Impact of egg white and soy proteins on structure formation and crumb firming in gluten-free breads

Regular or thermally treated egg white powder or soy protein isolate were included in the recipes of gluten-free (GF) breads based on rice flour (RF) or a mixture of potato and cassava starches (CS-PS) to improve bread texture and structure. Electrical resistance oven heating was combined with in-line measurements of batter height, viscosity and carbon dioxide release during fermentation and baking. Crumb firming during storage (6 days) of the different breads was studied by texture, moisture content, amylopectin (AP) crystal melting and proton nuclear magnetic resonance analyses. Drop shape tensiometry showed that both egg white types are highly surface active. Their use resulted in high gas cell stability and ultimately in high volume breads with fine crumb structure. Batters containing soy protein isolate, which had a lower surface-activity, had low stability. This resulted in low volume breads with inhomogeneous crumb structure. During the first 24 h of storage, the crumb firmness increase was mainly caused by AP retrogradation and later on by crumb-to-crust moisture migration. Crumb firming was less pronounced in CS-PS than in RF breads. Egg white decreased the extent of crumb firming while soy protein increased it. Most notably, differences in crumb firming between samples were largely related to differences in initial bread specific volume and crumb structure rather than to differences in AP retrogradation or moisture migration. Thus, inclusion of ingredients which improve bread volume and crumb structure in GF bread recipes may also strongly improve the shelf-life of such breads.