Bread staling: A calorimetric approach

Simple recipe breads with different water contents were allowed to stale in well-defined conditions. Bread crumb was investigated using differential scanning calorimetry, thermogravimetry analysis (TGA), and stress-strain determinations. Calorimetric investigations extended to subambient temperature allowed an exothermic signal to be recognized just about room temperature that appeared partially reversible on repeated heating-cooling cycles across the -10 to 35 degrees C range. The core spending thermal effect was maximum after aging 8-10 hr. According to the TGA investigations, the release of water on heating revealed two main binding states: water-1 and water-2. The relevant fractions were bread-age dependent; water-1 reached a minimum after aging 8-10 hr at room temperature, while the overall water content remained practically unchanged. These findings suggested a model for the extension of a crosslink network throughout the bread crumb. Water molecules would be displaced along polymer chains acting as sliders of an interchain zipper. The consequent direct interchain crosslinks would allow formation of a network that would justify the increasing firmness of the crumb. The same mechanism would also sustain the growth of amylopectin crystals. Accordingly, the observed correlation between starch retrogradation (evaluated from the endothermic effect of amylopectin fusion) and increased crumb firmness should be reconsidered in the frame of a more general picture where water molecules play a key role in the definition of the product structure.