Abstract
Staling of two commercial breads was studied by mechanical tensile testing, transfer damping function vibrator testing and Kundt's impedance tube sound absorption measurements. The staling process was found to be affected by the porosity as well as the elasticity and rigidity of the cellular structure of the studied breads. Thermal analysis revealed two successive thermal events in the bread during heating, i.e., moisture liberation accompanying gelatinization of starch components and thermal degradation of carbohydrate constituents. A linear or exponential growth increase of the stiffness of the bread matrix with staling time was apparent from the Young's modulus dependences. Concurrently, the transfer damping function dependence indicated higher stiffness, as confirmed by evolution of the first resonance frequency peak after staling. The primary sound absorption peak frequency position shifted from 900 Hz to 1070 Hz. Simultaneously the sound absorption coefficient increased from 0.72 to 1.0 after 336 h of staling.
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