Sound generated during eating of apples plays important role in its texture evaluation by consumers. In a plant tissue, a cellular skeleton which is responsible for certain mechanical properties, is created by bonded cells. The way of cracking of such structure, apart of sound properties i.e. crispness and crunchiness sense, determines also juiciness and mealiness. Therefore, knowledge about mechanism of cracking and physical laws governing this process is very important for further work on monitoring and improvement of apples quality and other fruits and vegetables.

The aim of this work was development a new method based on acoustic emission for evaluation of selected texture attributes of apples. In order to do this, a complex experiment was performed with using of new system for acoustic emission signal recording in three mechanical tests: puncture test, texture profile analysis (TPA), single edge notched bend (SENB). The experiment was performed on three cultivars of apples: Elstar, Gloster and Jonagold with taking into account storage way and storage time. A correlation between results of the instrumental evaluation and sensory analysis was found also. Additionally, an influence of turgor and temperature on cracking development during mechanical testing was investigated.

In the work an original and new methodological solution based on acoustic emission method was proposed for instrumental texture evaluation and for cracking processes analysis during different mechanical testing for apples. An approach of this research was focusing on practical aspects of apple texture evaluation by the acoustic emission method. The goal for the future is starting work on new device development for instrumental texture evaluation of apples. The results of this experiment have provided many methodological and practical conclusions.

The experiment has allowed to conclude that for analysis of apple tissue sound properties during mechanical testing its is enough to record acoustic emission signal in the 1-16 kHz frequency range. The sources of acoustic emission are cell walls ruptured in the process of deformation, cell debonding does not generate the acoustic signal. Analysis of the number of acoustic events during testing provides information about the number of ruptured cell walls. Values of the acoustic emission descriptors depends on pectin degradation level (as a result of storage or temperature) and on turgor level which have an influence on cracking mode of the tissue. With turgor increase the number of ruptured cell walls per deformation unit also increase, the tissue become more “brittle”. Temperature increase causes significant decrease of values of acoustic emission descriptors in each mechanical test as a result of change of cracking mode from cell wall rupturing to cell-cell debonding which is result of pectin degradation. It was noticed that during storage the number of acoustic events decreases which is caused by change of cracking mode, as a result of enzymatic pectin degradation, from cell wall rupturing to cell-cell debonding thereby decrease of number of ruptured cells. During storage, decrease of the mean acoustic emission amplitude is observed in the mechanical tests which is caused by decrease of the modulus of elasticity (increase of damping of the elastic wave) as a result of weakening of cell-cell adhesion and probably as a result of small decrease of the cell walls strength.

There are strong correlations among sensory attributes. Especially high correlation coefficients between attributes related to cracking processes (like juiciness and mealiness) and sound loudness detected by panelists (like crispness and crunchiness) were found. The moist important attributed for overall texture evaluation was juiciness, crispness and crunchiness.

In order to evaluate apple texture instrumentally, some of the mechanical parameters (from the puncture, TPA or SENB test) can be used as well. However, acoustic emission descriptors correlate better with sensory evaluation than the mechanical one. Therefore, acoustic mission provides more precise and direct information obtained instrumentally about apple texture. The number of acoustic emission events and the mean acoustic emission amplitude in any mechanical test can be used for evaluation juiciness, crispness and crunchiness which are related to cracking processes of the tissue. It was stated that the texture evaluation by the acoustic emission can be done independently on cultivar and one calibration curie can be used only.

In summary, it can be stated that a new method was developed. The method, using parameters of acoustic signal generated during mechanical testing. The method allows more precisely than the mechanical test only describing cracking processes in the material and allows monitoring of texture change of apples, especially attributes related to sound generated in during cracking.