlled by mechanical treatment of the dough and by dough additives and ingredients, such as salt, sugars, lipids, surfactants and alcohol produced by fermentation. Mechanical treatment greatly affects the structure—property relationship of doughs by: establishing an equilibrium between the co-existing phases; transformation of the continuous liquid protein—polysaccharide phase into the dispersed phase; orientation of polypeptide chains of the gluten phase, intensification of their hydrophobic interactions and formation of hydrophobic structural domains adsorbing lipids. Basic mixing of doughs includes: (i) deformation and breaking down of the liquid and gas dispersed particles; (ii) spinneretless spinning of gluten strips formed between oriented capillary-like particles of the liquid and gas phases; (Hi) decreasing the size of liquid, gas dispersed particles and the thickness of gluten strips; (iv) revolving starch granules in shear flow providing high fluidity of doughs due to a ‘ball-bearing’ effect; and (v) migration of starch granules towards a higher shearing gradient providing a decrease in water content of the central layers and the formation of ‘starch-empty’ surface layers of lower stickiness. Starch gelatinization upon heating results in dewatering of the protein phases and fixation of structure, shape and volume of the loaf A large body of experimental evidence supports the mechanisms of dough formation and functionality proposed here.