Gluten-free bread baked under reduced pressure characterized by TD-NMR

Abstract

International audienceBased on spin-spin T2 relaxation time measurements, the time-domain NMR (TD-NMR) spectroscopy has been used to provide relevant information on the water and biopolymer motion and transfer in bread [1]. This technique permits to characterize molecular interaction and transformations in a non-invasive and non-destructive way, in real time during a process (heating, freezing, hydration ...). In bread, proteins of gluten when hydrated form a viscous mass that confers to the dough, structure, viscosity, mixing tolerance and gas holding ability [2]. On the other hand, starch, in presence of water and increasing temperature, undergoes a series of changes known as swelling, gelatinization and retrogradation that induce variations in water distribution, in starch structure and interactions between them [3]. This study aimed at understanding and ranking the contribution of these biochemical transformations that contribute to the crumb structure and the textural properties of bread prepared with a gluten free mix (Schär). The water transfers and the extent of starch gelatinization in crumb were studied by TD-NMR after the heating/cooling process of dough hydrated at 55% and 48% (wet basis). Two baking processes were compared, one at the atmospheric pressure while the other was carried out at reduced pressure (-20 kPa). Bread baking using partial vacuum results in greater oven-rise and greater gas fraction in the crumb, giving an increased softness of the crumb for a more pleasant mouthfeel. Under reduced pressure, the boiling point of water decreases but, until now, no study was conducted to check if this baking condition modifies or not the starch gelatinization and protein denaturation. By comparing rheological measurements (modulus of elasticity using a compression stress relaxation experiment) with TD-NMR data, it was shown that the crumb softness was mostly driven by the gas fraction while the biochemical changes (starch gelatinization, protein denaturation), monitored by TD-NMR, were little modified when dough was baked under partial vacuum