Evaluation of the Crispness of Porous Foods Using the Acoustic Emission Technique

The acoustic emission (AE) technique was used to develop a method for the objective evaluation of the crispness of porous foods. The crispness of six types of commercial porous foods was changed by controlling their moisture contents at six levels by varying the humidity (i.e., dry, 22, 43, 56, 68, and 81 % relative humidity) of the storage environment at 25 degrees C. As the crispness of a sample with high water activity (a(w)) was expected to deteriorate, the physicochemical, mechanical, and acoustic characteristics of a(w)-controlled food samples were evaluated. The a(w) had no influence on the shape and porous structure of the foods. However, the number of force peaks in the uniaxial compression test appeared to be related to their a(w). Moreover, the breaking characteristics of a sample at different a(w) values changed depending on the porous food type classified according to the degree of starch gelatinization. Among the AE characteristics, the number of AE events and AE count corresponded well with the a(w) of the porous food. Therefore, for an objective evaluation of the crispness of porous foods, the AE events and AE count, which are highly sensitive, can be suitable indicators, rather than the number of force peaks.