Consumer appreciation of brittle cellular foods, like bread crusts, depends on textural properties such as crispness. This crispy character is lost above a certain water activity. It is not known what exactly is happening in these crusts when water enters. So is it unclear whether it is the change in the starch or the gluten that initiates the loss of crispness with ageing time. In this paper the effect of water on the glass transition of model bread crusts was studied using two complementary techniques: phase transition analysis (PTA) and temperature modulated differential scanning calorimetry (TMDSC). The mobility of water was studied with nuclear magnetic resonance (NMR). The results were compared with sensory data. Bread crusts prepared with different types of flour were tested to evaluate the effect of flour composition on the crispness of model crusts equilibrated at different relative humidities. In addition the single flour components starch and gluten were studied. Sensory crispness scores decreased with increasing aw from 0.55 upwards. At aw 0.70 sensory crispness was completely lost. Both DSC and PTA showed a transition point at an aw of 0.70-0.75. NMR gave a transition point in the mobility of the protons of water at aw 0.58. This supports the hypothesis that loss of crispness starts as a result of processes at a molecular level, before the macroscopic glass transition. This also suggests that the presence of water that is not directly attached to the solid matrix causes the loss of crispness at low aw. At higher aw increased mobility of the macromolecules will start to play a role. NMR experiments with the separate flour components indicate that the T2 transition point in starch samples occurs at a lower RH than for gluten. This could imply that starch loses crispness at lower aw than gluten. Increased mobility of small components and side chains might induce increased energy dissipation upon deformation of the material resulting in less available energy for fracture propagation and with that in a less crispy product.
- glass-transition temperature
- pulsed h-1-nmr